Lift is the aerodynamic force generated perpendicular to the relative airflow, produced mainly by the wing (aerofoil). It results from a pressure difference between the upper and lower wing surfaces - the modern explanation combines faster flow and lower pressure over the curved upper surface (Bernoulli's principle) with the wing deflecting air downwards (Newton's third law, reaction lift). Both effects contribute; neither alone fully explains it, and the old 'equal transit time' theory is now considered incorrect and outdated.
Lift = CL x 1/2 rho V^2 x S, where CL is the coefficient of lift (depends on AoA and aerofoil shape), rho is air density, V is airspeed (the dominant variable, since it's squared), and S is wing area. Doubling airspeed quadruples lift for a given CL - this is why speed control is so critical near the stall.
CL increases roughly linearly with AoA up to the critical angle of attack, typically around 15-16 degrees on a typical training aircraft wing, where the boundary layer separates from the upper surface and CL drops sharply - this is the stall. The stall is always caused by exceeding the critical AoA, regardless of airspeed, attitude, or weight. A common exam trap: an aircraft can stall in any attitude (even nose-down) and at any airspeed if the critical AoA is exceeded, such as in a steep turn or abrupt pull-up.
The boundary layer is the thin layer of air next to the wing surface slowed by friction (viscosity). It starts laminar (smooth, low drag, low energy) near the leading edge and transitions to turbulent (higher drag but more energy, so more resistant to separation) further aft. Boundary layer separation is what causes the stall - turbulators and vortex generators are sometimes fitted to encourage a turbulent boundary layer and delay separation, improving stall characteristics.
Drag is the aerodynamic force acting parallel to, and in the same direction as, the relative airflow - it always opposes the aircraft's motion through the air. Total drag is the sum of two main families: parasite drag and induced drag.
Parasite drag is produced by anything that is not directly related to producing lift. It has three parts.
Parasite drag increases with the square of airspeed - double your speed and parasite drag becomes four times greater.
Induced drag is the price paid for producing lift. It is caused by wingtip vortices - high pressure air below the wing spilling around the tip to the low pressure area above, which tilts the local airflow (lift vector) rearwards, creating a rearward-acting component of drag.
Induced drag is greatest at low airspeed and high angle of attack, and it decreases as airspeed increases - the opposite trend to parasite drag.
Because parasite drag rises with speed and induced drag falls with speed, adding them together produces a U-shaped total drag curve. The lowest point of that curve is the speed of minimum drag, often called Vmd. At Vmd the lift/drag ratio is at its maximum (L/D max) - this is the most efficient speed the aircraft can fly for its cleanest glide performance, giving the best glide range for height lost.
The lift/drag ratio (L/D) tells you how efficiently a wing or aircraft converts lift into forward progress for the drag it creates. L/D max corresponds to the best glide speed (published in the POH/AFM), giving maximum horizontal distance per unit of height lost in a glide.
A stall happens when the wing's critical angle of attack (AoA) is exceeded - not at a fixed airspeed. Airflow separates from the upper surface, lift collapses and drag rises sharply. The critical AoA is typically around 15-16 degrees on a conventional wing, though the precise figure varies by aerofoil design.
A stall can happen at any airspeed, any attitude and any power setting - straight and level, in a steep turn, climbing or descending. The only thing that matters is AoA exceeding the critical value. This is the single most tested concept on this topic.
Standard recovery: reduce AoA immediately by moving the control column forward (unload the wing), apply full power smoothly, level the wings with coordinated aileron/rudder, then ease back to level flight minimising height loss. Do not use aileron alone to pick up a dropped wing near the stall - it can worsen the situation and provoke a spin; use rudder to keep the aircraft balanced.
A spin is an autorotation that follows a stall where one wing is more stalled than the other, usually from being uncoordinated (yaw present) at the point of stall. The stalled wing produces less lift and more drag, so it drops and yaws the aircraft further into the spin - a self-sustaining rotation combined with a stall.
The standard recovery sequence (always follow the aircraft's POH/flight manual first): close the throttle, apply full opposite rudder to stop the yaw, then move the control column progressively forward to break the stall, and once rotation stops centralise the rudder and ease out of the resulting dive.
Thinking a stall is about airspeed rather than AoA; believing you cannot stall in a steep turn or with power on; using aileron to lift a dropped wing at the stall instead of rudder; freezing on recovery inputs and losing excessive height.
Stability is the aircraft's tendency to return to its trimmed condition after a disturbance, without pilot input. Static stability is the INITIAL tendency (does it move back towards equilibrium?). Dynamic stability is what happens over TIME as it returns (does the oscillation die away, stay constant, or grow?).
A classic exam trap: an aircraft CAN have positive static stability but negative dynamic stability at the same time (it starts to correct, but the correction overshoots more each cycle).
Controlled by the tailplane and the position of the Centre of Gravity (CG) relative to the Centre of Pressure (CP). For inherent stability the CG must sit ahead of the neutral point, giving a nose-down pitching moment that the tailplane's download balances in trim. Move the CG further forward and the aircraft becomes MORE stable but needs more elevator/stabiliser trim and control force; move CG aft and stability reduces. If the CG goes behind the aft limit, the aircraft can become longitudinally unstable and may be uncontrollable in pitch - this is why loading within CG limits is a strict legal requirement, not just good practice.
Provided mainly by dihedral, sweepback, and the position of the wing on the fuselage (high wing is more stable than low wing, keel effect helps too). Dihedral works because in a sideslip the lower wing meets the airflow at a greater effective angle of attack, generating more lift and rolling the aircraft back level.
Provided chiefly by the fin (vertical stabiliser) acting like a weathercock, keeping the nose aligned with the relative airflow. Excess sweepback and fin area both increase directional stability.
A combined yaw-and-roll oscillation caused by strong directional stability paired with weak lateral (or vice versa) - common on swept-wing types, corrected with a yaw damper.
Elevator controls pitch (angle of attack), ailerons control roll (via differential lift, causing adverse yaw - countered by rudder), rudder controls yaw. Adverse yaw happens because the down-going aileron creates more drag than the up-going one.
Flaps are trailing-edge (and sometimes leading-edge) devices that change the wing's camber and, on some types, its area. Extending flaps increases the coefficient of lift (CL max) at any given angle of attack, which lets you fly slower for the same lift - useful for shorter take-off and landing distances.
More lift from flaps always comes with more drag. Early flap stages give a lift increase with only a small drag rise (good for take-off). Later, fuller flap stages add a lot of extra drag with little extra lift (good for landing, because you want a steep approach and a low touchdown speed).
Flaps increase CL max, which lowers the stalling speed - but they generally reduce the stalling angle of attack because the camber change alters the pressure distribution, causing separation to start sooner in terms of AoA (though later in terms of speed). Never confuse the two: stalling speed goes DOWN, stalling angle of attack goes DOWN too on most flapped wings.
Lowering flap moves the centre of pressure aft on most wings, producing a nose-down pitching moment - this is why aircraft commonly pitch down when flap is selected and the pilot must retrim.
Load factor is the ratio of the lift produced by the wings to the total weight of the aircraft. It is expressed as a number, written 'n', such as n=1 or n=2.
In straight and level, unaccelerated flight, lift equals weight, so load factor is 1 (often called '1g').
Any manoeuvre that changes the direction or speed of the flight path - turning, pulling out of a dive, hitting turbulence - increases the load factor above 1. The wings must generate more lift than weight to make the aircraft change direction.
In a level, coordinated turn, load factor depends only on angle of bank, not on aircraft type or weight.
Learn 60 degrees = 2g. It is the classic exam number and shows how quickly load factor rises as bank steepens - the increase is not linear.
As load factor increases, stall speed increases too. The relationship is:
new stall speed = normal stall speed x square root of load factor.
So at 60 degrees of bank (n=2), stall speed increases by the square root of 2, about 1.41 times the normal 1g stall speed. This is why steep turns near the stall are dangerous - the aircraft can stall at a much higher airspeed than the published 1g stall speed. This higher speed is called the 'accelerated stall' speed.
Va, the design manoeuvring speed, is the maximum speed at which full or abrupt use of a single flight control will not overstress the airframe, because the wing will stall before load limits are exceeded. Above Va, full control deflection risks structural damage. Va reduces at lower weights, so a lighter aircraft has a lower Va than book figures for max weight.
Certified in different categories, aircraft have different positive load limits:
Exceeding these limits risks permanent structural deformation or failure.