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TB500
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Joined: Sat Oct 05, 2019 11:48 am

Why does the wings center of pressure moves aft with increasing mach number?

Sun Jun 21, 2020 12:12 am

Hello,
I've been watching a video on mach tuck recently and wondered why does the cop (center of pressure) moves aft. I saw this video of 'ATPL theory' about high speed (and altitude) flight (link: https://youtu.be/_LYU03RErTU) and at 8:55 it says that 'as the mach number increases the effective airflow has to upwash to a greater angle and the effective aoa will increase.' (I'll attach to pictures of the leading pressure wave and resulting air flow from the video)

So I was wondering why the cop moves aft as I have learned that for increasing aoa the cop moves forward.

Greetings, TB500ImageImage

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Starlionblue
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Joined: Fri Feb 27, 2004 9:54 pm

Re: Why does the wings center of pressure moves aft with increasing mach number?

Sun Jun 21, 2020 1:10 am

At transonic speeds, shockwaves form above and below the wing. These shockwaves increase pressure gradients and "concentrate" the lift towards them. With increasing Mach number, the shockwaves move aft as the aircraft "outruns" them. This means that the CoL also moves aft.

This CoL movement causes Mach tuck, a downward pitch moment with increasing Mach number.

Image

Aerodynamics start becoming a bit counterintuitive in the transonic regime.

This old video has good wind tunnel footage. https://youtu.be/bELu-if5ckU
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