Re: Thrust/HP

Date:         27 Aug 97 03:57:55 
From:         inet@intellisys.net (brian whatcott)
Organization: Teleport
References:   1
Followups:    1
Next article
View raw article
  or MIME structure

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.

The root of these striking differences lies in the fact that the propeller
couples the engine power quite well to an accelerated column of air a little
faster than the airspeed so that not too much energy is wasted in useless
stirring of the airmass.

In contrast, at slow speeds, the narrow high speed column of jet efflux is
doing a lot of no good!

Perhaps this note will flesh out the bare bones answer you have been
given; that for any vehicle, the force acting in the direction of motion times
the speed of the vehicle is numerically equal to propulsion power if you
choose compatible units.

brian whatcott <inet@intellisys.net> Altus OK