On Wed, 30 Jun 2004 23:53:15 -0700, "PAW"
wrote:

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.

So the engine drives a pump and the pump drives a hydraulic motor, and
the motor spins the prop, right? I'm thinking of the homebuilt
designers maxim: "Build it light and simplicate." In addition, you've
got the prop spinning at 3200 rpm which is kind of high and will mean
a smallish, noisy propeller that doesn't produce much low speed
thrust, which means long takeoff runs.

In addition, the weight of the items you mention will likely be higher
because you will need some means of cooling the oil, as it will be
working hard. There will also be the weight of the oil lines.

I'm not an expert but since you asked, here's a guess: Airplane
designers are a particularly ingenious lot. If it were possible to
utilize such a prop drive as you describe, I think someone would
probably have tried it by now. After all, we've had virtually every
other manner of providing thrust including photovoltaic cells coupled
to electric motors.

My guess is that your idea might work, albeit extremely marginally and
heat rejection will be a major issue, as will be efficiency due to all
the pumping losses incurred building pressure and converting the
pressure to rotational thrust. All for, in my opinion, relatively low
propeller thrust.

I'll bet a good mechanical engineer could compute the relative
efficiency of this design vs. a direct drive prop or PSRU driven prop.
That should be relatively painless and you'd know before buying any
pieces if this would be worth it or not.

Corky Scott