EASA PPL Principles of Flight (P50) — Study Guide

Principles of Flight explains why the aeroplane behaves as it does — buffet before stall, back pressure in steep turns, heavy controls at high speed, stability differences between types. The P50 paper is 20 questions · 30 minutes · 75% (15/20). Flying stalls, steep turns, and slow flight while studying accelerates intuition enormously.

The four forces

Lift acts perpendicular to relative airflow; weight through the CG downward; thrust along the flight path; drag rearward. In straight-and-level unaccelerated flight: lift = weight, thrust = drag. Change any force and equilibrium shifts — climb, descend, accelerate, or decelerate.

How wings generate lift

Camber and curvature accelerate upper-surface flow ⇒ lower pressure aloft (Bernoulli). Downwash and momentum change contribute too (Newton). Exams expect you not to claim lift comes from only one story — both pressure field and turning of the streamlines matter.

Angle of attack (AoA)

AoA is the angle between chord and relative airflow, not necessarily pitch attitude. Lift rises roughly linearly with AoA until the flow separates — then lift collapses and drag spikes: stall. Critical AoA is essentially fixed for an aerofoil; the airspeed at which you reach it changes with weight, load factor, and configuration.

Lift equation (relationships)

L = ½ ρ V² S · CL

• Lift ∝ V² — double speed ⇒ quadruple lift at same AoA/CL.
• Lift ∝ air density ρ — hot/high ⇒ worse performance.
• Wing area S and CL (AoA, flaps) scale lift directly.

Drag

Induced drag

By-product of lift — tip vortices tilt the lift vector aft. Rises at high AoA / low speed; falls when AoA is small at high speed. High aspect ratio wings (long span) reduce induced drag for a given lift.

Parasite drag

Form, friction, interference — rises ~V². Dominates at high speed.

Total drag and best glide

Total = induced + parasite ⇒ a minimum at one airspeed — best lift/drag, minimum sink/glide in zero thrust (subject to POH wording). Slower or faster than that point increases total drag.

Stalling

Stall means exceeding critical AoA (~15–18° typical training wings), not “slow speed” by itself. Buffet, horn/shaker, degraded controls (especially ailerons) warn you. Recovery: reduce AoA first (unload), add power as appropriate, roll wings level, then climb away — power without unloading does not fix an established stall.

Load factor and accelerated stall

Vs(n) ≈ Vs1 × √n    (n = load factor)

Level steep turn: n = 1/cos(bank). At 60° bank, n = 2 ⇒ Vs rises ×√2 ≈ 1.41. Exam traps: stall speed scales with √n, not n.

Spin

Aggravated stall with asymmetric stall ⇒ autorotation. Classic recovery mantra: power idle, full opposite rudder, forward elevator to break AoA, neutralise rudder as rotation stops, recover smoothly from dive — follow your POH and instructor cadence.

Flaps

Increase camber ⇒ higher CL (lower stall speed for same weight), add drag — especially at large settings. Small flap may shorten ground roll; excessive flap can trash climb on take-off. Respect VFE; configuration changes shift trim and sight picture.

Stability

• Static stability: initial tendency after a disturbance.
• Dynamic stability: whether oscillations damp or grow over time.

Longitudinal

CG must stay ahead of the neutral point for positive pitch stability; aft CG increases sensitivity and reduces margins — respect envelope limits every flight.

Lateral & directional

Dihedral, sweep, high wing contribute to roll stability; fin provides weathercock yaw stability.

Spiral vs Dutch roll

Spiral instability: weak roll stability vs strong yaw ⇒ bank slowly tightens — common, pilot corrects. Dutch roll: strong dihedral vs weaker yaw damping ⇒ coupled roll/yaw oscillation; jets often need yaw damper — light trainers usually self-damp mildly.

Controls

• Ailerons — roll; create adverse yaw (more induced drag on rising wing) ⇒ coordinate with rudder.
• Elevator / stabilator — pitch ⇒ AoA management.
• Rudder — yaw; slip/skid coordination.

Control power follows dynamic pressure — authority fades near stall; avoid curing a dropped wing with aileron alone at high AoA (can worsen asymmetric stall).

Wake turbulence

Heavy wings shed strong vortices that sink and drift with wind. Riskiest low and slow; stay above glidepath of heavies, touchdown beyond their point if following same runway, rotate before their point if departing behind, use ATC spacing as minimum not comfort margin in VMC.

Where students lose marks

• √n stall-speed maths vs linear guesses.
• CG scenarios — aft limit ⇒ lighter pitch, less stability, nastier stall; forward limit ⇒ heavier stick, higher Vs.
• Confusing critical AoA (fixed) with stall IAS (changes).

How to prepare

Chain concepts: load factor ⇒ lift required ⇒ AoA margin ⇒ stall speed. Drill numeric bank ↔ n ↔ Vs problems until automatic. Pair every formula with something you have felt in the circuit — P50 rewards causal reasoning, but crisp recall on spins, flap limits, and wake rules still decides borders between pass and fail.