Geometry.
Circumference (c) = 2 x pi x radius (r)
or r = c / 2 pi
Rate 1 turn = 2 minutes
@ 100Kt, 2 minutes = (100 x 2 / 60) nm = c (approximately)
so r = (100 x 2 / 60) / 2 x 3.1428
on your calculator = 0.5303 or approximately 0.5
So at 100kts r = 0.5nm or 0.5% IAS
For mental arithmetic purposes assume TAS = IAS and use the formulae radius of turn at rate one = 1/2 % IAS
