The image at the right is a screen cap of the widely distributed mpeg fs14ss.mpg. This mpeg shows an F-14 generating condensation clouds during a transonic flyby over water. The full explanation of the physics behind the cloud formation can be found at my main Prandtl-Glauert condensation site.
There are several features of interest in this short mpeg. The most obvious is that the cloud forms, vanishes, and then reappears. A simple explanation could be that the pilot varied the throttle, perhaps unintentionally, resulting in a short slowdown of the aircraft.
An explanation which involves the physics is based on the idea that the cloud formation can cause changes in the lift and drag on the aircraft. In fact, Professor Schnerr at the Universität Karlsruhe has shown that the lift and drag coefficients can undergo large variations due to the condensation. These scientific results are consistent with the anecdotal evidence provided by pilots who report considerable banging and bumping when large clouds are formed. This is over and above the buffeting familiar to fighter pilots as they enter the transonic regime without condensation. The disappearance of the cloud may just be due to a decrease in speed due to the added drag at constant throttle.
A second feature is the fact that the condensation first appears above the aircraft when the cloud reforms (and possibly when it forms). Of course, it is expected that the cloud should first form in the low pressure region which is necessarily above any lifting aircraft. However, the largest pressure perturbations are expected to be very near the aircraft. Hence the lowest pressures and therefore the cloud formation ought to be at the aircraft surface. We can give the usual atmospheric turbulence arguments, but without much conviction. Bob Harrington did point out that there may be flow disturbances generated by the ship which could cause an unanticipated Mach number or pressure distribution. I like this better than the generic turbulence argument.
Another feature is the formation of the condensation cloud over the cockpit. You can see this just after the cloud reforms and just as the aircraft passes the photographer.
For more information on this video, including some comments on it's origin, some further observations of the behavior, and other neat images/videos, head to Jeff Wilkinson's page.
To download the mpeg, just click on the highlighted text. It is a 1.35 Meg file, so it should not be a major bandwidth sink.
If you have any problems with the download, try a right click on the link. If worse comes to worse, then please drop me an e-mail.
Enjoy it. I would also be interested in any similar videos. Just drop me a note if you run across anything.
The photo at the above right is a screen cap of the mpeg fs14ss.mpeg. The mpeg itself has been bouncing around the web and newsgroups since sometime in 1999. The mpeg also appears on the Sonic Boom Forum site. In spite of persistent questioning, I have not been able to locate the source and therefore the copyright status. Perhaps someone will come up with a lead now that it is posted here.
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