Knight Paladin said:
Milt - Sorry chum, A-level physics wins. Yes, the force produced by the air that acts on the aeroplane comes from the difference in momentum between the aeroplane and its surroundings, and yes, the only direct link between the Earth and the aeroplane is gravity, BUT, in just the same way that the Earth has it's own momentum travelling through space, so does the aeroplane. This momentum can most easily be referenced to that of the Earth itself, hence groundspeed is relevant. Of course, strictly speaking you need to consider the momentum of the aeroplane through 3-D space, probably referenced to orthoganal axes, but I fear that would be getting far too complicated.
Oh well, there goes the last vestige of any notion that A-level is the gold standard!
And also:
As I said, the actual change is very progressive throughout the turn.
That's as may be, but how do you account for the IAS you quoted for an instantaneous turn being 240kts in error (I assume by 'airspeed' you meant forward airspeed, i.e. IAS)?