Principles of Flight

Principles of Flight – Study Notes

1. Overview

The ability of an aircraft to become airborne and stay aloft depends on four fundamental aerodynamic forces and the physical principles that generate them. Understanding these forces, how they interact, and the governing equations is essential for pilots, engineers, and anyone studying aeronautics.

2. The Four Aerodynamic Forces

Equilibrium for steady, level flight:
Lift = Weight and Thrust = Drag

3. How Lift Is Produced

3.1 Bernoulli’s Principle

• Faster airflow over the wing’s upper surface creates lower pressure.
• Pressure difference (Δp) multiplied by wing area (S) yields lift:
  [ L = \Delta p \times S ]

3.2 Newton’s Third Law (Action–Reaction)

• Air is deflected downward by the wing; the reaction force pushes the wing upward.
• The downwash angle relates to lift magnitude.

3.3 The Lift Equation

[ L = C_L ; \frac{1}{2} \rho V^2 S ]

• (C_L) – Lift coefficient (function of airfoil shape & angle of attack)
• (\rho) – Air density (altitude, temperature, humidity)
• (V) – True airspeed relative to the wing
• (S) – Wing planform area

Implication: Doubling speed quadruples lift (since (V^2) term).

4. Angle of Attack (AoA)

• Definition: Angle between the chord line of the wing and the relative wind.
• Effect on (C_L): Increases roughly linearly up to the stall angle (~15° for many airfoils).
• Stall: Beyond the critical AoA, flow separates, (C_L) drops sharply, and drag rises.

Key point: Lift depends more on AoA than on speed for a given airfoil; pilots control AoA with pitch attitude and trim.

5. Drag Components

Total Drag:
[ D = D_{\text{parasite}} + D_{\text{induced}} + D_{\text{wave}} ]

6. Thrust and Propulsion