Principles of Flight

Principles of flight explains why an aircraft flies, how its controls work and what happens at the edges – the stall, the spin and the limits of the structure. Understanding it turns flying from a set of memorised actions into something you can reason about.

1. The four forces

• Lift: produced by the wings, acting at right angles to the airflow.
• Weight: the pull of gravity, acting through the centre of gravity.
• Thrust: from the propeller or jet engine, pulling the aircraft forward.
• Drag: the air's resistance, acting backwards along the flight path.

In steady, straight and level flight lift equals weight and thrust equals drag. A change in any one of them makes the aircraft climb, descend, speed up or slow down.

2. The wing

• The chord line runs from the leading edge to the trailing edge; camber is the curve of the wing section.
• The angle of attack (AoA) is the angle between the chord line and the oncoming airflow – not the same as the pitch attitude.
• The angle of incidence is the fixed angle at which the wing is mounted on the fuselage.

Air flowing over the curved upper surface speeds up and its pressure drops (Bernoulli), while the wing also turns the airflow downwards (Newton). Both explanations describe the same thing: a pressure difference that pushes the wing up.

The lift formula

Lift = ½ ρ V² S C_L – air density × speed squared × wing area × lift coefficient. The useful parts for a pilot:

• Lift depends on the square of the speed: double the speed, four times the lift.
• Thinner air (high, hot) means less lift at the same speed.
• The pilot controls C_L by changing the angle of attack – and with flaps.

3. The stall

As the angle of attack increases, lift increases – until the critical angle of attack (about 15–16° for most light-aircraft wings). Beyond it, the airflow separates from the upper surface and lift drops suddenly: the wing stalls.

• A wing always stalls at the same angle of attack, but it can stall at any speed and any attitude.
• Warnings: the stall warner, buffeting, sloppy controls, high nose attitude.
• Recovery: reduce the angle of attack (ease the stick forward), apply full power, level the wings with the rudder and ailerons once flying again, then climb away.

What changes the stall speed

Change	Stall speed
More weight	Higher
Turning or pulling up (higher load factor)	Higher: stall speed × √(load factor)
Flaps extended	Lower
CG forward	Higher
Ice or dirt on the wing	Higher, and the stall may come without warning

4. Turning and load factor

In a turn, part of the lift pulls the aircraft round, so the wing must make more lift to keep the height. The load factor (n, or "g") is lift ÷ weight = 1 ÷ cos(bank angle).

Bank	Load factor	Stall speed increase
30°	1.15 g	+7%
45°	1.41 g	+19%
60°	2.0 g	+41%

That is why a steep turn near the ground at low speed – for example a hurried turn onto final – is so dangerous.

5. Drag

• Parasite drag (form, skin friction, interference) grows with the square of the speed.
• Induced drag is the price of making lift: it is greatest at low speed and high angle of attack, from the wingtip vortices.
• Total drag is lowest where the two are equal: the minimum drag speed, close to the aircraft's best glide speed. Flying slower than it is "behind the drag curve": slowing down needs more power.
• Ground effect: within about one wingspan of the ground, induced drag falls – the aircraft "floats" in the flare and can lift off before it is really able to climb.

6. Flaps and slats

Flaps increase camber (and sometimes wing area): more lift at a given speed and so a lower stall speed, but also more drag. Small settings help the take-off; full flap gives a steep, slow approach. Slats on the leading edge let the wing reach a higher angle of attack before stalling.

7. Controls and axes

Control	Movement	About the axis
Ailerons	Roll	Longitudinal (nose to tail)
Elevator	Pitch	Lateral (wingtip to wingtip)
Rudder	Yaw	Normal (vertical)

• Adverse yaw: the down-going aileron makes more lift and more drag, yawing the nose away from the turn. Use co-ordinated rudder – keep the ball in the middle.
• Trim removes the need to hold pressure on the controls.
• The propeller adds effects too (slipstream, torque, P-factor), which is why you need right rudder in the climb in most single-engine aircraft.

8. Stability

• Longitudinal (pitch): mainly from the tailplane; better with a forward CG.
• Lateral (roll): from wing dihedral (wings angled up) and a high wing.
• Directional (yaw): from the fin, like the feathers of an arrow.

9. Spins

A spin is a stall with yaw: one wing more stalled than the other, autorotating downwards. It starts from an uncoordinated stall (for example a skidding turn onto final). Standard recovery (always follow the flight manual): PARE – Power idle, Ailerons neutral, Rudder full opposite to the spin, Elevator forward until the spin stops; then rudder neutral and recover from the dive.

Flying it at Levant

Airliners protect the stall with alpha floors, stick shakers and fly-by-wire limits, but the physics is the same: watch your speed in turns, especially when slow and heavy, use flaps on schedule, and remember that a high-altitude cruise leaves a smaller margin between the stall and maximum speed (the "coffin corner").

Check yourself

1. What makes a wing stall? Exceeding the critical angle of attack.
2. Can a wing stall at high speed? Yes – at any speed, if the critical angle of attack is exceeded (for example when pulling hard).
3. Stall speed 50 kt level. What is it in a 60° bank turn? About 71 kt (× √2).
4. Which kind of drag is greatest at low speed? Induced drag.
5. Which control moves the aircraft about its lateral axis? The elevator (pitch).

Simulation training material based on standard PPL theory. Always follow the flight manual for your aircraft, especially for stall and spin recovery.