Re: "Visible" Sonic shock wave

[Scott Rippetoe <rippetoe@TENET.EDU>]

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> From: David Simmons <dsimmon@UOFT02.UTOLEDO.EDU>
> Date: Tuesday, January 12, 1999 6:47 PM
>
> A colleague showed me a photo on the web today. It purports to show an
> F-4 Phantom at an air show "in the act" of breaking the sound barrier.
> It shows one large and a smaller "shock cone"? of perhaps condensed
> moisture radiating out from the aircraft.
>
> If this is really what is happening...

As the Phantom approaches the speed of sound, shock waves will begin to
form on the surfaces. It looks like this is happening near the engine
intakes and along the wing surfaces. They are more like shock planes than
cones, judging from the picture. The resulting pressure drop across the the
shock wave causes the condensation.

> I  Can anyone shed light on the cause? My guess is that the shock wave
> caused condensation in very humid air?
>
> II Would this really be a transient event? Since the shock wave is
> formed continuously as long the aircraft is exceeding the speed of
> sound, wouldn't this effect be continuous?

The aircraft shown is just below the speed of sound. The closer it gets to
the speed of sound, the farther forward the shockwaves will move on the
wing surfaces. When it exceeds the speed of sound, the shock wave will form
at the tip and form a cone. The angle of this Mach cone is dependent on the
speed of the jet. One way of estimating the maximum speed of a supersonic
aircraft is to determine the smallest angle of cone within which the
aircraft could fit. The tighter the cone, the faster the aircraft. The cone
moves along with the aircraft, like the wake of a high speed boat.
> I'll admit to being pretty ignorant of this area. My sound unit begins
> in a few weeks. Certainly could be an interesting introductory photo.

I used to have an old videodisc that showed an F-111 flying through a wide
desert canyon. You could see the trace of the Mach cone on the ground as it
stirred up dust following behind the jet. Since I knew it was caused by the
Mach cone, it was easy to imagine that I saw a hyperbolic section following
behind the jet.

Another place you commonly see shock waves is in the exhaust from liquid
fueled rocket engines. You can often see three or more following behind the
nozzle.

Scott Rippetoe
Academy of Science and Technology
27330 Oak Ridge School Road
Conroe, TX 77385
rippetoe@tenet.edu