Shocks on transport wings (was stalls)

From: (leishman)
Organization: University of Maryland, College Park
Date:         06 May 94 18:02:15 
References:   1 2 3
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>>Michael Drews writes:

>>My guess is that this was a shockwave associated with flow over 
>>the wing was at or near Mach speed.  I had never heard of shockwaves 
>>with commercial transports until this stall thread came up.  Opinions?

Yes, indeed! What you were seeing was, in fact, a shadowgram (or
shadowgraph) of the shockwave on the wing. I have seen this many many
many times from the windows of various aircraft including B-757, DC-10,
B-747 etc. 

You need to know what to look for, but if the sun is in the right
location (preferably above, so about mid-day is a good time), the
sunlight is refracted (bent) out of its original path as it passes
through the high density gradients at the the shockwave, and a shadow
(usually a fairly fine dark line) is cast onto the wing surface. There
may be several finer dark/bright lines since the shock is
three-dimensional, and there will be multiple light paths that undergo
refraction. You may also see a region of "distortion" off the wing
surface which again indicates refraction through the density gradients
in the flow near the shock, although this latter phenomenon is harder
to see under most lighting conditions.

If you are really lucky, you will see the shockwave shadow all the way
along the wing to near the tip and you will see how much more
three-dimensional the flow becomes in the vicinity of the engines, such
as on a DC-10. As the aircraft flies though turbulence you will also
see the shadow move as the shock wave repositions itself under the
(very mild!) unsteady flow conditions.

In my experience, the B-757 seems to show a fairly pronounced shadow
compared with say a 767 or a DC-10 but this may also be related to the
lighting conditions, viewing angle etc. For most modern transport
aircraft with supercritical airfoils the shock is quite far back from
the leading edge and you can certainly see the shockwave shadow under a
wide variety of conditions if your seat is over the wing and if you
look very carefully. I also have a couple of textbooks and reports that
document this observation. One even has a photo of a shadowgram of the
shockwave on a B-707 wing. 

I will admit however, that although I have seen the phenomena many
times, making a decent photograph has been difficult. Now, only if we
could convince Boeing to cover one wing of a 757 with 3M Scotchlite
retroreflective film, then we could have some real interesting stuff to
talk about!

Real science from your airplane window! Have fun!

J. Gordon Leishman
Associate Professor of Aerospace Engineering,
University of Maryland at College Park.