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reply posted on 21-6-2007 @ 09:57 AM by Figher Master FIN
Encountering Mach Number Difficulties

Extract from FLY

Until now, you've been flying straight-wing airplanes in a relatively low speed range, one in which airspeed was the primary factor. In the jet, however, you'll be flying at a significant fraction of the speed of sound, called Mach 1 (after Austrian physicist Ernst Mach, who dod much of early reasearch into high-speed fluid and gas flow). As you approach Mach 1, the behavior of the air changes: it becomes more like water, an incompressible fluid, than a gas. Since air can't readily move faster than the speed at which sound propagates through it, in a sense it "can't get out of its own way" fast enough. Instead of flowing smoothly over a wing, it "piles up" to form of shock waves.

The speed at which this occurs, for a given airfoil, is called its critical mach number, and it applies to the speed at which the air moves chordwise, straight from the leading ege to the trailing edge. If the wing is swept, so the air moves over it obliquely, the speed of the chordwise component is reduced, so the airplane can fly faster without encoutnering mach number difficulties.


-My first question is simple: Is there any truth to this extract or does it go in the same drawer with the longer path explanation?

Since air can't readily move faster than the speed at which sound propagates through it, in a sense it "can't get out of its own way" fast enough. Instead of flowing smoothly over a wing, it "piles up" to form of shock waves.


-I got a little confused reading this. How come air can't move faster than sound?

-You have swept wings to encounter Mach number difficulties, but what does the swept wing have to do with this and what are the difficulties caused by Mach 1 flight?


reply posted on 21-6-2007 @ 12:15 PM by The Winged Wombat
Figher Master FIN

Here is a link to a reasonable explanation of what Critical Mach Number means...
www.aerodyn.org...

As the air passes over the wing it is accelerated due to the airfoil shape, so on any wing shock waves begin to form at the point where the air is moving fastest when the air speed at that point reaches Mach 1 (slightly below Mach 1 actually as the air compresses), so that speed is less than the speed of the aircraft as a whole. The actual speed difference is determined by the airfoil shape - generally speaking the thickness of the wing - actually the ratio of thickness to chord. That's why different aircraft may have a different critical mach number.

Therefore, to delay the drag rise and the onset of shock waves, one must reduce the thickness to chord ratio - that means making the wing thinner. Obviously there are problems with that because it is structurally more difficult to make a thinner wing as strong as it need to be, and a thinner wing makes for less space to accommodate undercarriage, fuel tanks or weapons within it.

The swept wing concept is simply this. By turning a thicker wing to an angle to the airflow, the air has to travel further over the wing surface (in the direction of flight) presenting an apparent lower thickness to chord ratio to the airflow, even though structurally the t/c ratio hasn't changed. Of itself it doesn't solve the problem of transonic drag rise or loss of control effectiveness, but it does increase the speed at which these problems occur.

However, this also induces part of the airflow to travel spanwise to the tip (or to the fuselage with a forward swept wing) which creates handling problems, especially at low speed and high G (called tip stalling).

The early answers to the spanwise flow problem included wing fences, leading edge cuts, and sawtooth leading edges (the latter two creating vortices to help keep the airflow in line with the direction of flight).

The Delta wing solves the structural problem because of the very long root chord, allowing a wing with a low thickness to chord ratio to be quite thick at the root.

I hope that helps.

The Winged Wombat


[edit on 21/6/07 by The Winged Wombat]
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