Re: Swept wing for transonic flight explanation?

Date:         12 Dec 96 03:49:23 
From:         David Lednicer <dave@amiwest.com>
Organization: Analytical Methods, Inc.
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ifly wrote:
>
> I`ve never had a satisfactory (to me, anyways) explanation as to why
> sweep delays the onset of buffet. I`ve heard the explanations about the
> vector, but the "little golden book of knowledge" explanation was a bit
> too simple.
> My best understanding is: The sweep produces a more slender airfoil to
> the RW by virtue of the longer chord, thus producing less acceleration.
> Is this true? I can buy this, but it also sounds too simple.
> My understanding of low speed buffet is equally vague. Is the low speed
> buffet at altitude induced by the increased speed of the airflow over the
> wing as a result of increased AOA?

	When transonic freestream flow hits a swept wing, the surface
flow turns slightly and goes across the wing normal to the leading edge.
This is what the velocity vector diagram that you have seen is attempting
to show.  The magnitude of the velocity normal to the leading edge is the
freestream velocity, reduced by cosine of the sweep angle.  Hence, the
wing "thinks" that the freestream flow is going slower than it really is,
and Mach number effects (shocks) are delayed.

	Transonic buffet effects are caused by shock induced separation.
When the Mach number of the flow in front of the shock is greater than or
equal to approximately 1.3, the surface boundary layer cannot make it
through the rapid increase in pressure at the shock and separation will
result.  It is this separation that causes transonic buffet.  Low speed
(stall) buffet is caused by separation also, in this case, the rapid
increase in pressure, behind the suction peak, in the airfoil pressure
distribution at high angles of attack.

	The best argument I have ever seen for swept wing theory comes
from R.T. Jones, the American discoverer of this (as opposed to Busseman,
who discovered it in Germany 10 years earlier).  It goes like this:

	Suppose a wing is placed in an airstream at an angle of yaw -
	ie., it is swept back.  Now, even if the local speed of the
	air on the upper surface of the wing becomes supersonic, a
	shock wave cannot form there because it would have to be a
	sweptback shock - swept at the same angle of the wing - ie.,
	it would be an oblique shock.  Such an oblique shock cannot
	form until the the velocity component normal to it becomes
	supersonic.


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David Lednicer             | "Applied Computational Fluid Dynamics"
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