Originally Posted by Rivet gun
Well, OK maybe not really.
Perhaps theoretically would be a better adjective.. I must be a bit of a sad case to be into this kind of thing.
The numbers come from a textbook on light aircraft performance by a chap called John Lowry. For a better explanation you might ask Genghis on the Flight Testing forum who I am sure understands this stuff much better than I do.
The key word here is sustainable. (Almost) any aeroplane, and certainly the aeroplanes that most of us discuss on here have two different maximum level turn rates.
The first is that dictated by the amount of lift that your wing can generate. This creates an instantaneous lift when you bank then pull. For a few seconds, you can pull quite a lot of g - which might just get you out of the mire.
However, for sustained level turning performance you need to keep putting back the energy otherwise either you lose height, or you lose speed (or often both). This is why you increase power in a moderate to steep turn. The steeper the turn, the more power you need; but eventually you just don't have enough.
(Partly) because of this, I've seen the advice in several places that if a light (e.g. Cessna), comes head to head with a fast-heavy (e.g. a Tornado) whilst both should continue to turn right as usual, for increased safety the light should also (if possible) descend (using potential energy to sustain the turn safely) whilst the fast-heavy should climb since they have the thrust to perform such a sustained manoeuvre.
A bit off-topic but this is a huge issue in fighter design. The aeroplane with the greatest sustained turning performance, at the engagement altitude, will almost certainly win any prolonged dogfight since it can outturn, and thus win a turning fight with an opponent.
G