Pietenpol-List: Re: Corvair PSRU (long)
Posted: Wed Sep 30, 1998 6:45 pm
Original Posted By: TLC62770(at)aol.com
Craig,>One thing that I am concerned about is it looks as though the >prop is raised up almost 3". Will that affect the flying >characteristics of the Piet?Yeah, that's a good question, and I don't have an answer. I'm sure there are many things to consider when altering the geometry of the source of power. Many Pietenpols have been built with PSRUs on engines such as the Escort, Subaru, Geo, etc... Each of these builders has probably had to consider theseissues with each new power plant. Maybe its not critical, but I'll be sure tolearn more if I decide to pursue this Corvair PSRU idea.Just a couple of posts ago, I was giving advice to follow BHPs plans, and now I'm talking about reduction gears. I have a lot of faith in the design, and I'd feel comfortable mounting a prop in direct drive configuration. However, I'm interested in this because of some warnings I received from a Corvair mechanic regarding stresses on the engine for which it was not designed. Now,I know Corvairs have been powering aircraft for 3 decades in direct drive configuration, but I also know that there have been some power failures leadingto dead stick forced landings. I can't help but wonder if some of these powerfailures have been due to accelerated wear caused by thrust loads. Naturally, a PSRU would prevent that, but it may introduce other problems. Does anyone know of any Corvair engine failures? Has anybody disassembled a Corvair engine after many hours of flight?The following is an excerpt from a cautionary note I received from a Corvair mechanic who often posts over at the Corvair Center Forum. Its long, but pithy.The #1 aspect that makes me nervous about this project is attaching theprop to the crank nose-Theres some problems with a Corvair engine in that area, and I feel that itwill compound them considerably if anything that big and/or with a thrustload is attached there. It is not unusual for a Corvair passenger car witha manual transmission to break the gear off, or the immediately adjacentcrank main bearing, due primarily to fatigue. Also, the thrust surface ofthe bearing on a Corvair is barely adequate for intermittent thrust fromshifting gears, and the support for the bearing insert is rather flexible-One thing that is evident about Corvair engines is that they are rather'rubbery' in that things move around and expand/contract a lot. To illustrate the crank bearing arrangement a bit better, a Corvair has a17 lb. flywheel assembly stock. GM found it necessary to move the #4 mainbearing 0.0015" downwards from the centerline of the other three mainbearings to even out the wear caused by the drooping crank (it bends thatfar from the weight). The crank #4 bearing is also offset towards the #6cylinder (drivers side forward in a passenger car) by 0.0015" to removekeep a noise and fatigue problem caused by a harmonic vibration caused inthe crank that is excited by the firing of the #6 cylinder. This correctionis made in the bearing shell insert in some engines, and at the bore inothers.It used to cause a bonk-bonk noise in Corvair engines , and it tookGM until 1963 to figure out the cause.Fatigue failures related to the crankshaft inevitably manifest themselveson the #4 main bearing also-The Corvair engine was originally engineered to use only an automatictransmission, and the addition of a flywheel and manual transmission was anafterthought, and had a curious execution, I imagine you may have seen howa Corvair flywheel is made-it has a very unusual design to cancel harmonicvibrations - a solid flywheel does not work on the Corvair engine as anexample, and leads to rapid fatigue failure in most cases.The 1964-69 engine uses a very good forged steel crankshaft, and the 140 HPand Turbocharged engine have further refinements, in that it is tufftrided,(hardened) for maximum fatigue strength. It is a very good peice.The gearon the end of the crank that drives the camshaft is not a particularlyfancy item, its made of regular 4130 or similar material.Corvair engines work great at what they're designed to do- if you use it ina similar fashion you'll have no special concerns. They are very capableand efficient at providing a sustained 15-40 horsepower output, and shortbursts of greater power- but long bouts of high power output will hurtthem, so be conscious of that.To drive a Corvair automobile at 60 mph takes about 22 hp as example. thisis about 100lbs of 'thrust' to overcome aerodynamic and road drag- you cansafely assume that a propeller attached to a Corvair engine crank at say 80hp output will be tugging on the crank to the tune of 300-400 lbs. I thinkthis is far too much strain. a 100 LB load would be too much in my view,actually. I am aware that it has been done, and could work, withaccelerated wear, but feel its a false economy, and if it causes a failure(likely in my view) it could be a catastrophic economy.I would strongly encourage you to use a separate propeller supportarrangement, with its own bearings, and take drive off the crank nose only.Theres other ways to take the drive off the engine, and I'd look into it.An inexpensive and reliable source of reduction gearing is as near as aHydra Matic or similar big transmission- to adapt a planetary set out ofone of those would be extremely easy. A direct drive arrangement with aslightly flexible coupling might be a good idea also. Please think aboutdoing this, I get terribly nervous about the idea of anything hanging fromthat crank. :-)The messages on the Corvair Forum were essentially me telling someone thatthey should not direct drive off the crank, and a strong criticism of myoutlook from another posting correspondent that he felt it would work.Theres more to this aspect than meets the eye, and I feel its worth playingit safe. If you are interested in the fatigue characteristics etc. of thecrank I have more information- but under MAXIMUM load a new Corvair enginerunning at normal temperature will explode with catastrophiccrank/connecting rod failure in 11 hours, according to Chevrolet's fatiguetests.This is GM's passenger car standard, essentially- thats how theydecide what is an acceptable connecting rod beam size, as example, its nota shortcoming of the Corvair engine. This would be similar to a fullthrottle run for 11 hours installed in a car. Naturally, the less often theengine is run at high speed/load/temp the longer it lasts, but theresdefinite limits.>One thing that I am concerned about is it looks as though the >prop is raised up almost 3". Will that affect the flying >characteristics of the Piet?________________________________________________________________________________
Craig,>One thing that I am concerned about is it looks as though the >prop is raised up almost 3". Will that affect the flying >characteristics of the Piet?Yeah, that's a good question, and I don't have an answer. I'm sure there are many things to consider when altering the geometry of the source of power. Many Pietenpols have been built with PSRUs on engines such as the Escort, Subaru, Geo, etc... Each of these builders has probably had to consider theseissues with each new power plant. Maybe its not critical, but I'll be sure tolearn more if I decide to pursue this Corvair PSRU idea.Just a couple of posts ago, I was giving advice to follow BHPs plans, and now I'm talking about reduction gears. I have a lot of faith in the design, and I'd feel comfortable mounting a prop in direct drive configuration. However, I'm interested in this because of some warnings I received from a Corvair mechanic regarding stresses on the engine for which it was not designed. Now,I know Corvairs have been powering aircraft for 3 decades in direct drive configuration, but I also know that there have been some power failures leadingto dead stick forced landings. I can't help but wonder if some of these powerfailures have been due to accelerated wear caused by thrust loads. Naturally, a PSRU would prevent that, but it may introduce other problems. Does anyone know of any Corvair engine failures? Has anybody disassembled a Corvair engine after many hours of flight?The following is an excerpt from a cautionary note I received from a Corvair mechanic who often posts over at the Corvair Center Forum. Its long, but pithy.The #1 aspect that makes me nervous about this project is attaching theprop to the crank nose-Theres some problems with a Corvair engine in that area, and I feel that itwill compound them considerably if anything that big and/or with a thrustload is attached there. It is not unusual for a Corvair passenger car witha manual transmission to break the gear off, or the immediately adjacentcrank main bearing, due primarily to fatigue. Also, the thrust surface ofthe bearing on a Corvair is barely adequate for intermittent thrust fromshifting gears, and the support for the bearing insert is rather flexible-One thing that is evident about Corvair engines is that they are rather'rubbery' in that things move around and expand/contract a lot. To illustrate the crank bearing arrangement a bit better, a Corvair has a17 lb. flywheel assembly stock. GM found it necessary to move the #4 mainbearing 0.0015" downwards from the centerline of the other three mainbearings to even out the wear caused by the drooping crank (it bends thatfar from the weight). The crank #4 bearing is also offset towards the #6cylinder (drivers side forward in a passenger car) by 0.0015" to removekeep a noise and fatigue problem caused by a harmonic vibration caused inthe crank that is excited by the firing of the #6 cylinder. This correctionis made in the bearing shell insert in some engines, and at the bore inothers.It used to cause a bonk-bonk noise in Corvair engines , and it tookGM until 1963 to figure out the cause.Fatigue failures related to the crankshaft inevitably manifest themselveson the #4 main bearing also-The Corvair engine was originally engineered to use only an automatictransmission, and the addition of a flywheel and manual transmission was anafterthought, and had a curious execution, I imagine you may have seen howa Corvair flywheel is made-it has a very unusual design to cancel harmonicvibrations - a solid flywheel does not work on the Corvair engine as anexample, and leads to rapid fatigue failure in most cases.The 1964-69 engine uses a very good forged steel crankshaft, and the 140 HPand Turbocharged engine have further refinements, in that it is tufftrided,(hardened) for maximum fatigue strength. It is a very good peice.The gearon the end of the crank that drives the camshaft is not a particularlyfancy item, its made of regular 4130 or similar material.Corvair engines work great at what they're designed to do- if you use it ina similar fashion you'll have no special concerns. They are very capableand efficient at providing a sustained 15-40 horsepower output, and shortbursts of greater power- but long bouts of high power output will hurtthem, so be conscious of that.To drive a Corvair automobile at 60 mph takes about 22 hp as example. thisis about 100lbs of 'thrust' to overcome aerodynamic and road drag- you cansafely assume that a propeller attached to a Corvair engine crank at say 80hp output will be tugging on the crank to the tune of 300-400 lbs. I thinkthis is far too much strain. a 100 LB load would be too much in my view,actually. I am aware that it has been done, and could work, withaccelerated wear, but feel its a false economy, and if it causes a failure(likely in my view) it could be a catastrophic economy.I would strongly encourage you to use a separate propeller supportarrangement, with its own bearings, and take drive off the crank nose only.Theres other ways to take the drive off the engine, and I'd look into it.An inexpensive and reliable source of reduction gearing is as near as aHydra Matic or similar big transmission- to adapt a planetary set out ofone of those would be extremely easy. A direct drive arrangement with aslightly flexible coupling might be a good idea also. Please think aboutdoing this, I get terribly nervous about the idea of anything hanging fromthat crank. :-)The messages on the Corvair Forum were essentially me telling someone thatthey should not direct drive off the crank, and a strong criticism of myoutlook from another posting correspondent that he felt it would work.Theres more to this aspect than meets the eye, and I feel its worth playingit safe. If you are interested in the fatigue characteristics etc. of thecrank I have more information- but under MAXIMUM load a new Corvair enginerunning at normal temperature will explode with catastrophiccrank/connecting rod failure in 11 hours, according to Chevrolet's fatiguetests.This is GM's passenger car standard, essentially- thats how theydecide what is an acceptable connecting rod beam size, as example, its nota shortcoming of the Corvair engine. This would be similar to a fullthrottle run for 11 hours installed in a car. Naturally, the less often theengine is run at high speed/load/temp the longer it lasts, but theresdefinite limits.>One thing that I am concerned about is it looks as though the >prop is raised up almost 3". Will that affect the flying >characteristics of the Piet?________________________________________________________________________________
>> The messages on the Corvair Forum were essentially me telling someone that> they should not direct drive off the crank, and a strong criticism of my> outlook from another posting correspondent that he felt it would work.>> Theres more to this aspect than meets the eye, and I feel its> worth playing> it safe. If you are interested in the fatigue characteristics etc. of the> crank I have more information- but under MAXIMUM load a new Corvair engine> running at normal temperature will explode with catastrophic> crank/connecting rod failure in 11 hours, according to Chevrolet's fatigue> tests.This is GM's passenger car standard, essentially- thats how they> decide what is an acceptable connecting rod beam size, as example, its not> a shortcoming of the Corvair engine. This would be similar to a full> throttle run for 11 hours installed in a car. Naturally, the less> often the> engine is run at high speed/load/temp the longer it lasts, but theres> definite limits.>> >One thing that I am concerned about is it looks as though the> >prop is raised up almost 3". Will that affect the flying> >characteristics of the Piet?>________________________________________________________________________________