Jet Engines: Momentum, Pressure and Thrust

[ChrisHarvey]

I am just about to start my Aero. Eng. degree in a few days time and am currently ploughing my way through all the reccommended reading. A lot of it either gives textual explanations (rather than mathematical proofs or both) or gives a brief description, glossing over anything that may be considered more complicated - as a result I keep finding apparent contradictions in the text, no doubt because I am not familiar with all the processes at work in a system. This question has been bugging me for a while, and I can't find a simple explanation anywhere...

A jet engine works on the principle of momentum change, and pressure difference. Now... in order to produce forward thrust by momentum change, the air is heated in the combustion chamber, expands, and hence 'squeezes' it's way out of the engine through the turbines at a much higher speed. Surely the fact that you now have high velocity air behind the jet engine gives you a very low pressure there relative to the front, where the air is moving comparatively slowly? Shouldn't there therefore be a lot of drag, rather than thrust, as dynamic pressure increases with velocity squared, whereas momentum only increases linearly with velocity? Hence, doubling speeds, doubles force due to momentum, but quadruples force due to pressure, does it not?

I'd be very grateful for any explanations,
I hope I am not being stupid,
Thanks a lot,
Chris

 

[ChrisHarvey]

Oh yea, and I forgot to mention that I realize the design of the rim of the compressor is very important as you can make it act like an aerofoil, and give an area of low pressure around the front (because the stagnation point is underneath rather than in front - low pressure is therefore needed on the leading edge to provide the centripetal force to get the air to flow around).

Surely though the effect of this is negligilble in comparison with the faster air moving behind, the low pressure of which acts over a greater area?

 

[Owen]

Hello chris, not seen you here before.

My understanding is that the relationship between pressure and velocity assumes that the total energy in the flow is constant. in the case of a jet engine somewhere between the front and the back there is a lot of energy injected into the flow. i think I'm right in saying that the static pressure behind a jet engine is approximately the same as it is at the front, but it just happens to be moving a lot faster.

hope that helps (i may be completely wrong btw)

Owen

 

[ChrisHarvey]

Hi Owen!

I've known about this site for a long time, but I usually come up with a satisfactory answer myself before making the tremendous effort of switching on the computer, logging in, typing a well-worded question, and waiting for replies! This one had me though!

Thanks, your answer satisfies me! It makes a lot of sense and I'm cross with myself know for not working out - I presume you are reading the same book as you will be doing the same course - one of the previous chapters on drag says something similar in an unrelated topic on boundary layer normal drag... even if flow does not separate around a shape (2D flow), energy is lost in the boundary layer due to friction at the surface, lowering both static and dynamic pressure... meaning that there is in fact low pressure behind the shape. I initially thought (without reading ahead) that a slow boundary layer against an adverse pressure gradient (if it did not lead to separation) would slow the flow (by viscous drag) and lead to high pressure... I always forget ENERGY!

This was just the same thing in reverse.

 

[Owen]

glad i could be of some assistance

I have read the book and I'm slightly more convinced about pressure causing lift now (althought i still think this pressure gradient will deflect the air well above the wing). There are numerous references in the chapters on control and stability to D.P Davies book "Handling The Big Jets" which is a very worth while read as it deals with a lot of the complex systems and structures on a large aircraft from the point of view of how they are used (as well as how they actually work)

anyways i have work i should be doing, see you next week

Owen

 

[ChrisHarvey]

Have you read the entire book already? You can't have had it more than a week! I'm on chapter 6 (propulsion) and struggling to read a whole chapter everyday. Having said all that I am cross-referencing all the time to several other books and am doing a lot of maths along the way.

 

[Owen]

I read it in about 3 days, then went off and looked at things in more detail. just started re-reading "Handling the big jets"

 

[ChrisHarvey]

Finished! All chapters read and understood and proved to myself using my limited maths . It was a great book, although slightly spoilt I think by the relatively short amount of time I had (especially with work too!).

 

[ChrisHarvey]

Although... one other thing I didn't quite get my head around...

Why is it that on forward swept wings, they tend to bend upwards towards to tips, leading to more lift, etc. leading to structural divergence and tip-stall? I keep thinking that on a forward swept wing, due to the geometry, the tips will experience more downwash, because they don't have the 'cancelling-out effect' of upwash from the inboard stretch of wing. If the tips have a relatively greater downwash, surely lift will be decreased... so why the problem up structural divergence and up-bending?

Thanks,
Chris

N.B. On a back-swept wing, the opposite is true. Due to the geomtry, the tips experience upwash, which leads to tips stalling because inboard wing sections.