Prop shaft

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Bigshu

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As we wait for billski to say "It's not that easy"

Note all the homebuilts with driveshafts have driveshaft problems... Not 100% sure, but yet to see mentions of one running an extension that didn't have issues.

I really wish there was an easy way to get a 3-5' extension that's reliable. But even the short ones are problematic.
I'm not so sure. Lesher set a lot of records in Teal with no issues, and the drive train was almost the same as on Taylor's IMPs. Douglas went with it on the XB-42, and it worked great, they just abandoned it for the jet version (XB-43). Granted, those aren't run of the mill homebuilts by any stretch, but with the right numbers from the right guys, it seems to be do-able (I think the problems are with the wrong people using the wrong numbers...). It might just be a situation where you need a corporation or University backing your play!
 

Bigshu

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I'm totally forgetting my avatar! The Lockheed Big Dipper was a pusher with a drive shaft, as was the twin engine Douglas Cloudster 2. I'm also a fan of the Doak 16, which used shaft drive for it's tilt prop design. The drive shaft wasn't an issue on any of those, not that there weren't other issues, chiefly economic.
 

Vigilant1

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I really wish there was an easy way to get a 3-5' extension that's reliable. But even the short ones are problematic.
Yes, being able to move the engine some distance from the prop would make some configurations more practical (CG issues) and also improve aerodynamics (esp less aft-body drag for pusher designs).
 

wsimpso1

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Readers of this thread will find the following helpful:


Any system with an engine and flywheel, a shaft of much length, and a prop or rotor, will find that it really does behave as a two mass system with a spring between them. Even a helo tail rotor has an impressive mass moment of inertia, most props significantly exceed engine and flywheel inertia, and the shaft has significant springiness. The big issue becomes one of figuring out if the resonant modes lie safely below all of the forcing functions, or if any of the resonances coincide with forcing functions within or near the operating range. Yes, even helo tail rotor systems.

Usually, you can drive down resonant mode frequencies with a torsionally soft spring at the engine end or by designing the shaft to have a suitable spring rate. This is not something amenable to solving by simple trial and error...
 
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Dan Thomas

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What about a fluid coupling? Like nested shafts with a viscous but shear-thinning liquid between them.
A torque converter or fluid clutch does that, but it requires diameter to make it work, meaning that weight goes up. It also generates heat, an indication of lost efficiency.
 

Dan Thomas

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Usually, you can drive down resonant mode frequencies with a torsionally soft spring at the engine end or by designing the shaft to have a suitable spring rate. This is not something amenable to solving by simple trial and error...
Those who keep saying that "this is experimental aviation" won't like that. But experimenting has to be done wisely if it's to be done safely. The early history of aviation proves it. It was deadly. When it comes to redrives and extensions, there is already lots of history to review to see what worked and what didn't. One just has to go looking for it, or asking honest questions exactly like the OP here did. He's doing his research.

Many years ago I wanted to build an amphibious "twin" that had one engine buried in the fuselage, with some sort of drive to a prop on each wing. It seemed so simple. But shafts, belts, and chains all had serious problems. Cooling was a hassle. The weight of a converted car engine was too high, especially in the '70s. I soon realized that if I wanted to fly anytime soon I'd better do something else. I had seen some other people spend half their lives on such projects and didn't want to end up like that. Life is too short. In fact, I still don't know what I want to be when I grow up...
 

jedi

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......... In fact, I still don't know what I want to be when I grow up...
That is a good thing. There are those who learn what they want to be when they grow up and it is not what they are. So sad to find the answer when it is too late to do anything about it.

A good part of "growing up" is learning what it is that you do not want to be and then doing something about avoiding that.
 

Sockmonkey

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A torque converter or fluid clutch does that, but it requires diameter to make it work, meaning that weight goes up. It also generates heat, an indication of lost efficiency.
I was thinking that if the concentric shafts were both rather long, the surface area the fluid is in contact with would provide sufficient "grip" to keep slippage down to a reasonable level and give enough external surface area via the outer shaft to cool it enough.
I should clarify that I mean having only a millimeter or so of clearance between the inner and outer shafts.
 

Bigshu

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A torque converter or fluid clutch does that, but it requires diameter to make it work, meaning that weight goes up. It also generates heat, an indication of lost efficiency.
[/QUOTE
Isn't the heat build up a function of the stall speed range of the torque converter? If properly sized, in a single gear (1:1) application, how much heat would it generate? Seems like if you know the torque band of the cam, and the cruise RMP of the output shaft, a lot of heat could be avoided.
 

BBerson

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It would need a flywheel, much like the original big flywheel on the Wright Flyer with the prop shafts. Prop shaft failures were a major problem for the Wrights from the start. Some failed in seconds while others were 2 hours.
 

Dan Thomas

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It would need a flywheel, much like the original big flywheel on the Wright Flyer with the prop shafts. Prop shaft failures were a major problem for the Wrights from the start. Some failed in seconds while others were 2 hours.
Didn't know that. Thanks for the history. It shows that redrives, even for 12 horsepower and low RPM, can be tough to design.
 

wsimpso1

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What about a fluid coupling? Like nested shafts with a viscous but shear-thinning liquid between them.
23 years in automatic transmission design engineering, 18 of it in torque converters. I know these machines.

On the good side, fluid couplings (and torque converters) are excellent isolators of the downstream parts from the engine. They are still a two mass system with inertia at each end and a spring in between, and you will get as many pulses per prop rev as you have blades, so you still have to make sure your operating range is safely separated from resonance modes.

One the bad side, fluid couplings (and torque converters) are:
Poor efficiency - around the speed ratio across the device. Usually efficiency is around 75-90% under conditions like we run in airplanes, meaning 1/4th to 1/10th of the engine power goes into heating the thing;
Heavy - Typically 30-50 pounds;
You would need an oil pump to turn over the oil in the thing and send it through a cooler to reject the cooling, adding more weight and complexity.

These gadgets only support modern fuel economy by having a clutch that mechanically bypasses the turbomachinery, and they have a calibrated springs selected in concert with the inertia of the system to put resonance outside of the range of forcing functions.

Seriously, it is way better to have a selected amount of springiness in a designed system so resonance occurs away from the engine operating vibrations and other inputs of the system... Yeah, it means doing some serious engineering, but there are no coolers, no oil loops, no pumps, no seals, no turbomachinery, and lot less weight.

Billski
 
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