Flight

• Power lines kill. Fly kites and balloons in open ground well away from them. If a kite snags one, let go and keep away: kite line conducts, above all when wet (PSE&G). A hot-air balloon that hit power lines near Lockhart, Texas, in 2016 killed all 16 aboard (NTSB).
• Storms: never fly a kite with thunder about.
• Airspace: large kites and moored balloons are aircraft to the law. In the US, one over 5 lb may not fly above 500 ft, within 5 miles of an airport, within 500 ft of cloud, or in visibility under 3 miles (14 CFR 101.13); other countries set similar limits.
• Fire: never let a hot-air model fly free or with a flame aboard. Brazil makes releasing balloons that can start fires a crime (Law 9.605/1998, art. 42).
• People in the air: this site gives no construction procedure for any aircraft that carries a person. See the last section.

A surface tilted into moving air deflects it downwards, and the air pushes the surface up. The lift equation:

Lift = ½ × air density × speed² × area × lift coefficient

• Air density at sea level is 1.225 kg/m³; at 2,000 m about 18 % less.
• Speed squared: twice the speed, four times the lift.
• The lift coefficient grows with the angle of attack — for a thin wing roughly 0.1 for each degree — until the flow breaks away from the upper surface and lift collapses: the stall, commonly near 15°.
• Area: lift scales with it, and so does the weight of the structure that holds it.

A balloon lifts the weight of the air it pushes aside, less the weight of what is inside it. Air at 15 °C weighs 1.225 kg/m³; lift per cubic metre is in the chart.

• Hot air is free and safe to handle but lifts little: 820 m³ at 100 °C for 230 kg.
• Hydrogen lifts four times as much and burns at any mixture from 4 % to 74 % in air, lit by a spark of 0.02 mJ (US DOE).
• Helium lifts almost as much as hydrogen and does not burn; it comes from natural gas wells.
• Every figure falls with height and heat: about 18 % less at 2,000 m.

Kites were the first flying machines and did real work: the first line across the Niagara gorge in 1848 (Nebraska State Historical Society), the aerial for the first transatlantic radio reception in 1901 (IEEE), and weather observation until balloons took over.

• A kite's bridle sets its angle to the wind. Moving the towing point towards the head makes it fly higher and more overhead; towards the tail, lower and pulling harder (Kitecraft).
• A loose cover bags and steadies a tailless kite; a tight one makes it dart and dive.
• A tail adds drag low down, keeping the head up in a kite that has no bow or keel.
• Anything that flies free — a model glider, an aircraft — is stable only if its centre of gravity lies ahead of the point where its lift acts, and a tail behind holds it there. Too far aft, and it pitches up and stalls: the most dangerous single fault in anything that flies.

Each machine answered the question the one before had raised. Figures from the National Park Service's history (handbook).

Nobody flew a heavier-than-air machine until they had first learned, without a person aboard, what kept it up and what turned it. The order that worked:

1. Kites and models: does it fly, is it stable, which way does it turn?
2. A wind tunnel: measure lift and drag for each wing shape instead of guessing.
3. Full-size gliders flown as kites, empty, on lines.
4. Short glides from low slopes into steady wind, with a person aboard.
5. Power, only after control is solved.

The gliders, aeroplane and jets and helicopters steps continue this path with dimensioned drawings; the model glider and wind tunnel are the first two stages to build. This site gives no construction procedure for any aircraft that carries a person. The recognised route today is a proven design built from its designer's plans or kit, inspected as it is built (in the US, FAA AC 20-27G), and flown only after training at a school.

• Safety: in 2024, one accident for every 880,000 airline flights (IATA), from rules, training, maintenance and independent accident investigation.
• Height: oxygen for the crew above 12,500 ft for more than 30 minutes (14 CFR 91.211); airliner cabins no higher than 8,000 ft (14 CFR 25.841).
• Electric: the first type-certified electric aeroplane, a two-seat trainer, in 2020 (General Aviation News); batteries hold about fifty times less energy per kilogram than jet fuel.
• Fuel: sustainable fuel blended up to 50 % into jet fuel (US DOE).