LIFTGLYPH<.notdef> GLYPH<.notdef> STALL 8 12-16 B A Increased Local VelocityGLYPH<.notdef> (Decreased pressure) Decreased Local VelocityGLYPH<.notdef> GLYPH<.notdef> Downwash Upwash Axis of Rotation Reference Plane PitchGLYPH<.notdef> Angle C h o r d L i n e Angle of Attack R E L A T I V E W I N D Leading EdgeGLYPH<.notdef> Stagnation Point Trailing EdgeGLYPH<.notdef> Stagnation Point B A Figure 2-5. As the angle of attack is increased, the separation point starts near the trailing edge of the airfoil and pro- gresses forward. Finally, the airfoil loses its lift and a stall condition occurs. Figure 2-7. Magnus Effect is a lifting force produced when a rotating cylinder produces a pressure differential. This is the same effect that makes a baseball curve or a golf ball slice. Figure 2-8. Air circulation around an airfoil occurs when the front stagnation point is below the leading edge and the aft stagnation point is beyond the trailing edge. Figure 2-6. Angle of attack may be greater than, less than, or the same as the pitch angle. now have upwash ahead of the rotating cylinder and downwash at the rear. The difference in surface velocity accounts for a differ- ence in pressure, with the pressure being lower on the top than the bottom. This low pressure area produces an upward force known as the “Magnus Effect.”  This mechanically induced circulation illustrates the rela- tionship between circulation and lift. An airfoil with a positive angle of attack develops air circulation as its sharp trailing edge forces the rear stagnation point to be aft of the trailing edge, while the front stagnation point is below the leading edge. [Figure 2-8] The explanation of lift can best be explained by looking at a cylinder rotating in an airstream. The local velocity near the cylinder is composed of the airstream velocity and the cylinder’s rotational velocity, which decreases with distance from the cylinder. On a cylinder, which is rotating in such a way that the top surface area is rotating in the same direction as the airflow, the local velocity at the surface is high on top and low on the bottom. As exists where the airstream line that impinges on the sur- face splits; some air goes over and some under. Another stagnation point streams rejoin and resume at identical velocities. We Air flowing over the top surface accelerates. The airfoil is now subjected to Bernoulli’ effect.” As air velocity increases through the constricted portion of a venturi tube, the pressure decreases. BERNOULLI’S PRINCIPLE MAGNUS EFFECT LIFT 2-3