No, the elevator is rarely fully up in a steep turn, and it definitely doesn’t act as a rudder. It continues to do its normal job of controlling the wings’ angle of attack.
This is correct, but it’s true of all normal turns, steep or shallow.
What Kevbo is saying is that the total surface area and shape of a wing determines speed and climb and altitude and maneuverability for any given power.
It turns out that wood and fabric and wire is not as strong as titanium. Stress increases as the square of the wing span. Dividing the a wing in half decreases the stress fourfold and increases the roll rate similarly. Bracing the two wings together makes them stronger still. Adding a third increases climb and altitude at cost of speed.
Modern wings permit maximum tolerable roll rates without need of extra bracing.
r~
An airplane turns more like a bicycle than a car.
In an optimally designed airplane, maximum turn rate occurs with wings vertical, the nose slightly high, with full up elevator and neutral rudder. The rudder is used more to balance the forces in a turn.
r~
An aircraft in balanced flight won’t maintain altitude with the wings vertical. An aircraft turns by banking the wings which means part of the lift vector is pointing sideways. It is necessary to have enough of the lift vector acting vertically to counter the weight vector (otherwise the aircraft descends.) To increase the lift vector, you need to increase the angle of attack, i.e., apply up elevator. As the bank angle exceeds 60 degrees, large increases in angle of attack result in only small increases in the vertical component of lift. In most civil aircraft the angle of attack required to maintain altitude in a steep turn approaches the stalling angle at a little over 65 degrees.
The other determining factor in turning is having an adequate power to weight ratio. A more powerful aircraft will be able to fly faster which means it needs a lower angle of attack to maintain altitude in any given attitude.
It would be more accurate to say “otherwise the aircraft accelerates downward”. The point being that the vertical component of lift equals weight when the aircraft is ascending or descending at a constant vertical speed, just as it does in level flight.
It will if “the nose is slightly high”. The body and the thrust can provide lift whether or not the craft is turning.
r~
Then it’s not balanced flight anymore, also, it takes a lot of nose up to get all the required lift from fuselage and engine, a lot of “top” rudder, which means a lot of drag. None of this is an effective way to get a max rate turn. Not to mention that the wings will still stall when exceeding their critical angle of attack, except now you are going to quickly flick in to a spin thanks to being way out of balance.