Another Pandoria's box

There are several fairly successful auto conversions running in airplanes. One of the more prominent is the corvair. For a very reasonable cost you can have an engine roughly equivalent to an 0-200 or O-235. Direct drive runs from 2300 to 3000 RPM all day long, depending on the prop. There have been efforts to put gear reduction units on auto engines. At the power level of the engines I have experience with it is a trade off. You might gain some power and be able to turn a larger prop, and run the engine at 120 or 180hp vs derated to 100 or 130. But you add from 30 to 50 lbs forward of the crank snout, and therefore need a longer cowling to accommodate it. And then a little ballast for w&b reasons. So you lose useful load too. One very good compromise is to just derate to 100 or 130hp (depending on how it was built) keep it direct drive, as in bolt the prop to the crank, and add a bearing to support the driving end if the crank. This, along with a properly prepped, nitrided crank goes a long way to preventing crank issues.

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zodiacflyguy said:
There are several fairly successful auto conversions running in airplanes. One of the more prominent is the corvair. For a very reasonable cost you can have an engine roughly equivalent to an 0-200 or O-235. Direct drive runs from 2300 to 3000 RPM all day long, depending on the prop. There have been efforts to put gear reduction units on auto engines. At the power level of the engines I have experience with it is a trade off. You might gain some power and be able to turn a larger prop, and run the engine at 120 or 180hp vs derated to 100 or 130. But you add from 30 to 50 lbs forward of the crank snout, and therefore need a longer cowling to accommodate it. And then a little ballast for w&b reasons. So you lose useful load too. One very good compromise is to just derate to 100 or 130hp (depending on how it was built) keep it direct drive, as in bolt the prop to the crank, and add a bearing to support the driving end if the crank. This, along with a properly prepped, nitrided crank goes a long way to preventing crank issues.
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OK, I can buy that. Still seems like dropping two cylinders would make up for the weight and size of the gearbox.
 
arnoha said:
OK, I can buy that. Still seems like dropping two cylinders would make up for the weight and size of the gearbox.
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Possibly, but then the loss in output would negate the weight savings.

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Should we bring up the reputation of the GO-300-D? How about the G0-480 ? Seen any GO-720s around lately?
 
zodiacflyguy said:
You might gain some power and be able to turn a larger prop,
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The larger the prop, the slower you must run it, that becomes a vicious circle.
prime example Alison T56- Turbine runs 11,200 RPM while that huge prop spins at 1120. and the gear box that will handle that load and reduce those RPM, weighs 450 pounds.
want a viable alternative ?

The new CD-300 is a three-liter, turbocharged, direct-injection, liquid-cooled V-6 with single-lever power controls and electronic engine management. The Jet-A-burning engine joins the company’s CD-100 and CD-200 line of engines that were originally developed by TAE. From now on, Continental won’t be using TAE’s “Centurion” brand name; they’re all rebranded as CDs (Continental Diesels).
 
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455 Bravo Uniform said:
On these engines, are the connecting rods from opposing cylinders on the same crank journal, or are they separate?
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Think about how an opposed engine works. The opposing pistons move out towards the heads at the same time in order to balance each other out. This REQUIRES separate journals.
 
zodiacflyguy said:
Possibly, but then the loss in output would negate the weight savings.
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The point being that the higher RPM would result in the same power output. Think, for instance, taking a Corvair engine making 120 HP. Swap it for a Rotax making 115 HP. Same class of output. (Tried to think of an auto engine that's used in that class...couldn't. That may be the issue...no availability of a suitable engine.)
 
flyingbrit said:
Think about how an opposed engine works. The opposing pistons move out towards the heads at the same time in order to balance each other out. This REQUIRES separate journals.
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Here is an example of a crank with 2 rods on one journal. Odd fire Buick v6. The 3 wider journals carry 2 rods, and there are 4 mains (all narrow except the wider rear main. I figured a flat engine could be the same (but understand it’s not).

846AD5E5-C3CC-4DEA-BF28-81F7F551274D.png
 
455 Bravo Uniform said:
Here is an example of a crank with 2 rods on one journal. Odd fire Buick v6. The 3 wider journals carry 2 rods, and there are 4 mains (all narrow except the wider rear main. I figured a flat engine could be the same (but understand it’s not).

View attachment 62001
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Notice the huge counter weights to offset having two rods in one place?
Did ya notice the counter weights on the 0-300 crank I posted?

Weight,, aircraft engines must be light as possible.
 
Here is a picture of an OX-5 (WWI era aircraft engine) crank. It was a V-8 and looks like two rods per journal.
A19780417000cp02.jpg
 
flyingbrit said:
Here is a picture of an OX-5 (WWI era aircraft engine) crank. It was a V-8 and looks like two rods per journal.
A19780417000cp02.jpg
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Very cool. Flat plane V8!

(Note...this doubling on the journal would not work with a flat engine. Also, the flat plane of the crankshaft is why it doesn't need the heavy counterweights of a cross-plane V8.)
 
There is an auto engine that is roughly in that class output wise. The subaru EA81. Only thing is it cannot make much power at low RPM and thus needs a reduction drive. Also it's a liquid cooled engine and brings with it another pile of complications. Not saying liquid cooling is bad. Just that the output needs to be of a level to make the added complexity worth it, IMO. (Like a small block chevy in a P-51 replica...or two of them in a P-38)

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arnoha said:
The point being that the higher RPM would result in the same power output. Think, for instance, taking a Corvair engine making 120 HP. Swap it for a Rotax making 115 HP. Same class of output. (Tried to think of an auto engine that's used in that class...couldn't. That may be the issue...no availability of a suitable engine.)
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Yes the rotax does make somewhere in the same neighborhood hp wise...but the issue I have with it is that is not direct drive, and unless you have official blessing from Rotax, there is not a lot you can do to it. It is known as a fairly reliable engine, but for the cost, I would just go with an O-235 and then any decent A&P can work on it.

The reason I lean heavily toward the corvair is actually not to save money. That is a poor reason to buy any engine. What appeals to me is that the entire airplane is built by me. From spinner to position light. Including the engine. Buuuut I can totally rebuild my engine for about what it costs to buy 4 cylinders for a Lycoming. I am talking all six cylinders with rings. All bearings, gaskets, and any other wear parts. Hell a brand new (well, NOS anyway)crank is about 500-600.

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