Builders,
Because there is no “Factory” associated with the 90 year old Pietenpol design, the transfer of experience and ideas to new builders flows through many places, predominately social media. A really large chunk of this info is harmless, but some of it is not. Myths and misconceptions are shared and spread by often well meaning people who mean no harm, but they cause it anyway. The harm ranges from the lost opportunity for the recipient to actually learn something, to sending people on a time and money wasteful detour that leads to many people quitting, straight on through encouraging people to fly with passengers in aircraft that are unairworthy, by a standard Bernard Pietenpol himself stated.
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The number #1 way you can tell you are looking at “an opinion” , not a piece of data: It is delivered without a reference, and particularly without testing nor personal observation. If someone chimes in to say “Lean the cabanes back, it will be alright“ This is a near worthless opinion. If someone says “I have the same engine on my flying plane, N 177XW, my wing LE is 4.25″ aft of the firewall, my EWCG is 10.3″ and my EW is 737 pounds, it is in CG with a 194 pound pilot” , this is a useful piece of data to work with. The Weight and Balance data provided by Ryan Mueller and myself is that kind of data, for 20 different planes. Yet, some people will proceed down the building and flying path, armed only with opinions and no data.
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Some questionable ‘advice’ comes from people with flying planes. Its not bad data, but it is often not applicable to a different engine, or different size pilot. Much of the time, it is delivered as “This worked great for me” which is fine, but it doesn’t address the question, ‘Is this the best way it could be done on the new plane being built? This is most commonly done with CG comments. A person reporting that the plane flies ‘good’ at the aft CG limit, almost never has personally flown one near the front limit, far less flown the same plane, the same week, on each end of the limit. Enter the photo below:
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My Pietenpol, 1996, Edgewater Florida. The reason why the cowl has a 6″ wide expansion in it is simple. I carefully measured, and in a single day, made a mount 6″ longer and plugged the cowl for test flying. In the picture is Gus Warren who did a lot of the work with me and covered much of the flying. It was an instant improvement in safe flying behavior. I can comment on the difference between the same plane flying at 15″ and at 20″. This is what testing looks like, and this produces data, not opinion. Read more here: Evolution of a Pietenpol and here: Evolution of a Pietenpol pt. 2
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Common Pietenpol Construction Myths:
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“Having threaded sections on the diagonal cabanes will allow the plane to have the CG corrected later.” This is a myth. Study the weight and balance articles, and understand that many builders missed their target by several inches on the wing position. The articles show that moving the CG just 1″ requires moving the wing 1.3″. Builders need to just study examples of planes close to theirs, make a calculated fine tuning adjustment in the wing position, and make the diagonal cabanes rigidly attached to the front vertical cabanes.
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“I did a W&B measurement on my plane and it is 1.5″ from the aft limit, it will be fine when it is covered.” This is a myth. If you look at just the covering on the wing with its 60″ chord, the weight of the fabric on it will logically be near 30″, and this is 10″ behind the aft limit. Now think about the fuselage, which has almost no covering ahead of the front cockpit, but a lot of it 6′ aft of the wing, and then there is all the tail surfaces, all the way back. The covering on a Piet can easily weigh 35-40 pounds, and nearly every bit of it is going to drag the CG backward. The effect is strong, and not easily countered. A W&B check when uncovered is not a substitute for a plan right from the beginning.
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“Having an axle location near the wing leading edge will make the plane hard to fly” This is a myth. Look at the 35,000 American certified light planes which had tailwheels made in the 1940’s and they all have the axle close to the wing leading edge when they are in the flight level attitude. No one speaks of these aircraft as hard to fly.
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“Having the axle a few inches further back can’t make much of a difference in the plane’s potential to end up on its back” This is a myth. Ask any person who knows what a Cessna 120/140 axle extender is. Before them, if the Goodyear brake jammed a disc (an issue on floating discs) many planes ended up on their backs. This modification moved the axle a few inches forward, and very effectively prevented the airplane from going over, even with a locked brake on pavement. A few inches difference on axle placement makes a big difference. One of the few light planes of the 1940s to have the axle a few inches back from the leading edge is a Luscombe, and these are the most common light plane to go over.
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“My buddy Mongo has brakes on his A65 powered Pietenpol, the axle is 10″ back from the leading edge and he says it never feels like his plane is going to nose over.” This is a misconception. The reason why this isn’t good data because it fails to mention that Mongo weighs 265 pounds, and he is flying with the CG several inches behind the aft limit. On any plane operating within BHP’s specified CG limits of 15-20″ having brakes on an axle located 10 inches behind the leading edge in an open invitation with a filled out RSVP to put the plane on it’s back.
A few words on wood:
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in 2010, I took this picture of the awning outside BHP’s shop in Cherry Grove. The frame had been there 8 years earlier, on my first visit, and I suspect it was BHP’s personal work. You have to appreciate the values of a man who ended up with an apparently straight aluminum Piper spar and thought that its best use was an awning frame. If you look closely at the photo above, you can see the diagonal bolt holes where the lift strut used to be attached.
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OK, BHP liked wood so much he thought aluminum wing spars were good for awning frames. So why didn’t any of his planes have wood lift struts or wood cabanes? This is a question you should really ask yourself before using wood on your plane.
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I have all the volumes of Juptner’s “US Civil Aircraft” , it catalogs in detail, the first 800 aircraft certificated in the US. Volume one starts in 1927, when BHP’s was testing his first ideas. I have scanned it quite closely, and I don’t see any aircraft with wooden struts. I suspect that once metal airfoil shaped tubing became available, no one thought of using wood anymore.
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People always point out that WWI biplanes had wooden struts, but they almost never have considered that the interplane struts on a biplane are always in compression, and they are almost never longer than 4-1/2′. There are also dozens of biplanes in “US Civil Aircraft” , and none of them use wood struts.
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Wood has obviously worked before on Pietenpols. The issue I have with it is how people choosing wood struts gloss over that these are not in the plans, and they often downplay variations in the wood and the difficulty of drilling precision holes in wood to match the fasteners. I understand why people like the look, but you honestly have to ask yourself is appearance a valid reason the deviate from the plans, the most common material, and to do so with little or no engineering. Think it over.
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William.
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