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ChrisHarvey
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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 alot,
Chris
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 alot,
Chris
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