How Does a Kite Fly? A Wing on a String
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A kite is a wing that stays in one place because a string stops it moving downwind. That constraint is the whole trick: because the kite cannot run away from the wind, the air keeps flowing over it, and a surface with air flowing over it at an angle generates lift in exactly the same way an aircraft wing does while standing still relative to the ground.
The three forces
A flying kite is in equilibrium under three forces and understanding the balance explains every adjustment a flyer makes. The wind striking the inclined surface produces an aerodynamic force, conventionally split into lift, perpendicular to the airflow, and drag, along it. Gravity pulls down on the kite's weight. The string tension pulls towards the flyer, down and forward. For steady flight these three must sum to zero, which means the string tension has to balance the horizontal component of the aerodynamic force and the difference between lift and weight. That relationship explains why a kite flies at a particular angle above the horizon and not higher: as the line lengthens, the weight and drag of the line itself become significant, pulling the whole system down, which is why a kite on a very long line flies at a much lower angle than the same kite on a short one and why line weight is a serious consideration in altitude attempts.
Angle of attack and stability
The angle between the kite's surface and the oncoming air is the angle of attack, and it is set by where the bridle, the arrangement of lines connecting the flying line to the kite's frame, attaches. Adjusting the bridle attachment point forward reduces the angle, suiting strong wind, and moving it back increases the angle, suiting light wind, which is the main tuning available to a flyer. Too small an angle and the kite generates insufficient lift and sinks. Too large and the airflow separates from the upper surface, lift collapses and the kite stalls, which is what is happening when a kite climbs, hesitates and dives. Stability comes from making the kite naturally return to its flying attitude when disturbed, which is achieved by keeping the centre of pressure behind the bridle point, by dihedral, meaning bending the kite so the wings angle upward, and traditionally by a tail. A tail adds drag well behind the centre of mass, which damps the swinging and spinning that make an untailed flat kite unflyable.
The main designs
Kite forms solve the stability problem in different ways and suit different conditions:
- •The flat kite, the simplest form, which needs a tail because it has no inherent stability
- •The bowed kite, curved so the wings have dihedral, which is self-stabilising and can fly without a tail
- •The box kite, invented by Lawrence Hargrave in 1893, whose cellular structure gives excellent stability and lift and which fed directly into early aeroplane design
- •The delta, a triangular kite with a keel, which flies at a high angle in light wind and is extremely forgiving
- •The parafoil, which has no rigid frame at all and inflates into an aerofoil section from cells open at the leading edge, generating large force and packing into a bag
- •The dual and quad line stunt kite, which is deliberately unstable so the flyer can steer it, with control coming from applying different tension to each side
What kites were used for
Kites are far older than aviation and were serious tools before they were toys. They originated in China at least two thousand years ago, with early uses including signalling, measuring distances and, according to accounts, military reconnaissance and psychological effect. In the eighteenth and nineteenth centuries they became scientific instruments: Alexander Wilson used them to measure temperature at altitude in 1749, Franklin's 1752 experiment established that lightning is electrical, and meteorological services flew kite trains carrying instruments to record upper-air conditions daily until aircraft and balloons replaced them in the 1930s. Kites carried the first line across the Niagara gorge in 1848, which became the foundation of a suspension bridge. Box kites lifted observers and aerials in wartime and informed the Wright brothers, who flew their early gliders as tethered kites to learn control. Modern applications include kite surfing, traction kites pulling boats and buggies, and experiments in using large kites to tow cargo ships and to generate electricity by flying figure-of-eight patterns that reel out a tethered generator.
The takeaway
A kite generates lift like any wing, and the line is what keeps air flowing over it by preventing it from moving downwind, so the three forces of aerodynamic force, weight and line tension balance. The bridle sets the angle of attack, forward for strong wind and back for light, and too large an angle stalls the kite. Stability comes from dihedral, from keeping the centre of pressure behind the bridle point, and from a tail whose drag damps swinging. Line weight is what limits the flying angle.