----- Original Message -----
From: "Thomas A Simmons" <tasimmons@xxxxxxxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
Sent: Monday, September 16, 2002 12:05 PM
Subject: Re: [OM] OT Low Flying, was an OM weekend
> Well, a F14 was approaching for a high speed pass. Got my trusty OM-1
> with the 400 MM ready to follow it as it crossed in front of me when I
> noticed
> a bit of condensation on it's nose. I have this picture of that bird
> popping through
> the sound barrier just as it crossed over RT15 at about 500 feet. I'll bet
> that the
Tom, we'd love to see the shot.
One note. Nearly all of the photos you see of supersonic airplanes with
various clouds of condensing vapor surrounding them have captions claiming
to record the instant of "breaking the sound barrier!" They aren't. They
record the condensation of atmospheric water vapor as the local pressure is
reduced by the accelerating flow around the airplane. This flow can become
locally supersonic, though the airplane isn't. As the pressure drops, the
air temperature drops below the dew point and vapor condenses. The cloud
often terminates at a disk-shaped boundary near the middle of the airplane.
This is the transonic configuration shock - the boundary where the locally
supersonic flow shocks back down to subsonic speed. The instantaneous
increase in pressure across that normal shock raises the temperature above
the dew point and the visible water once again becomes invisible vapor. All
of this happens at aircraft speeds typically 5 to 10 or even 150wer than
the speed of sound. In really humid conditions, you can often see that cloud
around airliners even at approach speeds. When the aircraft reaches Mach 1,
the configuration shock has moved far aft and a new shock forms at the nose
tip. I have yet to see one of these pictures with the distinctive signature
of fully developed supersonic flow. If that Tomcat had really been
supersonic, I assure you, at that proximity the sonic boom would have been
an event you would never forget. And, as Chris Barker can tell you,
exceeding Mach 1 low and with witnesses is a good way for that pilot to
newer have that opportunity again.
Gary Edwards
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