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In aviation, a ski-jump is an upwardly curved ramp that allows a fixed-wing aircraft to takeoff from a runway that is shorter than the aircraft normally requires. By providing an upward vector from the ski-jump's normal force, the aircraft is launched at an elevated angle and lift-off can be achieved at a lower airspeed than that required for flat takeoff, as it allows the aircraft more time to continue accelerating while airborne after leaving the runway. Ski-jumps are commonly used to launch shipborne aircraft from of that lack catapults.
It is believed that the first use of the ski-jump occurred during the Second World War, when a temporary ramp was added to to help heavily laden attack the German battleship . During the Cold War, the concept was studied as a means of reducing the length of 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 VSTOL fighter, then added a ramp to its next generation of aircraft carrier, the .
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 conventional, tailhook-equipped naval aircraft; and Short Take-Off, Vertical Landing (STOVL) for V/STOL aircraft. Catapult-equipped aircraft carriers have become a minority in the 21st century in part because ski-jump operations are simpler and cost less.
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.
In the years following the Second World War, the prevailing trend of increasingly heavy carrier aircraft continued apace, leading to fears that eventually such increases would exceed the viable payload capabilities of any catapult system. Accordingly, research into alternative methods of assisting takeoff was conducted. A NACA study completed in 1952 proposed the use of a ski-jump following after the aircraft catapult to provide additional assistance to departing aircraft.Stille 2012, p. 5.
In his 1973 M.Phil. thesis, Lt. Cdr. D.R. Taylor of Britain's Royal Navy proposed the use of a ski-jump to help the Harrier jump jet take off. His ski-jump design, which featured a curve, was initially met with scepticism, but other officials endorsed trials of the proposal. Thus, initial testing using various ramp angles was carried out at RAE Bedford; the aircraft used was the two-seat Harrier demonstrator G-VTOL. The results were further verified via computer modelling techniques and simulations. These tests demonstrated that performance increased with ski-jump angle, but planners chose to select the minimum angle, allegedly the reasoning behind this choice was to avoid placing excessive stress on the aircraft's undercarriage.
During the 1970s, the Royal Navy was considering the construction of a through-deck cruiser or light aircraft carrier, and decided to integrate the ski-jump into the project. Accordingly, the aircraft carriers were constructed with ski-jumps, greatly shortening the distance required for Harriers to take-off even when burdened with a useful payload.Hobbs 2015, pp. 469–472.Bull 2004, p. 120. The ski-jump proved to be a relatively cheap and straightforward addition to the carriers, comprising steel construction without any moving parts. A ski-jump was added to the first carrier of the type, , while she was fitting out in Barrow; it was set at a conservative 7° angle. On 30 October 1980, test pilot Lt Cdr David Poole conducted the first ski-jump assisted Harrier take-off at sea. was also initially fitted with a 7° ramp; however, , was built with a 12° ramp from the outset, which was determined to be the optimum angle. The earlier two ships were subsequently retrofitted with 12° ramps to improve their aircraft's performance.
After the success of the Harrier, the ski-jump became a proven method for launching aircraft from ships without the complexity and expense of a catapult. Furthermore, later models of ski-jump feature refinements over the original design; it was determined that even relatively minor ruts or imperfections on an otherwise absolutely smooth surface were sufficient to precipitate cracking in an aircraft's landing gear. It is for this reason that the Royal Navy implemented more stringent design tolerances in the ramp specifications of the s. It is possible for a modern ski-jump to be built as a single removable structure placed upon the forward flight deck, rather than being fully integrated into a ship's bow.
Ski-jumps were added not only to aircraft carriers, but also to numerous amphibious assault ships and landing helicopter docks to better facilitate the operation of STOVL aircraft. The Australian and Spanish Juan Carlos-class landing helicopter docks (LHDs) have also been outfitted with ski-jumps to facilitate potential STOVL operations. Somewhat unusually, the United States Navy has not ever used ski ramps onboard its amphibious assault ships, despite them being heavily used by VSTOL aircraft such as multiple models of the Harrier jump jet and Lockheed Martin F-35B Lightning IIs; this has been stated to be due to their operations involving combined use of helicopters and boats.
By the start of the twenty-first century, the British, Chinese, Indian, Italian, Russian, Spanish, and Thai navies all possessed aircraft carriers equipped with ski-ramps. Following the retirement of the Brazil aircraft carrier during 2017, the United States and France were the only two countries that still operated aircraft carriers with catapults.
A MiG-29 launching over the ski-jump ramp on a can take off at a speed of about , instead of the usual (depending on many factors such as gross weight).Gordon 2006, p. 84.
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.
Ski-jump ramp takeoffs are considered to be safer than takeoffs over a flat-top carrier. When a Harrier launches from an American landing helicopter assault (LHA), it would finish its takeoff roll and begin flight at 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 above the water.
In 1988, a detachment of US Marine Corps McDonnell Douglas AV-8B Harrier IIs conducted a series of flight tests on the . It was found that takeoff conditions which would use all of a 's flight deck would only take with the Asturias's 12° ski-jump ramp; this dramatic improvement for a ship without catapults was described as "nothing short of amazing."
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