12 Generation of LiftFor a lifting airfoil, the surface static pressure varies from top to bottom and from front to back.For a lifting airfoil, the average pressure on the upper surface is lower than the average pressure on lower surface.The difference in pressure produces the lift.
13 Generation of LiftWhy does the surface static pressure vary top to bottom and front to back?The ideal surface velocity varies from top to bottom and front to back. The surface of the foil is a streamline, so Bernoulli’s equation relates the surface velocity to surface pressure.When we include viscosity, the surface velocity is zero, but the local velocity varies at the boundary layer edge.
14 Generation of LiftWhy does the surface velocity vary top to bottom and front to back?The flow must follow the surface contour (or the edge of the boundary layer). Flow can not pass through the airfoil.What determines the values for the local surface velocity, and therefore the local surface pressure, and ultimately the amount of the lift?
15 Generation of LiftWhat determines the values for the local surface velocity, and therefore the local surface pressure, and ultimately the amount of the lift?The flow around the airfoil must satisfy the conservation laws:Conservation of mass(continuity)Conservation of momentum(2 or 3 components)Conservation of energy
21 Prandtl’s Experimental Photo of Starting Vortex
22 Prandtl Lifting Line Theory Bound VortexTip VortexThree dimensional version of Bound Vortex TheoryA continuous line of bound vortices terminating at the wing tips with “tip vortices” that continue downstream to the “starting vortex”.