Re: Thrust/HP

Date:         09 Aug 97 02:28:29 
From: (Mark Drela)
Organization: Massachvsetts Institvte of Technology
References:   1
Followups:    1 2
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In article <airliners.1997.1727@ohare.Chicago.COM>, Wally <> 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.

There is also "shaft power", or "mechanical power", which is
defined as Shaft_torque * Rotation_rate.  This only makes sense
for engines which have an output shaft -- IC engines and turboprops.
This is not necessarily zero when sitting on the runway with
the throttle open.  Shaft power has no meaning for pure jets.

The propulsive efficiency for an IC engine-prop combo is usually
defined as  propulsive_power/shaft_power.  You can't define
a comparable efficiency for a jet.  I suppose you could define
an overall thermodynamic efficiency as

  propulsive_power / fuel_heating_rate

for any device.  But this is equivalent to SFC which is a more
practical measure of the same thing.

For turboprops, the picture gets more fuzzy since part of their
output comes through the shaft, and some comes out as direct
exhaust thrust.  You can get creative and define

"equivalent shaft power"  =  shaft power + thrust*velocity/prop_efficiency

or   "equivalent thrust"  =  prop_efficiency*shaft_power/velocity + thrust

to allow you to compare any type of powerplant in terms of the
equivalent thrust or equivalent shaft power.

  Mark Drela                          First Law of Aviation:
  MIT Aero & Astro          "Takeoff is optional, landing is compulsory"