Why the air holds you up

Four forces act on an aeroplane in flight: lift, weight, thrust and drag (FAA PHAK, ch. 5). In steady flight at a constant speed and height, the opposing pairs are in balance (FAA PHAK, ch. 5). Nothing is being won. The aircraft is holding a settlement between 4 numbers.

Lift acts perpendicular to the relative wind. Drag acts parallel to the relative wind and rearward (FAA PHAK, ch. 5). Weight acts downward through the centre of gravity (FAA PHAK, ch. 9). Thrust from the propeller acts roughly along the longitudinal axis (FAA PHAK, ch. 5).

In a climb, part of the weight acts backward along the flight path (FAA PHAK, ch. 5). Thrust must then carry that share as well as drag (FAA PHAK, ch. 5). That is why a trainer leaving FNA climbs at a fixed published speed rather than at cruise speed (FAA PHAK, ch. 10). Read the speed out of your own aircraft's flight manual and fly it.

A wing produces lift by turning the air that flows over it downward (FAA PHAK, ch. 4). The air pushes back on the wing by the same amount in the opposite direction (FAA PHAK, ch. 4). That reaction, resolved perpendicular to the relative wind, is lift.

Downwash is the visible half of that bargain. The wing leaves a mass of air moving downward behind it, and the size of that downward push sets the lift (FAA PHAK, ch. 4). A wing that deflects more air, or deflects it harder, carries more mass.

The turning shows up as a pressure difference. Pressure over the upper surface falls below the pressure under the lower surface, and the wing is pushed toward the low pressure (FAA PHAK, ch. 4). Both descriptions are the same event measured 2 ways.

Lift depends on aerofoil shape, angle of attack, air density, wing area and the square of the airspeed (FAA PHAK, ch. 5). Double the airspeed and, at the same angle of attack, lift goes up by a factor of 4 (FAA PHAK, ch. 5). That square term is the whole reason speed control matters more than muscle.

Angle of attack is the angle between the chord line of the wing and the relative wind (FAA PHAK, ch. 5). It is not pitch attitude, and it is not the angle to the ground. You can hold 10 degrees nose-up with a small angle of attack in a steep climb (FAA PHAK, ch. 5). You can hold the nose level with a large one in slow flight.

As angle of attack increases, the lift coefficient increases, up to the critical angle of attack (FAA PHAK, ch. 5). Past that angle the airflow separates from the upper surface and the wing stops producing usable lift (FAA PHAK, ch. 5). The critical angle for a typical light trainer wing is near 16 degrees to 18 degrees (FAA PHAK, ch. 5).

In the cockpit you trade angle of attack against airspeed to hold the same lift. Slow down and you must raise the angle of attack, which is what the trim and the yoke are doing for you (FAA PHAK, ch. 5). This is the whole of the approach: 1 lever, 2 numbers.

Many books explain lift with a single line about longer distance over the curved top surface and faster flow. The distance argument is wrong: there is no physical rule that 2 parcels of air must meet again at the trailing edge (FAA PHAK, ch. 4). Handbooks now describe lift using both the pressure difference and the momentum change of the deflected air (FAA PHAK, ch. 4).

Two facts kill the shortcut. A flat plate held at a small angle to the airflow produces lift with no camber at all (FAA PHAK, ch. 4). An aeroplane also flies inverted, where any argument resting only on the upper curve would fail (FAA PHAK, ch. 4).

Keep the working version: the wing deflects air downward, and pressure and momentum are 2 accounts of the same transaction (FAA PHAK, ch. 4). Neither account alone tells you the whole thing. What you fly is the angle of attack and the speed.

Lift falls as air density falls, and density falls as temperature rises or pressure drops (FAA PHAK, ch. 10). Density altitude is pressure altitude corrected for temperature, and it is the altitude the aircraft believes it is at (FAA PHAK, ch. 10). Performance is set by density altitude, not by the field elevation on the chart (FAA PHAK, ch. 10).

Take FNA on an afternoon in April: 34 C, QNH 1010 hPa, field close to sea level. That gives a density altitude near 2,400 ft (FAA PHAK, ch. 10). The wing is unchanged, but the air is thinner (FAA PHAK, ch. 10). True airspeed for a given indicated airspeed is higher, and the ground run is longer (FAA PHAK, ch. 10).

The same arithmetic runs at DNMM, DGAA and ROB through the monsoon months, when 30 C to 32 C with high humidity is ordinary. Humid air is less dense than dry air at the same temperature and pressure, which lengthens the take-off run further (FAA PHAK, ch. 10). Work the performance chart before every hot departure, not after.

Lift comes from turning air, and you control it with angle of attack and airspeed (FAA PHAK, ch. 5). The 4 forces balance in steady flight and stop balancing the moment you change power or attitude (FAA PHAK, ch. 5). Past the critical angle of attack the wing quits, whatever the airspeed reads (FAA PHAK, ch. 5).

Take the same 3 questions to every aircraft you fly: what angle, what speed, what air. The answers change with mass, flap and temperature, and the flight manual holds all 3 (FAA PHAK, ch. 10).

On the ground at FNA or DGAA, the number that decides your day is density altitude (FAA PHAK, ch. 10). Compute it, then compare the take-off distance in the flight manual against the runway you actually have (FAA PHAK, ch. 10).

Next lesson is drag: why the aeroplane has 1 speed that costs it least, and how far that speed carries you with the engine quiet.