This topic has cropped up from time to time in many threads but usually as thread drift. While I'm waiting for dope to dry I thought I'd throw this out there.
What I would like is some input about various ideas how best to go about this. I am always happy to see other people's ideas even if I usually end up doing my own thing; many times it has given me a partial solution to a problem at least.
I understand the principles involved here (and yes, I have read everything on this topic that was ever written on this forum) but only have the haziest grasp of the mathematics involved. It is entirely beyond me (at this point) to attempt a mathematical confirmation of this idea, so Experimental it is. If I can get close to making a ball park estimate from various numerical estimates, practical experiences and educated guesses, so much the better.
This is an offshoot from my quest to get a Buick 215 flying; I am also still considering my original option of a direct drive Chevy. I have looked at everything that is out there, including the LS engines and a stroked small block built entirely from parts. Leaving the actual choice of engine for the moment beyond the fact that it should put out around 200hp at no more than 2700rpm and concentrating on the drive end:
Simply hanging it an a spacer/prop hub off the flywheel is not an option I would choose. Yes, it has been done. It flew. Nobody died. But there are better ways.
I propose the usual idea of a bellhousing with a short output shaft that has a substantial bearing in a plate fixed to the business end of the bellhousing.
I understand the principles involved here (and yes, I have read everything on this topic that was ever written on this forum) but only have the haziest grasp of the mathematics involved. It is entirely beyond me (at this point) to attempt a mathematical confirmation of this idea, so Experimental it is. If I can get close to making a ball park estimate from various numerical estimates, practical experiences and educated guesses, so much the better.
There are two options here:
1: A stiff system where the shaft is firmly fixed to the flywheel/flexplate and thus the crank.
2: A soft system with a similar output shaft but with a torsionally flexible coupling between it and the flywheel.
A stiff system would rely on simply being extremely stout and precisely fitted. The fact that several aircraft have used something like and got away with it would suggest that any harmful resonances seem to be outside the operating range in which it would be used. Or that they simply haven't grenaded just yet...
I like the idea of using modified axle shafts simply because they are available everywhere and many shops around here can modify them properly. Also they can be easily fitted to a flexplate, since parts like that already exist or can be modified. The fact that splines are involved and that any backlash in a stiff system appears to be a big No-no, would this be a factor? I've heard opinions that they will destroy themselves in no time to a opinions that it matters not a bit, they can handle it. Opinions and conjecture but not a lot of fact. Splines can be clamped, for one thing.
Also, alignment becomes very critical. Maybe not impossibly critical. Simple and lightweight, if it all works.
Soft system: Not too difficult either, given the infinite amount of torsional couplings out there. You can drown in information on these things; I had no idea how vast that field really is. The mechanical aspects are are easily handled. Attachment options to the output shaft and the flywheel are infinite and fairly simple. It is however another lump of weight to be added to the engine; anything from a couple of pounds to maybe 10? ( Making the most hp per lbs is obviously not the goal of this project so let's ignore that.) The usual doughnut type of coupling seems to be universally and successfully used but there are interesting other options. The difficulty is hitting on one the right size and with the right spring rate. Published figures for the power capability of these coupling are all available, that is no problem. To make even an educated guess of the mass moment of inertia of the rotating assembly of a V8 is totally beyond me. It's hard to even find those numbers for props, but props are relatively easy to work out yourself. To make even a wild guess at the coupling characteristics required we would at least need these two numbers, no? I wonder if anyone has ever sat down and modeled the whole thing and got even a vague number to use as a starting point?
Both drives would assume a relatively light, fixed pitch wood prop. I always leaned towards the IVO prop but was warned off composite props for this particular application.
Over to you....
What I would like is some input about various ideas how best to go about this. I am always happy to see other people's ideas even if I usually end up doing my own thing; many times it has given me a partial solution to a problem at least.
I understand the principles involved here (and yes, I have read everything on this topic that was ever written on this forum) but only have the haziest grasp of the mathematics involved. It is entirely beyond me (at this point) to attempt a mathematical confirmation of this idea, so Experimental it is. If I can get close to making a ball park estimate from various numerical estimates, practical experiences and educated guesses, so much the better.
This is an offshoot from my quest to get a Buick 215 flying; I am also still considering my original option of a direct drive Chevy. I have looked at everything that is out there, including the LS engines and a stroked small block built entirely from parts. Leaving the actual choice of engine for the moment beyond the fact that it should put out around 200hp at no more than 2700rpm and concentrating on the drive end:
Simply hanging it an a spacer/prop hub off the flywheel is not an option I would choose. Yes, it has been done. It flew. Nobody died. But there are better ways.
I propose the usual idea of a bellhousing with a short output shaft that has a substantial bearing in a plate fixed to the business end of the bellhousing.
I understand the principles involved here (and yes, I have read everything on this topic that was ever written on this forum) but only have the haziest grasp of the mathematics involved. It is entirely beyond me (at this point) to attempt a mathematical confirmation of this idea, so Experimental it is. If I can get close to making a ball park estimate from various numerical estimates, practical experiences and educated guesses, so much the better.
There are two options here:
1: A stiff system where the shaft is firmly fixed to the flywheel/flexplate and thus the crank.
2: A soft system with a similar output shaft but with a torsionally flexible coupling between it and the flywheel.
A stiff system would rely on simply being extremely stout and precisely fitted. The fact that several aircraft have used something like and got away with it would suggest that any harmful resonances seem to be outside the operating range in which it would be used. Or that they simply haven't grenaded just yet...
I like the idea of using modified axle shafts simply because they are available everywhere and many shops around here can modify them properly. Also they can be easily fitted to a flexplate, since parts like that already exist or can be modified. The fact that splines are involved and that any backlash in a stiff system appears to be a big No-no, would this be a factor? I've heard opinions that they will destroy themselves in no time to a opinions that it matters not a bit, they can handle it. Opinions and conjecture but not a lot of fact. Splines can be clamped, for one thing.
Also, alignment becomes very critical. Maybe not impossibly critical. Simple and lightweight, if it all works.
Soft system: Not too difficult either, given the infinite amount of torsional couplings out there. You can drown in information on these things; I had no idea how vast that field really is. The mechanical aspects are are easily handled. Attachment options to the output shaft and the flywheel are infinite and fairly simple. It is however another lump of weight to be added to the engine; anything from a couple of pounds to maybe 10? ( Making the most hp per lbs is obviously not the goal of this project so let's ignore that.) The usual doughnut type of coupling seems to be universally and successfully used but there are interesting other options. The difficulty is hitting on one the right size and with the right spring rate. Published figures for the power capability of these coupling are all available, that is no problem. To make even an educated guess of the mass moment of inertia of the rotating assembly of a V8 is totally beyond me. It's hard to even find those numbers for props, but props are relatively easy to work out yourself. To make even a wild guess at the coupling characteristics required we would at least need these two numbers, no? I wonder if anyone has ever sat down and modeled the whole thing and got even a vague number to use as a starting point?
Both drives would assume a relatively light, fixed pitch wood prop. I always leaned towards the IVO prop but was warned off composite props for this particular application.
Over to you....
