Wings, engines or hot air: it's all physics.

A wing turns the oncoming air slightly downward; by Newton's third law the air pushes the wing up. The size of that push is L = ½ × ρ × v² × S × C_L: air density ρ, true airspeed v, wing area S and a lift coefficient C_L set mostly by the angle of attack. Making lift always costs some drag: parasitic drag (the pilot, struts, wires and skin friction) grows with speed, while induced drag (the unavoidable cost of making lift, seen in wingtip vortices) shrinks with speed. Their sum is smallest at one particular speed, which is where a wing glides best.

Glide ratio: metres forward per metre down

Without an engine, an aircraft is always sliding down through the air it's in. The glide ratio equals lift ÷ drag (L/D). Typical round numbers, which vary a lot between models:

Long, slender wings (high aspect ratio) cut induced drag, which is why sailplanes have such long wings. A headwind shortens the glide over the ground; sinking air can shorten it far more.

Your climb rate over the ground is simply air's vertical speed − your sink rate. Soaring pilots use three main sources of rising air:

• Thermals: the sun heats the ground unevenly; air over a warm patch (a dry field, a town, a rocky slope) becomes less dense and rises in bubbles or columns. Typical climbs are around 1–3 m/s, stronger on hot, dry days, and the tops are often marked by fluffy cumulus clouds (Ch 5). Gliders circle tightly to stay inside the core.
• Ridge (slope) lift: wind blowing onto a hillside is forced upward. As long as the wind is steady and roughly perpendicular to the ridge, a glider or hang glider can fly back and forth along it. The lee side has sinking air and turbulence.
• Wave lift: strong wind over a mountain range can set up standing waves in the air downwind, sometimes marked by smooth, lens-shaped clouds. Wave can carry sailplanes far higher than thermals, which is how many gliding altitude records are set.

The sinking air you don't see

What goes up must come down: around every thermal there is sinking air, often 1–2 m/s or more. A glider crossing it loses height much faster than its still-air sink rate. That's why pilots speed up through sink (to spend less time in it) and slow down in lift.

Archimedes' principle: anything immersed in a fluid is pushed up by a force equal to the weight of the fluid it displaces. Air is a fluid. At constant pressure its density falls as temperature rises: ρ = P ÷ (R × T), with R = 287 J/(kg·K) for dry air and T in kelvin (°C + 273.15). The envelope is open at the bottom, so the pressure inside is essentially the same as outside; only the temperature differs. The gross lift is V × (ρ_outside − ρ_inside).

How a balloon is flown

• Up: the pilot fires the propane burner to heat the air; each extra °C inside adds about 7 kg of lift to our example balloon.
• Down: let the air cool, or open a vent at the top to release hot air.
• Sideways: no direct control. A balloon moves with the wind, so pilots steer by changing height to find winds blowing in different directions, and plan landing areas downwind.

In steady level flight four forces balance: lift = weight and thrust = drag. A glider has no thrust, so gravity provides it: the glider tilts slightly nose-down and "spends" height to keep moving. A powered aircraft (microlight, light sport aircraft, paramotor, motor glider) replaces that with a propeller. Since drag = weight ÷ (L/D), the power needed is P = drag × speed, and any extra power becomes climb: climb rate = extra power ÷ weight.

What the engine changes, and what it doesn't

• Changes: you can take off from flat ground, climb without rising air, and fly when there's no lift. A paramotor needs only a field; a microlight needs a runway or strip.
• Doesn't change: the wing still stalls at the same angle of attack (Ch 6), turbulence and wind still matter, and an engine failure turns you into a glider, so pilots always keep a landing option within gliding range.
• Balloons: the burner isn't thrust at all; it's a heater for buoyancy (Ch 3).

Hot, high and thin: density altitude

Wings, propellers and engines all work less well in thin air. Air density depends on both elevation and temperature, so a hot day at a high airfield can feel like flying from a much higher one.

Clouds, thermals and storms

• Cloud base: rising air cools as it expands; when it reaches its dew point, cloud forms. A common rule of thumb is about 125 m of height per °C of spread between temperature and dew point.
• Cumulonimbus (thunderstorms): powerful up- and downdrafts, gust fronts that spread out from the storm, hail and lightning. They're a hard no-go for every aircraft on this page.
• Monsoon: low cloud, heavy rain and embedded storms. Balloon operators in India typically fly in the drier months; SkyWaltz, for example, lists a season from 1 September to 30 April.
• Wind: balloons need light winds to launch and land; gliders and hang gliders need the right wind for ridge lift but can be overpowered by strong, gusty wind. Air flowing over hills, trees and buildings leaves turbulence (rotor) on the downwind side.

Raise the angle of attack and C_L rises, until the airflow can no longer follow the top of the wing. It separates, lift drops sharply and drag jumps: a stall. From the lift equation, the speed at which you reach that maximum C_L is V_stall = √(2 × n × W ÷ (ρ × S × C_L,max)), where n is the load factor (g). In a level turn n = 1 ÷ cos(bank).

Three ways to control an aircraft