Pietenpol lift struts; $65, a free education, and fun with friends..

Builders,

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Below a story in pictures of making a set of steel lift struts for Terry Hand’s Pietenpol. In this previous story: Pietenpol Project – Terry Hand, we had an overview and mentioned he was measuring it for lift struts. Terry lives in Atlanta and drove down for welding assistance in my shop yesterday. He has done me a million favors so it was a chance to repay him in kind.

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The raw material for the struts were a set of pre-war J-3 cub struts, provided to Terry by fellow Corvair/Piet builder Bob “early builder” Dewenter. who found them for $40 at a fly mart. Cub struts are about 2′ longer than the ones on a Piet, so they can be cut down and re-formed. The smaller rear struts had slight bends near the bottom, but they came with good forks and barrels. $40 is a good deal, but not unusual at a flymart, particularly for bent struts which are essentially impossible to straighten, but are good raw material for shorter struts.

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There are several options for material on Pietenpol struts, steel, aluminum, and wood. All have proven themselves to work, but I’m not a fan of wood ones. People point out that biplanes like Jennys and DH-4s had wooden struts, but they are showing they don’t understand aircraft loads, because every strut on a biplane is always in compression, even if the plane is pulling negative G’s. Conversely a lift strut on a monoplane is working in tension almost all of its life, and the tension loads at the strut/fitting interface are a whole different story than a biplane. The weak link on most wooden strut installations are the fittings, and the tear out strength of the bolt holes in the struts. The ornate fittings on some planes look like decorative gate hinges, but they are weak particularly in compression. If you think a Piet will never see 2 negative G’s, think again, loads in turbulence can be that high. I have also seen a set of wooden struts break while a pilot ran down a strip in a wheel landing attitude at 65-70mph with the wing sticking the plane down hard. He ran over a slight rough spot and the lift struts broke in column failure.

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If I teach you one single thing, let it be this phrase I wrote 20 years ago;

“It isn’t the probability of being right, it is the cost of being wrong that matters”

If you are 50% sure your paint won’t flake off who cares; If you are 97% sure your lift strut will not break, it is unacceptable. The first has no serious cost, and the second would likely be fatal. Use my phrase to evaluate things you hear in experimental aviation and you will understand that much of the talk and priorities people have are a foolish misuse of their attention.

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The work on these struts boiled down to removing the bent section on the rears and doing a FAA legit crafty splice ( they are stronger than the original strut) and then putting new wing attach fittings on all four. The additional material as about $25. If a guy can weld, that is the cost for first class struts. If you have a friend who can, good; in Terry’s case he is friends with both Vern (Fun with Agkistrodon Piscivorus and Vern’s Aero-Trike, and Vern’s 5/8 scale L-4..) and myself, so he got two welders for the price of none. Even if you had to pay for the work, it would be worth $300 tops. In Terry’s case it was just some driving, a chance to learn stuff, and a fun day in the hangar, the stuff that makes being an old school traditional homebuilders fun, elements I have worked very hard to retain in our world of Corvairs.

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Above, working with approved engineering. I’m always stunned how people will go to the internet and ask people they don’t know, questions about airplane construction and engines. Often, the advice is dangerous misinformation. What these people should know is aviation is a vast library of known, proven ideas, and there is zero need to ask advice from people with no training nor respect for existing standards or methods. When you are building a plane, you need old and proven, not some fools idea misapplied to aviation. The drawing above is an FAA approved method of splicing lift struts. It is airworthy on certificated aircraft. We used this to take out the bent sections on the lower ends of the rear struts, while retaining the ends with the forks and barrels, which are in great shape in spite of being 78 years old.

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Above, the splice elements of the drawing above, prior to welding. The insert tube is 1″OD, and 6″long.

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Above, Vern on Left and Terry on right. Yes, Vern welds with two pairs of glasses on, I don’t get it but it works for him. To get a look at Vern’s credentials and experience, look at this story: Shop Notes, 10/26/14

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Above, weld splice done. -NEVER, repeat NEVER grind a weld anywhere in aviation. It is a rule from the very first page of the FAA book on aircraft welding, AC-43.13. I have seen many people post pictures of doing this on planes “to make it pretty” including doing this on exhaust systems. In discussion groups this gets lots of “likes” which tells me that people don’t read FAA books on how to do things. The above repair is just to be cleaned and painted. It is not airworthy if a grinder touches it, period. Either you are going to follow proven methods, or go for “Likes” on social media, pick your path, its your life.

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When I post things like this out, there is a certain kind of person who will instantly say ‘it worked on Joe blows plane’ as if that is the standard for how proven methods are developed. All they are doing is citing what one guy allegedly got away with. If someone wants to debate me on a topic, they should pick a one different than aircraft welding, a category I know pretty well.

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The wing attach parts of the spars were fabricated by edge welding 1″ long .188 wall 5/8″ bushings to the edge of 1/8″ 4130 plates. These plates are inserted into slots cut in the strut tops, where they are fine adjusted for alignment and welded in, and then the tops are formed around this core. To start the process, the upper part of the strut needs an accurate slot 4″ deep in the leading and trailing edge. This could be done on a mill, but it only takes a few second on a 14″ chop saw with an abrasive disc.

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Above, a picture of the slot cut into the leading edge of the top of the front lift strut. The plate with the pre welded bushing is slid down in the and checked for alignment before welding. It is an excellent and proven way of transferring the bolt loads from the bushing into the body of the strut.

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Above, Terry cutting the strut end to shape with an air cut off wheel while I clown around taking selfies. I’m wearing a tee shirt from Corvair College #4, which had the slogan “I got my crank polished at Corvair College”, politically incorrect humor for Motorheads and troglodytes.  Airplane building was supposed to be about learning and having fun. If your not getting your share of both, perhaps it is time to plan on hitting Corvair events this year.

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The Next Event on the Schedule:

FlyCorvair/SPA – Joint Workshop/Open house, May 18,19,20

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Also get a look at:

Build a 3.3L Corvair at the May 18-20 Workshop/Open house.

Read the links now and make a plan today.

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WEWjr