Monday, March 28, 2011

Power v. Thrust

There was a bold statement that said "Piston engines develop power but jet engines (gas turbines) develop thrust"

This is, broadly true.


Power, however it is measured, is the RATE at which WORK is done.  Work can be described as torque.

Torque is a twisting moment about a shaft.  If someone grabs your hand and rotates it there is a twisting moment in your forearm.  Work is being done on your arm.

Now imagine a mangle.  Oh.  You're not old enough to know what a mangle is!  Hmm.  Two sets of rollers between which you pass clothes and sheets, etc, to squeeze most of the water out.  The rollers are operated by a handle that cranks them around.
By turning the handle you are putting torque on the rollers.  If you apply sufficient torque to overcome the resistance of the rollers - and the wet clothes between the rollers, then they will begin to rotate.  The speed at which they rotate will depend on the effort you wish to put into the job.
The torque that you apply is the work done.  The rotation is the rate at which the work is done.

You are applying Horse Power to the mangle.

A piston engine applies torque to the wheels of a car or a propeller on an aeroplane.  In both cases the resistance to motion has to be overcome by the torque to get movement, after that the speed of rotation (rpm  =  revolutions per minute) is the rate at which the work is done.

Power can be high rpm and low torque or, for the same power, low rpm and high torque.

Power = RPM x WORK.

Jet engines apply a reaction force to the engine by the hot gases rushing out of the back of the engine.  This is Newton's Third Law  -  to every action there is an equal and opposite reaction.

The reaction felt by the engine is an applied force just like someone pushing you in the back but doing it continuously.  This force is described in pounds (lbf) or in kN (kiloNewtons).

1000 lbf of thrust  =  4.45 kN (Actually: 4.4482216 kN)

1 kN  =  224.81 lbf    (Actually: 224.8089431 lbf)

On a jet engine  -  or a rocket, there is only the 'push', there is no 'rate' at which this is done so a jet engine does not develop power.

No comments:

Post a Comment