Pietenpol Weight and Balance project

Builders,

Besides covering the world of Corvairs, I have done a number of additional projects in Experimental Aviation. One of the most important of these other projects was the Pietenpol Weight and Balance project, 2010 -2012. We did this project to serve all builders of this design, not just the builders using a Corvair. The work was covered in a series of five articles in the Pietenpol newsletter. There is information at the bottom of this story on ordering the back issues.

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Above, My Pietenpol at the last hours of Sun n Fun 1996. From L to R, Gus Warren, Steve Upson and a much younger version of me. I have been around Pietenpols my entire 25 years in aviation. Take a moment to look at all the aspects of this on our Pietenpol page at this link:

Corvair – Pietenpol Reference page

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The nature of the issue: People who know the design well, understood that a great number of these planes were being finished and flown near their aft CG limit or behind it. This is a dangerous situation. The problem was driven by a number of factors: people using light engines like A-65’s, pilots who are far past the 170lb FAA example, and the fact many people had no examples to follow, and operated on old wives tails. The Aircamper is extraordinarily sensitive to poor planning because the pilot sits entirely behind the rear spar of the wing, much further aft than a typical tandem cockpit light plane such as a J-3.

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Additionally, many planes were built with their main landing gear too far back. This lead to several airplanes being put on their backs. Combining axle placement from 1930s drawings with modern brakes caused this.

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Although there had not been any fatalities directly attributable to CG and axle placement, there had been significant preventable damage done. I also suspected that the poor utilization of a great number of finished planes was due to the undesirable handling caused by these issues.

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A weight and balance document that Bernard Pietenpol developed in the 1960’s with an example of showing correct axle placement for brakes, an example of W&B that allowed a 290 pound pilot to be in limits, and stating in all capital letters that the CG of his design was 15″ to 20″ and that it was never to be flown aft of this, was available, but largely ignored by builders. Additionally, I weighed “The Last Original, ” Bernard’s personal plane, confirming his design data. I can think of no other design where builders routinely ignored designers CG limits. Our goal was to demonstrate that there is no reason to.

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The project was done with the support of many well known Pietenpol people like Doc and Dee Mosher and Bill Knight. We also had great participation for pilots who allowed us measure their aircraft and weigh them. The project had broad support.  A gentleman who was personal friends with BHP told me that it was the single most constructive project undertaken since Bernard had passed in 1984.

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The data collection: In 2010,2011 and 2012, I brought a set of very accurate electronic scales to Brodhead and weighed more than 30 different Pietenpols with Corvair Ford and Continental  engines. All of the measurements of the aircraft, such as fuselage length, motor mount length, landing gear location and wing to fuselage location were accurately taken. I used the same set of scales every year. A number of the aircraft we weighed had very poor bathroom Scale type W&B reports. Several planes had not been weighed in years, or were purchased second hand and had W&B data that was clearly copied from a different aircraft. About 1/3 of the aircraft had flown to Brodhead at or beyond the designs 20″ aft CG limit. All of these pilots expressed thanks at learning the situation and made plans to correct it.

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The data showed many parts of builders lore to be foolish myths; Both long and short fuselage models were shown to be equally prone to CG issues;  Fuel and passenger weight was shown to have little effect; we proved that building a longer engine mount had very little effect on CG compared to wing placement; Lighter was not better, as the lightest planes as a group had the most aft CG.

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We did additional measurements on planes in 2011 and 2012. I used the same scales in Florida and South Carolina to measure several other planes. The total data set is now 33 aircraft, enough to cover the design thoroughly. For an example of a specific CG change and performance change in a Pietenpol going from a 65Hp engine to a Corvair read: Pietenpol Power: 100 hp Corvair vs 65 hp Lycoming.

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Bob Lester strikes the “Intrepid Aviator” pose with his Pietenpol.  Bob weighs 210 fully dressed up for open cockpit flying. with his Lycoming, his plane was flying near the back of the CG range at 19.1″. With the Corvair we moved it forward to 15.9″ This is a dramatic shift, and it would now take a pilot over 320 pounds to move his CG to the aft limit. This is a much better position to be in.

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The project was started at Brodhead 2010. With the help of the above crew,  we performed a weight and balance on 14 Piets. From left to right above, Ryan Mueller, Jess (whose shirt says “Real men fly  Pietenpols”), Emory Luth and myself.  Gathering the data was a quick process,  taking less than 10 minutes per plane once we had the drill down.

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The calculations and results: Technically every pilot and A&P must know how to do a weight and balance calculation to pass his test. The reality I know is 50% can’t do a weight and Balance calculation to save their lives. On the other hand, I am particularly good at this, especially the complex variable of adjusting the wing fuselage location to correct the issue.

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One of the first things Ryan and I did was develop a complex computer algorithm that we could plug each planes data into and it automatically spit out the maximum pilot weight that the plane could take before it went out the aft limit of the design at 20.” There were several planes we measured that had A-65 Continentals that could only take a pilot of 130-135 pounds before going out the aft limit of the envelope. Several of these were being flown by 180-190 pilots. You can get away with this as long as you have the engine running creating high air flow over the tail, but if the engine quit and the speed decayed, the plane would be very prone to an unrecoverable condition.

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Rational people do not build airplanes to see what they can get away with, they do it to effectively master the skills and utilize the design correctly. Anyone arguing that it is “OK” to build and fly a plane at or beyond the CG limit because he has evidence that it has been gotten away with before isn’t a person who should be taken seriously, and their judgment can rightly be called into question. Using our data, any plane, even one with a light motor can be set up correctly to fly with a pilot of 220 pounds. Planes using a Corvair, Ford or O-200 can be set up to fly 300 pound pilots in CG.  Several of the examples we weighed could have pilots over 305 pounds and still be in CG. There is no reason to build your plane and not have it operate in the designers CG limits.

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Where you can find the full data set. Click on this link:

Pietenpol Weight and Balance article source

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 Most people don’t view it as their task to correct negative or dangerous things others advocate. They value “getting along.”  For a reason explained below, my loyalty lies elsewhere.

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I earned my A&P license from Embry-Riddle in 1991. It was in an era when the department was run by men who were former military, who had come of age in WWII,  Korea, the Cold War and Vietnam. They took aviation very seriously, they all had seen its potential costs. They were tough.  I am biased, but I do think the program was without peer.  At the end of training, a handful of select students, I among them, elected to take a solemn oath in a private ceremony  to swear our unwavering allegiance to aviation safety.

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We did not swear to protect our employers, nor to defend the FAA or their rules, nor did we swear to defend our friends, careers or egos. We didn’t even take an oath to protect pilots. The only people we were taking an oath to protect was unwitting passengers who would fly in planes, people who had supreme trust and the belief that their fellow man, an aviation professional, was trustworthy with their very life.  The critical element of the oath is that we might be the passengers last line of defense, and if it was so, we were to “forsake every other consideration to protect them.”

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To have some understanding of my perspective, spend an evening reading my Risk Management reference page.  If you only have some time, just read this story: Effective Risk Management – 2,903 words ” It include the statement “This was the first time I can clearly say I understood the cost of keeping your mouth shut. This was the first step to me becoming the kind of “Bastard” who publicly points out people doing dangerous things.” At the conclusion of the CG project I wrote the paragraph below when a builder sent me a photo of an 8 year old kid flying in a plane with an aft CG. Few people outside professional circles understood the tone, but I did get one short note from a guy who graduated from Embry-Riddle before I was born, when the school was still in Miami. He knew without asking I had taken the oath as he had, and the tone made perfect sense to him.

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” Many builders over 160 pounds with light engines are actually flying behind the aft CG limit, which is a great idea if you feel you have already accomplished every thing you wanted to do in this life. In my book if, you want to knowingly fly out the aft cg limit of a homebuilt, it’s your choice, I don’t base my happiness on the actions of others. If someone wants to tell other people this is a good thing to do, then they will find me disagreeable. If a guy wants to go a step further and fly passengers who know nothing about CG, like little kids, they will find me to be a vocal opponent of theirs, no matter who they are. When it comes to speaking up for the safety of unwitting passengers, I am not intimidated by any combination of the offending pilots wealth, experience, popularity or physical size far less peer pressure or being thought of as a mean spirited sob.”

About William Wynne
I have been continuously building, testing and flying Corvair engines since 1989. Information, parts and components that we developed and tested are now flying on several hundred Corvair powered aircraft. I earned a Bachelor of Science in Professional Aeronautics and an A&P license from Embry-Riddle Aeronautical University, and have a proven 20 year track record of effectively teaching homebuilders how to create and fly their own Corvair powered planes. Much of this is chronicled at www.FlyCorvair.com and in more than 50 magazine articles.

6 Responses to Pietenpol Weight and Balance project

  1. Gary Boothe says:

    William,

    An excellent subject…and worth repeating many times.

    Gary Boothe

  2. Sarah Ashmore says:

    As an engineer with a degree from the same school as yours I find this article to be very shocking. Calculating W&B as well as CG limits are done following mathimatical formulas and when you get the answer it is concrete. The setting of CG limits is absolute and is needed to ensure that the aircraft remains controlable. I remember covering these subjects in class and the discussions on what being to far forward or aft does to the controllability of the aircraft. I understand that few in the homebuilt world have such an engineering background but there are plenty of A&P’s out there and I would think their training is sufficient to enforce the importance of proper W&B calculation as well os proper calculation of an aircrafts CG and how to take the measurements that go into that calculation. Even pilots receive training on W&B so they should understand what CG limits are all about. For those reasons it astounds me that anyone would knowingly fly outside the calculated limits for CG. Maybe it would be OK in a single seat aircraft, but to take unsuspecting passengers along on those death cheating flights is simply not acceptable. How can aviation, espically in the EAB catagory, ever be considered safe if such blatent disregard for safety is allowed to go on.

  3. Bill Rotenberry says:

    Pietenpol articles are always of interest to me, not only because of the Corvair engine history, but also because of the similarity to the Fly Baby (my project). Short comparison of the two are at http://www.bowersflybaby.com/stories/piet.html

  4. Jon Coxwell says:

    William,
    I want to thank you for your work on weight and balance for the Pietenpol. My project is a GN-1. I am finishing someone else’s project who chose to put a corvair engine on the project. At the time of purchase I was not for or against the corvair but accepted the fact that my project came with a corvair engine and so I needed to work through that. At this point I have attended 2 colleges and seen my engine run. I believe it will be a good engine for the plane and I am excited about the possibility of flying it.

    What I really want to talk about is the GN-1 with a corvair. GN-1’s are welcomed at Brodhead the same as Piets. In fact when asked what I am building I tell people that it is a Pietenpol because they may have seen or heard of a Pietenpol but not a GN-1. The GN-1 was derived from a Piet and was supposed to be an improvement. That last statement is questionable, but I have what I have and I intend to fly it safely.

    The main impetus for the GN-1 design was a Pietenpol look alike that used off the shelf aircraft parts from Cubs and Tailorcrafts and Aeroncas including a Cub landing gear and the popular 65 – 85 hp Continental engines. Unless you know what you are looking for it is easy to mistake a GN-1 for a Piet. However, outward appearance is where the similarity stops. The GN-1 was not designed with a moveable wing like the Piet. It was designed for a lighter engine, different location for the fuel tank, the firewall is further forward, the fuselage is narrower, and in general the total geometry is different and in general it is a heavier airplane. It stand to reason that this will impact the weight and balance. The GN-1 wing is very similar to the Piet airfoil and the CG envelope for the Piet does apply.

    Your work with the Piets did not yield numbers that were useful to me, but the concepts of gear location and the number of examples you found that were out of CG limits opened my eyes. As a result I did a pretty thorough simulated weight and balance with as much of the air frame in place as possible and the rest of the weights calculated pretty closely. I generated a computer spreadsheet for weight and balance and then plugged in actual weights along with calculated weights. Here is what I found:
    1. I had to shorten the motor mount (basically build a new one) and bring the engine as close to the firewall as possible to provide for stability on the ground. My corvair weighed in at about 245# with all the stuff I could hang on it. With the previous builders motor mount the tail weighed about 5# with simulated weight for the wings hanging in place.
    2. I had to redesign the cabanes to allow for the wing to move back 4″ to get into CG limits for today’s heavier pilots and passengers ( I designed for 220# & 120# in each seat, and checked for min and max fuel in all senarios) Even with that, the gear is still 4″ behind the leading edge and you and BP recommend 3″ max.

    I did not have access to electronic scales but will when the plane is finished. Had you not done your work and published the results I likely would not have know I had a problem until the very end of construction when it was expensive and time consuming to fix. Again I thank you.

    Jon Coxwell

    • Jon,

      Thanks for a thoughtful letter. First let me say that I have been to Brodhead about 20 times, and GN-1s are totally welcome there. ! out of 100 guys is some type of purist who may only like pre-war Ford powered Piets, but just about everyone else is totally relaxed about the difference between the two designs. I myself try to call a GN-1 and “Aircamper’ and leaver the name “Pietenpol” to the original, but this is just as note of respect to the family name.

      A few years ago I stopped calling BHP “Bernie’ and switched to writing of him as “Bernard” because I was listening to Vi Kappler, who was personal friends with the man, and I realized that I would rather use his formal name and leave the nickname for men who were his personal friends, just to acknowledge that their understanding was first hand and mine is second. I don’t think other people care about details like this, but they are important to me.

      I have a lot of respect for any builder who goes back and makes a part like a mount over again instead of trying to make do with something that really isn’t right. I get tired of people trying to rationalize why something is “going to be OK”, it is always refreshing to hear of people making things right. I look forward to hearing of your airplanes first flight.

  5. john says:

    many years ago, I built a pete with a ford engine. I was told at the time by an a7p freand ,that the cg should fall about 30% of the airfoil.(18 inchs) long story short I made a teter todder saw horse ,set the plane on it and slide the engine back and forth until it balanced with 200 lbs in the aft seat. and then bolted the engine down. I flew it 100 hrs. one time I got into a spin with it and was almost on my back,but was able to recover. also got flipped ove in the wind once. this is all great info for the first time builder,that knows very little about these things. I always felt the engine was to far forward,it the nose seemed to float around on it. other than that it was a great little plane. thank you for all your work. john

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