Maneuvering speed vs weight
FAA says maneuvering speed INCREASES with weight. Why?
This question has been bugging me ever since my PPL days (which is only a few months ago).
I gave the FAA answer on the written test, and passed, but since I'm an engineer, I'm going to figure this one out.
It doesn't make sense to me at all.
Maneuvering speed is the speed at which full and abrupt movement of any ONE control surface can be applied without overstressing the airframe.
Or in other words, the control surface will either stall before a force big enough to overstress the airframe can be generated, or end of travel will be reached.
Why does it INCREASE and not DECREASE with weight?
The stress on an airframe is the sum of 2 forces - gravity pulling the plane down, and lift pulling the plane up.
Lift generated by the wings is proportional to airspeed squared, and the angle of attack (roughly, before stall). That means, at the same speed, and same angle of attack (critical angle of attack), it doesn't actually matter how heavy the plane is. The wings will ALWAYS generate the same amount of lift (the plane could be climbing or descending depending on the weight, but we don't care about that).
Force of gravity, on the other hand, is proportional to mass.
What that means is, at the same airspeed and critical angle of attack, a heavier airplane will stress the airframe more.
So why is it that maneuvering speed increases with weight?