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Critical Mach Question

nimtz

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Okay I understand that as you pass Critical Mach the center of lift shifts aft, because the first appearence of a shockwave occurs. What I'm not understanding is exactly why? Every book I've read basically says 'the shock wave on the upper surface upsets the lift distribution chordwise.' This statement doesn't answer the why part of the equation to me.

Now I understand the compressibility affects associated with shockwaves in terms of the drag coefficent. But it seems to me that where the air is supersonic, it has the greatest speed thus lowest pressure. Therefore why wouldn't the center of lift be located where the air is supersonic? In other words if the shockwave is located just aft of the leading edge, shouldn't there still be lift produced at the leading edge. Why can you not have a center of lift ahead of a shockwave?

Sorry if its confusing question. I have no formal training in this stuff, so I'm largely reteaching myself what I taught myself a few years ago.
 

SkyWestCRJPilot

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My understanding is that the center of lift is at where the shock wave is because that is where the air is moving the fastest (not just aft of the leading edge). As the aircraft continues to accelerate beyond critical mach the shock wave moves farther aft, and thus so does the center of pressure following the shock wave. I too am not an expert on this so if I'm wrong I'll let others correct me.
 

Monkeyfist

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I don't have my books or notes with me right now, but I seem to remember two contributing factors to center of pressure moving aft during transonic flight

1. Behind the developing shock wave is an area of seperation sometimes called "mach stall". The bottom of the wing does not form shock waves a early as the top, and when the bottom does produce a shock wave it is less intense and further aft of the upper wave. The lower surface's center of pressure is not changed during the early shock wave formation. the upper surface airflow begins to separate causing less lift. the result is the total center of pressure moving aft.

2. Swept wing aircraft will have a total center of pressure near the wing root when viewed from the top, (due the most wing area being closest to the wing root, and less 3 dimentional airflow near the wing root).
As the wave begins to form, the area behind the wave is considered a stalled region. More total lift is lost near the wing root than wing tip, moving the the center of pressure further from the aircrafts center line when viewed from the top. As center of lift is moving further from the aircrafts centerline on a swept wing, you will also move further aft.


This is the best I can do at the moment, I will dig up my notes and try to give a better answer
 

Dangerkitty

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nimtz said:
Okay I understand that as you pass Critical Mach the center of lift shifts aft, because the first appearence of a shockwave occurs. What I'm not understanding is exactly why? Every book I've read basically says 'the shock wave on the upper surface upsets the lift distribution chordwise.' This statement doesn't answer the why part of the equation to me.

Now I understand the compressibility affects associated with shockwaves in terms of the drag coefficent. But it seems to me that where the air is supersonic, it has the greatest speed thus lowest pressure. Therefore why wouldn't the center of lift be located where the air is supersonic? In other words if the shockwave is located just aft of the leading edge, shouldn't there still be lift produced at the leading edge. Why can you not have a center of lift ahead of a shockwave?

Sorry if its confusing question. I have no formal training in this stuff, so I'm largely reteaching myself what I taught myself a few years ago.

Do you happen to have the book "Aerodynamics for Naval Aviators"? I used that book for my interviews at AA and DAL. I have never seen a book go into so much detail as that one when it comes to high speed jet aerodynamics. If you can get your hands on a copy it might help you out.
 

Cows Go Moo

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Lots of good info in here already, but I thought I'd add what I think I know.

Your question was, "What makes the center of pressure move rearward?"

A wing experiences lift (pressure, really) in different amounts at different locations along itself. The forward part may lift a little more, the rearward part may lift a little less. The wing root may lift a little more, the wingtip a little less.

The center of pressure could be thought of as your average location of lift. When airflow over the wing becomes supersonic, a shockwave is formed at the location where subsonic air becomes sonic. Airflow separation occurs and hurts the wing's lifting ability (and control surface effectiveness). So where on an airplane will airflow be the fastest and become supersonic first? On most airplanes, it will be nearer the wing root where, for lack of a better description, the wing is big and fat. Now if you consider a bird's eye view of a sweptwing airplane, the wing root is more forward and the tip is more aft. If airflow reaches supersonic speeds at the wing root first, the lift nearest the wing root will be the first to be effected (in other words, go away). If a sweptwing airplane's wing progressively loses lift from the wing root to the tip, the average lift (or center of pressure) will move rearward.
 

Cubgrad

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Nailed it

Cows go moo hit it right in the head.

It is usually a lot simpler than we think.

Cheers.
 
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