Principles of Flight — PPL(A)

Four forces act on an aircraft in flight: lift (up), weight (down), thrust (forward), and drag (backward). In straight and level flight at constant speed, lift equals weight and thrust equals drag. Any imbalance causes acceleration in the direction of the resultant force.

• Lift: generated mainly by the wings. Acts perpendicular to the relative airflow.
• Weight: acts vertically downward through the centre of gravity.
• Thrust: produced by the engine and propeller. Acts approximately along the longitudinal axis.
• Drag: opposes motion through the air. Acts rearward along the flight path.

An aerofoil (wing cross-section) is shaped to accelerate airflow over the upper surface and slow it on the lower surface. By Bernoulli's principle, faster airflow means lower pressure — the pressure differential creates lift. Additionally, the wing deflects air downward (Newton's third law), and the reaction pushes the wing up.

• Angle of attack (AoA): angle between the chord line and the relative airflow.
• Increasing AoA increases lift — up to the critical angle.
• Critical angle: approximately 16°. Beyond this, flow separates and lift collapses (stall).
• Camber: curved upper surface increases lift at a given AoA.

• Parasite drag: form drag + skin friction + interference drag. Increases with speed squared.
• Induced drag: produced as a by-product of lift. Increases as speed decreases (higher AoA).
• Total drag: the sum of parasite and induced. Minimum at a specific speed (best L/D ratio).
• Best glide speed: the speed of minimum total drag — gives maximum glide range in a glide.

A stall occurs when the critical angle of attack is exceeded and airflow over the upper wing surface separates. It is an angle of attack phenomenon, not a speed phenomenon — an aircraft can stall at any speed or attitude if the critical AoA is exceeded.

• The stall speed shown in the POH is at 1g, wings level — this is the minimum clean stall speed.
• Load factor increases stall speed: in a coordinated 60° bank, load factor is 2g, so stall speed × √2 ≈ 1.41.
• Signs of approaching stall: buffet, mushy controls, stall warning horn (if fitted).
• Recovery: reduce AoA (push forward), add power, level the wings.
• Accelerated stall: pulling hard in a high-g manoeuvre can cause stall well above the 1g stall speed.

• Static stability: the initial tendency after a disturbance. Positive (returns toward equilibrium), neutral (stays displaced), negative (diverges).
• Dynamic stability: the long-term response. An aircraft can be statically stable but dynamically unstable.
• Longitudinal stability (pitch): affected by horizontal tail and CG position. Aft CG reduces stability.
• Lateral stability: dihedral, keel effect, and swept wings contribute to roll stability.
• Directional stability: fin and rudder provide weathercock stability (yaw).

• Torque reaction: propeller turns clockwise (from pilot view) — aircraft tends to roll left.
• Slipstream effect: rotating slipstream hits left side of fin — aircraft yaws left at high power.
• Gyroscopic effect: on tail-wheel aircraft, raising the tail during take-off causes left yaw.
• Asymmetric blade effect (P-factor): at high AoA, descending blade has more pitch and produces more thrust — yaws left.