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Newsgroups: sci.aeronautics.airliners
From: (Robert Dorsett)
Subject: Re: Stalls
Date: 24 Apr 94 22:23:43

In article <airliners.1994.1164@ohare.Chicago.COM> (Kyle D. Jackson) writes:

>I had the understanding that the commercial transport aircraft were designed
>with supercritical airfoils to completely avoid the chance of a sonic shock
>forming on the upper wing surface...

As I understand it, one purpose of supercritical wings is to delay shock-
wave formation, pushing it further back along the wing, and to make sure it
appears only at higher speeds.  The advantage being primarily drag reduction.
Supercritical wings (memory tells me) delay the onset to about M 1.0.

But the shockwaves still exist.  You can see them when the airplane is
flying 90 degrees to the sun; the shockwaves change the refractive property
of air in the vicinity, and can be seen as little span-wise
reflections dancing along the middle to rear of the wing.

Robert Dorsett

Newsgroups: sci.aeronautics.airliners
From: (leishman)
Subject: Shocks on transport wings (was stalls)
Date: 06 May 94 18:02:15

>>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.

Newsgroups: sci.aeronautics.airliners
From: (leishman)
Subject: Re: Stalls
Date: 12 May 94 13:19:37

In article <airliners.1994.1180@ohare.Chicago.COM> (Nitin Gupta) writes:

> I looked into this, and i'm not so sure that the faint "reflections" are
> due to changes in refractive index...
> ...I do not see n changing enough to manifest
> enough contrast to actually be visible on a sunny day. I'm not into
> airfoil dynamics, so I have no idea what the nature of schockwaves are in
> terms of their temporal pressure.

Careful here!! The observation of flowfields containing shocks and
other density variations are routinely examined by means of a class of
density gradient flow visualization methods known, in general, as
schlieren methods. A simple schlieren system is direct shadowgraphy -
which is essentially what is being described by the various observers
of shockwave images on transport aircraft wings.

Note that the refractive index varies if the density in the flow
changes. For practical purposes, the refractive index, n, is related to
the density, rho, by the equation

n-1 = k * rho

where k is a constant for a particular gas and wavelength of light.
This equation can be written as

n-1 =(n_0-1)(rho/rho_0)

where _0 indicates the quantities at a reference temperature and
pressure. For air, n_0=1.000292 at 0 deg C and 760 mm Hg and for 5893A.

Consider a beam of light (could be from the sun) passing through a flow
with a density variation (a shockwave being a good example), and this
beam of light eventually falls on a viewing screen (the wing of an
airplane, say). If the density changes (at the shock, for example) then
the time of arrival of a particular point on the screen on a light wave
will change because the velocity of light, c, is related to the
refractive index, n, by the equation

c=(1/n) c*

where c* is the velocity of light in a vacuum.

If there is a gradient in refractive index normal to the light rays,
then the rays will be deflected because the light travels more slowly
where the refractive index is larger according to the above equation.
The deflection of these light rays is a measure of the first derivative
of the density with respect to distance, that is the density gradient,
and can be observed using various schlieren techniques (which require
lenses or mirrors and a knife edge or graduated filter for a cut-off).
If the refractive index gradient normal to the light rays varies, then
deflection of adjacent rays will differ, so they will converge or
diverge giving regions of increased or decreased illumination on a
viewing screen (dark or bright bands). This is the basis of the direct
shadowgraph method. It requires no lenses or mirrors and is essentially
a measure of the second derivative of the density field.

These schlieren methods are routinely used in the laboratory when
examining high speed flows containing shockwaves. Turbulence and
vortices can also be observed, such as those behind propellers and
helicopter rotors. In the field, obviously it is much more difficult to
visualize such flows, but the example of the "natural" shadowgraph of
the shockwave on a transport wing has been cited in the literature for
many years. It is indeed interesting to me that so many of our friends
on the internet have also observed such phenomena.

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

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