1. How Airplanes Fly Forces
Marc Masquelier

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
Airplane - a heavier-than-air aircraft kept aloft by the upward thrust exerted by the air passing over its wings
Wing - airfoils attached transversely to the fuselage of an aircraft that provide lift

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
Wings keep airplanes aloft by pushing the air down
Every action has an equal and opposite reaction

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” Lift Coefficient
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 Show CL alpha curve
Affected by camber (more drag, higher CL/a), thickness (thicker = more drag, benign stall)

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

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 = 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 Best L/D
Add a bunch of drag
L/D reduces  slower max range speed 
More thrust required
Best L/D
“approaching stall”
Cl – Lift Coefficient
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 Flap Slat Lift Coefficient - CL Angle of Attack - α

37. Flap

38. Wing Fence

39. Recommended Reading
Stick and Rudder by Wolfgang Langewiesche