Supersonic Lift?

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Supersonic Lift???

Lift appears to be from the angle of attack, due to the deflection of flow-stream in supersonic flight but then in subsonic flight the pressure difference causes lift. So what is going on here that i am missing? Why such a change? Its starting to confuse me.
 

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  • #2
Matterwave
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Lift is always dependent on the angle of attack, I don't think it has anything to do with supersonic vs subsonic. Newton's third law is what is essential for creating this lift. The buoyancy effect due to the pressure difference (Bernoulli's principle) only works with airfoils oriented the correct way (and would not work, for example, on airplanes flying upside down). I don't really know how much of the lift if due to which effect though, for that, ask an aerospace engineer.
 
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Okay this is way to long and involved to go into it requires a full degree :-)

Basically the lift process of a normal sub sonic wing will still work it just isn't the most important thing when you go supersonic.

The lift-to-drag ratio drops massively once you go beyond mach 1 (http://en.wikipedia.org/wiki/Lift-to-drag_ratio) so lift is rather trivial unlike sub sonic flight.

The biggest problem with supersonic cars is keeping them on the ground.

Controlling shock waves forming and heat at all the contact surfaces become much more important than the actual lift and thats what brings about the change of wing shape etc.
You can also use the shock waves to provide extra lift etc via airfoil design

http://en.wikipedia.org/wiki/Supersonic_airfoils should give you enough background.

Once you get up around mach 5 it gets even more fuzzy you enter the realm of hypersonic
http://en.wikipedia.org/wiki/Hypersonic

if you look at the table in that it will give you a basic view of the wing changes and why.
 
  • #4
cjl
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Lift is always dependent on the angle of attack, I don't think it has anything to do with supersonic vs subsonic. Newton's third law is what is essential for creating this lift. The buoyancy effect due to the pressure difference (Bernoulli's principle) only works with airfoils oriented the correct way (and would not work, for example, on airplanes flying upside down). I don't really know how much of the lift if due to which effect though, for that, ask an aerospace engineer.
Bernoulli has nothing to do with buoyancy. It simply describes the pressure along a streamline as a function of the local velocity, assuming an inviscid, incompressible flow. For subsonic airfoils, it isn't really a case of Bernoulli vs Newton - they're actually equivalent ways of looking at the problem, and either will give you the correct answer for low speed airfoils (Oh, and a momentum balance [Newton] always works, regardless of speed and geometry).

In supersonic flow, shocks exist, and that completely messes up the assumptions for Bernoulli. They feature high viscous dissipation, so the flow as a whole is no longer inviscid, and it certainly isn't incompressible. At supersonic speeds, lift is still generated by the pressure difference between the lower and upper surfaces (this is pretty much always true). However, that pressure difference is caused by the different shocks generated at the leading edge - the shock going down from the leading edge is stronger than the shock going upwards (which doesn't even always exist - sometimes you'll have an expansion fan going upwards, but I digress...). Because of this, the pressure jump across the shock going down from the leading edge is larger than the one across the shock going up. This causes a higher pressure beneath the wing, which causes lift.
 
  • #5
Matterwave
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I shouldn't have used the word "buoyancy", that's my bad.
 

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