Pietenpol Power: 100 hp Corvair vs 65 hp Lycoming

Friends,

Bob Lester flew his Pietenpol, newly converted to Corvair power, over to our airstrip this morning. I wanted to get a look at it, and run a weight and balance with the same electronic scales that I used to do the calculations last year when the plane still had its original Lycoming 65 hp engine installed. The information is interesting because it is data that dispels misconceptions that many people have about aircraft performance.

Above, Bob stands beside Grace in our front lawn in December of 2009, when the plane was still powered by the 65 hp Lycoming. The 2009 photo caption said “On an overcast, blustery Sunday, we were surprised with a visit from Bob Lester. He lives on the other side of Northern Florida, and bundled up and flew over. His Piet was built in the 1970s, and sports a Lycoming 65 hp power plant. Bob found the aircraft in Arizona last year and flew it back on a 25-hour adventure. From my first days in aviation, I really wanted to live on a grass strip. Fifteen years of hard work later, we made it. Having old friends taxi into the front yard is one of those moments that makes working past midnight on two thousand nights and saving our pennies worthwhile”

Although Bob really liked his Piet, it had marginal performance on the original engine. In spite of being we built and fairly light, the plane barely had a 300 fpm climb rate solo on an average day. With two people on a very hot Florida day,  Bob said that he had found the plane to just barely have a positive rate of climb. If you live in the rural midwest, you might not think of this as critical, but Florida has the combination of hot and humid weather, short grass strips, and some types of terrain and restricted airspace that makes a plane much more useful if it can climb to 2,000′ agl in two and a half minutes rather than 10 to 12 minutes. The biggest issue with any plane with this poor climb, especially a four-cylinder one, is that is you have any problem, like having one mag cut out, the aircraft will not climb, and you are going to have a forced landing.

  Additionally, Bobs Lycoming, like it’s 65 Continental brethren, was a hand prop engine, and it was problematic to start when hot. I looked at the details of Bob’s old engine, and there was nothing wrong with it, the impulse on the mag worked, and the carb was set correctly. It just that heat soaked mags on a 6:1 compression engine with a wood prop isn’t always an easy starter. This issue was an additional factor in Bob’s choice to go with a Corvair. Bob had been flying with a Corvair since 2001 when he removed the Subaru from his KR-2 and replaced it with a 2700cc Corvair, so he is not new to the movement. Bob had built a 2700/Dan bearing engine and run it at CC#17 and briefly flown this on his KR, and now had it available for transplant. A few months ago Vern and I made Bob a custom motor mount and an intake for a Stromberg carb, and he went to work on the engine swap.

The results: Lycoming 65 hp; cruise 65 mph, top speed 72, gross climb (at 59F) 250 fpm. With Corvair: cruise 75, top speed 95 full gross climb (at 75F) 800 fpm. Solo climb rate 1,100 fpm. Bob installed a battery and electrical system with his Corvair and changed his fuselage fuel tank to aluminum from fiberglass, and made a very quiet muffler system. The empty weight went from 644 to 739 pounds. Yes, the above measured performance increases are with an aircraft that weighs nearly 100 pounds more than before. 35 extra hp is worth far more than 100 pounds less weight. I can not count the amount of times in the last 25 years I have heard someone pontificate in a hangar flying session just the reverse of this reality. Yes, building things light is important, but the empty weight of the plane is not the single factor in climb rate. If you have an ‘expert’ at your local airport that debates this, just know that he has never done a test like we just did with Bob’s aircraft.

First question is how if the plane got 35 hp added to the original 65, an increase of 54%, does the plane now climb at several times the original rate? Planes climb on excess power, not total power. If the climb speed is 60 mph, first consider how much power it takes to fly the plane straight and level at this speed. In Bob’s case, it might be 50 hp. From that point, the Lycoming only had 15 hp available to climb. At the same 60 mph, his Corvair has at least 50 more horses in reserve for climb, a 330% increase in climb power. Just saying this, I know will generate disagreements from net, but the people who don’t understand it don’t own a well-worn copy of Aerodynamics for Naval Aviators, far less a B.S. in Aerodynamics and Performance. If your new and having a hard time picturing it, think of any plane at top speed in level flight.  What is the possible rate of climb? always zero. The plane is using 100% of its power to go forward, it has none for climb. If the pilot pulls back on the stick, yes, plane climbs, but first it instantaneously slows down. A planes best climb speed is near the L/D speed, (the best glide) because this is the point in the envelope where the plane requires the least power to fly straight and level, thus having the most power in reserve for climb.

Also very important in a Pietenpol is the CG location. The range of the design is 15-20″ from the leading edge. Ryan Mueller and I have done a lot of work accurately documenting the CG of examples of the design. 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.

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. Piet builders interested in comprehensive CG info can get the 5 article series in back issues of the Brodhead Pietenpol association newsletter http://www.pietenpols.org/.

Bob will have his plane on hand at CC#24. He is retired now, and he plans on doing lots of flying in his bird, traveling the country as barnstormers did. He now has a good simple smooth and powerful engine to serve him. If you have similar ideas, sign up for the event and come get to know people who share the same dream.-ww