How wings make lift

An airfoil is a structure shaped to obtain a reaction from the air moving past it. A typical wing cross-section shows a rounded leading edge facing forward, a narrow, tapered trailing edge, and a curvature called camber that is usually more pronounced on the upper surface than the lower one.

The chord line is a straight reference line drawn from the leading edge to the trailing edge; angle of attack is always measured from this line, not from the fuselage or the horizon. The mean camber line runs from leading edge to trailing edge, equidistant from the upper and lower surfaces at every point -- it is what defines how much camber the airfoil has.

When the relatively flat lower surface of a wing meets the air at a small positive AOA, the air is forced to rebound downward, producing an upward reaction: this is Newton's third law, for every action there is an equal and opposite reaction. The airflow over the curved upper leading edge is also deflected downward. The result, the PHAK states plainly, is that if a wing produces a lift force greater than the weight of the airplane, the airplane flies.

If all the required lift came only from this downward deflection off the lower surface, an airplane could fly with a flat wing, like a kite. It cannot generate enough lift that way alone: the balance of the lift comes from the airflow above the wing, which is where the second explanation is needed.

Bernoulli's principle states that as the velocity of a moving fluid increases, the pressure within it decreases. A venturi tube demonstrates this directly: air speeds up at the narrow throat, and its pressure drops there, then slows and its pressure rises again past the constriction.

The upper surface of a wing acts like the throat of a venturi. Air moving over the curved top surface speeds up, and by Bernoulli's principle, the pressure there drops. This pressure drop is a real component of total lift, and the PHAK is explicit that the pressure differential between upper and lower surfaces does not by itself account for all of the lift -- Newton's downward deflection is also happening, at the same time, on the same wing.

Lift varies with AOA and with airspeed. A large AOA at low airspeed can produce the same lift as a small AOA at high airspeed -- this is why the nose sits noticeably higher in slow flight than in cruise, and why, at very high cruise speeds, some airplanes fly at a slightly negative AOA.

As AOA increases from zero, the coefficient of lift (CL) increases -- up to a point. The PHAK calls that peak CLMAX: the amount of lift produced drops sharply after CLMAX is exceeded, though the wing does not stop producing lift altogether. The AOA at which CLMAX occurs is the critical angle of attack, and every airfoil has exactly one, typically somewhere between about 16 degrees and 20 degrees depending on the design -- and, crucially, this critical AOA does not change with weight, bank angle, temperature, density altitude, or center of gravity. That single fact is the foundation of the entire next lesson.

Because lift depends on a smooth, attached airflow over the wing, any surface contamination that disrupts that flow costs lift out of proportion to how thin it looks. The PHAK gives a specific figure: as little as 0.8 millimeter of ice on the upper wing surface increases drag and reduces lift by 25 percent.

The mechanism is that contamination causes the airflow to separate from the wing at an AOA lower than the clean-wing critical AOA -- in effect, a frosted or iced wing stalls sooner, at a higher airspeed, than the same wing clean. This is why preflight guidance never treats a light frost as cosmetic.

Without an angle of attack indicator, AOA is invisible to the pilot -- you infer it from airspeed, attitude, and control feel. An AOA indicator measures the current AOA directly and displays how close it is to the critical AOA, independent of weight, bank, or configuration.

The FAA has actively promoted AOA indicators as a general aviation safety improvement, because loss of control is the leading cause of fatal accidents in general aviation, and roughly half of those accidents involve a stall or spin. An AOA indicator does not replace airspeed and stall-warning training -- it adds a second, more direct source of the same information.