Re: Thrust/HP

Date:         17 Aug 97 01:57:22 
From:         Patt <click-on-address@below.net>
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Wally wrote:
> 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.

Wally.. although Mark's answer is quite technical, an oversimplified,
'rule of thumb' is.. 1 horsepower is about 2.5 lbs of thrust. For
example, your Cessna 150 with 100 hp would perform about the same if it
had a 250 lb thrust jet engine installed.

On the other hand, a CFM-56 high bypass fan engine (Boeing 737) rated at
25,000 lbs thrust is, for comparison purposes, about the same as a
10,000 hp reciprocating engine.

This conversion is not terribly accurate, but the rule of thumb of    1
hp = 2 1/2 lbs thrust is more than adequate for 'hangar flying'.

By the way, as Mark stated, the conversion gets more inaccurate and
confusing when you talk about turbo-prop engines because now mixing
apples and oranges, ie: a propeller connected (either by a shaft or free
turbine) to a jet engine.  On the Allison 501-D13 engine, for example
(Electra/P-3/CV-580), power is displayed in the cockpit on
torquemeters.. which are run thru a mini-computer and you read
'horsepower'.  The torquemeters read from 0 to 4,000 hp. There is a
small benefit from the thrust out the tailpipe, but if I remember
correctly its only about 300 lbs thrust.

So... for horsepower to thrust (and vice versa) you'll be in the
ballpark using 1 : 2.5 ratio.

           Patt
 mailto:pattmcd@swbell.net