Pietenpol Box Spar Construction, 6/27/13


About 25% of our builders are Pietenpol guys. Almost all of them subscribe to the printed Brodhead Pietenpol Association newsletter, published by our friends Doc and Dee Mosher. It is an outstanding publication. In my opinion, the quality of the newsletter comes from the fact that Doc is one of only a handful of aviators who hold both the FAA’s highest awards, the Master Pilot and Master mechanic awards. The task of editing other newsletters often falls on a nice guy who is good with websites or graphic arts, but doesn’t have depth of aviation experience. Doc is just the reverse of this. I mention this so that everyone understands my comments here are not a critique of the caliber of the BPA newsletter.  Although Doc is a guy who has worn many hats in aviation, when it comes to producing the newsletter, he adheres to the role of ‘Chronicler, ‘  Which means publishing all the info that people are interested without passing judgment on the concepts. I am in a different position, and I want to draw our Piet builder’s attention to a serious error in a spar testing article in issue 13-03, which is just hitting Piet builders mailboxes this week.

Above is a cross sectional drawing of Pietenpol spars that I took off an internet site. The left and the middle are US Piet spars, on the right is the spar that Piet builders in the UK use. It is an open ‘C’ section. They all work, but they are not interchangeable, because the UK spar is based on moving the lift strut further out, and it’s dimensions are not proven with standard US length lift struts.

In the newsletter, Steve Williamson, a friend of our who is flying a Corvair powered Pietenpol, wrote an article about how he and several other guys built a set of full box spars for his Pietenpol, motivated primarily by trying to save money. In the story, Steve mentions the tests they did, and feels that his tests proved his design. I have met Steve in person a number of times, and he is a good guy, but his work here and the conclusion he is drawing is a based on a huge error.

OK, I can’t sing nor dance, but I know something about aircraft construction. Steve states that the test load they picked was based on the belief that the two spars in the wing pull an equal load in flight, so his test only subjected the spar to 1/4 the gross weight times the load. This is a serious error. Although the spars are the same height, they are not pulling the same load, the front one is doing way more that 50% of the work. The calculation of the percentage of load on a two spar wing has a lot of factors in it, too many to detail here, but follow this: Many classic aircraft have wing cords right around 60″ like a Piet. They often have 31″ spar spacing (The Piet is 29″) and have the spars at 15 and 65% of the cord. I have seen calculations that illustrate that front spar can be pulling 83% of the lift load on classic aircraft. I do not know the exact number on the Piet, and you would even have to know factors on the specific plane like CG location to know exactly, but 50% is way too low a number, and the person who suggested this as a valid test needs to stop offering structural advice to airplane builders.

To compound the issue, Steve only tested the wing to 3 G’s. This is too low to verify anything. That would only be considered to validate the wing to 2 G’s with the traditional 1.5 to 1 factor of safety. When you throw in the incorrect load assumption, you might be down to 1.25 G’s. Now the plane is flying, and I am sure it has already seen more than this, but I want to point out that it is a very dangerous and incorrect to conclude that the test run on the spar proved anything more than this. A lot of factors go into an actual test, I have been part of several of them, both for metal and composite wings. There are very important details like having the spar at the angle of attack that generates the maximum lift, not level. No serious designer draws a conclusion from a 3 G load test. The last test I saw was the Panther wing, and they loaded it to 10 G’s before they felt that it was approved for 6 in flight.

The UK spars of no use to US builders for two reasons. First, they will not sell you the plans for them. Second, they are based on using longer lift struts, and to use them with US length lift struts would invalidate their engineering. Although C section spars obviously can work, if you are going to the work to build this type of spar, you might as well build a full box spar, there are a number of advantages to them.

What is needed is a US based design for a full box spar, based on The Original strut location, a design that takes in to account the actual load and can have this validated in a real test. This is simply not that difficult. Bruin’s analysis of flight vehicles and the values of the wood are all that is needed. The job is greatly simplified by knowing that the original ‘plank’ spar works just fine, so the new box would just have to match or slightly better the strength of the original.

In previous years we did a lot of work to develop a great deal of exact data on Pietenpol weight and balance. By comparison, this is a much smaller project.  After the calculations are done, I will be glad to share the data with Steve, and his wing may prove strong enough for him, but he can make this conclusion based on better info. As a reality check, if you are only going to save 25% on the cost of spars, I don’t think most people would find that worthwhile. Routing the spars saves 16 pounds on the plane, and a well designed set of box spars might only beat this by 5-10 pounds. There is another option, using extruded aluminum spar blanks that look like Piper spars. They take comparatively no work, they are strong and only cost about $100 each. There are a number of options in the long run, but with some work there can be a good box spar option for US builders.-ww

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