Ski-jump (aviation)

A Royal Navy Sea Harrier taking off from the ski-jump on the deck of HMS Invincible in 1990.

In aviation, a ski-jump is an upward-curved ramp that allows aircraft to take off from a runway that is shorter than the aircraft's required takeoff roll. By forcing the aircraft upwards, lift-off can be achieved at a lower airspeed than that required for sustained flight, while allowing the aircraft to accelerate to such speed in the air rather than on the runway. Ski-jumps are commonly used to launch airplanes from aircraft carriers that lack catapults.

History

Early aircraft carriers could launch aircraft simply by turning into the wind and adding the ship's own speed to the airspeed experienced by the aircraft. After World War II, naval aircraft became so heavy that aircraft catapults were needed to accelerate aircraft to takeoff speed, and it was feared that even a catapult might one day not be enough. A NACA study from 1952 proposed the use of a ski-jump after the aircraft catapult to assist aircraft in taking off.

In 1973, Lt Cdr D.R. Taylor of Britain's Royal Navy proposed the ski-jump for use with the Harrier Jump Jet, and initial testing with various ramp angles was carried out at RAE Bedford, using the two-seat Harrier demonstrator G-VTOL.

Principle

A fixed-wing aircraft must build up forward speed during a lengthy takeoff roll. As the forward velocity increases, the wings produce greater amounts of lift. At a high enough speed, the lift force will exceed the weight of the aircraft, and the aircraft will become capable of sustained flight. Since the aircraft must reach flight speed using only its own engines for power, a long runway is required so that the aircraft can build up speed. On an aircraft carrier, the flight deck is so short that most aircraft cannot reach flight speed before reaching the end of the deck. Since lift is less than gravity, the aircraft will lose altitude after the wheels leave the flight deck and possibly fall into the sea.

A ski-jump ramp at the end of the flight deck redirects the aircraft to a slight upward angle, converting part of the aircraft's forward motion into a positive rate of climb. Since the aircraft is still traveling at an inadequate speed to generate enough lift, its climb rate will start to drop as soon as it leaves the flight deck. However, the ski-jump launch has given the aircraft additional time to continue accelerating. By the time its upward velocity has decayed to zero, the aircraft will be going fast enough for its wings to produce enough lift. At this point, the aircraft will be in stable flight, having launched from the carrier without ever dipping below the height of the flight deck.

Many modern aircraft carriers lack catapults, so heavy aircraft must take off using their own engines. Ski-jumps make it possible for heavier aircraft to take off than a horizontal deck allows. However, ski-jump launches cannot match the payloads made possible by high-speed catapult launches.

Aircraft carrier operations

STOBAR

A Su-33 "Flanker D" launches from the Russian Navy aircraft carrier Admiral Kuznetsov

On Short Take-Off But Arrested Recovery aircraft carriers (STOBAR), conventional aircraft are launched using a ski-jump. The pilot increases the aircraft's thrust by switching on the afterburners, while holding the plane by braking. Two panels are raised from the deck of the aircraft carrier in front of the aircraft's main landing gear, ensuring the plane remains motionless. Upon command, the pilot releases the brake; the panels from the deck drop back into their slots; and the aircraft rapidly taxis forward under maximum thrust. Rolling over the ski ramp launches the plane both upward and forward.

A MiG-29 launching over the ski-jump ramp on a Kuznetsov-class aircraft carrier can takeoff at a speed of about 70 kn (81 mph; 130 km/h) instead of the usual 140 kn (160 mph; 260 km/h) (depending on many factors such as gross weight).

With the exception of the United States and France, every navy in the world that currently operates naval fixed-wing aircraft from carriers uses ski-jump ramps.

STOVL

A Sea Harrier launches from the Indian Navy aircraft carrier INS Viraat

Short Take-Off, Vertical Landing aircraft (STOVL) make a conventional rolling takeoff, with the jet exhausts set to provide maximum forward thrust. As the plane nears the ski-jump ramp, the jet exhausts are rotated to provide lift as well as forward thrust. Such takeoffs allow a larger takeoff weight than an unassisted horizontal launch, because the ski-jump ramp provides a vertical impetus when most needed, right at takeoff at the slowest takeoff speed.

Ski-jump ramp takeoffs are considered safer than takeoffs over a flat-top carrier. When a Harrier launches from an American LHA, it would finish its takeoff roll and begin flight at 60 ft (18 m) above the water. It might not have a positive rate of climb, especially if the ship had pitched nose down during the takeoff roll. Using a ski-jump ramp, a Harrier will certainly launch with a positive rate of climb, and its momentum will carry it to 150 to 200 ft (46 to 61 m) above the water.

For example, an AV-8B Harrier with a gross weight of 29,000 lb (13,000 kg) on a 59 °F (15 °C) day and a 35 kn (40 mph; 65 km/h) wind over the deck would require 400 ft (120 m) to takeoff using the 12° ski-jump ramp on the Spanish aircraft carrier Príncipe de Asturias. The same plane would use the entire 750 ft (230 m) length of the Tarawa class's flat flight deck. U.S. Marine aviators who experimented with takeoffs from the Spanish aircraft carrier found the improvement to be "nothing short of amazing." The United States is the only country which operates STOVL aircraft from carriers without a ski-jump ramp.

Land operations

The U.S. Air Force has examined the use of ski-jumps on land to enable short-field takeoffs. This was seen as "a possible solution to the runway denial problem in Europe" during the Cold War. When a ski-jump with a 9 degree exit angle is used, the takeoff roll of an F/A-18 Hornet can be cut in half.

See also

• Australia
  • Canberra-class landing helicopter dock
• Italy
  • Italian aircraft carrier Giuseppe Garibaldi
  • Italian aircraft carrier Cavour
• Spanish aircraft carrier Príncipe de Asturias
• UK
  • Queen Elizabeth-class aircraft carrier
  • Invincible-class aircraft carrier
  • Centaur-class aircraft carrier
• USSR (Russia / China / India)
  • Kuznetsov-class aircraft carrier
  • Kiev-class aircraft carrier
• Vikrant-class aircraft carrier