Re: Thrust/HP

Date:         17 Aug 97 01:57:22 
From:         Steve Lacker <look@the.sig>
Organization: Applied Research Laboratories - The University of Texas at Austin
References:   1 2
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Mark Drela wrote:
> In article <airliners.1997.1727@ohare.Chicago.COM>, Wally <gordow@cts.com> writes:
> > At the expense of sounding stupid, could anyone explain the
> > mathematical relationship of thrust(rocket or jet) and horsepower.
> > More precisely, how does one convert from one to the other.  I can't
> > begin to relate how long and often I've pondered this question. Any
> > help would be greatly appreciated.
>
> Depends what you mean by "power".
>
> "Propulsive power", also called "thrust power" is defined as
> Thrust * Flight_Velocity.  This definition can be used for any
> isolated propulsive device.  So the thrust power of a powerplant
> depends on the flight speed.  When sitting on the runway, the
> thrust power is zero.

Isn't it also true  that it should be possible to define a power rating
for a jet or turbofan engine sitting in a test stand or on an aircraft
with the brakes locked in terms of how much power it is producing in the
form of moving air, assuming all the air was at rest prior to being
ingested by the engine? Basically something along the lines of this:

Energy in a moving mass= 1/2 MV**2
Power=energy/unit time, thus:

Power= (1/2)*(mass of air moved / second)*(velocity of air)**2

If the mass moved is in kg/second and the velocity is in meters/second,
the power rating would be in watts. To get HP, take the rating in watts
and divde by 746.

But the horsepower the engine _applies_ to the airframe (the "thrust
power" from the previous post) is still thrust*velocity of the
_airframe_ not the _air_

Definitions, definitions... :-)

--
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