How Airplanes Fly Forces

1. How Airplanes FlyForces Marc Masquelier

2. Your Ideas… • What is an airplane? • What are wings? Aheavier-than-air aircraft kept aloft by the upward thrust exerted bythe air passing over its wings Airfoils attached transversely to the fuselage of an aircraftthat providelift For many forces on an airplane, wing area (S) is a major reference number

3. Some Terminology • Knots  1 kt = 1.15 mph • Angle of Attack  AOA, or alpha, or α • The angle of the wind relative to the wing AOA

4. Forces • Lift • Weight • Thrust • Drag Lift Thrust Drag Weight

5. Before We Start on Forces We need to understand Pressure

6. Pressure • Two types of pressure • Static (surrounding air) • Dynamic (speed) • Total pressure = static + dynamic pressure

7. Pressure • Total Pressure = Static Pressure (p) + Dynamic Pressure (1/2*ρ*V2) = constant for a given flight condition Flow accelerates over the top – static pressure decreases Flow remains constant – static pressure stays constant

8. Forces • Lift • Weight • Thrust • Drag Lift Thrust Drag Weight

9. Lift • Mostly created by the wings • Lift = CL * q * S • Where q = dynamic pressure = 1/2*ρ*V2

10. Lift Net Lift Pressure distribution on upper surface Flow accelerates here

11. Lift Higher AOA  higher lift … until the wing stalls

12. Lift Stall AOA is controlled by the pilot!

13. Lift – Airfoil • Angle of Attack “AOA” or “α” • Relative wind • The angle where the wing meets the air • Lift Coefficient • Lift = CL * q * S, or if you turn it around: • CL = Lift / (q * S) • Is a function of angle of attack (as shown on last chart)

14. Lift – Wing • Aspect Ratio • Tradeoffs

15. Net Lift So now we have: • Lift = CL* q * S, where • CL = function of wing design, AOA • q = dynamic pressure = 1/2*ρ*V2 • S = wing area • And recall that AOA is controlled by the pilot • So you get more lift by flying faster, or increasing AOA (until you stall)

16. Forces • Lift • Weight • Thrust • Drag Lift Thrust Drag Weight

17. Weight • What contributes to weight? • Can it change?

18. Weight • Counteracts lift (generally) • 1 lb extra on an airplane requires 8 lb extra other “stuff” to support it (stronger structure, bigger wing, extra electrical power, more cooling, more powerful engine, more gas…) • Additional weight means • aircraft stalls at a higher speed  higher approach/landing speed  longer runway/bigger brakes/harder on gear • higher AOA required to maneuver  less stall margin  less maneuverable • higher AOA at a given speed  more drag  more thrust required  more fuel consumption • Aircraft designer’s #1 enemy

19. Forces • Lift • Weight • Thrust • Drag Lift Thrust Drag Weight

20. Thrust • Generally provided by jet or prop • Pushes the airplane forward • Generally directed along aircraft waterline • A function of throttle position and airspeed • Props – max thrust when stationary – good for low-speed applications • Jets – max thrust when moving – better for high-speed applications

21. Propellers • “Rotating wings” • Push the air backwards • Reaction is … • Usually powered by a gasoline engine similar to a car engine, or a gas turbine

22. Jet Engines • Smash the air down (compressor) • Toss in some fuel • Ignite (combustor) • Make the burning air do some work (turbine) • Expand and accelerate the hot gases out the back (nozzle)

23. Forces • Lift • Weight • Thrust • Drag Lift Thrust Drag Weight

24. Drag • What is drag? • What contributes?

25. This May Have Some Extra Drag…

26. This One Also

27. Drag • LOTS of sources of drag • Drag due to lift (induced drag, typically the biggest drag source) • Flight controls • Fuselage • External stores • Sensor packages

28. Induced Drag Induced Drag Lift Net Force

29. Induced Drag Airflow Low angle of attack Low induced drag What can you say about these two flight conditions? Airflow High angle of attack High induced drag

30. Net Drag • Drag = CD * q * S • CD is a composite of all drag sources • Can be a function of AOA • “drag counts” – 1 drag count = 0.0001 CD • q = dynamic pressure = 1/2*ρ*V2 • And remember S = aircraft wing area (ft2)

31. Another Note about Drag • Putting something external on an airplane is just like selling a house… • How you condition the airflow is a Big Deal • Flat plates are ugly – unless parallel to the airstream • Fairings are important

32. A Quick Side Story LANTIRN Pods Ventral Fins

33. Lift versus Drag • Function of aircraft configuration Best L/D “approaching stall” Best L/D Cl – Lift Coefficient Add a bunch of drag L/D reduces  slower max range speed  More thrust required Zero lift line Cd – Drag Coefficient

34. Summary • Weight and drag are overcome by lift and thrust • Weight increases wreak havoc on aircraft performance • Adding stuff on the outside of the airplane must be carefully done to minimize drag and turbulence • Aircraft design is always a compromise between vehicle performance and onboard systems (weapons/ sensors/ avionics/ fuel/ cargo) • Best if requirements are known from the start

35. Stall • White Board

36. Flaps and Slats Lift Coefficient - CL Angle of Attack - α Lift Coefficient - CL Flap Angle of Attack - α Lift Coefficient - CL Slat Angle of Attack - α

37. Flap

38. Wing Fence

39. Recommended Reading • Stick and Rudder by Wolfgang Langewiesche