R = TAS^^2 / (g x tan AoB)
And that's it.
The 'g' loading is equal to L/W and thus = sec AoB.
The question was asked when I was going through the CFS course. They used the example of the BBMF Lancaster, with the Spitfire (on the outside) and Hurricane (on the inside) in formation with it and asked which a/c had the greatest AoB.
The answer they wanted was that they would all have the same AoB. But that is incorrect as the radii of turns differ, even though the turn rate must be constant to stay in formation. Quite complicated to work it out - first assume a bank angle and TAS for the Lancaster to obtain the radius of turn and turn rate. Then assume the formation separation distance for the 2 fighters. Work out their radii of turn, substitute into the turn rate and obtain their TAS. Then from TAS and turn radius for each, recalculate the AoB for each......
Woody (RIP) changed the question after that as he didn't really understand algebra - much less trigonometry!