# Is the gas velocity of a jet engine relative to the engine speed or is it absolute?

Let's say we are testing a jet engine on the ground by keeping it still. We measure the inlet and outlet velocity of the gas at a specific operation point. Let's assume that we attach this engine on a aircraft that is of course moving. In a random time the operation point is the same as before. If we measure the absolute inlet and outlet velocities are the same as before or only the relative ones(aircraft velocity+gas velocity) are the same?

And one more similar question. In the case of the engine attached to the aircraft all the fuel energy that can be used is transformed into changes of the kinetic energy of both the aircraft and the air passing through the engine.Right? In the first case of the test, the kinetic energy of the engine isn't changing, so all the used fuel energy is changing the gas kinetic energy, so in this case the difference in the kinetic energy of the gas is greater. Does this makes sense since we talk for the same operation point?

To conclude, is the thrust the same regardless if the jet engine is moving or not?

• Seems like you have a lot of questions here... it may help to break them down into bullets/paragraphs, or even separate questions. Also see existing questions, such as thrust vs. speed and exhaust velocity. Aug 23, 2016 at 19:46
• There is no such thing as "absolute" speed, except for speed of light, which is not reached here. So I suggest rewording as "aircraft-relative" and "free-stream-relative" (free stream is the air mass not (yet) affected by the aircraft). There can also be "ground-relative", but that is irrelevant, since ground does not come into any significant interaction here. And to be honest, I am not sure which of the speeds you actually meant by "absolute". Aug 25, 2016 at 21:45
• Energy is a bit unusual in that it is conserved in all inertial reference frames, but has different values in each. So you can calculate the energies for aircraft in flight relative to free stream or relative to the aircraft and the energy will add up in both, but in very different ways. Aug 25, 2016 at 21:49

The answer is NO, but it is important to understand why. I will explain the conditions, and answer your questions at the end one by one.

First point you are never testing the same engine:

• Aircraft manufacturer usually introduces its inlet and its nozzle so both are not usually the same in a test and in the real conditions. (that will mean is partially installed)
• You are not including the general effect of the A/C aerodynamics.
• You are not including the air bleeding and oil and fuel pump (also, a partially installed engine)

Essentially you are asking about the effects between installed and uninstalled effects on an engine.