AngleofAttack,DragandPerformance

The angle of attack is the angle between the wing’s chord line and the relative wind — the direction the air is actually arriving from, which is always directly opposite the aircraft’s flight path.

It is not the aircraft’s pitch attitude. An aircraft can have its nose well above the horizon and a low angle of attack, if it is climbing steeply. It can have its nose below the horizon and a high angle of attack, if it is descending steeply. Attitude is measured against the ground; angle of attack is measured against the air.

As angle of attack increases, lift increases — up to a point. Beyond the critical angle of attack, the airflow can no longer follow the curve of the upper surface, it separates, and lift falls away sharply. That is a stall.

Here is the sentence that matters most in this chapter, and it is worth reading twice: a wing always stalls at the same angle of attack. Not at the same airspeed. Not at the same attitude. Not at the same weight or bank angle. The critical angle is a property of the wing’s shape, and it does not change with anything the pilot does.

Because in one specific condition — wings level, unaccelerated, at a stated weight — reaching the critical angle of attack happens at a predictable airspeed, and an airspeed indicator is a great deal cheaper than an angle of attack indicator.

Change any of those conditions and the number changes. Increase the load factor and the wing must produce more lift, which means a higher angle of attack at any given speed, which means the critical angle arrives sooner. The relationship is exact: the stall speed increases with the square root of the load factor.

Work an example. In a level turn the load factor is 1 divided by the cosine of the bank angle. At 60 degrees of bank, the cosine is 0.5, so the load factor is 2. The square root of 2 is about 1.41, so the stall speed is 41 per cent higher than the wings-level figure. An aircraft that stalls at 50 knots straight and level stalls at about 71 knots in a 60-degree level turn.

At 75 degrees of bank the load factor is nearly 4, and the stall speed has doubled. This is why steep turns at low altitude are treated with the seriousness they are — the margin above the stall that felt comfortable in level flight has quietly disappeared.

Drag comes in two kinds and they behave in opposite directions with speed, which is the single most useful fact in aircraft performance.

Parasite drag is the cost of pushing an aircraft-shaped object through air: skin friction over the surfaces, form drag from the shape, interference drag where components meet. It grows with the square of airspeed. Double the speed and parasite drag quadruples.

Induced drag is the price of making lift. A wing producing lift leaves rotating air behind its tips — wingtip vortices — and the energy in that rotation had to come from somewhere. It comes from the engine. Induced drag is greatest when the wing is working hardest for its speed, which is at low speed and high angle of attack, and it falls away rapidly as the aircraft accelerates.

Add them together and you get a curve with a minimum. At that speed, total drag is at its lowest and the wing is producing the most lift for the least drag — the best lift-to-drag ratio the aircraft has. It is the speed to fly for maximum glide range with the engine stopped, and close to the speed for maximum endurance.

Within about one wingspan of the ground, the surface interferes with the formation of the wingtip vortices, which reduces induced drag substantially. This is ground effect, and it is why an aircraft floats down the runway in the flare when the approach was a few knots fast.

It is also a trap on takeoff. An aircraft can become airborne in ground effect at a speed at which it cannot climb out of it, particularly when heavy or at high density altitude. The aeroplane flies, the pilot raises the nose to climb, the aircraft leaves ground effect, induced drag increases sharply, and it settles back toward the runway with the nose high and the speed decaying.

Everything an aircraft does depends on air density. The wing makes lift by accelerating a mass of air downward; thinner air means less mass per second and less lift at the same speed. The propeller works on the same air. A normally aspirated engine makes power by burning fuel with the oxygen in the air it can draw in.

Density altitude is the number that captures all of it: the altitude in the standard atmosphere at which the air would have the density the aircraft is actually experiencing. It is what performance charts are computed against.

Three things drive it. Pressure altitude — how high you are and what the barometric pressure is. Temperature — hot air is less dense than cold. Humidity — water vapour is lighter than the nitrogen and oxygen it displaces, so humid air is slightly less dense than dry air at the same temperature and pressure.

Of the three, temperature is usually doing most of the work, and this is what catches people. A field at 1,000 feet elevation on a 35°C afternoon has a density altitude around 3,700 feet. The airport is not in the mountains and the aircraft performs as though it is.

A well-designed aircraft returns toward its trimmed condition after being disturbed, without the pilot doing anything. That is static stability, and it is designed in rather than being a happy accident.

Longitudinal stability — resistance to pitch upset — comes largely from the horizontal stabiliser and from where the centre of gravity sits relative to the centre of lift. The centre of gravity is deliberately placed forward of the centre of lift, so the aircraft has a nose-down tendency, and the horizontal stabiliser produces a downward force at the tail to balance it. Disturb the aircraft nose-up and the angle of attack at the tail changes in a way that pushes the nose back down.

This is why loading matters so much and why every aircraft has a centre of gravity envelope. Load it too far aft and the stabilising moment weakens; the aircraft becomes twitchy in pitch, stall recovery becomes harder, and past a certain point it stops being recoverable at all. Too far forward and the aircraft becomes heavy in pitch, needs more elevator to flare, and may not have enough authority to raise the nose for landing.

The consequence for the wing is subtle and worth knowing: because the tail pushes down, the wing has to carry the aircraft’s weight plus the tail download. An aft centre of gravity requires less tail download, which means less lift required, which means slightly better performance — one of the reasons airliners manage their fuel distribution in flight.