Re: Thrust/HP

Date:         28 Aug 97 15:40:35 
From:         Steve Lacker <look@the.sig>
Organization: Applied Research Laboratories - The University of Texas at Austin
References:   1 2
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brian whatcott wrote:
>
> In article <airliners.1997.1727@ohare.Chicago.COM>,
> gordow@cts.com says...
> >
> >.,.. could anyone explain the
> >mathematical relationship of thrust(rocket or jet) and horsepower.
> >More precisely, how does one convert from one to the other....
>
> By this time, you will have seen the profusion of responses.
> No one quite fingered the root of the difficulty, which is this:
>
> pure jets produce (somewhat) constant thrust
>  BUT
> prop engines produce (somewhat) constant horse power.

>This has the rather strange consequence that
>a prop plane produces progressively LESS thrust force with increasing
>speed,
> BUT
>pure jets produce progressively MORE horse power with increasing
>speed.

But you are really tiptoeing around here is a consequence of how we
choose to define the measuement. Propellor engines are only rated in
"horsepower" because it is quite easy to connect the output shaft to a
dynomometer and measure how much power is being delivered *to* the
propellor- and that is the rating.

We could just as easily measure the *thrust* produced by the propellor,
in which case the ratings for a jet and propellor powerplant would look
much more similar. A propellor engine, *just like a jet* delivers ZERO
horsepower to the *airframe* if the airframe is sitting still, even
though the engine may be delivering 5000 horsepower to the propellor via
the shaft. Those 5000 HP are going into moving *air* not into the
*airframe,* same as with a jet.

--
Stephen Lacker
Applied Research Laboratories, The University of Texas at Austin
PO Box 8029, Austin TX 78713-8029
512-835-3286	slacker@arlut.utexas.edu