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Marc Masquelier How Airplanes Fly Forces. Your Ideas… What is an airplane? What are wings? A heavier-than-air aircraft kept aloft by the upward thrust.

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Presentation on theme: "Marc Masquelier How Airplanes Fly Forces. Your Ideas… What is an airplane? What are wings? A heavier-than-air aircraft kept aloft by the upward thrust."— Presentation transcript:

1 Marc Masquelier How Airplanes Fly Forces

2 Your Ideas… What is an airplane? What are wings? A heavier-than-air aircraft kept aloft by the upward thrust exerted by the air passing over its wings Airfoils attached transversely to the fuselage of an aircraft that provide lift 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 Weight Drag Thrust Lift

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*V 2 ) = 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 Weight Drag Thrust Lift

9 Mostly created by the wings Lift = C L * q * S – Where q = dynamic pressure = 1/2*ρ*V 2

10 Lift Pressure distribution on upper surface Net Lift 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 = C L * q * S, or if you turn it around: – C L = 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 = C L * q * S, where C L = function of wing design, AOA q = dynamic pressure = 1/2*ρ*V 2 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 Weight Drag Thrust Lift

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 Weight Drag Thrust Lift

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 Weight Drag Thrust Lift

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 Lift Induced Drag Net Force

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

30 Net Drag Drag = C D * q * S C D is a composite of all drag sources – Can be a function of AOA – “drag counts” – 1 drag count = C D q = dynamic pressure = 1/2*ρ*V 2 And remember S = aircraft wing area (ft 2 )

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 Ventral Fins LANTIRN Pods

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

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 Lift Coefficient - C L Angle of Attack - α Flaps and Slats Lift Coefficient - C L Angle of Attack - α Lift Coefficient - C L Angle of Attack - α Flap Slat

37 Flap

38 Wing Fence

39 Recommended Reading Stick and Rudder by Wolfgang Langewiesche


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