Sighted a shock wave above airliner wing

Date:         21 Sep 98 00:31:57 
From:         pchapman@ionsys.com (Peter Chapman)
Organization: Psychic Friends Network, Inc.
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A couple days ago I saw a fascinating sight I had never yet come
across -- seeing the shockwave of local transonic flow above the wing
of an airliner in flight.

This isn't the visualization of low pressure because of condensing
water vapour, a more commonly observed phenomenon.

I'd like to hear comments on this. While my undergrad was in aero
engineering (making this all the more fascinating), I'm not in the
business now.

During a flight in a B-757, what  I first saw  were a number of
tangled lines of shadows that basically formed one line extending
outboard for about 10 feet from the fuselage, on the upper wing
surface, at roughly the 1/2 chord point.

These shadows danced fore and aft by about 30 cm total with any little
bit of turbulence the aircraft reacted to.

The reason I could see the shadows this time, but not on any other
flight I've been on, was because the sun was on the other side of the
aircraft, direcly in line with my view from a window seat outboard
onto the center of the wing root area. The light would be shining
right down along a plane perpendicular to the airflow direction, if
the shock was barely at Mach 1.

Then when I moved my head so that I was right inline with the shadows,
the view out to the wingtips was distorted along a thin vertical line,
that also moved in unison with the shadows.

This was now a more direct viewing of the shockwave itself, or at
least its effect on light passing through it. The effect was only seen
when the light refracted at a very shallow angle almost directly
inline with the shock wave with its abrupt change in air density.

Schlieren photos work on the basis of density changes too, or am I
mistaken?  The airliner might be travelling at  0.8 Mach. Generally
one doesn't want any shock waves -- with large drag increases around
the speed of sound.  Is it reasonable that locally, at least in the
inboard area of the wing a slight shock wave could form during normal
cruise conditions? I haven't tried working out what pressure
coefficients would be appropriate, or learned whether there's an
increased thickness to chord ratio at the root that would make a
undesirable shock wave more likely.

It was fascinating to be there in flight, seeing aerodynamic effects
that are normally invisible. Took a couple photos too.

Peter Chapman
Toronto, Canada