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Lift, Drag, and Airfoils

Objective​

To understand basic aerodynamic concepts like lift, drag, and the nomenclature we use to describe it.

Motivation​

Forms foundational knowledge for more advanced aerodynamic topics and provides a useful mental model for pilots so they can better anticipate and understand the operation of an airplane.

Timing​

1 hour

Format​

Elements​

Basics​

  • Airplane components [Airplane diagram]
    • Fuselage
    • Wings
    • Ailerons
    • Elevator
    • Horizontal stabilizer
    • Rudder
    • Vertical stabilizer
    • Empennage: Entire tail
  • Newton's laws of motion [List]
    1. Momentum
    2. Fnet = mass * acceleration
    3. Equal and opposite reaction
  • Four forces of flight [Force diagram]
    • Lift
    • Weight
    • Thrust
    • Drag
  • Lift theories
    • Newton: Barn door / hand in the wind [Barn door force]
      • Equal and opposite reaction
    • Bernoulli [Constricted tube]
      • Pressure in a tube constricted in the middle
      • Pressure is lower in the faster air in the middle
  • Basic airfoil [Chord vs camber lines]
    • Chord line
    • Mean camber line
  • Lift on an airfoil
    • Newton [Wing with rear/up force]
      • Air deflected downward causes for up and back
    • Bernoulli [Wing with stream tubes]
      • Stream tubes get condensed on the top of the wing
      • Lower pressure induces a force
  • Angle of attack [Angle of attack + relative wind]
    • Chord line vs relative wind
    • Difference between flight path and relative wind
    • Where momentum wants to take vs where the wing is pointed
  • Lift equation
lift=12ρV2SCl2\text{lift} = \frac{\frac{1}{2} \rho V^2 S C_l}{2} Where:ρ=air densityV=velocityS=surface areaCl=coefficient of lift\text{Where:}\\ \rho = \text{air density}\\ V = \text{velocity}\\ S = \text{surface area}\\ C_l = \text{coefficient of lift}
  • Critical angle of attack
    • Coefficient of lift vs angle of attack (Cl vs AoA) [Graph]
    • Critical angle of attack
    • Stall, turbulent airflow, detaching of the boundary layer [Stall and wing]
  • Flight at slow airspeeds [Two airplanes, high vs low speed with AoA]
    • To maintain level flight as we slow: increase AoA

Pitching Moment​

  • Center of pressure vs center of gravity [Force diagram]
    • Cl should always be behind Cg
  • Elevator downforce

Thrust and Drag​

  • Thrust vs drag [Forces]
    • Engine provides thrust
    • Drag works opposite
  • Parasite drag
    • Difference shapes hit more or less wind
  • Induced drag
    • Low vs high pressure on the wing
    • Theses pressures meet on the wing tip
    • This causes a vortex
    • This vortex creates a slight downward on the air just ahead of the wing
    • Tilting of the lift vector
    • Wingtip vortices
  • Ground effect
    • More lift, less drag because vortex is disrupted
    • "Cushion of air"
  • Parasite drag vs induced drag
  • Best L/D, power required above and below

Wing Design [Commercial]​

  • Aspect ratio: Ratio of wing span to wing chord
    • Low aspect ratio: Concord, fighter jet
    • High aspect ratio: Glider
  • Wing taper: more lift and less drag
  • Wing loading: how much force each square foot of the wing needs to produce

References​

Pilot's Handbook of Aeronautical Knowledge pg. 4-1 Pilot's Handbook of Aeronautical Knowledge pg. 5-1