9.2 Aerodynamics and Flight Controls

9.2 Aerodynamics and Flight Controls

More AI points come from aerodynamics and flight controls than from any other topic, so master this first. The concepts chain together: lift depends on angle of attack, exceeding angle of attack causes a stall, and the flight controls are simply the means by which a pilot changes attitude and therefore angle of attack and bank. If you understand the chain, you can reconstruct most answers even when a question is phrased unusually.

Lift and the four forces

Lift is generated as air flows over a wing (airfoil). The curved upper surface speeds the airflow and lowers its pressure, while higher pressure beneath pushes the wing up — an application of Bernoulli's principle combined with Newton's deflection of air downward. In steady straight-and-level flight, lift = weight and thrust = drag. Increase thrust and you accelerate until drag rises to match it again.

Angle of attack (AOA) is the angle between the wing's chord line (an imaginary straight line from leading edge to trailing edge) and the relative wind (the oncoming airflow). Do not confuse it with angle of incidence, the fixed mounting angle of the wing to the fuselage set by the manufacturer. As AOA increases, lift increases — up to the critical angle of attack, typically 15-20 degrees.

Stalls

Beyond the critical AOA the airflow separates from the upper wing surface, lift collapses, and the wing stalls. The single most-tested fact here: a stall is a function of AOA, not airspeed. An aircraft can stall at any speed and any attitude if the critical AOA is exceeded — including in a high-speed turn (an accelerated stall). Recovery means reducing AOA, usually by lowering the nose.

The three axes

Ailerons move opposite to each other: one up, one down, creating differential lift that banks the aircraft. The elevator moves the nose up or down. The rudder swings the nose left or right and coordinates turns; it does not turn the airplane by itself — that is the job of bank plus elevator.

Secondary and high-lift devices

• Flaps — extend from the trailing edge, increasing wing camber and area to produce more lift at low speed, enabling slower takeoff and landing speeds. They also add drag.
• Slats/slots — leading-edge devices that delay the stall by re-energizing airflow at high AOA.
• Spoilers — panels on the upper wing that destroy lift and add drag to slow down or steepen a descent; on the ground they help braking.
• Trim tabs — small surfaces that relieve the constant control-stick pressure a pilot would otherwise hold.

How a turn actually works

A frequent AI misconception is that the rudder turns the airplane. It does not. To turn, the pilot banks with the ailerons so a component of the wing's lift pulls the aircraft toward the turn, then adds back-pressure on the elevator to hold altitude. The rudder only coordinates the turn, offsetting the adverse yaw the lowered aileron creates. Picture lift as an arrow perpendicular to the wings: when you bank, that arrow tilts, and its horizontal part becomes the turning force. This is why steeper banks turn faster but demand more lift and load the wing more heavily.

Drag types

Drag splits into two families. Parasite drag comes from the aircraft pushing through the air — skin friction, form drag, and interference — and it rises with the square of speed, so it dominates at high speed. Induced drag is the by-product of producing lift; it is greatest at low speed and high angle of attack, such as during a slow climb. The combined total drag is lowest at a middle speed, which is why every airframe has a most-efficient cruise speed. Expect AI to ask which drag dominates when slow (induced) versus fast (parasite).

Mach and load factor

Mach number is the ratio of true airspeed to the local speed of sound (Mach 1 = the speed of sound). Because the speed of sound falls with temperature, Mach changes with altitude even at constant true airspeed; an aircraft holding a steady true airspeed can edge closer to Mach 1 simply by climbing into colder air. Speeds below Mach 1 are subsonic, near Mach 1 transonic, and above it supersonic. Load factor ("G") is the ratio of lift to weight; in a level 60-degree bank it is 2 G, meaning the wing carries twice the aircraft's weight.

Stall speed rises with the square root of load factor — a wing that stalls at 60 knots straight-and-level stalls noticeably higher in a hard turn, which is exactly how an accelerated stall happens. Tie this back to the core rule: more G means more required lift, which means a higher angle of attack, which brings the wing closer to the critical AOA at any given speed.