Thrust - Thrust To Propulsive Power

Thrust To Propulsive Power

A very common question is how to contrast the thrust rating of a jet engine with the power rating of a piston engine. Such comparison is difficult, as these quantities are not equivalent. A piston engine does not move the aircraft by itself (the propeller does that), so piston engines are usually rated by how much power they deliver to the propeller. Except for changes in temperature and air pressure, this quantity depends basically on the throttle setting.

Now, a jet engine has no propeller. So let's find out the propulsive power of a jet engine from its thrust. Power is the force (F) it takes to move something over some distance (d) divided by the time (t) it takes to move that distance :

In case of a rocket or a jet aircraft, the force is exactly the thrust produced by the engine. If the rocket or aircraft is moving at about a constant speed, then distance divided by time is just speed, so power is thrust times speed:

This formula looks very surprising, but it is correct: the propulsive power (or power available ) of a jet engine increases with its speed. If the speed is zero, then the propulsive power is zero. If a jet aircraft is at full throttle but is tied to a very strong tree with a very strong chain, then the jet engine produces no propulsive power. It certainly transfers a lot of power around, but all that is wasted. Compare that to a piston engine. The combination piston engine–propeller also has a propulsive power with exactly the same formula, and it will also be zero at zero speed –- but that is for the engine–propeller set. The engine alone will continue to produce its rated power at a constant rate, whether the aircraft is moving or not.

Now, imagine the strong chain is broken, and the jet and the piston aircraft start to move. At low speeds:

The piston engine will have constant 100% power, and the propeller's thrust will vary with speed
The jet engine will have constant 100% thrust, and the engine's power will vary with speed

This shows why one cannot compare the rated power of a piston engine with the propulsive power of a jet engine – these are different quantities (even if the name "power" is the same). There isn't any useful power measurement in a jet engine that compares directly to a piston engine rated power. However, instead of comparing engine performance, the gross aircraft performance as complete systems can be compared using first principle definitions of power, force and work with the requisite considerations of constantly changing effects like drag and the mass (of the fuel) in both systems. There is of course an implicit relationship between thrust and their engines. Thrust specific fuel consumption is a useful measure for comparing engines.