Hydrostatic transmissions (hydraulic pump driving a hydraulic motor) are used all over the place for heavy equipment.
These are best for high torque, relatively low speed operation. A good aerospace quality pump will give you about 90%
efficiency, and the motor will be about 85%. Industrial pumps and motors typically are much less efficient. As someone
else said, the heat rejection will be an issue (we cool our pumps through heat exchangers inside fuel tanks sometimes).
You will be better off using direct drive from a couple of small engines than using a big engine and driving a pump then
driving a motor...

Our smaller pumps can spin up to 13,000 rpm (Apache helicopter) and deliver as much as 85 gpm @ 4500 psi (B2 bomber).
Our motors can deliver full torque at very low speeds (100 rpm)...

http://www.parker.com/ag/pdf/abexbrochure.pdf

--
Dan D.
http://www.ameritech.net/users/ddevillers/start.html


..
"PAW" wrote in message ...
This is a BS question, but I'm curious.

I was looking at some hydraulic motors the other day and was wondering if
a pump and motor could be used to drive a prop. A crazy example; two
hydraulic motors and a couple pumps (powered with a mazda 13b maybe ??) to
power something like a Cessna 337 in-line thrust type aircraft.
Understanding weight would be an issue, I'm wondering how it would, or could
,work. I was looking at an Eaton motor that was rated at (up to) 3200 RPM @
about 120 ft. lb of torque. Weight was 20 lbs. They have a pump (48 lbs)
that moves 42 gpm @ 4000 psi.

Is it possible? Single place would be fine.