Sonic Rainboom speed
I went back to think it through again, and realized that @Ace did solve the problem. I did not realize that RD was in freefall both times when she performed a sonic rainboom, but that explains things perfectly.
First, we do not know the air density around Cloudsville. That might affect the speed of sound in the area. Anyway let's not dwell on that 'cause I've got more important points to make.
Second, my estimate of terminal velocity was based on a human's, however, drag force changes if the subject is more aerodynamic. Apparently people -have- broken the sound barrier in freefall under special circumstances, and it is not unreasonable to assume that pegasi are -very- aerodynamic, so they can increase their velocity while falling a lot longer, reaching higher speeds in a shorter time.
But the most important thing I didn't realize that downward speed in freefall becomes cumulative. So if RD is falling AND speeding downwards, she can keep piling on the extra acceleration from her own speed. Let's say she has a natural flying speed max of 10 m/s. If she is speeding downwards for 20 seconds, she can reach an extra 200 m/s in addition to what she gains from gravity by her own power.
So I guess that makes sonic booms by pegasi possible. Actually, it turns out you don't even need to be Rainbow Dash to do it! What's more, an Earth pony with proper sky diving skill and guts might be able to achieve the same.
edited 14th Oct '11 12:57:16 PM by Vree