OK, so now take a fully flat, symetric wing, with ailerons, apply ailerons, release. What happens ? You have a bank, no turn. what the plane do ? Following the degres of the bank, the plane loose altitude, more or less quickly. Why ?

I think you will agree with me that it is because the projected surface of the whole wing is smaller than normally, hence you need to apply elevator, or ailerons in reverse to fly straight and level ( lower) again.

Now, why is that, that something true for the whole wing would not be equally true for half of it ?

Or is there also another explanation for the need to apply elevator when you bank ?

Bernard