Some Thing on the Wing

Quite a shock out my airplane window

Ever since my first trip with my Dad, who worked for National Airlines and Pan American, I’ve always loved when the engines throttle up and the runway lights start to track by. I don’t understand dropping the shade for a lousy movie or snoozing in the aisle seat. A window, always.

See that faint, upside-down "Y" running vertically down the middle of the photo?

A few times I’ve seen what I believe to be a shock wave forming on the wing of the jet I was traveling on. Ernst Mach told us that at sea level, the speed of sound — Mach one, or the ratio of the speed of sound at a given altitude and the speed of the vehicle — is around 750 miles per hour. The outside air temperature, or OAT, is important here (which I also remember being a factor during test engine runs as a crew chief) because the density of air at a given altitude affects the speed of sound.

Recently, during an American flight from Atlanta, I saw what I thought was a shock wave dancing along the upper surface of the wing; it rode back and forth, divided, re-combined, and stayed visible until we began to descend. I even got a picture.

Based on a quick, and quite possibly faulty, Internet search, my best guess at the speed of sound at our announced altitude — 39,000 feet — is about 660 mph, or 574 knots. The cruising speed of the latest 737s (Next Generation) is 514 mph, with a max speed of 544 mph. Theoretically, the difference between the speed of sound at that altitude and the speed of our jet was as low as 116 mph — a few feet outside the window of a near-sighted airplane geek with his tray table down in seat 15A.

I checked with Gordon Leishman, a professor of aerospace engineering at the University of Maryland in College Park, who emailed back:

Yes, what you have is a photo of a shock wave (well, it is the shadow of the shockwave cast onto the wing). So, we call such images shadowgraphs or shadowgrams. Seen it many times, have many photos, generations of my students have taken photos, etc. Science from your airplane window! Of course, we also use the technique in the laboratory.

Commercial jet aircraft cruise at transonic speeds (Mach 0.8 to 0.86), so there is generally always a shock or a series of bifurcated (i.e., the upside down “y”) and interconnected shocks) over the upper surface of the wing. On early jets, the shocks were nearer to the leading edge (727 is a good example), but on newer generations of aircraft with their better transonic airfoils, you will see the shocks further aft on the wing chord (777). You can also sometimes see shocks on the engine nacelle or between the nacelle and the fuselage. If the lighting is right, you may even see the optical distortion of the shock extending well off the wing surface.

Getting a good photo of the shocks is all about lighting (in this case where the sun is relative to the aircraft) so that the light rays refract and cast a shadow of the shocks on the wing. Often it is luck. In this case you can clearly see the bifurcated shock pattern as the flow over the wing interferes with the fuselage flow.

Whew. And, I thought what I saw was just due to the free drink coupons!