I went looking for a Black Redstart. It turned out to be under this Vulcan bomber on display at Carlisle Airport! I'd always thought that Vulcans were supersonic but the data I've found suggests not. I've seen a top speed quoted as 645mph. This is 287 metres per second. I teach the speed of sound as being 330 metres per second. But then I remembered that the speed of sound varies with temperature. And surely it must also vary with pressure since it must be harder to transmit the vibration when the particles are more thinly spread. So I found this: http://en.wikipedia.org/wiki/File:Comparison_US_standard_atmosphere_1962.svg The Vulcan bomber got up to about 15km max so the speed of sound would be down to 295 metres per second at that altitude. Still sadly not low enough for supersonic flight.
