Principles of Flight

drop in pressure. This lowered pressure is a component of Low angle of attack total lift. The pressure difference between the upper and lower surface of a wing alone does not account for the total lift force produced. The downward backward flow from the top surface of an CP airfoil creates a downwash. This downwash meets the flow from the bottom of the airfoil at the trailing edge. Applying Newton’s third law, the reaction of this downward backward flow results in an upward forward force on the airfoil. gl n e A of a t c ta k 8 ° - High Pressure Below A certain amount of lift is generated by pressure conditions underneath the airfoil. Because of the manner in which air flows underneath the airfoil, a positive pressure results, Normal angle of attack particularly at higher angles of attack. However, there is another aspect to this airflow that must be considered. At a point close to the leading edge, the airflow is virtually stopped (stagnation point) and then gradually increases speed. At some point near the trailing edge, it again reaches a velocity CP equal to that on the upper surface. In conformance with Bernoulli’s principle, where the airflow was slowed beneath t t th h h e e e p f a l r i u e r i f s d o s i u s l p r , e e a e d p d if o f d s e e i r t c e i r v n e e t a i a s u l e p s b w , e t t a h w r e d e p e p r n e r e t s h s s e s u u r u e r p e m p w e u r a s a s t n i c n d r c e l r o a e w t a e s e d e r ) ( s . i u . S e r i f ., n a c c a e e s A n gl e of a tt a c k + 4 ° of the airfoil increases, total lift increases. Both Bernoulli’s Principle and Newton’s Laws are in operation whenever lift is being generated by an airfoil. High angle of attack Pressure Distribution From experiments conducted on wind tunnel models and on full size airplanes, it has been determined that as air flows along the surface of a wing at different angles of attack CP (AOA), there are regions along the surface where the pressure is negative, or less than atmospheric, and regions where the p p o r r n e e t s s h s s u e u r r w e e i i o n s n g p t o t h h s e i a t n i u v p i e s p , e o c r a r s u g u s r r e e f a d a t c e b e r y t c h t r h a e e n a p t a e o t s m s a i o t r i s v e p l e h a p e ti r r v i e c e s . l s y T u h l r a e is r g r n e e e s r g u f a l o t t i r i n v c g e e A n gl e of att a c k + 1 0 ° from the air striking the lower wing surface. Figure 4-7 shows the pressure distribution along an airfoil at three different angles of attack. The average of the pressure variation for any given AOA is referred to as the center of pressure (CP). Aerodynamic force acts through this CP. At high angles of Figure 2-8. Pressure distribution on an airfoil & CP changes Figure 4-7. Pressure distribution on an airfoil and CP changes attack, the CP moves forward, while at low angles of attack with an angle of attack. with AOA. the CP moves aft. In the design of wing structures, this CP travel is very important, since it affects the position of the air loads imposed on the wing structure in both low and high lift is a complex subject. The production of lift is much more AOA conditions. An airplane’s aerodynamic balance and complex than a simple differential pressure between upper controllability are governed by changes in the CP. and lower airfoil surfaces. In fact, many lifting airfoils do not have an upper surface longer than the bottom, as in the case of symmetrical airfoils. These are seen in high-speed Airfoil Behavior aircraft having symmetrical wings, or on symmetrical rotor Although specific examples can be cited in which each of blades for many helicopters whose upper and lower surfaces the principles predict and contribute to the formation of lift, 4-8