Brodhead Pietenpol Association

Original Posted By: "Ernie Moreno"
Subject: Pietenpol-List: corvair fifth bearing upgradehello all, after talking with Jack Phillips the other night about my upgrade to the Roy Szarafinski fifth bearing and the reason for it, he thought with so many people considering the corvair for their piets,that I should share my experience with the list. after 188 hours engine time on my piet I had a crankshaft break,luckily it happened during the run up on the ground.but before anyone getsnervous about their decision to use the corvair I should tell everyone that before the plane was finished and while I was doing the engine test runs I didnot have the tail secured properly and it nosed over and got the prop in the pavement.I ruined a beautiful brand new tn. prop,(what a sick feeling) I got the tail back down and the engine shut off before it choked the engine down.so I thought maybe being a wood prop and that I didn't kill the engine it would be o k. so I didn't tear it down and have the crank checked. So my feeling is had it not been for the prop strike there probably would not have been a crank failure in my pietenpol however I had been considering upgrading to a fifth bearing just for the extra peace of mind. andI think if I were at that stage of building I would go ahead and make the investment in one. I really like my corvair and since the addition of the fifth bearing it is really a smooth running engine,and I would not hesitate on using one again,as a matter of fact I plan on using one on the sonex I am building now. And let me add any time you have a prop strike of any kind play it safe and get the crank checked! Randy Bush NX294RB ( Miss Le Bec ) ________________________________________________________________________________

Original Posted By: Rick Holland
Subject: Re: Pietenpol-List: corvair fifth bearing upgrade

Original Posted By: owner-pietenpol-list-server(at)matronics.com
Thanks for posting this, Randy. I think many Pietenpol builders using orcontemplating using the Corvair have the VERY mistaken idea that excessiveprop loads are more of a problem with faster aircraft like a KR-2 or aZenith than they are with a Pietenpol, and that the 5th bearing for aPietenpol falls into the =93nice to have=94 category, but is not necessary.This is absolutely ludicrous.The reason for large bearings supporting the crankshaft near the propelleris the presence of assymetric loads produced by the propeller duringoperation, and these loads are produced (as any pilot knows) by two factors:Gyroscopic forces and =93P=94 Factor. Of these two, I suspect that the biggestproblem is the gyroscopic loading. Gyroscopic loading occurs when aspinning object (in this case, the propeller) is moved by a force exerted onits axis. In an airplane, such movement is caused by raising or loweringthe nose in maneuvers, or by simply turning the airplane. One of thepeculiarities of gyroscopes is that if a load is applied to the axis orrotation, the gyroscope responds by producing a reaction force 90=B0 from theapplied force. This is one reason that raising the nose of the airplanemakes the plane want to turn left (right, if powered by an engine that turnsthe =93wrong way=94, such as a Corvair). This reaction force is proportional instrength to the RATE of turn imposed on the gyroscope. Again, as any pilotknows, the slower the airplane, the greater the rate of turn possible. Ithoroughly enjoy putting my Pietenpol into a very tight turn at fullthrottle, standing the airplane on its wingtip and seeing just how fast Ican turn it. The turn rate is astounding (and fun!), and it is possible todo a complete 360=B0 turn in just a few seconds ' something I simply can=92t dowith my RV-4 because it is too fast.Such a high rate of turn puts loads on that prop that a fast airplane simplycan=92t match. A Pietenpol is capable of inducing HUGE loads on itspropeller.I question if even the 5th bearing gives sufficient bearing area to resistthe gyroscopic loadaing of a Pietenpol propeller. If you look at the 5thbearing available from Weseman=92s, which is external to the crankcase, orfrom Roy=92s garage, which is internal, neither has a terribly large bearingarea ' maybe 1-1/2=94 of crankshaft length at best. Compare that to the frontmain bearing of a 65 Continental aircraft engine as shown below:That bearing is very large compared to any of the bearings on a Corvair. Inaddition, the Continental crankcase is much stronger, since it completelyencircles the crankshaft, giving good support to the bearings ' particularly=93up front=94. The Corvair case on the other hand, is open top and bottom andsimply is not as stiff as the Continental.Anyway ' the purpose of this email is not to disparage the Corvair, but totry to correct the erroneous impression that many Piet builders seem to havethat the 5th bearing is not needed on a Corvair in a Pietenpol. Randy=92sbroken crank may or may not have been caused by his prop strike, or the propstrike just did quickly what would otherwise have taken a few hundred hoursto do ' overload the crankshaft and cause it to fail due to inadequatesupport for large moment loading.If you are building a Pietenpol and planning to use a Corvair engine, pleaseput a 5th bearing in. And as John Hofmann would say, consider adding the6th and 7th bearings as well. If you haven=92t yet decided on an engine butare leaning towards the Corvair, put the cost of adding the 5th bearing intoyour calculations and you will probably find the total cost of the Corvairto be much the same as for a Continental.Jack PhillipsNX899JP =93Icarus Plummet=94Raleigh, NC _____

Original Posted By: airlion

Original Posted By: Rick Holland
Here's a very interesting video demonstrating why a Pietenpol is so slow.Think of all the wires on a Piet after watching this video. Wire spokesdon't help much either. http://www.youtube.com/watch?v=ftq8jTQ8ANEJack PhillipsNX899JP "Icarus Plummet"Raleigh, NC________________________________________________________________________________Date: Mon, 10 Jan 2011 11:08:13 -0700Subject: Re: Pietenpol-List: corvair fifth bearing upgrade