Hydraulic coupling PSRU?

OK, I've been following the alternative engine threads for a while and I must admit that I like the idea of having more choices. But the issue of the PSRU seems to be the limiting factor.

I really don't have real life hydraulics experience (and my questions here might just show that) beyond what I learned in Physics in college, but would it be possible to:

1. Use a fluid coupling to isolate the PSRU from the torsional pulses of the engine? I keep thinking of an automatic transmission torque converter type thing.

2. What about a hydrostatic type of PSRU, with a variable displacement either pump or motor. (I just saw that Honda has come up with an motorcycle automatic transmission that uses this principle)

I know that weight of off-the-shelf components is an issue, but I'd like to know about the concept.

Any bright minds want to chime in? I'd appreciate the education I'd get from this discussion.

Torque converter would undoubtedly reduce torsional vibration, but at the expense of complexity and weight. A 150 hp hydraulic pump and motor would weigh at least 125 lbs, and would require a heat exchanger of its own.

Seems to me someone fitted a modified Powerglide from a Corvair back in the '70s into some airframe but I can't recall the details.

These days most of the new drives are being fitted with Lord or Lovejoy type elastomeric couplings which are designed for this job and can be scientifically matched to the application. They are light, reliable and inexpensive. I see MPS' new drive uses a Lord part.

A former collegue of mine is working on a hydraulic drive for wind turbines (3-5 MW) using the pump-motor principle. It obviously is an alternative, and will be both lighter and smaller than standard drives. A hydraulic motor with 150 HP could fit inside the spinner. Complexity and cost is another matter altogether, at least while keeping a reasonable efficiency. And efficiency is the problem.

Hydraulic Coupling

I investigated several motor/pump combinations in the past. The reason was the posibility of placing the engine at CG for a design a bud was doing. An interesting concept but the pump and motor were HEAVY. Most of the hyd. motor design for high HP has been for large terrain vehicles (mining dump trucks for instance) where weight has been a smaller consideration. The idea is interesting, think of a P39 type aircraft where the engine might be mounted behind the pilot, but without the driveshaft harmonics problems. Nobody I found was making or interested in making a lightweight pump/motor combo, atleast at that time. (About 3 years ago)
Bill
PS The reason he was interested in a hyd. motor idea was to mount 2 props to counter rotate and improve efficiency. (No P factor) Think a P38 with the motor in the center pod and 2 super thin booms. This was a really interesting concept, but it fell off my radar a while ago.
WRJ

SvingenB said:

A hydraulic motor with 150 HP could fit inside the spinner.

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Not after it was made capable of handling the prop loads, I suspect.

There is a huge difference in how the bearings and shaft would have to be designed-------end thrust comes to mind, and gyroscopic loads.

Still, may have some merit if a clean sheet design was done.

Honda's hydraulic trans

I found a link to the system developed by honda. It is really neat. They don't mention weight though, but still, pretty cool.

http://world.honda.com/news/2007/2071004Human-Friendly-Transmission/

Mike S said:

Not after it was made capable of handling the prop loads, I suspect.

There is a huge difference in how the bearings and shaft would have to be designed-------end thrust comes to mind, and gyroscopic loads.

Still, may have some merit if a clean sheet design was done.

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I agree. I think a hydraulic coupling is really only worth while for very special purposes. The good thing about it is the infinitely variable gear ratio and the possibility of decouple the engine from the drive unit. For an airplane none of these things are of any great importance.

Another dream dashed

Thanks a lot; Y'all have destroyed my dreams and candy cane wishes with your logic and facts. I was always wondering about a hydraulic coupling. Why not? That would take care of the harmonics. Your neg and Con comments answered that. I guess doable but not practical, i.e., size/weight is not trivial. If possible, it would not be off the shelf parts. Not to beat the subject to death, but direct drive does have its charm (compact/light).

Just difficult, not impossible

gmcjetpilot said:

Thanks a lot; Y'all have destroyed my dreams and candy cane wishes with your logic and facts. I was always wondering about a hydraulic coupling. Why not? That would take care of the harmonics. With your neg and Con comments answered that. I guess doable but not practical and size/weight is not trivial. If possible, it would not be off the shelf parts. Not beat the subject to death but direct drive does have its charm (compact/light).

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Hey George,
Funny how negitive some of the people on the forums can be huh?
In all seriousness though this is something that COULD be done. My comments in particular were not saying it wasn't possible, because it IS. The real difficulty is there aren't any lightweight pump and motor combinations made now. (As you mentioned) If somebody wanted to invest either the time or the money it could be done. You would need to use a efficient combination. If you checked out the Honda reference in a previous thread you might use a varible ratio pump so that you could set the engine to the exact RPM range you wanted. Direct drive does have its charm, but it is interesting that the biggest Lyc ever made not only had a gearbox, but in fact a 2 speed transmission! http://people.virginia.edu/~rjr/engines/

Bill Jepson