Lecture: Lift Forces on Aircraft

Recap

• Previous lecture focused on forces in horizontal flight at constant speed:
  • Weight, Lift, Drag, Thrust
  • Equilibrium: Lift = Weight, Drag = Thrust

Focus: Lift Force

• Lift Equation: Lift = CL * 0.5 * ρ * V^2 * S
  • CL: Lift coefficient (dimensionless)
  • ρ: Air density
  • V: Air speed
  • S: Wing area (projected on X-Y plane)

Dimension Analysis

• Left side: Newtons (kgs m/s²)
• Right side: kg/m³ * m/s² * m²
• Conclusion: CL is dimensionless
  • Influenced by airfoil geometry and airflow

Airfoil Characteristics

• Angle of Attack: Influences lift
• Airfoil Geometry:
  • Front: Leading edge
  • Rear: Trailing edge
  • Camber: Curve in the middle line between surfaces
  • Chord Line: Straight line with angle to airflow

Historical Development

• Early airfoil experimentation
• NACA Profiles: Systematic airfoil nomenclature
  • 4-digit code (e.g., NACA 2412)
  • Describes camber, position, thickness

Lift Factors

• Wing Surface Area & Airspeed: Design parameters
• Air Density: Affected by altitude & temperature

Lift Generation

• Bernoulli’s Principle: p + 0.5 * ρ * V² = constant
  • Increased speed = decreased pressure
  • Pressure differences create lift
  • Applicable for low airspeeds & incompressible media

Pressure Distribution

• Higher speed over airfoil creates lift
• NASA Foil-Sim: Tool to simulate effects

Measurement of Airspeed

• Pitot Tube: Measures total and local pressure
• Use pressure difference to calculate velocity

Lift Coefficient & Angle of Attack

• Lift Curve: CL vs. angle of attack (α)
  • Initially linear, then non-linear
  • Stall: Decrease in lift after max angle

Low Airspeeds Adjustment

• Use high lift devices:
  • Flaps & Slats: Increase lift coefficient and wing area

Wing Geometry Parameters

• Wing Span: Tip to tip length
• Wing Surface Area
• Root & Tip Chord
• Taper Ratio: Root to tip chord ratio
• Sweep Angle: Leading edge to Y-axis angle

Next Steps

• Next lecture: Focus on drag force