Nopoop! You're such a nice chap, and fun to argue with too.
You said it was not a "turn indicator". I'll give you the benefit of the doubt, and assume you thought I was still talking about the ball. I wasn't, and had moved on to the turn indicator instrument, as depicted in my picture - with comment.
It is clearly marked as a TURN indicator, but you said it wasn't a turn indicator.
Virages explanation is a good one.
Read the last two sentences of it 100 times.
Your "turn" gauge shows exactly what the stick and ball does. A bank to the left and a slip to right. Requiring left rudder to center the ball.
I'll have to give you the benefit of the doubt here also; that may be how it's set up in AH, but having done my own tests, I can tell you that it's not set up properly, and does
not provide valid rate of turn information in AH.
Indeed, as Virage's C&P reminds us, it is a useful instrument in the event that the vacuum system fails - the turn ind. will still work because it has a different source of power. Some aircraft don't have a vacuum system, and the instrumentation is "partial panel" - no artificial horizon or direction ind., just the turn and ball.
Virage's C&P said
The turn needle of the turn-and-bank indicator gives an indirect indication of the bank attitude of the aircraft.
That statement is clouded by the words "indirect indication". The fact is, the turn indicator will not provide an absolute indication of the angle of bank, because there's another factor that enters the equation. Speed. A plane like your C150 travelling at 100mph can make a Rate 1 turn of 3° per second with a modest angle of bank. Indeed, taxiing on the ground, it can make a Rate 1 turn without having to bank at all! But a faster plane, travelling at say 250mph, will need to bank more steeply to achieve that Rate 1 turn.
So yes, the turn indicator could be used as a loose indication of angle of bank - in coordinated flight and at a predetermined speed.
One could also argue that a clock that stopped at 8:44 is dead accurate twice in each 24 hour day. True, but not a lot of use.
