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01/11/03 RECOMMENDED STAPLERS

What staplers and staples are recommended for rib asssembly?
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One of our builders, now completed, George Twente used an electric stapler to assemble his ribs. I am pretty sure it was a Duo-fast brand that he found at a local distributor. The staple size you want is 20-22ga. wire with 3/16" to 1/4" crown (width across the top of staple). Lengths are 1/4", 3/8", and 1/2".

Otherwise, we recommend a Senco SJS-A stapler or a Duo-Fast 3400 series unit. We use 1/4", 3/8", 5/8" long staples (A04, A06 & D10) in building the wings of a Model 12. See Staple Chart KK

12/22/02 PLYWOOD TYPES SPECIFIED IN PLANS

I've been going through the plans wing section to cost out the materials and have a question. Regarding the material description for the plywood used in the wing ribs and wing assembly, it seems that the descriptions for mahogany and birch plywood are reversed on the plans. For example:

Rib gussets are listed as .063 plywood Nose ribs are .25 mah (mahogany I assume) plywood, reinforcement plates (12-420-28, -34, -35, -36, -37) are listed as .25 mah 3 ply.

I presume the thin pieces like rib gussets and sheer skins are .063 mahogany plywood while thicker pieces like nose ribs and structural parts like reinforcement plates are .25 birch plywood.

Also, its not mentioned in the plans, but what about Okoume ply? - Dave
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Dave, the plans were created with all Mahogany ply in mind. Mah ply is the most common for wood wing construction as it has been from the 1930's on. Mah is lighter than the same thickness birch, accepts glue into its grain better, etc., as compared to birch. However, it is not as hard as birch. For this reason in our HP wing kits, we supply birch crushplates and Mah rib gussets. Nose ribs can be made of either as you see fit. Typically, birch ply is accepted as interchangeable for specific mah ply in wooden aircraft wings such as these where it is used for gussets, crush plates, etc. The main reason most people use birch instead of mah is the lower cost of birch. The penalty is weight but a benefit is added toughness.

As I mentioned above, as you look at photos of our HP wing wood kit, you will see Mah rib gussets and webs while the crush plates are all birch. Please note that the spars Must be Douglas Fir and cannot be Sitka spruce.

Okoume ply is neat stuff. It does not meet the mil spec for aircraft ply however. We use 1/8" for the skins on the McCullocoupe wings. It weighs about the same as 1/16" birch aircraft ply yet makes nicer skins. This is the ply used on lots of acro wings for the skins.

Okoume is not a substitute for structural ply in your wings. It can be used in wings if the wings are designed for it like in a one design and the McCullocoupe for skins. It should not be used for crush plates or gussets etc. You would not want to use it for aileron nose ribs as you need the weight in the nose anyway and the lighter weight of the okoume would be a negative. KK

12/16/02 AILERON BALANCE

I have one of your HP wing kits with the aluminum ailerons and would like to know if I need to balance them?
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Nope, we fully balance ALL of our factory built model 12 aileron assemblies PRIOR TO SHIPMENT to our customers. The ailerons have 1/2" x .049 4130 steel tube in the aileron which is fully filled with lead. This is just enough lead and steel to balance the aileron SYSTEM. Note I wrote aileron system not aileron. The mass we use will work for the heaviest of cover jobs while still being good for a light cover job too. This way, our builders do not have to go through the following balancing process we used to determine the balance of our ailerons:

Build the ailerons with empty steel tubes in them.
Build the slave struts.
Cover, dope and paint the ailerons.
Paint the slave struts.
Weigh the slave struts including end fittings and bolts. Call this "Ms" for Mass of strut. Determine the value of 1/2Ms. In other words, what is half the mass of the slave strut. Weigh the strut assembly with bolts, end fork/bearing etc and divide by two.
Install the aileron on a set of bearing points on its hinges. Easiest way to do this is to install it in a wing that is on saw horses. It will hang trailing edge low.
Tie a string to the slave strut attach arm on the now mounted aileron and hang a mass equal to 1/2Ms on the string. This assigns half the weight of the slave strut to the aileron at the point where the strut attaches to it. The other half will be assigned to the aileron that connects to the other end of the strut.
Now, get a bean can and punch a couple of holes thru it near the top such that you can hang it from a string. Attach that string to the aileron such that it hangs over the leading edge of the aileron and the can dangles below under the leading edge of the aileron a few inches. Add lead to the can until the chord line of the aileron become parallel to the floor (assuming your floor is flat).
Remove the string/can/lead assembly and weigh it. This weight is the amount of lead you need to ad to the tube. Obviously, you have to add more lead than that which is in the can to account for the mass of the string and can.

08/04/02 VARNISH, GLUE & EPOXY

I have been asked a few questions about the different types of varnish and glues available, and what we use on the Model 12's we build. So here goes:

First, all wood should be sealed with a varnish of some type. See AC 43.13 chapter 1 for more details. Epoxy varnish or other epoxy sealing coating have become standard since the introduction of Stits and urethane covering systems. In the cotton and dope days, ordinary 1 part spar varnish was all that was needed and used for sealing aircraft wood structures. But, with the newer "chemically hotter" covering system coatings, 1 part varnishes can be attacked causing them to lift and expose the wood to mosture. Also, if you use Polytak fabric cement to attach the fabric to a 1 part varnishcoated part, the varnish will wrinkle up like you used paint stripper on it. Then your fabric is bonded to loose wrinkled varnish instead of being soundly attached to the wood structure.

Another common old practice was to varnish all the wood structure with varnish except the areas that touch the fabric like ply leading edges and walkways. These areas were sealed with a few coats of dope. This too was done to avoid incompatability between the varnish and the fabric system being used. Today, we coat all wood with 2 part epoxy varnish such as Polyfiber EV400 or System Three Clear Coat. We prefer the EV400 as we use the polyfiber fabric system and it is part of that approved set of substraits.

It is interesting to note that in 43.13-1A, two wood glues were approved. Resourcinol and plastic resin glue. The plastic resin glue is commonly know as weldwood and is a light brown powder mixed with water. This is the glue used in just about all wood aircraft structures from the late 1940s to current time and it is part of thousands of approved production methods such as propeller lamination, wings, etc. Both plastic resin and resorcinol glues require PERFECT fits of the 2 parts being assembled as they have no gap filling abilities. Clamp pressures are much more critical as well. The requirement for super nice fits is usually not fully met resulting in joints that are not as strung as the parent wood. I have not done a test with a water soaked epoxy joint and a poorly fitted resorcinol joint to see which fails first but I'd bet that the epoxy joint would win the strength test.

Some of the confusion in wood glues comes from the new 43.13-1B. There are many glaring errors in the latest release of this publication as compared to the -1A release. Errors in recommended welding filler rod, fabric system usage, wood structures are the ones that affect us. The welding section was changed and now states that 4130 rod is to be used on 4130 aircraft structures....WRONG! They ignored the data in the mil specs for such structural assembly. Plastic resin glue was lumped into the same undesirable catagory as the old Casine glues used from the 20s thru WWII. In -1A, casine glue was not recommended as it does break down over time and exposure.

-1B states:

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1-4. ADHESIVES. Because of the critical role played by adhesives in aircraft structure, the mechanic must employ only those types of adhesives that meet all of the performance requirements necessary for use in certificated civil aircraft. Use each product strictly in accordance with the aircraft and adhesive manufacturer's instructions.

a. Adhesives acceptable to the FAA can be identified in the following ways:

(1) Refer to the aircraft maintenance or repair manual for specific instructions on acceptable adhesive selection for use on that type aircraft.

(2) Adhesives meeting the requirements of a Military Specification (Mil Spec), Aerospace Material Specification (AMS), or Technical Standard Order (TSO) for wooden aircraft structures are satisfactory providing they are found to be compatible with existing structural materials in the aircraft and the fabrication methods to be used in the repair.

b. Common types of adhesives that are or have been used in aircraft structure fall into two general groups: casein and synthetic-resins. Adhesive technology continues to evolve, and new types (meeting the requirements of paragraph 1-4a) may become available in the future.

(1) Casein adhesive performance is generally inferior to other available products. These adhesives should be considered obsolete for all repairs.

CAUTION: Casein adhesive deteriorates over the years after exposure to moisture in the air and temperature variations. Some modern adhesives are incompatible with casein adhesive. If a joint that has previously been bonded with casein is to be rebonded with another type adhesive, all traces of the casein must be scraped off before the new adhesive is applied. If any casein adhesive is left, residual alkalinity may cause the new adhesive to fail to cure properly.

(2) Synthetic-resin adhesives comprise a broad family which includes plastic resin glue, resorcinol, hot-pressed Phenol, and epoxy.

(3) Plastic resin glue (urea-formaldehyde resin glue) has been used in wood aircraft for many years. Caution should be used due to possible rapid deterioration (more rapidly than wood) of plastic resin glue in hot, moist environments and under cyclic swell-shrink stress. For these reasons, urea-formaldehyde should be considered obsolete for all repairs. Any proposed use of this type adhesive should be discussed with the appropriate FAA office prior to using on certificated aircraft.

(4) Resorcinol adhesive (resorcinol-formaldehyde resin) is a two-part synthetic resin adhesive consisting of resin and a hardener. Resorcinol is widely used in wooden aircraft structure and fully meets necessary strength and durability requirements. The appropriate amount of hardener (per manufacturer's instruction) is added to the resin, and it is stirred until it is uniformly mixed; the adhesive is now ready for immediate use. Quality of fit and proper clamping pressure are both critical to the achievement of full joint strength. The adhesive bond lines must be very thin and uniform in order to achieve full joint strength.

CAUTION: Read and observe material safety data. Be sure to follow the manufacturer's instructions regarding mixing, open assembly and close assembly times, and usable temperature ranges.

(5) Phenol-formaldehyde adhesive is commonly used in the manufacturing of aircraft grade plywood. This product is cured at elevated temperature and pressure; therefore, it is not practical for use in structural repair.

(6) Epoxy adhesives are a two part synthetic resin product, and are acceptable providing they meet the requirements of paragraph 1-4a. Many new epoxy resin systems appear to have excellent working properties. They have been found to be much less critical of joint quality and clamping pressure. They penetrate well into wood and plywood. However, joint durability in the presence of elevated temperature or moisture is inadequate in many epoxies. The epoxy adhesives generally consist of a resin and a hardener that are mixed together in the proportions specified by the manufacturer. Depending on the type of epoxy, pot life may vary from a few minutes to an hour. Cure times vary between products.

CAUTION: Some epoxies may have unacceptable thermal or other hidden characteristics not obvious in a shop test. It is essential that only those products meeting the requirements of paragraph 1-4a be used in aircraft repair. Do not vary the resin-to-hardener ratio in an attempt to alter the cure time. Strength, thermal, and chemical resistance will be adversely affected. Read and observe material safety data. Be sure to follow the adhesive manufacturer's instructions regarding mixing, open and closed curing time, and usable temperature ranges.
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West systems and T88 were both developed for the marine industry where lots of water exposure is the norm. It is safe to assume that a boat would have a greater exposure to moisture than an airplane with wooden structure included as most airplanes of this type spend most of their lives in hangars protected from the elements. Refer to paragraph 1-4a (1) and (2) above. Note that the glue used and recommended by the manufacturer is the prefered one for repairs and any adhesive found to meet the stated requirements in (2) can be used in repairs as long as they are compatible with the existing glues in the sturcture.

What does all this mean? Does any of it apply to your construction of a model 12? 43.13 is titled: ACCEPTABLE METHODS, TECHNIQUES, AND PRACTICES -- AIRCRAFT INSPECTION AND REPAIR. It is not called "required manufacturing techniques for aircraft structures." It is simply a guide for mechanics to use if they cannot determine how the factory would want the repair made. 43.13-1B is NOT approved data itself. It is simply a general guide. In our case with the model 12, we recommend using T-88 as the adhesive for wing assembly. This is based on our tests with the product and many years of inservice experience with it in the most severe conditions known. Hundreds of aerobatic airplanes are built with T-88. One being, 4 seasons and several thousand hours of +10-8 g acro in a black pitts style biplane flown by one of the nation's leading airshow performers. If I were to be in the market for a set of model 12 wings and had the choice to buy a set constructed with T88 and a set with Resorcinol, I would choose the T-88 set. The one greatest freedom we have in homebuilding is to ability to use materials or methods that are far superior to the old ways of doing this yet are not FAA approved. KK
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Kevin, Great post, thank you. It is interesting to me that you use the Stitts varnish rather than the System Three clear coat. Do you varnish the ribs first before assembly on the spars? I was concerned about the T-88 compatibility with the EV-400 at compression struts, leading edge skins, and other points of contact between the varnished rib and new work. Have you done any tests to see if the clear coat interferes with Polytak? How about tests to see if the EV-400 interferes with the T-88? - Dan
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Dan, We have used the stits epoxy varnish for over 20yrs. Our prime biz is antique aircraft restoration and because these are certified airplanes for the most part, we need to use approved products throughout. So, our use of EV400 comes from our long use of it and the success we have had with it. I have never used the system three clear coat and therefore not done any tests with its compatability with other top coat systems. We do not precoat wing ribs prior to installing them on the spars. Again, coming from the certified airplane side of things, you are not allowed to have any sealing coatings in a glue area. Typical practice for the past 100yrs of aircraft wood work has been to varnish the structures AFTER they are fully assembled. We build a wing, complete, then varnish it. This makes for much less work than having to protect or reclean the glue areas when dipping the ribs before assembly. We seal any internal areas that are closed off at the time of assy but otherwise, the entire structure is coated with EV400 after it is complete. Because we use this practice, we have had no need to test the adhesion of T-88 over EV400. It is just not done here in our shop. We have restored/built over 80 airplanes in 20+yrs. All but 2 have had the stits/polyfiber system used. None have had compatability problems with the EV400 and the covering systems even the 2 (travelair speed wing A14D and Gee Bee Z replica) that did not use stits as the covering. I have seen many airplanes done by others over the years that had the final color coats stained from inside by the varnish leeching up through the finish. Not pretty. By the way, many antique airplanes actually had their wooden structures dipped in large vats of varnish after assy. WACO did their wings this way, Beech dipped the staggerwing wings and tail. These airplanes have lots of holes drilled in the ply boxed areas to allow the varnish to enter and drain out.

This glue discussion is quite different however. We are not talking about a newly invented technology in w/t/f construction. The discussion is between a 60yr old glue, a 50 yr old glue and a 30yr old glue. Specifically, the latter two being resorcinol and epoxy respectively. Both have been in use in w/t/f aircraft construction for more than 20yrs. Yes, new variations of epoxy have been created since it was first introduced resulting in varying work times, strengths, temp resistance, etc. But, it is still the same technology. With decades of use of both glue types in this discussion, it is not a huge leap to conclude that the proper use of each has been determined for w/t/f construction. Both glues have their merits. If we look at the airplanes of the hard acro airshow pilot I refered to in my earlier post and compare the wings in them. Initially, he used stock S2 wings built with plastic resin glue. Glue joints failed. Next he used modified factory wings with resorcinol glue and joint failed as well. The next set of wings were epoxy assembled which made the joints strong enough but now he begain to have structural wood failures. The next version of his wings used epoxy for assy and were redesigned for added strength in the wooden members and now have gone 4 seasons without a failure in the epoxy or wood being found at each yearly uncover, inspect, revcover. All the while, this airplane was painted black except for the past year as it is now red. Most would agree that this airplane sees higher than average abuse in flight with more than 500hrs a year for the past 4 yrs.

The moisture issue really does not apply to our type of airplanes. Wing wood is sealed and most are not tied down outside all year round. Rarely are our wings wully submerged in water for extended periods of time with raw wood in contact with the water. Even boats have the wood sealed.

The temp issue can apply to epoxy in wood airplanes. The Extras have a temp sensitive decal on the wing spar in the cockpit. If the temp of the spar get high enough to soften it and damage the wing, you must fly around gently and cool it off until the sticker says it is OK to pull hard g's. This is a certified airplane.

T-88 for example begins to give up some strength at 165�F glue temp but only a slight amount. That is glue temp not air temp or fabric surface temp. All glue joints as well as weld joints in the moel 12 are designed at 80% efficiency meaning that if the joint is attached perfectly but with a 20% void area, the joint will still hold to design ultimate loads.

A black wing surface in FL summer sun gets up between 150�F to 175�F. The prime concern in wings is the spar glue joints. The spar joint glue is not in contact with the black paint on the fabric. The heat must transfer thru the black paint, UV coatings, fabric, and air space, then wood both ply and solid to get to the glue. I have measured the internal wing air temps on such a hot day and have found them to be 120�-130�F, far below the 165�F needed to affect T-88. Even if the internal air temp was equal to the softening temp of the glue, it must still transfer this heat thru the wood and a full 100% efficiency to affect the glue joint and as we know, wood is a great insulator (ref wooden handles on pots and pans). In short, the temp issue with epoxies in our type wing construction has been thought thru and tested in most severe conditions than you will see in your model 12.

T-88 lacks one piece of paper which keeps it from being universally accepted as an FAA approved wood adhesive. It does met a mil spec for metal adhesion. AC 43.13-1B states that epoxies can be used if it meets a mil spec for wood adhesion. A good project in glue would be to determine the requirments for a wood assembly adhesive mil spec that T-88 matches. Find a mil spec that lists epoxies and wood in the same text. Then , we can use it in certified airplanes.

It seems as though many subjects in aircraft construction tend to get "over thunk" these days. "Wheel reinvention" if you will. When to varnish, how to glue or clamp and cutting tubes for example. Granted, many of these new ideas can make life easier for that very small part of the project. But, in many cases, the time saved at point A can create 3 times the work at point B in the project. Precoating ribs for example seals a very small part of the wing structure. The vast majority needs to be sealed after construction like spars, play areas tip bows etc. So, squirting another pint of varnish on the ribs at that time takes only a minute or 2 more than coating the rest of the wing. It takes much more than a couple of minutes to indivdually coat each rib with a brush or make a dip tank and dip each, sand off drips and glue areas, etc.

There is an old joke that I cannot fully recall but the main idea in it is that a mathmatician and an engineer had to determine a numerical value of something. The mathmatician worked frantically while the engineer sat and watched him. When a period of time when by, the math man decided that he could not come to a finite value as the trend extended to infinity. The engineer quit working at 3 decimal places and added a statement to his worksheet that said " acceptable within engeering Tolerences." I think too many of us these days lean toward the mathmatician and try to out think the engineers who created the methodologies we employ. Sorry for straying from the subject here. It just seems that many people tend to spend more time trying to invent new or better ways to perfom the 100yr old processes used in wood/tube/fabric airplane construction than it would take them to do the job using standard methods and tools available to them now. We don't cast our own tires, make our own bolts, weave the fabric, etc. It is the "not invented here syndrome" at its best.

To sum up:

We use weldwood plastic resin glue on certified airplanes.
We use T88 on experimentals and will use in on certified airplanes if it gets approval.
We use EV400 epoxy varnish on all wood structures applied after construction is complete.
We use the polyfiber covering system. KK

06/06/02 LOWER WING COMPRESSION RIBS

On the compression ribs for the lower wings, I think they are skinned one side with ply, gussetted oppposite, one left, one right?

I had planned on coating the whole ply skin one side with T-88, then applying to the sticks. I occurs to me that it's gonna be real tough to staple to the sticks between the upper and lower capstrips. I'm considering gluing the skin on and weighting it instead of stapling. What has everyone else done?

On the same skin there is an ovoid cutout, I believe for the control rod to the aileron, and 2 holes 3/4" , apparently for the bay wires. Seems to me I should wait to cut these holes until wing assembly. Any input? - Jerry
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Nope, all compression ribs on the lower wings are skinned both sides. Rib 6 is a truss rib that is gussetted on the outboard side and skinned on the inboard side. This is a rib made of 1/4" sticks and is the one located at the outboard edge of the walkway. The skin is on the rib to aid in attaching the rail support for the walk plywood. This is the 1L, 1R requirement.

On the compression ribs, both sides are skinned. On these, you should mark the staple location lines with a pencil on the skin. Then flip the skin over and coat with T-88, place on rib jig and sticks, staple along lines. Drawing lines on the jig above and below the rib outlines showing the location of the sticks can help too. After stapling the first skin on, remove rib from jig, flip over, coat sticks and inside of second skin with T88 and staple second skin on as first. It is not a good idea to leave the rib in the jig with weights on it as the jigs are wood and you might bond the rib into the jig.

Only the 2 rib # 6s get the 2 holes 3/4" in them. This is for drag wires and should be drilled in the rib before the rib is placed on the spars. Do not drill these holes in any compression ribs made with 5/8" wide capstrips. Only the 1/4" wide #6 ribs of which there are only two. KK

03/25/02 DRAG WIRE TENSIONING

I am about to tension the drag wires on my upper wing. I have the instructions included with Kevin's wing kit and it seems pretty straight forward. Any words of wisdom, advice or possible pitfalls for someone who has never done it before would be appreciated.
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Tramming the top wing is pretty simple. Follow the steps in the instructions and take your time. It takes a few runs through it to get it all to the correct spot. We like to tension the wires such that if you hook your 4 fingers at the first knuckle under one wire near where it crosses another and press the thumb on that hand on the oposite wire, you can spread the wires apart about 1/2". More than that is too loose, less too tight.

On the lowers, the tension is checked by hand like above. However, the tramming is done with a trammel bar and points starting with the inboard bay and working out. KK

03/25/02 DRAG WIRE BLOCK DRILL SIZE

What size drill should I use for drilling the holes in the drag wire blocks?
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It is 13/64 (.203). This gives you some room for sealing the wood etc. KK

02/18/02 AIRFOIL DESIGNATIONS

What are the airfoils used on the model 12?
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The model 12 uses NACA 63A(2)-015 for the upper airfoil and NACA 0012 for the lower airfoil. Same as Eagle and S-2B. KK
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I once knew what the airfoil designation code meant. For instance 63(2)-A-105. Does anyone know where I can look to decifer this? Thanks, Phil
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From Hary Riblett's GA airfoil book:

A four digit airfoil number: Turbulent airfoils.
A five digit airfoil number: Turbelent, zero camber airfoil.
The 6 series airfoils (e.g. 63-xxx): Laminar airflows.
The 'A' in the 6 series airfoil indicates that there is no cusp in the airfoil.

For your example:

6= 6 series airfoil (=laminar)
3= maximum thickness of wing is at 35% chord length (4=37.5%, 5=40% and 6=45%)
A=indicates cusp
4th digit= Indication of maximum camber in tenths of the cord length.
Last two digits (05): The maximum thickness of the airfoil as a percentage of the chord length...Ides
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I'd like to add to the above info from Ides. The (2) that is in the upper wing airfoil designation, 63(2)A-015, is refered to as "sub 2". It is actually a subscript 2. This sub 2 airfoil was added to the list in the 70s after Curtis Pitts' patent on his symetrical wings expired. Curtis modified the airfoil and refered to it as "63A-015 Modified". The standard version of the airfoil has a cusp or reflex in both upper and lower surface from just ahead of the rear spar to the trailing edge. Curtis removed the cusp by drawing in a straight line from the trailing edge forward touching the curvature of the airfoil, upper and lower. This gave him a rib that was easier to build and, more importantly, made the rear spar taller. So, the designation for the upper airfoil in a Model 12 is read as NACA Sixty-three, sub 2, A, Oh, Fifiteen. KK
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The upper wing is 63(2)-A-015. Where the 2 is a subscript. I once read somewhere what all those numbers meant, but I can't seem to find it now. I think the dimensions for the upper wing are from the following site. http://amber.aae.uiuc.edu/~m-selig/ads/coord/n63015a.dat Reason I think this, is because it gives the exact same coordinates. Obviously each x should correspond to each y, but I mean they use all the same x's no more, no less. One heck of a coincidence. Phil

10/02/01 STOCK vs. HP RIBS

Kevin, I have built up several ribs using your jig L4, which I borrowed from Darin's HP wing kit, and have discovered a problem.

I have noticed that ribs fom this jig are longer than the 42.00" specified in the plans for the lower ribs. In addition, they do NOT have the same coordinates as the plans for NACA 0012, at least for a 42.00" cord length. If one uses a ~42.60" cord length then the jigs are a NACA 0012 airfoil.

It concerns the dimensions of the lower wing ribs. According to the plans, part number 407-3, full cord truss rib, is 42.00" long. More importantly the center of the rear spar to the trailing edge is quoted as 12.60". The length of the aileron is 11.50", from tip to tail.

Looking at drawing 12-400 sheet 4 of 4, view G-G, I see the following problem. The distance from the back of the rear spar to the trailing edge of the truss rib is 12.60" - 0.687"/2 = 12.256". Then subtract the cove block, 400-58, and we are down to 11.256. Next allow for the aileron nose-cap to cove spacing, 0.188"-0.200", and we are down to 11.068-11.056". Now substract off the full length of the aileron and we have about -0.444". So either the trailing edge of the aileron sticks out about 1/2" from the trailing edge of the main wing or the aileron nose rib interfers with the cove block.

So here are my questions?

1.) Is there an error in the plans ribs? 2.) Are the HP jigs a NACA 0012 airfoil with a 42.60" cord? 3.) Or are the plans aileron dimensions messed up?

Thanks in advance, for the help, Phil
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Phil, there are too many differences in the HP wings and the plans stock wings to list. I changed most all of the parts in some way aiming for added performance and increased strength. Styling was a factor as well. This is why the HP wings are intended to be built from complete kits purchased from us rather than trying to mix and match info from the plans with parts from an HP wing kit. There are fit and function considerations that cannot be satisfied by trying to scratch build our HP wings verses purchasing our actual HP kit parts. This was done to protect our investment in the development of the kits.

As you know, the way we sell the model 12 stuff is 2 fold. Build from plans or build from our kits. You can mix and match fuselages and wing sets but not scramble within a sub kit. For example, you can buy and build HP wings from our kits or scratch build wings that conform to the plans. Partial HP, partial plans wings are not possible as the parts and dimension vary between the 2. You can scratch build a fuselage and bolt HP wings to it or just the opposite. But you can't bolt stock plans elevator pushrods into an purchased HP fuselage.

Using the jigs from Darin's wing wood kit are causing some of the confusion. The HP ribs are specified to be built at 42.25" chord not 42.6 as you think. This is in the instructions made available with a purchased HP wing wood kit. This is actually making the rib a little too long so it can be shortened as needed to match the prefabricated aluminum alierons we sell. Our wings come out right at 42.25" chord just like the stock plans wing will including the trailing edge material. Yep, the stock ribs will come out to be 42.25" chord with most of the added chord being at the trailing edge. How? They were designed with a thick trailing edge, not a razor point. Then you add on the trailing edge material which adds some length and can actually be used to match the wings and ailerons up a bit if needed.

As you can see from the details in the plans for an aileron bay rib, the rib extends aft of the rear spar rear face .625" and that is .969" aft of the centerline of the rear spar. The idea is to build the wing ribs and aileron ribs as in the plans. Put the wing and aileron ribs on their spars, mount the ailerons and adjust the 1" dimension of the 400-58 cove block as needed to get the proper clearance of .188-.200. So, for example, the trailing edge adds some 1/8"to 3/16" length to the wing ribs that makes the center rear spar to TE dim 12.75 ". 12.75"-.75-.339=11.661". 11.661"-11.5"=.161. Pretty close for had made wood ailerons and wings. The tolerence is 1/16" so that is within the limits. It all comes together on assembly.

If you decide to switch over from scratch built wings and go with an HP wing kit from us, the dozens little tricks are built right in and beginners are doing them without even knowing about it. This is why we have the ribs built at 42.25". Assemble the wing, then mount the aileron with a preset gap as instructed. The aileron is alum and premade by us so it cannot be changed by the builder. The wing ribs are trimmed and sanded as needed to get the wing trailing edge to match the trailing edge of the ailerons. One reason for this is that I personally don't like the trialing edge material to hang off the end of the rib like Curtis designed it and as many factory wood wings are built. I like to have FULL contact of the rib tail and the inside of the trailing edge material. Leaving the ribs 42.25 long allows for this good fitting.

Yes the Ordinates are different, slightly, for the HP wings and the stock wings. Again, this comes from some of the design changes we made. The plans Ordinates build the plans stock wings. Our HP kits build the wings I designed for which we do not provide Ordinate values. Note that the plans show the thickness of the rib at 42.0 chord to be 1/8" (1/16" top and bottom). Ours are about the same at 42.25". Maybe just a bit thicker.

There are 2 basic techniques to use when drawing ribs. Draw it to a razor point at the trailing edge at the required chord length. Then thicken the trailing edge to be a practical 1/8" and taper this forward tangent to the airfoil curve. This is the way the plans were done. The way I do it is to draw the rib 10% longer than desired then nip the taip off at the proper chord which leaves the needed trailing edge thickness. This is how I did the kit ribs as it creates slightly taller spars which are factored in the increased strength of the HP wings. Every little bit helps.

The HP ribs have a different truss layout than the plans ribs. Plans ribs will not work with HP spars. HP ribs will not work with stock spars. So, I suggest you settle in on one or the other types of wings and follow the construction of the type chosen. All plans version with plans ribs, from plans jigs, etc. or buy a kit from us and build the HP version. KK

09/06/01 JAVELINS

Another VERY important point that needs to be reviewed...

NEVER USE ALUMINUM JAVELINS ON STAINLESS STEEL FLYING WIRES!!!! That's NEVER, not even if you put tape on the wire or protect it. NEVER!! Why? Because the wire WILL break!! IT has happened hundreds of times and one of the most recent that I heard about was on Sean D Tucker's Challenger II red biplane. He broke a wire at the javelin during practice a month or so ago.

Javelins should be made of wood, garolite or plastic. We use wood for the model 12s as all the old airplanes we mess with use wood. I know that some guys like to make shinny beautifully machined alum Javs but the trade off is that it can cause a major structural failure that might cause you to test out your relationship with the heavens or your parachute. KK
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More good stuff. Mere curiosity, any idea about the mechanics of failure? If surface protection doesn't prevent it, then apparently it's not dissimilar metals or fretting. I dunno, perhaps aluminum javelins are too stiff, wire vibration creates a bending load? Aluminum javelins too heavy, lowers the frequency of the wire assembly into the range of engine exciting frequencies? Dan
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I think it is a combination of all the above. The stiffness is bound to cause a bending load at the javelin. The cycling of the loads can cause the protective tape or whatever to move and allow the alum to contact the wire resulting in dissimilar metals problems. The Eagle uses alum javelins but as Mike points out, they are many pieces that allow for the front and rear wires to 'rotate' individually. They also have machined or molded nylon or delrin inserts at the wires I think.

This is a good place to say 'never' so that it will prevent a homebuilder or other person who is not aware of the problems with alum javelins and strainless flying wires from doing the wrong thing. KK

09/05/01 DRAG WIRES

Can I make my own drag wires, and if so what should I use? Mike
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Mike, store bought 316 stainless steel drag wires are fine. If you make your own, 4130 is weaker than the store bought 316 stainless ones. If you use 416T stainless, you will have drag wires that are stronger than the store bought units. It must be 416T stainless rod that is 140,000psi. If you get plain 416, it is untempered and is only 55,000psiand would Not be strong enough for the loads the drag wires see.

We make the drag wires for the Model 12 kits here from 416T stainless 140,000PSI material. the threads are cut on a lathe. We make them for a bunch less than the Bruntons price for our kits. Yes, cut and that is more than good enough as all flying and drag wires used to be cut threads until the Most recent import of Bruntons wires by Hale.

So, yes, you can make your own but be sure you get the correct material. Finding 416T heat treated stainless rod is the trick. We have to buy it in large quantities. LArge enough to make approx 15 or 20 sets of drag wires for the model 12. The stuff is TOUGH so use a high grade fresh die when cutting the threads.

A bit of tech info for you guys on wires sizes and ratings:

If you look at the Bruntons drag wire sizes as listed in their chart, you can see that, for example, a 3/16" 10-32 drag wire is rated at 2100lb. If you assume(incorrectly) that the wire is 3/16" dia., that will show that the material the wire is made from is only 75,000PSI stainless. But, we do know that the typical tensile strength of 316 stainless rod is 90,000PSI. If you use 90,000PSI and 3/16"dai rod, you get a tensile rating of about 2550lb. So, this adds up to show that Bruntons rods, drag rod, flying, etc are actually slightly undersize on diameter. If you measure the dameter of a Bruntons 3/16" drag wire, it will measure approx 0.174"-0.175". Do the math with that and you get a bit over 2100lb for the rating. Why does Bruntons use undersize rod on their wires? Simple. The rolled thread process does not remove metal like a cut thread set up. The die mashes or plows a grove in the rod that pushes the excess metal up next to the grove kinda like a farm field and a plow. The final max thread diameter is to be 10-32 in this example so the rod is somewhat smaller to allow for the growth in diameter at the threads. The threads have to fit a given threaded end and or nut. Same is true for the flying wires by Bruntons, they are slightly undersize at the round section between the flat and the threads.

All this is not a problem as Bruntons wires still meet the required specs set forth in the Mil Spec. which are calculated at the smallest crosssectional area. But, you need to know what you are using in special cases like anti drag wires in the Model 12, skybolt and Super stinker upper wings. A big engine skybolt or model 12 at 2000lb can see anti drag wire loads of well in excess of 3000lb! If you have a 2100lb wire in that spot, you've got a big problem.... In the case of the Model 12, the 416T wires we make are good for about 2700lb each. But, Curtis Pitts did one better on that high stress anti drag wire in the upper wing.............He put 2 of 'em there! That's 5400lb of capacity which equals stiffness!

Mike, the super stinker was designed for 416T drag wires and Curtis used the same on the Model 12. He likes the material. If you use .155 to .160 as the root dia of the threads, that will give you the cross sectional area, useable for the 140,000PSI material. KK

07/13/01 SPARS, SPRUCE, DOUGLAS FIR, PLANS WINGS

MODEL 12 Builders!!!! Occasionally the question about spars brings up a problem that has popped up a few times in the near 180 sets of plans that we have sold for the Model 12. Read the plans carefully. The spar material is specified as douglas fir not sitka spruce. If you buy spars from Wicks or Aircraft Spruce, it will be sitka spruce not douglas fir.

The Model 12 must have DOUGLAS FIR SPARS.

While most wood wing airplanes were designed for Sitka Spruce spars, the Model 12 was not. Curtis Pitts designed the model 12 wings for douglas fir and if you substitute Spruce for the fir, your wings will NOT be strong enough for safe flight!!

It is commonly known that sitka spruce spars can be replaced with douglas fir as the fir is stronger but slightly heavier. You CANNOT go the other way. So, all you model 12 builders out there, or others who have purchased doug fir, who have found doug fir spar material, how about posting the source so others can know where to go. We sell fully machined douglas fir spars in our wing kits. IF you have one of our wing kits, you have D. Fir spars and no worries about this issue. Scratch builders needs to be sure you have vertical grain Douglas fir spars that meet the specs for vertical grain spar material. We can sell fully machined stock style spars too but our vendor does not sell raw blank wood. KK

06/21/01 RIB GUSSET GRAIN DIRECTION

When I am building the ribs does it matter which direction the grain on the gusset goes, relative the the chord? I checked the Kimball site and all the gussets seem to have the grain perpendicular to the chord, while on the 2wings site, the rectangular and triangular gussets' grain seems to parallel the chord. Question is, since I have 90� plywood anyways, does it matter? Phil
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The idea is to have the majority of the grain in the direction of the load. That is, the face grain( 3 ply material has 2 plies of good stuff, one middle ply of crappy filler wood). So, the gussets on the truss ribs, rect and triangle should be vertical or on a 45deg angle. The 1" wide strap gusset at the spars should be vertical. The gusset at the extreme tail end of the long ribs should be chordwise. The ply on the compression ribs should be chordwise. KK
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Kevin, Phil sent me your reply to the grain direction question, which has caused me to run full speed from my garage after looking at my ribs that Steve Wolf built. But now that I've checked my pictures of your wings, I know mine are at least as strong as yours. How big of a deal is the grain direction, hopefully not big enough to rebuild 2 sets of wings! Darin
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A damned if you do damned if you don't type of situtation. You caught me on it.....:) Basically, the grain directions I spelled out for Phil are the ideal perfect/best way to do it. I have found in most cases, it is easier to give a precise direction to those who ask rather than a vague or general one. Kinda like the folks at Polyfiber saying ALWAYS do this or NEVER do that. It is easier for a newbie to digest a specific instruction rather than a general suggestion. That being said, I give Phil the info that will best use the ply to its fullest extent as gusset/rib building material. Heck, he has to cut them anyway so he might as well go for max theory while doing it as it is no more work on his part. Another way to think if it is something I recall from my First Aid/CPR class back in college. We were trained to single out one person and give them a task like CALL 911. If you merely yelled "Will someone go call 911?" nobody would move.

SO, this brings us to the gussets we sell in the wings kits for the model 12. There ain't a thing worng with having the grain near parallel to the chord line of the rib as the gusset size has been calculated based on the weakest orientation of the plywood and the size of the gusset exceeds the shear load requirements on the glue joints.

So, the moral of the story is, Phil, it don't matter..............But, the way I told you is better.......even though you don't need it to be better. How's that for squirming my way out of this thread? KK

06/18/01 RIB JIG ACCURACY

Here's a valuable lesson that I learned tonight. Recently I began cutting the capstrip and gussets for the lower wings after finishing up on the Top Wing capstrips and gussets. It was at this time that, out of curiosity, I decided to measure my rib jigs for accuracy. After all, I had built them right on top of the plans. The Rib Jig for the Top Wing Typical was 1/8" too short in total length but less than 1/16" error between Spars. I decided that this was ok. Check your plans. If you're building directly on top of the blue prints, you may be in store for a very rude awakening when you decide to assemble the wings. Luckily for me, the top wing came out ok. It seems that it would almost be better to just use the plans measurements to design the fixtures from the ground up!
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The problems you encountered with size errors in your full size rib drawings is not unusual. A 4ft long of paper can change size as much as 1/8" or so with changes in humidity. Also, the process of making the copies can introduce errors in the actual size vs printed size of the part. For these reasons and the resultant mis-made parts, I eliminated the full size rib templates from the Model 12 drawing set after we bought the rights from Mid America Aircraft. I added the rib ordinates to the rib drawing set so that the ribs could be drawn accurately by the builder.

If full size templates are to be used, they should be printed/drawn on mylar which is temp and moisture stable. After printing, the template should be checked for accuracy as some of the errors you have found may be in the original hand drawing. KK

06/10/01 EPOXY TESTS

I'm curious though about epoxy tests. Should I make samples with each epoxy mix? Any advice as to what they should consist of? I'd think to be representative they should at least include capstrip material glued to gusset material and stapled as the rib is. But how big? Say 1"x1" of ply gusset glued to 2-3" of capstrip? Am I on the right track?
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While it is a nice extra to have some test samples for the Feds to break, it is not required and most inspectors out there today wouldn't know if the break was good or bad. Some might think that the fact that they can break to wood is a bad sign and the airplane unsafe. In 30 + yrs, we have never had to provide test samples for the feds. I do know of one guy who did provide test coupons but they were for a belanca viking wing 25yrs ago fixed with epoxy before epoxy was approved. I would be cautious about giving them too much info unless you are specifically asked for it. KK

06/10/01 RIB BUILDING

Setting up for Rib production and was wondering how fanatical to be about plywood gusset overhang. That is, should I spend alot of time before assembly getting the gussets perfectly trimmed to the outside of the capstrip, or is this something better done after the rib is finished with say sandpaper or a piloted flush bit in a router. Thanks Guys, Jerry
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Let the gussets hang off the edge a bit and trim the afterwards. Look closely at the rib drawings in your wing instruction pack and you will see that the gussets are drawn in such a way that indicates they do hang off the edge and are trimmed. I believe you noticed this. The way we do it is quite simple. Build all ribs with gussets hanging off. Use the band saw to trim them close. Sounds scary but is quite easy if you let the trailing edge of the blade ride on the capstrip with the rib at an angle to keep the teeth away from the capstrip. After that, over to the 6"x48" belt sander and slick them right off flush. An entire set of Model 12 ribs can be trimmed in about 1 or 2 hrs using this method. KK

06/08/01EXTERNAL COMPRESSION RIB STITCHING

Here is a quick sketch of how we stitch around external compression ribs: StearmanRibStitch.gif This is a sketch I did for our guys on the Model 4 Stearman we are restoring. This is far easier than a hook and loop method. Uses a standard needle. Just stitch the comp rib first so you can use the holes of the adjacent ribs as turn-around points as I have drawn before the stitches are there. Quite easy and fast. Note the staggered pattern of the comp rib sticks on this rib. This is how the stearman delt with the spar rotation. A waco has the 2 sticks on the chord line. KK

06/01/01 SPAR BUTT FITTINGS, EAGLE SPAR DAMAGE PHOTOS

Kevin, Jim, others, may I ask some questions about a point of design? Typical wood spar butt end fittings are a pair of simple steel straps with a row of cross bolts through the spar. Generally the straps angle up across the face of the spar web. I assume that's to ensure that the cross bolts are not in a single line parallel to the wood grain, and to align with the applied loads? It's what you find on everything from Cubs to Skybolts.

The M-12 lower wing uses a pair of plates, with plate width being the full height of the spar web. The cross bolts are placed in what might be described as a modified diamond pattern.   Am I correct in thinking that the strap fitting with all the bolts in a line concentrates stress at the first spar cross bolt hole nearest the butt, while the plate fittings have no such drawback? Other reasons for the design approach?

The full height plates do require bolting in the butt compression rib, which adds a little weight and complication. Is the plate-style fitting and bolted butt rib typical for Pitts wings?   Dan
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You wrote "Generally the straps angle up across the face of the spar web. I assume that's to ensure that the cross bolts are not in a single line parallel to the wood grain, and to align with the applied loads?"

Yes. Generally speaking, the axial spar load is resolved to the fastener pattern centroid. Because the spar attach bolt is low with respect to the spar centerline there is necessarily an induced moment. This small concentrated moment and the moments induced from airloads must be accounted for in a beam-column analysis of the wing spars.

You are correct that the spar butt straps should be tapered or should consist of two or more staggered straps to get a uniform distribution of fastener loads if they are only intended to transfer axial load. The strap(s) would be thicker inboard and would step/taper down to a thinner section as more of the load is transferred into the spar. Moreover the spar butt plywood should be tapered 12 to 1 where it transfers the load to the spar section (outboard). Refer to ANC-18 Chapter 4 if you have it. Having said this, I didn't do either! I built the wings per the drawings. As you said the design works on everything from Cubs to 'Bolts. Curtis Pitts used alot of what is found on a J-3 Cub in his S-1 design.

The lower wing spar butt design of the M-12 yields a better load distribution but then it uses almost twice the number of fasteners and it is heavier. It's doesn't appear to be optimized for an even load distribution but I suspect that with all those fasteners the bearing stresses are so that peaking effects are negligible. It's interesting that Curtis made such a radical departure from what he used on his previous S-1 and S-2 designs. I think it underscores the difference between two airplanes of approximately the same size: one designed from the outset to handle 2300 pound gross aerobatic loads and one that is designed for 1650 pound gross aerobatic loads per the drawings. You can't hang an 0360 on a M-12 and expect it to perform well and you can't hang an M-14 on a stock Skybolt airframe and expect to acro at the same max gross as an M-12...apples and oranges in my opinion. The Skybolt design will handle all you can give it within the gross weight it was intended to be flown. If you fly at a higher than intended gross weight you should either:

a) modify the structure for the higher gross weight or
b) modify your flying to prevent these higher loads.

It isn't uncommon for an airplane to have an aerobatic gross weight and a normal gross weight. This is what I intend to do. I decided on the M-14P after I was well into my wing structure. The drawings say "1650 pounds gross aerobatic" and 1800 pounds gross. Hale would often say that he knows it is much more capable than this. I'm sure that he based this on actual experience. Typically analysis is conservative and structures are generally very forgiving. Nonetheless I'm analyzing the wing structure to find out what the limits are for my airplane.   Jim Doyle Stress Analyst

Christen Eagle with spar damage   Pitts 12 spar butt fitting at 2wings.com

05/07/01 SOURCES OF DOUGLAS FIR FOR SPAR, WOOD SPECIFICATION

Does anyone know a source for a/c grade Douglas Fir? This would be for a spar. Aircraft Spruce only seems to have Spruce (I guess I should have figured that from the name, huh?) Thanks, Phil
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Try Richard Walton at R.A.W. Resources. His web page is at: http://sites.netscape.net/rawresources/homepage I contacted him about a month ago looking for spar stock for my Model 12, and he was quite helpful, and actually had the material available to ship immediately. I'll be ordering my spars from him shortly, unless someone can point me towards a better source. Kevin? -Cory
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Our wing wood kit supplier will not sell raw douglas fir stock. He will sell spars machined to the stock form as found in the plans. If R.A.W. Resources has the douglas fir, you can get it from them.

The spec for A/C grade D. Fir is quite loose. I specify a tighter spec on quality than what the feds will accept as this is not a t-craft but rather a high performance aerobatic machine. We like to see a grain count of 16 to 24 grains per inch. The feds accept 8 and up. Less than 16 indicated very fast growth and over 24 indicates very slow growth. Use the standard specs for grain run-out along the length of the spar but add a spec for the end grain angle. While 12:1 or 15:1 is good along the length, we prefer to use boards that we can follow the grain from one end to the other with near zero run out. If you look at the board from the end, a perfect board would have the end grain perpendicular to the tall face of the board. This is quarter sawn, perfectly. By definition, this grain must not have an angle that exceeds 45 degrees to the face. If it does, it is not quarter sawn. We spec that this angle not exceed 30 degrees to the face. Hope this helps. KK

05/06/01 T-88 GLUE MIXING I bought some T-88 & the System 3 mixing kit (that looks like a twin caulking gun). Prior to gluing up my ailerons, I tried some tests on scrap wood.

I used the same spruce & plywood left over from cutting out the aileron parts. The first time I tried this, I cranked the heat in my garage to 70 degrees. I then glued the parts (with NO clamping pressure) & let them sit for 3 days. My theory for using no clamping pressure is that I don't see a way to apply pressure to a spruce spar threaded through a plywood rib.

Surprise, surprise -- the parts fell apart in my hands when I tested them. There was almost no glue penetration.

I tried a second time, using various "joints" - face to face plywood, edge to edge, edge to face, etc. This time I used spring clamps & let the glue dry for a week (no exaggeration). It worked a little better - when I broke the pieces apart (again, with my bare hands), I did get some wood tear out. Unfortunately, it was less than 1/16". (Garage was still at 70 degrees). If I didn't know any better, I'd be questioning the use of T-88. However, I know it is the correct glue to use. Therefore, I must be messing up somewhere. Any ideas?
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First, I found the "gun" to a piece of crap. Sorry, I have no other way to describe it. I used it once (the one from Aircraft Spruce, not from System Three, but I assume they are the same) and tossed it in the trash can. I use the small molded plastic measuring cups from System Three and have found they word very well. The 1 and 3 ounce sizes.

T-88 reaches, at room temperature, about 90% of its final strength within 24 hours after mixing. So overnight is all it should need to cure for shop purposes.

If you want to try thinning T-88, use a little heat. EMPAHSIS on the LITTLE! The warmer you make it, the faster it cures. Do NOT add ANYTHING to thin it.

I used T-88 at room temperature on my wings with no problems. I made numerous test strips as you have before starting. I found clamps to be desirable (some clamping pressure---weights, etc.) because it helps force the glue into the fibers. Caution: too much clamp pressure is as bad as too little. Too much pressure starves the glue joint. When you test the samples to destruction, you are looking for failure IN the wood fibers. NOT at the glue joint interface. I recommend, before you go any further, that you contact Dick Anderson at System Three and see what he has to say. He is their head tech guy and very willing to answer questions. (800)333-5514. Andy
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I've used T-88 on test pieces down to about 55 degrees with spring clamps and broke wood on all samples. I use small unwaxed plastic cups from the supermarket and cheap wood craft sticks to mix it up. Usually you don't sand pieces to be joined, however, if your doing mahogany gussets for wing ribs you might try scuff sanding the ply a bit with course paper. Just rough it up a bit. Hope this helps some. Cheers, Steve
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Hmmm...I've had no problems with the "gun", and have found it great for mixing small amounts of T-88 without having to worry about measuring out exactly 1 oz. in a cup. After the first two or three ribs, I figured out exactly how many "squeezes" I need to epoxy a rib, set of gussets, etc. I've had *very* little waste this way, and so far I have yet to mix more than an ounce at a time. I'd be wasting more than I was using with the measuring cups.

I've also found that to get the best results, I need to fill one of the cartridges, and then stand it vertically for a day or two until *all* of the air bubbles work their way to the tip. Then place the cartridge in the gun, and squeeze out any excess air and resin/hardener until you get a steady flow from both sides. Make sure you wipe the tip off before re-applying the cap, and also check that the tip is clean and clear of "gummies" (I just use a toothpick) before squeezing it out into a cup for mixing . I've been making test pieces from each batch (couple of 2-3 inch capstrip scraps), and have yet to see a failure in the epoxy.

Try a warm water bath - stick your little plastic mixing pot into a pie tin, coffee cup, whatever, full of warm water...it'll thin up nicely after a minute or so. I've also tried nuking it in the microwave on the lowest power setting for about 5 seconds. It worked, but you tend to get "hot spots" if you're not careful. Personally, I prefer the water bath, as it warms the epoxy more
evenly and gently. Cory

04/22/01 POLYESTER BODY FILLER

In some of my wing pictures I saw the filler and noticed you use polyester autobody filler, is there any advantage to that over epoxy / microballons? Probably just easier and cheaper. And you spray the EV-400 over it too?
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We have found that the high quality polyester body filler works great in spite of what the salesmen of polyfiber superfil or similar products will tell ya. We have airplanes that are championship winner from 20 yrs ago that still look great. Use a product like RAGE from Fiberglass evercoat. Avail at auto body paint supply stores. Advantages include local availability, speed of drying, sands easier than the mahagony ply, sticks to everything, etc. etc. Cost under $20 per gal. Works great on wood, fiberglass, steel alum etc. If I had to wait for epoxy and balloons or epoxy superfil to dry everytime I needed to fill something, I'd only be working on our 3rd airplane right now. KK

04/21/01 AILERON COVE RIBS

Quick question on the aileron cove ribs (truss vs. compression) for the lower wings. Plans show truss ribs with a 0.625 "vertical aft of the rear spar with the capstrip extending 0.875" into the bay, while compression ribs show a 0.750" vertical member and the capstrip extending 1" into the bay. Also, there is a 1" spacer block placed horizontally mid-spar for attachment of the aluminum cove material. Should all of the ribs extend the same amount into the bay, or does it not matter as the cove material is attached to the spacer block anyway? Does the amount of space between the cove material and the ribs vary slightly for the two types? Cory
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This is how Curtis did the drawings for the ribs. I cleaned it up a bit for the kit wings we sell. I made all the ribs, truss and comp, .75 aft of the spar. I also made the cove block .75" thick to match the ribs as Curtis did. This gives better clearance for the ailerons. Hope this helps. KK

04/19/01 RIB CONSTRUCTION, GUSSETS, GLUE, EPOXY, STAPLER, CLAMP, JOINT, CLAMPING SPAR CRUSH PLATE

Hi all, This is a question about building wooden ribs. I have heard that some people say that the gussets should only be clamped in place during glueing (T-88) and not nailed. Others say nail, but then remove them. And yet others say go with staples and leave 'em in. So the $1M question...what do I want to do one MY plane? I recently saw some ribs with staples in them. It looks like it would be a heck of a lot easier than smacking in all those tiny nails, but then again maybe just strickly clamping during the cure is sufficient. In the event I go with staples the questions are...what size, what material, were do I get them and what about a staple gun? Mechanical, pneumatic, electric? Thanks for the help. Phil
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There is no significant harm done to the wood in the ribs if you use mechanical fasteners to hold them together why the glue dries. Aircraft nails or small 22Ga staples. We recommend a Senco SJS-A stapler or a Duo-Fast 3400 series unit. We use 1/4, 3/8, 5/8 long staples (A04, A06 & D10) in building the wings of a Model 12. See Staple Chart

One factor you need to consider in deciding how to hold the ribs together is jig construction. A really good wooden jig like those (7) we provide in our wing kits, require that the rib NOT dry in place as it may become a permanent part of the jig! Some jigs are made so that leaving the rib in to dry is not a scary idea. If you want to make 1 rib a day, use weights. If you want to make 3 ribs a day, use nails. If you want to make 10-15 ribs a day, use staples. Using the stapler is like having 3 extra hands available. With one hand, you hold the gusset, the other the stapler, aim and shoot. If nailing, you need one had to hold the gusset, one for the hammer, one to hold the small pliers that hold the nail, etc. We use nails on the restorations to keep them original. But, we don't use epoxy glue on retorations either. The epoxy glue really acts like a lubricant helping the gusset to slide all over the place while you try to nail it down. KK
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I'm currently in the midst of building up the ribs for the lower wing of my Model 12. I'm using T-88 and have been very pleased with it so far. I highly recommend digging around on System-3's website and reading the tech info available there if you haven't already www.systemthree.com. Also, I'd suggest ordering one of their redi-mix "guns", and a mess of empty cartridges for it. Makes measuring out small amounts a breeze, especially since T-88 is kinda thick (like honey). Using graduated cups for small batches just results in lots of waste, and runs the risk of slightly goofing the ratio. Using the redi-mix unit, I've never had a bad batch of epoxy (each batch is tested on scrap and then broken). As far as the clamping/nailing/stapling question, here's my $0.02...I'm not using any nails or staples in the ribs as they add nothing structurally except weight (Think light, build light!). I've got a single jig built which will allow me to build all of the lower ribs with only slight mods for the compression ribs, and the only clamping that is done is to hold the forward edge of the capstrip flush against the leading plywood "nose-piece", and to hold the trailing edges together on full-length ribs. No clamping is used for the gussets, just some light pressure to ensure a solid coating of T-88 on both the gusset and rib during application (Boy...I'm gonna get it for that one!). I've never had any of them "slide around" on me...use enough epoxy to do the job, no more, no less (epoxy is fairly heavy stuff comparatively speaking). A little experimentation is all it takes to get the hang of it. Also, according to the T-88 tech data sheet: "glue line thickness is not critical and clamping is not necessary if the joint is undisturbed during set-up of the adhesive." I've seen pictures on the web of someone building a Pitts wing that had a half-dozen spring clamps on a spar-reinforcement plate. Ouch. I'd be worried about squeezing all of the epoxy out and winding up with a starved joint. It used to be that such clamping pressure was necessary for the glue to perform correctly...but not so with epoxy, where a distinct glue line is necessary for the joint to develop full strength. I also saw Kevin's follow up to your original post, and have to agree about the time required. In my case, it takes 3 days to fully build a rib...one day for the "rib", one day for the first set of gussets, and a third day for the second set of gussets. Once you get going, though, you wind up with a nice little queue of them. I've gotten to the point where I set everything up beforehand, and can build one rib in the jig, do the first set of gussets on "yesterday's" rib, and do the third set of gussets on "the-day-before- yesterday's" rib in an hour (i.e. one batch of epoxy). For me, that's not a big deal, but it may be for others. Another day...another part... -Cory
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Good info. I concur that epoxy needs to have a film thickness for a proper joint. In the case of rib gussets, the final width or thickness dimension is not critical. Spar crush plates are a different matter. In our wing kits, all the fittings and wood parts are built to a high level of precision. We need the final dimension of the glued up spars to be correct or the fittings and ribs will not fit properly. I recommend the use of many clamps on the crush plates along with the use of clamping plates. This is to guarranty a dimensionally correct part as well as a proper glue line. These plates are large enough with many square inches of area so the PSI is not that high. We have made test coupons and measured the glue lines to determine the techniques that best work for the Model 12 wing assembly.

We used to manufacture certified wooden aircraft propellers. In these, we had to use Weldwood plastic resin glue to meet the TC data sheets. The glue line for this was spec'd at .003" and we had to check each prop blank with an inspection loop eyepiece. We used the same tools to set up the glue lines for the model 12 kits aiming for .010" glue lines for the epoxy. Fittings we weld up in jigs are set up for this. If the spar splices, for example, are left to just float together, the joint could be as much as .050" to .100" too thick for the cabane saddle fittings to be slipped on. This would require the ply crush plates to be sanded thinner reducing their strength. Same is true for the spar crush plates at the aileron hinge bracket attach points. If the plates are not clamped to the spar, the likelyhood of having a fitting mating surface that is properly aligned to the spar face is low. As the System Three T-88 sheet states, "glue line thickness is not critical", so why not control it? KK
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I re-read Cory's post, It does sound as thos Cory is gluing the sticks together, then adding gussets to each side. This would indicate that the sticks are custom miter fitted on each end, then glued. This is NOT required on any gussetted aircraft rib. The size of the gusset is engineered to have adequate shear glue strength to transfer the loads and more. I recommend that the sticks be cut as long as you can to fit the location with 90 deg cuts on the ends such that one corner of the end of the stick contacts the adjacent one. Factory Pitts, stearman and staggerwing ribs have no inter-stick contact in places relying on the gussets only. Just like the roof over your head. This can save bunch of build time. Ribs built in this manner are still stronger thant the required loads.

Scratch builders be careful with gusset sizes too. Make them the size shown on the plans. Larger ones only add weight of wood, glue and varnish. The extra wood in each rib miter fitting each stick along witrh the glue on the ends of those longer sticks will make for a near even swap of weight as the staples we leave in. KK

04/19/01 RIB CONSTRUCTION, SPAR OPENING

Since we are on the subject of epoxy film thickness here is the next question. The Model 12's (as well as other A/C's) upper wing is swept back a few degrees.,7�. So the "spar-hole" in the rib needs to be modified from its normal 90� angle to 83�. Question is, is this done before the ribs are put together or after they are assembled? If after, how is this typically done? For 1/4" thick ribs the spacing is only off by ~0.03", but for 5/8" thick ribs it is nearly 0.08". Phil
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The ribs on the 12 don't get special treatment, the upper wing picture (wings/wingup2/P4112023.JPG) I shot at Kevin's last week had the ribs just glued on. In fact they seemed to have a little gap here and there. Darin
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One thing to keep in mind is that the ribs built from our kits are different in the spar opening area than those made from the plans. We build the ribs as Curtis Pitts did with a spar slot that requires only 1/2 of the rib width to be sanded at 7 Deg. For example, the right half of the front face and the left half of the left face. Note a lack of gap at top and bottom of the spar where it is more critical.
BTW, the gap on the front of the spar will be filled with epoxy prior to leading edges going on.

If the ribs are built to the spec in the plans, the 7 deg angle will have to be sanded across the full width of all ribs both front and rear face of the spar slots. This was a mod to Curtis' original drawings by Mid America when they converted the drawings to CAD before we owned the rights. Both are good ways to build ribs, the plans way is just more work.

It is possible to pre cut the vertical sticks at the spar opening slots to 97 and 83 degrees before building the ribs. Just be careful to maintain spar spacing. KK

04/19/01 AILERON FITTINGS, SLAVE ROD ARMS, SEAN TUCKER'S WINGS

I've been drawing up a materials list for wing and aileron fittings, and I'm having difficulty locating certain thicknesses of 2024-T3 and -T4 stock (i.e. I can find 0.250" -T3, but not -T4). What's the difference between -T3 and -T4 from a structural point of view, or is the difference simply due to working of the material during the manufacturing process? -Cory
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T3 alum is solution heat treated then cold worked.
T4 alum is solution heat treated then naturally aged.

The .25" thick alum parts in the ailerons are hard to bend without cracking if T3 alum is used. By specifying T4, as Curtis did on the Model 12, the parts can be heated, quenched and then bent to shape. After bending, the parts will naturally harden again.

We found these parts, the slave rod arms, hard to make. The bearing is hard to keep staked into it as well. We redesigned the slave rod arms to copy those used in Sean Tucker's wings and make them out of.125 4130 with doublers and tube sleeves welded in them.

04/04/01 PLYWOOD LEADING EDGES, SCARF JOINT, INSTALL

Could anyone tell me what kind of plywood is best to use for leading edges? I'm thinking of using 1/16" mahogany plywood, but am also wondering what works best for softening the wood for bending. - Mike G.
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Mike, 2 layers of 3 ply .8mm finnish birch ply is the best. The ultimate way to do this is to buy a set of super stinker leading edges from Steve Wolf that are pre molded to the correct shape. The Wolf Leading edges, which we include in our wing kits, are not vacuum formed. Steve has a solid wood form of nearly the shape of the leading edge nose rib of each airplane like Model 12, S1, S2, Super stinker etc. I say near the shape because he imperically determined thru trial and error, the required shape to yeild a great fitting part AFTER it springs out a bit when removed from the mold. This also makes a tougher leading edge than the single layer of 1/16" mah ply wet formed in place. The premolded units are made from 2 plies of .8mm 3 ply birch ply glued together in a male/female mold form. That gives 6 plys of birch ply vs 2 plys of mah and one of poplar in the 1/16" mah. ply.

The plywood is pressed tightly to the wood form with a layed up heavy fiberglass top clamping unit that is clamped tightly every inch along the length of the form with mating flanges on both the form and top clamp cover. Steve found that using straps or bungees or only straping at a few points resulted in a wavy part not good enought for a black airplane. The top clamp cover has to completely cover the laminates. Trying to use the actual wing and its widely spaced nose ribs will not give as good a result as full length forms. Steve has many tricks to get the glue on the ply, fitting the plys together, and keeping them aligned while pressed into the form etc. I suppose that knowledge is part of the price for his parts. While I know these tricks, I don't think it is fair for me to leak them out. As for weight, Wolf's leading edges weigh exactly the same as factory S2 alum leading edges foot for foot. That is pretty light.

He also makes them in sections that vary in length depending on the airplane. Typically, 2 pieces for each lower wing, 5 for the upper (1 c/s 2 ea for L and R wing). He can scarf splice these together to make one long piece for each wing. He and I choose to scarf join the sections rather than a stagger joint like Jim Doyle mentioned. No matter what you do, you will see a line at the butt joint of the ply after a few months. The scarf takes care of that. I find mounting the leading edges in sections half a wing at a time enough of a job and would not want to have a 8 or 9 foot long one to deal with in one glue session.

On the Model 12, the premolded leading edge sections are to be scarf spliced over the compression rib with double nose ribs on it at mid span on each wing and at the center section. The leading edges fit so well that all we use is bungee cords and spanwise pressure distribution sticks to hold the leading edges in place.

As far as bending your own leading edges, while water works good to soften plywood for bending and molding, Janitorial or industrial strength Aqueus(sp?) Amonia is much, much better. The amonia penetrates the wood better, faster and evaporates faster. Have good ventilation when using it though! Using amonia comes from the 1930's and 40's wood work. Water is just easier to get. BTW, amonia from the supermarket is not the right stuff to use. A section of 1/16 ply soaked in amonia overnight wrapped in plastic will be soft enough to fully form around the nose ribs without lots of rigging and pulling. It gets posterboard soft. Staple it in place and let dry over night. Next day, remove and reinstall with glue etc. This is a staggerwing, stearman way of doing things. KK

04/04/01 HOW TO DO LEADING EDGE SCARF JOINTS

So what is the basic procedure for doing the leading edge scarf joints? How do you get them cut at the same angle all around after they are airfoil shaped? -Darin
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Darin, scarfing the leading edges is easy and not as critical as you may think. Why not critical? Because we use T-88 epoxy.

First, lets look at how I designed the Model 12 HP wing kits. I have set up the leading edge nose ribs with the required joint in LE sections in mind. The C/S butt ribs and a compression rib that is mid span have double nose ribs making the nose rib 1/2" thick rather than 1/4" thick. The give a 1/2" wide area to glue/scarf the ply leading edges over. No need for an added doubler skin or nose rib mid way between 2 others.

The routine goes like this.........

On the lower wings, trim and fit the inboard LE section to fit flush with the inboard face of the butt nose rib and flush with the outboard face of the mid span double nose rib. Glue it on. We squeegie and thin coat of T88 inside the entire LE skin and brush on T88 to the nose ribs and spar caps. Install the LE using bungees, or straps and staple it to the spar caps thru tack strips so you can remove the staples after the glue dries. No staples in nose ribs.

When dry, remove straps, staples, etc. Use a sanding block and or a 1" wide Makita electric belt sand or equiv. to sand a 1/2" (same width as the double nose rib under it) scarf tape on the outboard end of the skin as wing mid span. Cut trim fit the outboard section like you did the inboard one. Make sure to have 1/2" overlap for the 1/2" scarf joint at mid span. Use the makita or drum sander to sand the scarf on the inside of the OTB LE skin. Easy to do. Fit it to wing again and place witness marks across the scarf joint area to align when gluing. Glue on like inboard one except cover the joint with wax paper and use tack strips to staple at the joint as well. When dry sand scarf joint with only minor filler required if you didn't get it perfect.

Top wing works the same way except install the C/S LE first and work outboard.

Sanding the scarf joints is very easy to do. You have 6 plies of alternating color as well as the glue between them. It is simple to read the color change lines as you sand. One other tip for you guys using ply leading edges, be sure to have your wing set up with ZERO TWIST in it when you glue the ply on. If you don't your wing will NOT be straight. We have one customer who did not watch this and he had to router off the outb LE skin on his top wing and redo it. KK

03/18/01 RIB FINISHING, VARNISH, SYSTEM THREE CLEAR COAT vs. POLY-FIBER EV-400

I just finished building my ribs for both wings and I'm ready to seal them with System Three clear coat. Has anyone tried to dip them in the clear or spray the clear coat on them?
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I used the Polyfiber EV400 on mine, stuff works great. One thing to keep in mind is epoxy won't adhere to anything that's been sealed with EV400 which is why it's recommended you do your varnishing AFTER all the ribs are on the spars. However one good tip is to varnish under your gussets before putting the ribs to spars, again just make sure you don't get varnish where you still need to glue. Steve
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I use System Three "epoxy clear coat" and finish the ribs before installing them on the spars. Use 2 coats. Much easier (my opinion) to coat each rib separately and hang it from a wire to dry, than try to do them after they are on the spars.

Do NOT coat the areas where they will slide over the spars or you will have to do a lot of sanding to get them to fit again.

System Three also makes T-88, so the clear coat and T-88 are 100% compatible. If you need to T-88 to an area that has been clear coated, just lightly scuff sand and then glue normally. Andy
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I know that this System Three Clear Coat (STCC) vs. PolyFiber EV-400 discussion has been fully thrashed out in our forum in the past. However, here are a few things to consider. EV-400 is compatable with Polyfiber covering system while STCC is not approved by polyfiber to go under their stuff. EV-400 has a longer working time based on Denis' post. EV-400 can be sprayed for smoother finishes. Brushing leaves brush marks. I can promise you that brushmarks in the clearcoat, sand scratches more course than 120 grit, and low spots not filled will show up as huge flaws in the finish in a fabric covered structure.

With the limited working time, being forced to brush it, etc., STCC seems to be more trouble than it is worth. Sounds like it takes a long time to CC the ribs this way. I can spray EV-400 on a pair of fully assembled wings, 2 coats,both sides in about 2 hrs using a small touch up gun. I'd put my rib coverage and gussett coverage up against anyone's brushed work. Our standard proceedure for over 20yrs has been to spray a double coat of EV-400 on, sand smooth with minimal bust thru. Then recoat. Before covering, we sand again with 220grit paper making the surfaces that touch the fabric furniture smooth. Then cover. Keith, I'd consider EV-400 and varnishing the wings after they are fully assembled as has been industry standard. As an engineer, I consider the greater surface area interaction of the fabric to wing vs. a few glue joints (if precoat ribs) of greater importance. While I have not tested STCC and polytak fabric attachment cement for compatability, I have tested dozens of other varnishes, clearcoats, urethane and epoxy paints and primers. Most all of them dissolve or wrinkle up when the glue is applied to it. Most all the fabric systems use a glue similar to Polytak all of which are thinned with MEK. A good test for STCC would be one like I did for the powdercoat we use on our parts. Take a coated sample part and place it in a can of MEK and leave it there for 30minutes. If it is unharmed, you are good to go with polytak glue. If it wrinkles or gets gooie, don't use it under the fabric.

BTW Darin, that wing you took pics of had a bunch of mud (polyester filler) smeared on it and not yet sanded off. That wing and the mate to it are finished with the bodywork and will get varnished soon. The the top wing will follow. Note that the filler goes on before varnish. KK
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For what its worth, I dipped all of my ribs in a thinned varnish bath and let them dry before I assembled the wings. I didn't have any problem sanding away EV400 in the areas where it interfered with bonding. Test samples with T-88 broke wood, not the bond. Incidentally, I coated the entire inside of the gussets and members covered by the gussets with glue when as I applied them. This removes any concern about varnishing inside the gussets. Jim Doyle
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I just received a FAX reply back from Dick Anderson, the head tech at System Three. MEK has no effect on either cured T-88 or their cured "epoxy clear coat". So anyone using epoxy clear coat from System Three rest easy. I had this as a verbal several years ago, before I decided to use it. But now I have it in writing as well.

Kevin, and anyone else doing "commercial" covering: I understand you have no choice but to use all "Polyfiber" products so you do not have the possibility of finger pointing if something goes wrong. You have to do it.

For me, that is not a concern. I like the deep penetration of 2 coats of the epoxy clear coat, and the bonus that it strengthens the wood significantly in the area of the penetration.

On "thinning" T-88 or their epoxy clear coat----do NOT thin by adding anything. PERIOD! The correct way to "thin" T-88 is with a LITTLE heat to each component (if you apply heat to the mix, you will accelerate the cure - yes, heating the components will also accelerate the cure), but not much over about 100 degrees F.

Epoxy clear coat from System Three is almost water thin and requires not thinning. Andy

03/02/01 AILERONS, SPADES, BALANCE

Yesterday I got talking with someone about ailerons and rudders. He mentioned that when he builds his 12 he plans on putting spades on both wings, which should save weight since he'd get rid of lot of lead. He also talked about weight in the slave struts, bending bellcranks and other topics. I wish I knew enough to understand all he said or could remember it all. He was a little suprised when I sent him a picture of the red 12 showing spades on the lower wing, he didn't think spades had been tried before on a 12.
I don't think I'll be getting into aerobatics like that, or flying my airplane that hard. I also plan to continue building like plan and you do as much as possible, but I'd like to know your methodology on your wing/aileron design and the tradeoffs of weights (60lb?) vs spades and anything else you have to say on the subject. Also what was the experience with the spades on the red one? Its more of an educational topic for me, so there would be no hurry on answering it.
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The ailerons on the HP model 12 wings are 100% balanced with 0.8lb of lead in each aileron. There is no(zero) added weight in the slave rod. So, the total added weight to the aileron system for balance needs is 3.2lb. Why? Because of the nearly 30% hinge location in the aileron and the fact that our HP ailerons are about 5lb each rather than 9-10lb each. So, adding spades as a subsititute for lead is not needed. We have spades available for those who think they need them. The ailerons in the HP wings have pressures that closely match that of a SU-29 that belonged to a good friend of mine. He had Nikolay Temofeev set up the ailerons which made them slightly heavier than stock. The spades we sell are set up to allow high speed fully aileron deflection without being so big that they snatch. The biggest issue with this airplane is that the typical lycoming pilot will roll left. He has to learn to roll right which is a harder pull on the right arm than going left. So, the lyc pilots think a right roll is a lot harder than it is to a suk pilot. Also, rolloing left doesn't let the engine torque help with the roll. Both Ben's red 12 and our yellow 12 have spades on them now. We have just built 24 sets of spades for those who want them. I personally prefer it without spades.

So, there is no 60lb vs spades trade off. 3.2lb and add spades if you need them is the actual set of #s. KK

03/02/01 T-88, GLUE

I got a small pack of t-88 glue from the local woodworker store yesterday, as I have some pieces of wood for a stairway railing to glue together. Thought I would try to use some of it and get a feel for what the product is like.

Mixed in my low heated garage last night, and boy is it thick. Temp was maybe 50F. When mixed, it was a whitish color, and spread very hard. Is this normal or is it a factor of the temp. I took the glue inside last night, and will try another glue job tonight, but will do it inside where it is 70F and see what the difference is. I seem to think that I have heard that sometimes you thin it out to get penetration with several coats like when you are putting the upper wing spars together at the scarf joint. I think I read that in Mac's newsletters but not sure. I do so much reading that you tend to forget.   What do we use to thin it out with?

Can it be thinned out a lot and used as a sealing varnish, or is there a separate product for that?    Thanks, Paul
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It's temperature. Hale used to talk about microwaving (ever so slightly!) the resin and hardener before mixing for this reason. I also know he routinely cut it with lacquer thinner for fiberglassing. I don't believe he used it for bonding when he thinned it this way. I read an article about thinning epoxy in the Experimenter (I think) that really showed a serious bond compromise for thinned epoxy. For what its worth, I use T-88 for joints where I need a little gap filling quality, but I really prefer FPL-16A (Hughes epoxy) when I need to bond large surface areas. It has a consistency like coffee cream (and looks like it too!). You can get it from Aircraft Spruce. Incidentally, Hale used FPL-16A for the splice joints. -Jim Doyle
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T-88 has the acceptable temp range of use right on the bottle. As I recall, (I don't have a bottle of T-88 here at home), min temp is 50f not 70f and it can be used at much higher temps than 80f. Temp changes working time.

As for adjusting the viscosity, it is easy to do with the microwave as recommended by System Three. On a 50f day, approx 15-20 seconds on high for the clear part and about 25-30 seconds for the amber part. Make the stuff think it is summertime! We stick the 2 squeeze bottles in the micro and heat them up, then squeeze out what we need.

On a second note, if your T-88 or other 2 part epoxy gets kinda sugary looking, it has gotten cold in storage. It is perfectly fine and only needs to be heated up to return to the non-sugared form. Use the microwave or boiling pot of water to heat the affected component until it is nice and clear again. Then let cool. Ready to use. (this from tech guy at System Three and we have done it and it works).

Poly Fiber EV400 is a great product for sealing the wood.    KK
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Paul, My 2 cents on T-88: I keep it in the house all year. DEFINITELY easier to use, and penetrates better if at 70 degrees or above. NEVER thin T-88 with anything, or by any means other than heating it VERY slightly above room temperature (to maybe 100 degrees or so). BUT also keep in mind that as with any chemical reaction, the higher the temperature - the FASTER the cure! According to System Three's head tech guy:
- if you need to THICKEN T-88 so it does not run on a vertical surface, for example, use "silica" (white, kind of like talcum powder). If used in small amounts, silica does not affect the joint strength. I use this when glueing ribs to spars so it does not run out of the vertical joint. Works very well. Mix to about the consistency of toothpaste, or cake frosting. - if you need to reduce the WEIGHT of the T-88 like when it is used as a non-structural fillet, use "micro balloons" (usually purple). I've not found a need for this type of application on the 'Bolt yet so cannot comment here. If one part crystallizes (speaking only of T-88) just set your oven on the LOWEST possible setting and put the container in for maybe 30 minutes. It will go back to liquid. There is no shelf life for T-88. It never goes bad. Yes, use something much thinner for the big scarf joints. System Three makes "epoxy clear coat" which is water thin, penetrates VERY deep and easy to use. Also dries nearly water clear so it looks very good too. The great thing about their epoxy clear coat is that since they also make T-88 the 2 products are 100% compatible. If you want a very deep bond you can first apply a coat of epoxy clear coat. Allow that to cure. Then scuff sand and bond to that with T-88. I've done this on many different "lab" test samples I have made up, and ALWAYS the failure occured where the epoxy clear coat stopped in its penetration (in other words, VERY deep into the wood). Also, due to the very deep penetration of the first coat, a second coat will be required in order to fully seal the wood. This offers 95%+ protection against moisture migration into or out of the wood. And another benefit of System Three's clear coat is that it is 100% solids, so after it cures the wood is actually STRONGER than before it was applied. This is NOT true for ANY epoxy sealer that contains any form of "thinner", which most others on the market do. And no, I am not paid by System Three. I've just had outstanding experience with their products and their service for many years now.     -Andy

02/08/01 VARNISH, SYSTEM THREE CLEAR COAT vs. POLY-FIBER EV-400

I like the Stits Polyfiber 2 part epoxy varnish. The guarantee it to be compatible with the fabric systems including their coatings and fabric cement. The polyfiber coatings will wrinkle 1 part varnish like paint stripper does. The Poly Tak fabric cement will melt wrinkle and or dissolve many cheap primers like the spray can chromate stuff.

So, I recommend the EV-400 varnish. You can buy it in qt or gal kits including thinner and catalyst. This is the coating product we recommend for your wings when you build them. It ain't cheap but then again, it is cheaper than having to do it all over again when the cheap stuff craps out on ya. KK
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I like System Three's "epoxy clear coat". It's water thin so it penetrates DEEP. It has no volatile components (it's all solids). It actually strengthens the wood, unlike varnish which does nothing to strengthen and very little to moisture proof either (I'm talking about "spar varnish", urethane varnish, and all one-part "varnishes" here). Also, since it is made by System Three who also makes T-88, the 2 products are 100% compatible (in other words, no problem glueing to a surface that has been coated with epoxy clear coat). 2 coats are necessary because the first coat soaks in so deep.

I have not used Polyfiber's 2 part epoxy varnish, but according to Mike Grimes, who the FAA enlisted to rewrite the wood section in AC 43.13, a 2-part epoxy sealer out performs all the one part sealers by a large margin.

One thing to watch for when buying a 2 part epoxy is to check the label to see if it has any volatile components. The best has no volatile components. The volatile components are used to thin the epoxy so it will penetrate. System Three's is naturally thin, so they don't use any volatile components.

(Yes, I've had lengthy discussions on this subject, and others, with the head tech had System Three over the 6 year course of my project, BUT I am not, nor do I claim to be, any kind of expert on the subject. I'm just passing on what I know).

System Three's web site is at http://www.systemthree.com Later, Andy

12/04/00 RIB JIGS, RIB CONSTRUCTION, NAILS, STAPLES, WEIGHTS, ROUTED RIBS, TRUSS RIBS

I am getting ready to start the wings on my S1-11b. Is it possible to construct a wing jig by routing the wing shape into a piece of plywood? Is there a better or easier way? Thanks, Stan
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I made a simple jig out of 3\4 ply. I had my local copy shop make some copies of my full sized rib plans and glued them to the ply. Andy has a really neat jig he built for his ribs and I think he has some pics of it too. Cheers, Steve
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I think one of the most easy to use and simple to make rib jigs is the Plywood board well varnished and waxed with 3/16" hardware store rivets forming the holders. You just lay out your rib structure on the plywood and then drill the 3/16th holes where you need them to hold all parts exactly in place. Be sure you do not put any rivets under the gusset and removing the rib is very easy even if you spill a little glue. You only need to punch the rivet out with a drift and if needed slice the rib off the board. If you can find 3/16th by 1" rivets you can use 3/4" plywood and have just the right amount of rivet showing. Countersink them so they don't stick out the back. If you use T-88 you can make three or four ribs a night. I like to use weights to hold the gussets for drying but staples are the next best way. You pull the staples when the glue is dry. If you use nails you will probably leave them in and that adds unnecessary weight and may eventually cause the wood to rot from corrosion, plus those 1/4' nails are a bitch to handle.   Keenflyer
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On the issue of weights and rib building, is there an optimum weight for holding the gussetts in place while the glue dries? I would think that too much weight will force out too much glue leaving a weak joint. Also, the question came up some time ago as to how the ribs should be made for the upper wing so that they will fit snugly against the spars. The question is listed in the vault but I couldn't find an answer. Any ideas? To use square stock where the ribs straddle the spars would leave gaps. It would seem necessary to bevel some 1/4x1/2 stock to the correct angle and dimension but maybe there's an easier way? Bart
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All you need to do to fit the ribs is sand a little off the inboard side of the fwd. upright and the outboard side of the rear upright. Use epoxy to fill any loose fit when you glue rib to spar. This goes much easier than it sounds, only takes a few minutes per rib. With about 6 degrees of sweep over a 1/4" upright, you won't have much fitting to do anyway. There is a sanding bar on the market that does a real good job on this. I can't think of the brand name, but it is about 1"x8" and 1/16" thick and coated with some sort of grit that last a long time. Some of these joints that need strength ( compression ribs, nose ribs ) have triangular strips cut to proper bevel and glued to reinforce the joint. Never used weights to clamp, so I can't help with that question. I like to use staples and remove them after glue dries. Bill
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Bart, You are being too carefull in this. Build the ribs to the plan and when installing you can bevel a little on each side to fit the 6 & 1/2 degree angle. An Exacto knife works good for this cut. T-88 is pretty thick and will fill the gap, if any. You could add some micro balloons to make it thicker and lesslikely to run. Remember epoxy is heavy so use only the minimum amount and wipe off any that squeezes out. On the amount of weight again I think you are trying to be more precise than is needed. You should try to get a thin layer of glue on all of the 1/4" square spruce and then any reasonable weight will do just fine. If you take a 2" round billet of mild steel and cut 1" slices off of it those make great weights for this operatioin. You need twenty or so of these. Any local machine shop should be able to supply these for a small price.     Keenflyer
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Bart: I poured lead into a form to make my weights. My buddy took old wheel balance weights and melted them for weights. He got them for free a the tire store. A machineshop may have cut-off that you can get for nothing or scrap price also. Charles in Kansas
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In the course of teaching cabinet making at Garret Aviation (biz jet 737's), and from over 50 years in the Woodworking School of Life, I will pass on what I know to be facts about glueing. I have also talked at length over the years with Dick Anderson, the head tech at System Three (makers of T-88), on this very topic. Too little clamping pressure is as bad as too much. Too little pressure and the glue line is too thick, which weakens the joint. On the other hand, excessive clamping pressure squeezes too much glue out of the joint resulting in a glue line that is too thin for optimum strength. Novice woodworkers mistakenly believe that high pressure forces the glue into the wood---wrong! I strongly recommend, and so do the glue manufacturers, that you make test samples with whatever clamping/weight system you plan to use. Allow these to FULLY cure (at leat 72 hours for T-88) and then test them to destruction. Save these samples (bag and tag) to show the FAA inspector when it comes time to have your aircraft inspected. A good glue joint will fail well into the wood, NOT at the glue line between the parts. If you get a failure at the glue line, investigate and find out why! Failure at the glue line is UNACCEPTABLE with any glued joint. Also, I have learned that "time" is important when gluing. Glue does not instantly penetrate into the wood fibers. It takes time. I found it best (by making test samples, and testing to destruction) to, in the case of a rib for example, apply the epoxy to all gusset areas first, then lay the gussets in place and gently push them down, and THEN go back and apply the clamps. Always work from one end of the rib to the other, then go back to the start for the next step. This allows the epoxy time to penetrate before the clamp pressure forces excess glue out. My tests conclusively proved deeper penetration with this method as compared to applying the epoxy to one gusset area and immeditely clamping the gusset in place.
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Our Skybolt was built with routed ribs. Sure saves a lot of work. I really don't know why guys want to build builtup ribs when the routed ones are stronger and much less work intensive. There is a slight weight penalty but it is only a few pounds.   Keenflyer
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Actually, as proven by tests, routed sparcraft type ribs made of 1/4" aircraft ply are NOT stronger than the equvalent truss rib they replace. A Model 12 truss rib is good for 850lb of static load. And, as you wrote, the ply ribs are heavier. They ply ribs break at lower loads than the truss ribs do. This has been shown time and time again with sparcraft pitts wings. Add to this that most builders who want to save time by using ply ribs also want to save $ by using marine ply, and you get ribs that can be up to 40% weaker than the original truss rib. A solid ply rib can get close to the load capacity of a truss rib but it is VERY HEAVY.

The beauty of the truss rib is that it places the wooden member and its grain in the line the load is applied. A ply rib with the typical rectangular holes or round holes does not have a proper load path arrangment and the grain is in at least 3 directions. If a builder is going to sweat a pound or 2 for front instruments which have good usefulness and function, why throw in dead weight with ply ribs?

Our rib jigs are routed boards with ejector buttons. We have used these for years and have over 50 builders using them on Model 12 ribs. We recommend using staples, 3/16" crown 20ga. to install the gussets. The weight program for rib building to eliminate nails or staples is not needed. To quote one of my Dad's favorite sayings: "That's alot of sugar for a nickel". All the staples/nails left in a set of skybolt or 12 wings weighs only a small percentage of the added weight using plywood ribs would. KK

12/04/00 JOINT PREPARATION BEFORE GLUING, SANDING

On sanding-- FAA AC 4313 states that wood to be glued shall never be sanded for proper fit. Machining (plane or sawing) is acceptable. Reason is that sanding debris clogs the pores with saw dust which reduces glue penetration and can weaken the joint.

Shop tip: disc sanders are intended for end grain sanding, belt sanders are for parallel to the grain sanding (correct use of production wood shop tools is one of the many courses I teach). Later, Andy
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Hhmm ... the AC says "softwood" must not be sanded prior to bonding. Is this also true for the sort of wood we use mostly (spruce, mahogany - "hardwood")?    Tom
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On the subject of sanding, it is not only the dust that interfers with the glue joint you are also polishing the wood to a certain extent and it affects both softwood like spruce and hardwoods alike.   Gary
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With regards to the sanding of rib members, if one were to sand-to-fit the ends of rib truss members, would it really matter as the strength seems to be in the gussets? Bart
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No it would not matter. The shear load path determines the joint strength. Often the capstrip does not match evenly and it is necessary to block sand (locally) to level out the joint members before bonding the plywood gussets. Certainly you want to blow away any of the saw dust before you bond. If any of this really bothers you, make a couple of samples and break 'em. If the failures are in the wood (they will be, especially if your using a good epoxy like T-88 or FPL-16a) you don't have a problem.   Jim Doyle, Stress Analyst
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Starting to read the posts and on the sanding point here's a neat tool for every tool box. It's a razor blade plane. Great to tune up the fuzzies before gluing. to keep in compliance with those fedrallies. Cheep and better than a bic razor. Picture   Later, Denis
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Forgot to add about plywood: Birch plywood must be scuff sanded before gluing. Mahogany plywood needs no scuff sanding, but it wouldn't hurt. Andy

10/30/00 WING, DRAG WIRES, DRILL, TENSION

I just finished drilling my drag/anti-drag wires and I mis-drilled two of the holes. The hole that goes through the spar 0.125 inch below the centerline crept up while I was drilling to the point where it now interferes with the other wire where they cross in the spar. Is it safe to fill the incorrect holes with a dowel and re-drill them? Mike G
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You did a GREAT JOB if you only had 2 of the holes intersect! To quote Curtis Pitts on this subject, "Just waller 'em out 'til you get both wires to go through". In other words, you can fix it easily without a dowel job. First, what size hole did you drill? Should be .200 to .203, we use .203. 3/16" or .188 is too tight. Here is how you fix it. Stick an old bolt or some scrap 3/16" rod thru one of the holes. Run your drill thru the other so that it hits the bolt/rod and claws it's way past it. This will slot the hole away from center a bit. Repeat swapping the bolt/rod and drill. This will clear the holes so that the wires pass each other. Yeah I know it sounds crude but that is how the "Man" said he does it, the factory does, it , we do it etc. KK
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Once again this group has helped me out. I drilled mine, coated with epoxy and reamed to 3/16". I had no intersecting wires but had one at a bit of an angle. I egged out the hole and lost sleep over this for months (and seriously considered redoing the wing). Thanks, Charlie
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Now that's enough you're starting to get me real worried here!!! The answer is a Jig (Period) To attempt this free hand is a gamble at best. Andy I've seen you have built one so if you have a chance could you explain your set up? And how well it worked . Your construction Hints etc. Any one else put your 2 cents in also . Ya it's simple enough and I hate to be loosing sleep over a miss drilled hole. My instructions indicate 25# to 35# pull on the wires IS THIS FOOT OR INCH POUNDS??? And does this wire have to be inspected often (inspection Covers).   Later, Denis
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I made a hardwood block with the holes drilled at the correct angle and spent a little time aligning the block on the spar before drilling. I agree with Kevin, that the holes should be oversized. I drilled them with a 3/16 bit and then enlarged them.   It helps to not have the adjacent ribs glued in place - the trailing edges of the ribs get in the way of the drill. The 25# to 35# pull is kind of a nebulous thing if it's not accompanied with a deflection. Basically, what your trying to do is pre-tension the wires approx. 10% to 15% of their rated strength. The pull should be accompanied by a deflection. For a given pull, the deflection is a function of the tension. The assumption is that the foundation that the wire is attached to is relatively flexible so that when you pull on the wire the tension in the wire remains unchanged, in other words the wire doesn't stretch. Let P= 25# pull (fishing scale), L= length of the wire, T=tension in the wire (10% of the rated strength). The deflection in the middle of the wire is PxL / (4xT). I made a little gauge when I did mine. It's a fussy operation. Ben Owen wrote a couple of pretty good articles on this a couple of years ago but his math was wrong. I think he divided by 2. Hope this helps. Later, Jim Doyle
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I machined my drill jig straight from McKenzie's newsletter sketch. Worked like a champ. If anyone needs the info I'll dig it out of McKenzie's stuff and send it.   Later, Andy
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Mike and I have emailed back and forth a few times on this since his post. I explained to him how to fix it and why it happens. Even with a jig (we have a very nice jig we built that we use and have loaned to builders), a doug fir spar will cause the drill to wonder and require "pulling" a few holes. Mike brought up the fact that one bit he used was dull. The holes should be .203 in dia not 3/16". The bits should be VERY sharp, and an airdrill should be used. Turn the bit as fast as you can and use light pressure with a peck drill technique. Peck drilling is to ease the dit in then pull it back. Repeat again and again until thru.   KK
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This was another one of the things I had fits about. Ended up having one intersecting set on my lower wings, called Hale with my tale of woe and was told the same thing Kevin said. So it's great to know that it's nothing to loose sleep over. On tension I used a fish scale and Hale's instructions, 30lbs with a 1/2 inch deflection. One other thing, it's a good idea to have a "buffer" where they cross so they don't rub on each other. I just used some vinyl tubing and ty wraps. Cheers, Steve

10/09/00 AILERON FLUTTER

There's a very interesting page on Kurt Haukohl�s Musclebiplane site. Seen that I�ll definitely build my ailerons as light and balanced as possible!   Here is the URL:
copy of http://musclebiplane.org/htmlfile/spardam.htm

Anyone some more info/knowledge/pointers/tips regarding aileron flutter?

Later, Tom
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Best tip for flutter is to operate the airplane within the design parameters. That eagle had been flown hard, a lot. At the time of the flutter, he was diving into the box about 30 mph over Vne speed. The friese aileron design is used on the eagle. It is easier to balance than the symetrical style of the S2B yet not as easy to balance as the Model 12/super stinker ailerons. The eagle ailerons are not mass balanced. Model 12 ailerons are. I don't know if Skybolt ailerons are shown balanced in the plans or if builders do it. We balance all the ailerons we sell so the customer doesn't have to do it. Building the aileron light is a good thing to reduce total system weight and inertia. Lighter ailerons take less lead to balance them. The aerodynamic bal aileron design of the 12/ super stinker is not as prone to flutter as the friese style on the eagle and skybolt. None the less, we do balance them. KK
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Charles Dupois and I discussed this a little back in early September. I sent the following to Charles. It's pretty general. The point is that while light is always good for control surfaces, flexibility is not. Do not sacrifice stiffness for weight in your control surfaces.

...I'm no aerodynamicist. Having said this, as I understand the flutter monster your friend is correct in some cases. Flutter is a harmonic cycle in which two moments compete (one decays as the other increases and static equilibrium is never reached).   Many things can keep equilibrium from ever being reached, for example, angular momentum. Due to the dynamic nature of the system, the surface is constantly being accelerated and decelerated so these moments must accelerate and decelerate a mass moment of inertia. It's not hard to see how a really heavy control surface could have some problems. By balancing the surface about the hinge line the mass moment of inertia is reduced and the surface is less likely to develop a harmonic. A flexible surface can have flutter problems. The two competing moments are aerodynamic and structural. As the surface is loaded aerodynamically it flexes until the AOA reduces the aerodynamic load and the elasticity of the structure pulls the surface back. It is possible for the structure to bend in to opposite direction (due to momentum like a fishing rod) a harmonic ensues and the flutter monster raises its ugly head!

Generally, you do not want to balance 100% because overbalancing can aggravate the flutter problem. This is where things get kind of touchy feely. In general you want to load the surface somewhat to take the slop out of the control system. 90% to 95% balance comes to mind.

Jim Doyle Stress Analyst
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Charles and the gang,   Just my opinion but what Mac says to do is wrong. You do not want the ailerons 100% balanced. This causes flutter, not prevents it.   Hale's response to us on this subject was that the slave struts themselves are sufficient to do the job. I have exceeded 240MPH with our Skybolt smacking the stick and rudders in all axis and no flutter.   We have no weight in our ailerons.    Of more importants in avoiding flutter is to have all systems with minimum slop. With the stick locked in place you should have less than 1/16" play in the elevators and aileron trailing edges. Also be absolutely sure you have no slop in your trim tabs.   This is why Hale always told everybody not to use the Vernier control recommended by Mac.    A push pull set up like that will always have a good deal of slop. When you use pull-pull cables you can take the slop out with the turnbuckles. For those of you who are making modifications to your airplanes, keep in mind that lightness of all control surfaces is paramount so don't go covering those ailerons with plywood. Keep them light and you won't need any excess weight. Do put the 2 &1/2 lbs of lead in the elevator counterbalances of course, but again do not attempt to balance them 100%.   Keenflyer

10/01/00 WING, AILERON HINGES, TORQUE ARMS

No torque arms on Kimball wings? Good catch!! I did stress 'stock model 12' in reference to the torque arms. In the stock wings for the 12, the aileron hinges are not close to compression ribs. Some are mid bay some are near truss ribs. The ailerons are driven at the inboard hinge. The torque arms are used to transfer the the twisting loads to the front spar as I described in my last post. On the prototype 12, Curtis left off the outboard arms using only the ones at the inboard driven hinges.

On our wing kits we sell, we eliminated the torque arms saving a few pounds. How?? Well, we have 3 hinges per aileron with the middle hinge being the driven one. The hinges are all located at compression ribs with the center driven hinge being located at the I strut. The I strut IS the torque arm for the driven hinge. Having 3 hinges per aileron rather than the stock 2 reduced the % load at each hinge. When we did the calcs on this, we found that adding some corner blocks to the compression ribs in the wings would more than double the required strengths needed to handle the twisting loads. Basically, we redistributed the torsional load on the rear spar making the torque arms unnessary.

So, on the stock wings, use the arms. Our kit wings, don't use them. KK

09/03/00 WING, RIB CONSTRUCTION, EPOXY

I didn't use any staples or brads on my ribs, just clamps and weights while the T-88 cured. Nothing fancy on my jig either, used some cheap 1" thick particle board shelves from the local Home Depot. The rib plans come full size so I had my local copy shop make some direct 1 to 1 copies which I trimmed down and glued to the particle board. Trimmed off the excess and made room for clamps, screwed in some pieces of 1/16 ply to support the rib material and started making ribs. For my nosepieces I made one "master nose" and used my router table with a pattern bit to make all the rest. This way there all the same.

You might have to use my email address to view the pictures, sgossett@erols.com. Hope this helps some. Cheers, Steve
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I don't use any brads, staples or anything else to hold the parts together until the T-88 cures. Just clamps. Any mechanical fastener is redundant after the epoxy cures, and in fact could possibly weaken the joint. Weights work just as well as clamps (which is what Hale always used, steel slugs cut off a piece of solid steel bar stock).

My rib fixture is in my site. It is ejector style. On my last iteration I used 1/4 x 1/4 aluminum bar stock for the top and bottom airfoil shapes. The fixture is oak plywood, 3/4 thick. Just keep the oak waxed (every 3 rd rib works fine) with Turtle paste wax and the ribs pop right out. Melamine can be used for the base of the fixture, as no glue sticks to melamine. But it's expensive. And remember, the fixture is like a race car---it can fall apart after the last rib is made.

I bought my 1/4 ply nose ribs from Hale, and that saved me a LOT of time as they are routed in an aluminum fixture so all are identical. Jere Larson has all that tooling now, and I assume he will sell just the nose ribs, but you'll need to contact him on that.

Another thing I did was to buy one built up rib from Hale for the top and bottom wings, and then I literally built my rib fixture around those ribs. I did not use the drawings. So my ribs are identical to Hale's. I found it easier, too, to make the fixture using a rib than making it from the drawing. Later, Andy
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Here's what I did for building my ribs. I took the rib sheets to the local blueprint store and had copies made. I checked the dimensions for the ribs, especially the front and rear spar locations from the nose of the rib and then laid the cutout pattern on an oak stair tread. I then laid a piece of polyethylene dropcloth over. I bought some 1/4" polyethylene flat stock from one of the local plastics companies...get some scrap at least 5' long...and then ripped off some long pieces about 3/8" wide. Then I screwed the 3/8 x 1/4" pieces just on the outside line of the upper and lower capstrips on the rib plan and use some #5 screws to hold them into place...any small screws will work here. I also cut some short pieces to use for holding the rib upright and diagonal bracing in place. I also made 1/4" thick spar pieces that where screwed into the appropriate place on the plan. Using this jig it took me about 45 minutes per rib. I've only done the lower wing ribs so far. The wife has had me building plantation shutters for the house and I'd probably get hollered on if she caught me out in the shop building the upper ribs <g>. The T-88 won't stick to the poly and all you have to do is just pull a few screws loose to get the rib off the plan. A poly drop cloth from Home Depot works just fine for the cover between the plan and the wood being glued. Happy building. Bill Hibbing

08/24/00 WOOD STORAGE

The ideal moisture content for most woods is around 7 to 9%. You live in Indiana. High humidity country, especially in summer (I grew up in Chicago). The most important thing is to season or cure the wood before you use it. No matter what whoever I buy my lumber from says, I let it sit in my shop for a minimum of 2 weeks before I do anything to it. Long pieces should be stored nearly vertical, allowing for air circulation all around the pieces. Air circulation is the single most important factor in storing lumber of any type. Warping is caused by uneven moisture loss, or gain. If only one or two faces of a board are exposed to air circulation, warping can result. You bought a wing kit, so I assume everything is pretty close to final size. If you have lumber that is considerably oversize, however, it is best to take it down in steps to its final size, allowing it to sit at least 2 weeks between steps. Removing significant amounts of wood (especially by ripping, or planing) changes the stress in the board. The fibers must equalize to the new stresses, and that takes time. Back to the moisture content, for our purposes don't worry about it. If your moisture is a little higher than 10% it does not significantly affect the strength. If you store it in a basement in your part of the country, a dehumidifier is a good idea. Let it run continuously as long as you store wood down there. Store your lumber in the place where you will be using it. Don't store it in the garage if your shop is in the basement! This applies to spruce, hardwood, and Home Depot stuff. Let it set at least 2 weeks before you make anything out of it! Which leads to your other question, how much should you have on hand? Since curing is an important step, and the longer the cure time the better, buy whatever you can afford when the price is right, and carefully store it. Go into any lumber supplier that stocks the exotic hardwoods. You will never find their wood stored horizontally, except for very thick lumber like 8-quarter stuff and bigger and then it will have spacer blocks to allow for air circulation. They always store their wood vertically in bins. Hope this helps! Later, Andy

07/24/00 SKYBOLT ROUTED SPAR, COMPRESSION RIBS

Dennis, I did not get any of the options. I'll give you my take on the options. I don't buy the routed spar weight savings. Depending on when you talked to him, Hale would tell you he saved something like 17 pounds by routing the spars. He justified routing by first bringing up the fact that lots of early biplanes had routed spars and second, that a "DER looked at the numbers". I don't think the DER looked very closely. I seriously considered routing. Three things made me decide against it.

1) Using a density of spruce of 27pcf I couldn't find 17 lbs of wood to cut out of the spar. If you don't count the bays with doublers and route 10" by .25" and leave .5 caps I calculate about 13 pounds saved. If you leave 1" caps I calculate 8 pounds saved. I've never seen the routing pattern they use but the weight savings seems very optimistic.

2) Spars in externally braced wings are typically sized by three things. Axial compression due to internal and external brace wires, compression due to bending from external airloads about the minor axis , and lateral buckling about the major axis due to compression from internal and external brace wires. Take, for instance, the top wing spar. The axial compression capability is reduced by 27%. The minor axis bending capability is reduced by 8%. The lateral buckling capability of the spar with 1" caps is reduced by 68%!   I calculate a routed top wing spar failure in lateral buckling for a 2000 pound Skybolt in a 9g positive high angle of attack maneuver.

3) The Pitts (all of them I think) uses sheeted compression ribs (box ribs) with the compression members inside the sheeting.   Curtis Pitts told me that he went to this design in the S-2A when he noticed twisting (wrinkling in the fabric) of the rear spar on the upside wing (down aileron) when the aircraft was rolled vigorously. The box rib supplies a torsion constraint that makes the spar much more torsionally rigid locally. A side benefit to the box rib is that it is much easier to rib stitch. If you route the rear spar of the Skybolt and stick with the compression strut design you will compromise the torsional capability. Incidentally I didn't build box ribs but I would recommend them.

Many early airplanes did indeed utilize routed spars but they also employed very thick spars due to the thin airfoils. Routing is fine if the spar is DESIGNED for it. You could make a lighter spar without compromising strength. One easy way would be to use Douglas Fir and reduce the thickness. My conclusion was that routing my wing spars was not a good idea. I'm particularly glad I decided this since I'm using the M-14P and will probably want to fly with a heavier gross weight.

Laminating would make a stiffer spar in at least one direction depending on how they do it. I think it would end up being unnecessarily heavy.   Jim Doyle, Stress Analyst

07/06/00 WING DRAG WIRE BLOCKS

The Model 12 uses Douglas Fir as the drag wire blocks and we can use the maple as a substitute if needed. Poplar and spruce are just too soft. We don't use AN970-3 washers for the drag wires as they are too soft also. Factory S2, Super Stinker, and Model 12 all use .100" 4130 cut in a small rectangle. The piece has a small hole in it to to accept a nail to keep it from spinning as you tighten the wire. This type washer ended most of the spruce drag block problems on S2 aircraft. KK

06/13/00 SQUARING WINGS, LEADING EDGES

Charlie, a 1/8"tolerence is a perfect airplane. A tolerence that tight makes a few assumptions. You must assume the fuse is straight and that the wings are both trammed perfectly. I'm not saying that it is impossible to have such fine parts, but it ain't the norm.

SO, when he says 1/8" difference, that means using a tape measure measure from some convenient point on the wing, says the rear I strut bolt hole, to a point on the tail post, say at the lower longerons. Repeat for the other side of the plane. If these numbers are within 1/8" of reading the same left to right, that is a PERFECTLY SQUARE lower wing set.

What is good enough? I our shop, I have seen as much as 1.5" difference in this measurement. Our personal limit is 1/2" difference. Keep in mind that we deal with alot of very old, previously wrecked airframes. In the Model 12 world, we don't give a number for the builders to look for when measuring the squareness of the wings. We tell them to measure the lower wings and write down the measurements. What ever the difference, set the upperwing to the same difference which will set the upper wing directly square with the lowers which is more important than how they square with the tail.

What are some tricks to use for a Skybolt or S2 Pitts type top wing with a center pylon?? First, do this measuring BEFORE you install you leading edges. IF you can, do this before you weld on the lower wing attach fittings, measure the squareness and clamp perfect then weld on the fittings. Or if your fittings are welded on and leading edges are off, retram the lower wings to get the measurement good enough to suit you. Then install the leading edges.

Top wings on a center pylon airplane are tough. You have to get the pylon on perfect and the fitting perfect of wait and drill the fittings when you install the wing for the first time and measure the squareness to the tail. Again here, if the LE is not on, you can do the best you can with the fittings, then retram the wings to get them to have a satisfactory measurement. Here use the EXACT SAME POINT to measure the upper wings as you did the lowers. You can have some slight lean in the tail post if it is not rigged perfect with the tail wires. So, eliminate the chance of error when aligning the upper and lower wing by using the same EXACT reference point, say the tail post at the lower longerons. Keep in mind that the diag measurement for the upper wing will differ from that of the lower wing.

Hope this helps answer your question KK

04/30/00 WING KIT

Darin, you can do about 2 months of spare time or 1 month of full time work on the wing wood prior to needing the wing metal. I normally advise ordering the wing wood, wing metal, and ailerons at the same time. The wood usually arrives first and then the meatl get there by the time you need it. As it stands now, we have a set of wing metal in the current batch that is not sold. This batch will be ready in a few weeks as will the wood kits. Ailerons should be avail in about 2 months. We are printing more plans sets this week and should have them avail by next week. KK

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