How Does a Parachute Work? Drag, Terminal Velocity and the Shape of a Canopy
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A person falling from an aircraft does not accelerate for ever. Within about twelve seconds the air resistance on the body equals its weight and the fall settles at a steady speed, around 200 kilometres an hour, which is survivable on landing by nobody. A parachute changes one number in that balance, the area presented to the air, and changes it by a factor of a few hundred, so that the same balance is reached at 20 kilometres an hour instead. Leonardo sketched one, a Frenchman jumped from a balloon under one in 1797, and the modern version is not a canopy at all but a wing that can be flown to a chosen spot.
Terminal velocity
Air resistance, drag, on a body moving through it rises with the square of the speed, with the area the body presents, and with the density of the air, and it acts against the motion. A falling body accelerates under gravity until the drag grows to match its weight, and from then on the forces cancel and the speed is constant: terminal velocity. For a skydiver spread flat it is about 55 metres a second; head-down and streamlined it is nearly double. The equation makes the parachute's job plain. Weight is fixed, so to bring the terminal speed down by a factor of ten the drag at that speed must be the same, which means the area times the drag coefficient must go up by a factor of a hundred, and a round canopy of 8 metres across provides just that.
The round canopy
The classic parachute is a hemisphere of fabric, originally silk and since the 1940s nylon, held open by the air it traps and connected to the harness by suspension lines. It works by drag alone: the canopy is a bag of air that the falling body must drag down with it. A hole at the apex lets some air escape so that the canopy does not spill from one side and swing; without it a round parachute oscillates violently. The descent rate under a military round is about five metres a second, a jump from a wall a metre and a half high, which is why paratroopers roll. Round canopies can barely be steered and land where the wind takes them, and they are now used mainly for cargo, for emergency escape and for slowing spacecraft and dragsters.
The ram-air wing
Almost every sport parachute since the 1970s is a ram-air canopy: a rectangular or elliptical wing made of two layers of fabric divided into cells that are open at the front, so that the air rushing in inflates them and the canopy takes the shape of an aerofoil. It then flies rather than falls, generating lift like a wing and gliding forward at three or four times its rate of descent, and the jumper steers it by pulling toggles that deform the trailing edge, turning, slowing and, at the last moment, flaring to a near standstill for a standing landing. A skilled jumper can land within a metre of a target. The parts of a modern rig:
- •The main canopy, packed in a container on the back and deployed by a small pilot chute thrown into the airflow
- •The reserve, a second canopy packed by a certified rigger, deployed by hand or by an automatic device that fires if the jumper is still falling fast below a set altitude
- •The slider, a square of fabric that slows the canopy's opening so that it inflates over a second or two rather than instantly, which would injure the jumper and tear the fabric
- •Suspension lines of braided polyester or Spectra, and risers connecting them to the harness
- •The harness, which spreads the opening shock, around three times body weight, over the thighs and shoulders
What can go wrong
The parachute is the safest it has ever been, with a fatality rate in sport jumping of about one in 200,000 jumps, and the failures are known. A canopy can fail to open cleanly, twisting its lines or catching a line over the top, and the drill is to cut it away and deploy the reserve, which takes seconds and a few hundred metres of altitude; the automatic activation device exists for the jumper who is unconscious or has lost track of height. Most modern deaths come not from equipment but from flying: a fast, small canopy turned low to the ground for a swooping landing, which converts height to speed and leaves no margin. The old fear, of the parachute that does not open, was answered by the reserve; the new one is the wing that is flown into the ground.
Beyond people
The same physics slows anything that must come down through an atmosphere. Cargo drops use clusters of rounds; the rovers landed on Mars hung under supersonic parachutes 20 metres across, deployed at twice the speed of sound in air a hundredth as dense as Earth's, before rockets took over for the last stretch; and the capsules that bring astronauts home from orbit descend under three rounds each 35 metres wide. Drag racers deploy parachutes at 500 kilometres an hour because brakes would not hold, and the recovery parachute has become standard on light aircraft, where a canopy that lowers the whole aeroplane has saved several hundred lives since 2002. Every one of them is the same trade: give the air enough to push against and it will hold you up, at a speed set by the square root of your weight over your area.
The takeaway
A parachute works by increasing the area a falling body presents to the air by a factor of hundreds, so that the drag, which rises with the square of speed, balances the body's weight at a terminal velocity of a few metres a second instead of fifty. Round canopies do it by drag alone and land where the wind sends them; ram-air canopies inflate into a wing that glides, steers and flares to a standing landing, backed by a reserve and an automatic device, and the same principle lands cargo, capsules and rovers on Mars.