Quote:
Originally Posted by winkosmosis
I don't get the top diagram. Air hitting the top leading edge pulls the wing forward, but air hitting the bottom leading edge pushes it rearward? How does the air know the difference?
|
Those little lines are vectors and do not represent the total aerodynamic force. The vector diagram below the airfoil section represents a sum of the forces acting on the wing. It does not show the relative wind, which is the angle of the arrow at the leading edge of the wing.
Air is a fluid and does not compress below supersonic flow, so the effort required to pull that fluid along the upper surface of the wing becomes greater by the square of the velocity of the relative wind. That means it takes four times as much thrust to maintain level flight at twice the speed increase. If the air is forced to follow an airfoil at too high an angle of attack (the angle opposite the arrow), the flow will pull away from the wing and just tumble, that is called stalling. The airflow is no longer producing useful lift, but mostly drag. Because the drag also increases by the square of the velocity of the relative wind, there is not enough power to overcome the stall.
So the increasing angle of attack does not produce more lift because of Newton "pushing" the air downward. That would cause so much drag that in order to keep the wing from stalling, the aircraft would be forced to descend, regardless of power setting... That is how flaps and other drag-producing wing modifications help slow an airplane down for landing without stalling the wing.
A glider has no engine, but produces thrust by keeping the local flow of air over the wing as it descends. They stay aloft by carefully taking advantage of updrafts and rising warmer air. There is still Lift, weight, drag and thrust acting on the aircraft, thus the term "total aerodynamic force." Keep that force vector pointing forward and the airplane flies (regardless of whether it is climbing or descending).
Hope that didn't confuse things more...