Basic Aerodynamics

1. Basic Aerodynamics Dartmouth Flying Club October 10, 2002 Andreas Bentz

2. Lift Bernoulli’s Principle

3. Energy • Definition: Energy is the ability to do work. • Energy cannot be created or destroyed. We can only change its form. • A fluid in motion has (mainly) two forms of energy: • kinetic energy (velocity), • potential energy (pressure).

4. The Venturi Tube and Bernoulli’s Principle kinetic energy(velocity) potential energy(pressure) velocity increases pressure decreases

5. Lift: Wing Section • Air flows toward the low pressure area above the wing: upwash and downwash. • Newton’s third law of motion: to every action there is an equal and opposite reaction. • “The reaction to downwash is, in fact, that misunderstood force called lift.” Schiff p. 8 relative low pressure upwash downwash

6. total lift chord line average relative wind Angle of Attack • The angle of attack is the angle between the chord line and the average relative wind. • Greater angle of attack creates more lift (up to a point).

7. induced drag total lift effective lift chord line average relative wind Lift and Induced Drag • Lift acts through the center of pressure, and perpendicular to the relative wind. • This creates induced drag.

8. Got Lift? Flaps • Flaps increase the wing’s camber. • Some also increase the wing area (fowler flap). • Almost all jet transports also have leading edge flaps.

9. Too Much Lift? Spoilers • Spoilers destroy lift: • to slow down in flight (flight spoilers); • for roll control in flight (flight spoilers); • to slow down on the ground (ground spoilers).

10. Side Effects There is no such things as a free lunch.

11. 1,400 1,200 1,000 800 600 400 200 max. lift/drag best glide Drag (lbs) 50 100 150 200 Indicated Airspeed (knots) Drag: Total Drag (Power Required) Curve • induced drag • parasite drag • resistance • total drag

12. Wingtip Vortices and Wake Turbulence • Wingtip vortices create drag: • “ground effect”; • tip tanks, drooped wings, “winglets”. relative low pressure

13. Stability Longitudinal: Static, Dynamic Lateral

14. lift down lift weight Longitudinal Stability • Static stability (tendency to return after control input) • up elevator increases downward lift, angle of attack increases; • lift increases, drag increases, aircraft slows; • less downward lift, angle of attack decreases (nose drops).

15. lift down lift weight Aside: CG and Center of Pressure Location • Aft CG increases speed: • the tail creates less lift (less drag); • the tail creates less down force (wings need to create less lift). • This also decreases stall speed (lower angle of attack req’d).

16. relative wind relative wind Lateral Stability • If one wing is lowered (e.g. by turbulence), the airplane sideslips. • The lower wing has a greater angle of attack (more lift). • This raises the lower wing.

17. Directional Stability • As the airplane turns to the left (e.g. in turbulence), the vertical stabilizer creates lift toward the left. • The airplane turns to the right.

18. max. endurance ca. 75% of max. lift/drag Speed Stability v. Reverse Command • Power curve: • Power is work performed by the engine. (Thrust is force created by the propeller.) • Suppose airspeed decreases. • “Front Side”: Power is greater than required: aircraft accelerates. • “Back Side”: Power is less than required: aircraft decelerates. 1,400 1,200 1,000 800 600 400 200 100% 50% Percent horsepower Drag (thrust required) 50 100 150 200 Indicated Airspeed (knots)

19. Turning Flight Differential Lift

20. Turning Flight • More lift on one wing than on the other results in roll around the longitudinal axis (bank). • Lowering the aileron on one wing results in greater lift and raises that wing.

21. Turning Flight, cont’d • More lift on one wing than on the other results in roll around the longitudinal axis (bank). • Lowering the aileron on one wing results in greater lift and raises that wing. • This tilts lift sideways. • The horizontal component of lift makes the airplane turn. • (To maintain altitude, more total lift needs to be created: higher angle of attack req’d) Centrifugal Force

22. Adverse Yaw and Frise Aileron • However, more lift on one wing creates more induced drag on that wing: adverse yaw. • Adverse yaw is corrected by rudder application. • Frise ailerons counter adverse yaw: • They create parasite drag on the up aileron.

23. Stalls Too Much of a Good Thing

24. Stalls • A wing section stalls when its critical angle of attack is exceeded. • Indicated stall speed depends on how much lift the wing needs to create (weight, G loading).

25. lift weight Stalls, cont’d • The disturbed airflow over the wing hits the tail and the horizontal stabilizer. This is the “buffet”. • Eventually, there will not be enough airflow over the horizontal stabilizer, and it loses its downward lift. The nose drops: the stall “breaks”.

26. Stalls, cont’d • The whole wing never stalls at the same time. • Power-on stalls in most light singles allow the wing to stall more fully. Why? • Where do you want the wing to stall last? • Ailerons

27. Stalls, cont’d (Stalls with one Engine Inop.) • Stalls in a twin with one engine inoperative lead to roll or spin entry: • Propeller slipstream delays stall.

28. Stalls, cont’d • Stall strips make the wing stall sooner.

29. Stalls, cont’d • Definition: The angle of incidence is the acute angle between the longitudinal axis of the airplane and the chord line of the wing. • Twist in the wing makes the wing root stall first: • The angle of incidence decreases away from the wing root.

30. Preventing Stalls • Slats direct airflow over the wing to avoid boundary layer separation. • Slots are similar but fixed, near the wingtips. • Delays stall near the wingtip (aileron effectiveness).

31. Stalls and Turns • Greater angles of bank require greater lift so that: • the vertical component of lift equals weight (to maintain altitude), • the horizontal component of lift equals centrifugal force (constant radius, coordinated, turn)

32. acrobatic 6G Normal 3.8G Stalls and Turns, cont’d • Load factor (multiple of aircraft gross weight the wings support) increases with bank angle. limit load factor: • Stall speed increases accordingly.

33. Turns • As bank increases, load factor increases. • But: as airspeed increases, rate of turn decreases. • In order to make a 3 degree per second turn, at 500 Kts the airplane would have to bank more than 50 degrees. • Uncomfortable (unsafe?) load factor. • This is why for jet-powered airplanes, a standard rate turn is 1.5 degrees per second.

34. High and Fast In the Flight Levels

35. High and Fast • Mach is the ratio of the true airspeed to the speed of sound. • Speed of sound decreases with temperature. • Temperature decreases with altitude. • At higher altitudes, the same indicated airspeed leads to higher Mach numbers. • Conversely: at higher altitudes, a certain Mach number can be achieved at a lower indicated airspeed. • The indicated stall speed increases with altitude (compressibility).

36. High and Fast, cont’d • At high subsonic speeds, portions of the wing can induce supersonic airflow (critical Mach number Mcrit). • Where the airflow slows to subsonic speeds, a shockwave forms. • The shockwave causes boundary layer separation. • High-speed buffet, “aileron snatch”, “Mach tuck”. velocity increases velocity decreases, shockwave forms boundary layer separates

37. High and Fast, cont’d • Vortex generators delay boundary layer separation.

38. High and Fast, cont’d • With altitude: • indicated stall speed (low speed buffet) increases; • indicated airspeed that results in critical Mcrit decreases. • coffin corner

39. References • De Remer D (1992) Aircraft Systems for Pilots Casper: IAP • FAA (1997) Pilot’s Handbook of Aeronautical KnowledgeAC61-23C Newcastle: ASA • Lowery J (2001) Professional Pilot Ames: Iowa State Univ. Press • Schiff B (1985) The Proficient Pilotvol. 1 New York: Macmillan • U.S. Navy (1965) Aerodynamics for Naval Aviators Newcastle: ASA