In aviation, a ski-jump is an upwardly curved ramp that allows a fixed-wing aircraft to take off from a runway that is shorter than what conventional takeoff requires. By providing an upward vector from the ski-jump's normal force, the aircraft leaves the runway at an elevated angle similar to how a ski jumper takes off. This elevated launch angle provides the aircraft a longer airborne time despite a lower speed, where it can continue accelerating under its own engines until reaching enough airspeed to obtain the aerodynamic lift needed to overcome gravity and sustain flight. In modern naval aviation, ski-jumps are commonly used to launch shipborne jet aircraft from flight decks of aircraft carriers that lack aircraft catapults for assisted takeoffs, where the runway lengths are too short for conventional takeoffs for these aircraft (which are much heavier than the older propeller aircraft) and the aircraft therefore likely crash into the water after launching off the ship. It is believed that the first use of the ski-jump occurred during the Second World War, when a temporary ramp was added to HMS Furious to help heavily laden Fairey Barracudas attack the German battleship Tirpitz. During the Cold War, the concept was studied as a means of reducing the length of flight decks required for aircraft carriers and to facilitate ever-heavier aircraft at sea. The Royal Navy took a particular interest in the ski-jump during the 1970s, conducting tests with the new Hawker Siddeley Harrier V/STOL fighter, then added a ramp to its next generation of aircraft carrier, the Invincible class. Numerous naval services have since adopted the ski-jump for their own aircraft carriers and amphibious assault platforms, while land-based uses have been examined as well. Ski-jumps can be used in two ways: Short Take-Off But Arrested Recovery (STOBAR) for tailhook-equipped naval aircraft capable of arresting gear deceleration; and Short Take-Off, Vertical Landing (STOVL) for V/STOL aircraft capable of vertical thrust vectoring. As aircraft catapults are much more sophisticated, costly and energy-intensive systems, currently only three countries (the United States, France and China) are capable of operating Catapult-Assisted Take-Off but Arrested Recovery (CATOBAR) carriers, while all other countries that still have aircraft carriers in the 21st century use STOBAR or STOVL carriers with ski-jumps.
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. While aircraft such as the F/A 18 that are normally catapult-launched can make use of a ski-ramp, this typically comes at the cost of a reduced capacity for either fuel or munitions, and thus negatively impacting mission scope significantly.
History
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