Sky Diving

Skydiving aerodynamics

Typical jump altitudes in modern times for experienced skydivers range from 7,500 to 15,000 feet (2,300 to 4,600 metres) above ground level, yielding a freefall time of between 40 and 85 seconds. The length of the freefall (the time between exiting the aircraft and deploying the parachute) is dependent upon such factors as exit altitude, opening altitude, and fall rate. The fall rate is determined by the jumpsuit the skydiver wears and the way in which the suit conforms to the body during the freefall (a looser suit offers more resistance to the air and slows the fall) and by the diver's body configuration relative to the ground. Freefall speeds for skydivers falling “belly to earth” (the standard arched position) range from 110 to 130 miles per hour (180 to 210 km per hour). Descent speeds in excess of 330 mph have been recoded for those skydivers in speed skydiving competitions (in which the diver falls headfirst with the body in a streamlined position), and those as slow as 40 mph have been noted for skydivers using extended-wing “birdman” suits. The altitude at which a parachutist opens the canopy varies, but 2,500 feet is usual. With most parachutes, the skydiver initiates deployment by throwing a pilot chute into the airstream (other parachutes have pilot chutes that are automatically released by pulling an attached cord). The pilot chute is a small chute opened by air resistance that acts to pull the main parachute out from the bag in which the parachutes are encased. The main parachute increases the skydiver's air resistance and slows the diver's fall to the ground to a speed of about 10 mph. Modern ram-air parachutes are made of seven to nine nylon cells that inflate and act as a wing, or glider; these canopies allow the parachutist to steer and sail gently to the ground.
To maximize safety, sport skydivers wear two parachutes, one main and one reserve. This is prescribed by law in the U.S. Federal Aviation Regulations and in the aviation regulations of many other countries as well. Additionally, skydivers use a variety of altitude-sensing devices in order to know when to deploy their parachutes.