Only produces drag??? Whoever came up with that wasn't thinking at all. What do flaps do? LIFT!

It is however true that down aileron produces more drag than up aileron. This is because the down aileron is going into the high pressure, while up aileron into low.

So, both of the theories are both semi accurate, however the one where down aileron does nothing was definetly not thought through.

This is where differential comes in. In computer radios, differential is a mix in which the down aileron travels less than the up aileron. This helps stop adverse yaw, which is when the plane yaws towards the down aileron due to its higher drag.

Hope that helps

Alex