The net thrust of a jet engine
 

hi everyone,

I just need some advice about the variation of thrust of a jet engine with speed.

I know increasing speed increases the intake momentum drag, which reduces the thrust. Can anyone explain me what this mean? What is the "intake momentum drag"?

Thanks a lot


Best
subsidence

As far as I know increasing speed (= intake velocity V0) actually increases a jet engine's thrust, and ofcourse drag increases as well.

The "intake momentum drag" you are talking about is nothing I've ever heard of, but I reckon it is associated with the slowing of intake air as well as the associated increase in static air pressure in the intake at very high speeds, particularly with supersonic aircraft.

Additionally, Google would have told you this and this. :ok:

I believe the term comes from using a frame of reference relating to a moving engine rather than a fixed airmass.

In either frame you can approximately relate the thrust of an engine to the change of momentum (mass flow * velocity) of the air as it passes through. This ignores cowl drag etc..

In a fixed air mass frame of reference, the momentum of the air is nil on entry and is (mass flow (air+fuel) * jet velocity) on exit.

If you solve the problem relative to a moving engine, it appears that the velocity of the incoming air is the same as the aircraft's forward speed, but the jet velocity is also increased by this amount.

The nett result of subtracting the two figures is called momentum thrust and is the same whichever reference frame you use, but in a moving frame of reference there is a large negative term for the air momentum on intake called momentum drag.

This follows directly from Newton's laws, as the force accelerating the air is required to equal the thrust reaction on the engine.

Ok, the matter's closed as far as I'm concerned.

I just found this explanation:

The air entering the front of the engine is slowed down and loses momentum before it is again accelerated.This is intake momentum drag and decreases the overall thrust of an engine.

Thanks for your patience; thank you Mark1
for your help

suby