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While calculating air pressure and temperature during cruise of jet engines, we consider that air comes into the jet engine inlet with our flight velocity.

But why? In fact, air stops stagnant in atmosphere and aircraft moves with a flight velocity into air.

During calculations, we say: "Static pressure of air is 1, and it has a velocity of flight velocity. Then air comes to rest, so its static pressure increases!".

But the thing I don't understand is actually air doesn't have that velocity, what has that velocity is jet engine/aircraft.

Can you explain this assumption?

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    $\begingroup$ Velocity depends on frame of reference. If you were in the vacuum of space, and an asteroid was flying towards you, from the asteroids perspective it is static and you are flying towards it. Same thing is happening between the plane and the air, you're just using the nearby reference point of the earth to confuse yourself. $\endgroup$
    – James T
    Commented May 17, 2023 at 9:28
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    $\begingroup$ For a turbo-engine what ultimately counts is not the proper velocity of air, nor the velocity of the aircraft, but air velocity relatively to compressor rotor blades (or fan blades), which spin, that makes a big difference. Note when tests are made in wind tunnels, this is air which is moving, not the surfaces. $\endgroup$
    – mins
    Commented May 17, 2023 at 16:16

4 Answers 4

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Both the air and the engine are moving very, very fast. Our galaxy and everything in it is moving at a blistering speed of several hundred miles per second.

Of course, we don't feel this speed. Velocity is relative. Try sticking your hand out the window while driving (or flying, if you are brave and your POH permits it). The air pushes against your hand exactly the same as it would if you were sitting in a wind tunnel and your car wasn't moving at all.

This is the only reason wind tunnels are useful to begin with- because velocity is relative, the physics is the same whether the plane is flying through the air or whether it is sitting in a wind tunnel with the air being blown over it.

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  • $\begingroup$ "Our galaxy and everything in it is moving at a blistering speed of several hundred miles per second." moving relatively to what? The Universe expands, but there is no center of expansion. $\endgroup$
    – mins
    Commented May 17, 2023 at 16:21
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    $\begingroup$ Actually, if the observable universe is in consideration, I am the center of it. $\endgroup$
    – Jpe61
    Commented May 17, 2023 at 17:25
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    $\begingroup$ @mins It's the peculiar velocity of the Milky Way. Every galaxy would have a particular velocity if the expansion of the universe were perfectly isotropic, and the peculiar velocity is the difference between this velocity and the galaxy's actual velocity. You can also think of it as the velocity relative to the cosmic microwave background. This is the closest thing that the universe has to a rest frame and so the closest thing to an "absolute velocity" we can define. Of course, there is nothing actually privileged about this frame- the point of my answer is that only relative velocities matter $\endgroup$
    – Chris
    Commented May 17, 2023 at 17:25
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    $\begingroup$ I originally had some of these details in my draft answer, but I think they detract from the actual point. $\endgroup$
    – Chris
    Commented May 17, 2023 at 17:26
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    $\begingroup$ @Chris: In my hubble opinion, you're correct. $\endgroup$
    – mins
    Commented May 17, 2023 at 18:00
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Because it doesn't affect the math, and because it helps with keeping relative change in perspective if we use the engine as a frame of reference instead of the airmass.

Also because you are incorrect in assuming that air "stops stagnant in the atmosphere". The air is in constant motion and has velocity relative to the earth. However, if we used a fixed earth reference then we'd have to account for wind in our calculations and that would complicate things unnecessarily. (because where do you stop? i.e. what about planetary motion?!)

We all understand that outside of a wind tunnel it is actually the engine that is moving, but it just makes a lot more sense to keep things simple and consider the engine as a stationary reference for doing things like describing the function of an engine and calculating airflow.

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  • $\begingroup$ Whether the engine is moving is a matter of choice, both inside and outside the wind tunnel. Motion is relative and no reference frame is better than any other (well, reference frames in free fall are a bit better, but on the Earth those don't tend to be co-moving with anything useful). $\endgroup$
    – Jan Hudec
    Commented May 16, 2023 at 21:33
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    $\begingroup$ @JanHudec, I disagree completely that "no reference frame is better than any other" for the reasons given in my answer. Consider the verbal gymnastics needed if you used ground track with a stiff crosswind as your reference when explaining the path of airflow through a centrifugal flow compressor. Why not eliminate unneeded variables? $\endgroup$ Commented May 16, 2023 at 22:23
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    $\begingroup$ Well, we have to distinguish between better for describing particular situation and “always” better. The former definitely exists (and is the reference frame attached to the engine in this particular case, which is the point here). The later does not, that's the whole point of relativity. $\endgroup$
    – Jan Hudec
    Commented May 17, 2023 at 5:44
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    $\begingroup$ I quite like the answer as written (and upvoted it before I even wrote that comment). I only commented—and admit I was very unclear—the bit that the engine ‘is actually moving’. You can always choose a reference frame in which they are moving, but it isn't actual, it's still just a choice, and it would be a bad choice here. $\endgroup$
    – Jan Hudec
    Commented May 17, 2023 at 18:50
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    $\begingroup$ @MichaelHall: In this context, "better" is (probably) not being used to mean "more useful," but rather to mean "more correct." And it is absolutely true that no reference frame is more correct than any other reference frame. That probably seems like an obvious truism to you, but it can be a bit of a revelation for physics students who have not yet encountered it. $\endgroup$
    – Kevin
    Commented May 18, 2023 at 0:22
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The answer to the last question is "Galilean relativity", the assumption that the same laws of physics hold in any inertial frame of reference. In the example, it is presumably easier to do the analysis in the engine's frame of reference.

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    $\begingroup$ Welcome, you win for succinct! $\endgroup$ Commented May 17, 2023 at 16:46
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Our universe is so constructed that it doesn't matter if the air is moving and the model is standing still, or if the model is moving and the air is standing still. In either case, air is coming into the jet engine.

Note also that when a jet-powered plane is operating at its design point for cruise conditions, the air right inside the engine inlet is moving at the same speed as the air just outside the inlet. That is, the streamlines of air parcels entering the engine are not perturbed by the engine cowling. There is no spillage nor suction effects at design point cruise.

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