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#71
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Corvair conversion engines
clare at snyder.on.ca wrote in message
... On Wed, 25 Jan 2006 19:47:08 -0500, "Morgans" wrote: "Peter Dohm" wrote Interesting. I had assumed they were originally flying with 80HP or less, but had no idea it was that much less. Thanks. You could find many people that would argue that low of a HP figure. I think your 80 is closer, and in might be a few more than that. Original flying corvairs were the little engine - 145 cu inch, IIRC, producing 128 ft lb gross torque at 2300 RPM. So, at 2300, 56 hp. If run at 2700 rpm, torque approx 125 ft lb, and 64 hp. The 164 inch engines produced up to 160 ft lb torque at 2600 or 2800 rpm depending on the engine, for 80 hp at 2600, or 85 at 2800. Mine produces 90 at 3000. With a fancy cam and a bit of rework they will put out closer to 170 ft lb - and at 3200 RPM with a small prop, that is 103 hp almost 115 at 3500 rpm. The factory 140 hp engine supposedly produced 140hp at 5200 rpm and 160 max torque at 3600. That means the torque dropped off to 140 at 5200. The 110 does not breathe nearly as well at speed, so the 14% torqe drop of the 140 would be more like 20% on a 110 - or 122 ft lbs at 5200 for 120 hp if you ran a 2:1 PSRU for a 2600rpm prop. And that's being optimistic. Assuming 170 peak torque at 3000 RPM (likely pretty close with OT10 cam and properly prepared for aircraft use) it is pretty close to a 100 hp engine .For the extra 10 HP there is a couple hundred dollars worth of Camshaft etc required over and above what I've got -so I'm satisfied, so far, with what I've got. We'll see what 90 HP does in a Pegazair when we get it together. My engine has 180 degree equal length headers and a short smooth equal length intake with a 50mm carb, and it's a 140 based engine, so it breaths a bit better than a "stock" 110 at 3000 RPM Thanks, Clare, for a lot of excellent specifics and history. It seems that those original engines were smaller than I remembered, and produced less maximum torque even for their size. That shouldn't surprise me, considering the power levels of the compact cars the Corvair was designed to compete with and the people it was designed to serve. I always tend to think of these little engines in terms of installing them in go-fast machines, for their power. To me, that means turning a 52 inch propeller about 3500 to 3600 rpm. So 95 to 100 hp may not be all that crazy--especially with a rear drive installation such as offered by Great Plains for their VW based engines. That is similar to Steve Whittman's V8 canversion, and presumably to his Formula-Vee racing installation as well. Thanks to Richard Lamb for the link to Great Plains earlier in this thread. OTOH, before someone else posts yet another recitation that more propeller disk area equals more thrust and therefore more performance... I took a quick look at a set of posted specs for the Pegazair, on UltralightNews.com, and suspect that you are just about at the top of the horsepower and rpm range for that installation. It would not surprise me at all, using your numbers above, it the 80 hp version gave identical performance to the 90 hp version in the Pegazair's speed range. (Discalimer: I am not qualified to make this observation.) Peter |
#72
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Corvair conversion engines
"Peter Dohm" wrote I always tend to think of these little engines in terms of installing them in go-fast machines, for their power. To me, that means turning a 52 inch propeller about 3500 to 3600 rpm. Willim Wynne does not reccomend turning the Corvair engine at more than 3000 RPM, and pretty much says right out, that doing so has been shown to break the crank, in all of the cases he has studied. So 95 to 100 hp may not be all that crazy--especially with a rear drive installation such as offered by Great Plains for their VW based engines. Some pretty experienced people (in VW engines) have said that the valves are the weak link, and that much more that 45 HP will melt the valves down, if run at that level for more than a few minutes. -- Jim in NC |
#73
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Corvair conversion engines
"Morgans" wrote in message ... "Peter Dohm" wrote I always tend to think of these little engines in terms of installing them in go-fast machines, for their power. To me, that means turning a 52 inch propeller about 3500 to 3600 rpm. Willim Wynne does not reccomend turning the Corvair engine at more than 3000 RPM, and pretty much says right out, that doing so has been shown to break the crank, in all of the cases he has studied. So 95 to 100 hp may not be all that crazy--especially with a rear drive installation such as offered by Great Plains for their VW based engines. Some pretty experienced people (in VW engines) have said that the valves are the weak link, and that much more that 45 HP will melt the valves down, if run at that level for more than a few minutes. -- Jim in NC That is an interesting assertion. I really don't know. The number would conform quite nicely to a 1600cc/96cid engine producing around 60hp; as well as most older normally aspirated aircraft engines, which seem to have maximum continuous power figures in the 0.5 hp/cid range. There is even a rather famous dissertation, supposedly published in Contact!, which suggests failure due to heat dissipation problems in the valves and/or heads of automotive engines operated above continuously that power level. As it happens, I am really on both sides of this issue--and may not be technically qualified on either. (Required disclaimer as I am neither a mechanical engineer nor a mechanic) On the "this is unadulterated manure" side: there is a dissertation attributed to an engineer at one of the big three auto makers and posted here a couple of times, possibly by Corky, asserting that manufacturer testing includes a 100 hour run at full rated power--and that the failure about which they are concerned is the harmonic dampener. That is in keeping with articles I read more than 30 years ago in my school days, which stated that the exhaust manifolds glow incandescent during this proceedure. However, the colant and oil are maintained within their normal temperature range during that portion of the testing proceedures. On modern automotive engines, this equates to more than 1.0 hp/cid; and 100 hours is clearly much more than a few minutes. However, my real problem with the valve assertion is that I really don't know anyone who managed to run one of these little air cooled engines long enough and hard enough to burn a valve. I do know of two broker cranks on Corvair conversions (same person) and at least one, and possibly two, broken cranks on VW conversions (same other person). Both are mentioned on the FlyCorvair site, so I am really not adding much that is new. I am convinced that all of the failures were torsional damping issues. The only burned valve that I know of on an automotive conversion was on a liquid cooled Geo/Suzuki engine and was traced to a carburetion problem--which was run at a much higher power level. I was told that the carburetion problem was corrected and has not recurred. On the other hand, I strongly suspect that very high power levels equate to accelerated wear; and I really dislike very short TBOs. So all of my own scratch pad doodles are based on continuous power levels of less than 0.5 hp/cid, and usually significantly less. Peter |
#74
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Corvair conversion engines
"Peter Dohm" wrote in message
. .. However, my real problem with the valve assertion is that I really don't know anyone who managed to run one of these little air cooled engines long enough and hard enough to burn a valve. The 140 hp Corvair engines tend to lose valve seat inserts if overheated. Rich S. |
#75
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Corvair conversion engines
On Fri, 27 Jan 2006 21:49:21 -0500, "Peter Dohm"
wrote: "Morgans" wrote in message ... "Peter Dohm" wrote I always tend to think of these little engines in terms of installing them in go-fast machines, for their power. To me, that means turning a 52 inch propeller about 3500 to 3600 rpm. Willim Wynne does not reccomend turning the Corvair engine at more than 3000 RPM, and pretty much says right out, that doing so has been shown to break the crank, in all of the cases he has studied. So 95 to 100 hp may not be all that crazy--especially with a rear drive installation such as offered by Great Plains for their VW based engines. Some pretty experienced people (in VW engines) have said that the valves are the weak link, and that much more that 45 HP will melt the valves down, if run at that level for more than a few minutes. -- Jim in NC That is an interesting assertion. I really don't know. The number would conform quite nicely to a 1600cc/96cid engine producing around 60hp; as well as most older normally aspirated aircraft engines, which seem to have maximum continuous power figures in the 0.5 hp/cid range. There is even a rather famous dissertation, supposedly published in Contact!, which suggests failure due to heat dissipation problems in the valves and/or heads of automotive engines operated above continuously that power level. As it happens, I am really on both sides of this issue--and may not be technically qualified on either. (Required disclaimer as I am neither a mechanical engineer nor a mechanic) On the "this is unadulterated manure" side: there is a dissertation attributed to an engineer at one of the big three auto makers and posted here a couple of times, possibly by Corky, asserting that manufacturer testing includes a 100 hour run at full rated power--and that the failure about which they are concerned is the harmonic dampener. That is in keeping with articles I read more than 30 years ago in my school days, which stated that the exhaust manifolds glow incandescent during this proceedure. However, the colant and oil are maintained within their normal temperature range during that portion of the testing proceedures. On modern automotive engines, this equates to more than 1.0 hp/cid; and 100 hours is clearly much more than a few minutes. However, my real problem with the valve assertion is that I really don't know anyone who managed to run one of these little air cooled engines long enough and hard enough to burn a valve. I do know of two broker cranks on Corvair conversions (same person) and at least one, and possibly two, broken cranks on VW conversions (same other person). Both are mentioned on the FlyCorvair site, so I am really not adding much that is new. I am convinced that all of the failures were torsional damping issues. The only burned valve that I know of on an automotive conversion was on a liquid cooled Geo/Suzuki engine and was traced to a carburetion problem--which was run at a much higher power level. I was told that the carburetion problem was corrected and has not recurred. On the other hand, I strongly suspect that very high power levels equate to accelerated wear; and I really dislike very short TBOs. So all of my own scratch pad doodles are based on continuous power levels of less than 0.5 hp/cid, and usually significantly less. Peter On VW engines the problem is a lack of fin area on the heads, combined with limitted thermal mass. Anything over 40HP produces more heat than the heads can dissipate, and the thermal mass is low enough to limit any operation above that level to a matter of a very few minutes. Corvairs do not suffer this lack of fin area. |
#76
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Corvair conversion engines
Peter Dohm wrote:
Some pretty experienced people (in VW engines) have said that the valves are the weak link, and that much more that 45 HP will melt the valves down, if run at that level for more than a few minutes. -- Jim in NC For what it's worth, I'd have to agree, with the thought that you can operate at higher power - until - the heads are heat soaked. Then it's 45 HP or bust. I think the thinking is that the engine needs to turn up fast to make maximum engine power. Which may be true drag racing dune buggies. Grab a gear and spin that puppy up! Fixed pitch props don't do that. Think one speed automatic transmission - with a lot of slip. Pure torque converter. IIRC, and it's been a while, the VW factory specs said max rpm ws 4200? The torque curve peaked about 3000 (?) Swinging a propeller at 2500 to 3000 RPM puts us on the rising part of the curve - approaching peak. The faster it turns - the more torque it makes - the faster it turns. That's fun! I think that's why VW powered parasols and biplanes tend to be faster than their two-stroke powered brothers. For a VW to turn a propeller ~~3000 rpm means... 1600cc 54-56" prop diameter 1835cc 56-56" 2180cc 58-60" Our little low and slow airframes lean toward the long end. Swing as big a stick as you can. It pays off in prop efficiency, disk area, and tip circumference. (compared to a 66"~68" prop on a Rotax 503, VW's climb like sea slugs) Faster airframes tend to use shorter props and throw the excess torque into pitch to go faster! But at high cost in prop efficiency (again). Well, like I said, for what it's worth... Richard |
#77
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Corvair conversion engines
Rich S. wrote:
"Peter Dohm" wrote in message . .. However, my real problem with the valve assertion is that I really don't know anyone who managed to run one of these little air cooled engines long enough and hard enough to burn a valve. The 140 hp Corvair engines tend to lose valve seat inserts if overheated. Rich S. Same with VW. Probably the same with any shrink in valve seats... Richard |
#78
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Corvair conversion engines
"Peter Dohm" wrote That is an interesting assertion. I really don't know. The number would conform quite nicely to a 1600cc/96cid engine producing around 60hp; as well as most older normally aspirated aircraft engines, which seem to have maximum continuous power figures in the 0.5 hp/cid range. There is even a rather famous dissertation, supposedly published in Contact!, which suggests failure due to heat dissipation problems in the valves and/or heads of automotive engines operated above continuously that power level. You have no doubt read ditties from VeeDubber here, and one of them was on VW engines, and how many HP they really can sustain. Seem as though (from nearest I can remember) the amount of cooling fins in the area of the valve guides and seats, and the cross area is insufficient to carry away the amount of heat that is flowing through that area, if it is run flat out with a big enough prop for very long. Remember, heads in most auto engines nowadays, are cooled by water circulating through the head. Flow some oil past the critical areas, and heat can be dealt with. Air cooled engines must depend on cooling fins, and enough metal to carry the amount of heat produced to the fins. Perhaps VeeDubber will be kind enough to repost that treatise? g -- Jim in NC |
#79
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Corvair conversion engines
clare at snyder.on.ca wrote:
On Thu, 19 Jan 2006 15:35:13 -0500, "Morgans" wrote: "Bret Ludwig" wrote in message groups.com... I just think hanging a prop on a crank directly is a non-starter in the first place...especially on a crank and case not specifically designed for this in the first palce. Maybe a good redrive and flywheel would be a better way to go? That is my opinion, also. What isn't there can't break. That's my reson for a direct drive 'vair insted of a geared Soob - same weight - same HP. Exactly right Clare. The soob has a bulletproof interior but the use of liquid cooling plus a drive system adds two complete failure modes that aren't there at all with the Corvair. With the Corvair if you take care of the systems design aspect, basically by using sound aircraft design practices for carburation and ignition, you address the vast majority of the reliability issue and the only open question left is how strong are the basic mechanicals and that is something that is finally being addressed. I'm kinda glad that these crank failures have come about because it was always clear to me that the crank configuration should be considered "marginal" when subjected to prop gyro loads at higher power outputs, since simply by looking at it you see that bending loads can't be absorbed by the 1st bearing and bending is happening. One bit of good news is that the failure mode contains the end of the crank and does not result in the prop leaving the aircraft. I thought from the get go that the guys using extension shafts were nuts. However they have unwittingly provided a service by finally uncovering the crank's weak point in what amounted to a severe service qualification endurance test, ending that uncomfortable sense that nobody really knew just how strong the crank was or wasn't, or exactly where its weak point was. It's a great credit to William that he immediately responded by publicizing the issue and conducting further testing. As someone with a job that provides a ringside to seat to qualification, certification and continuing airworthiness of components on regional jets, I found his approach to be very much like, and sometimes superior to, the commercial world (in terms of letting it all hang out and responding to crises). Anyway, mistakes in calculations or engineering judgment in certification of commercial airliner components sometimes results in certified parts that are not up to snuff and fail in service well before predicted (I see this all the time). Truth is, sometimes the only thing that keeps commercial jets raining down on peoples' heads is double and triple redundancy, not the super duper construction of their components. There is still an unknown though. What I'd personally like to see William do is send the fracture results and the metallurgical data on the crank to a metallurgical and dynamic stress specialist who can calculate the loads/cycles that it took to initiate and propagate the cracks, then work backwards to establish the gyro forces and torque forces required to generate those loads, then apply a safety factor and establish safe propeller weight/length/horsepower limits for the existing configuration with a nitrided crank (the calculated limits may make a lot of people unhappy though). Builders need to know just where the safety boundaries are for the existing config. Myself I am still a big fan of the Corvair but will probably adopt the extra bearing mod he's working on if I ever get to that point. John Kahn Montreal |
#80
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Corvair conversion engines
On Fri, 27 Jan 2006 19:13:28 -0800, "Rich S."
wrote: "Peter Dohm" wrote in message ... However, my real problem with the valve assertion is that I really don't know anyone who managed to run one of these little air cooled engines long enough and hard enough to burn a valve. The 140 hp Corvair engines tend to lose valve seat inserts if overheated. Rich S. ANY aluminum head engine can loose valve seats. The secret is not to expect more of the engine than it can safely produce. They guy who did mine pinned them, but I don't plan on putting the pins to the test. |
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