Ski-jump (aviation)

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.

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 (47) to help heavily laden Fairey Barracudas attack the German battleship 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 VSTOL fighter, then added a ramp to its next generation of aircraft carrier, the Invincible-class aircraft carrier.

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.

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


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. During World War II, carrier aircraft became so heavy that assisted take-off became desirable. Deck catapults were used to accelerate aircraft to takeoff speed, especially when launching heavy aircraft or when it was inconvenient to change course. An early use of the ski-jump occurred in 1944, when the British aircraft carrier HMS Furious (47) launched a strike against the German battleship GERMAN BATTLESHIP Tirpitz. A relatively crude ski-jump ramp was temporarily installed on the end of the flight deck, which helped the heavily bomb-laden Fairey Barracudas take off.

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.

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 Invincible-class aircraft carrier aircraft carriers were constructed with ski-jumps, greatly shortening the distance required for Harriers to take-off even when burdened with a useful payload. 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, HMS Invincible (R05), while she was fitting out in Barrow; it was set at a conservative 7º angle. On 30 October 1980, test pilot Lt Cdr David Poole conducted the first ski-jump assisted Harrier take-off at sea. HMS Illustrious (R06) was also initially fitted with a 7º ramp; however, HMS Ark Royal (R07), was built with a 12º ramp from the outset, which was determined to be the optimum angle. The earlier two ships were subsequently retrofitted with 12º 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 Queen Elizabeth-class aircraft carriers. 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 Brazilian aircraft carrier BRAZILIAN AIRCRAFT CARRIER São Paulo during 2017, the United States and France were the only two countries that still operated aircraft carriers with catapults.

STOBAR
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 take off at a speed of about 70 kn, instead of the usual 140 kn (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
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 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 60 ft 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 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 SPANISH AIRCRAFT CARRIER Príncipe de Asturias. It was found that takeoff conditions which would use all 750 ft of a Tarawa-class amphibious assault ship's flight deck would only take 300 ft with the Asturias's 12° ski-jump ramp; this dramatic improvement for a ship without catapults was described as "nothing short of amazing."

Land operations
During the early 1990s, the United States Air Force examined the use of ski-jumps on land to enable short-field takeoffs; the approach was viewed as "a possible solution to the runway denial problem in Europe" during the Cold War. It was determined that, when using a ski-jump with a nine degree angle of departure, the distance required for an McDonnell Douglas F/A-18 Hornet to takeoff would be reduced by roughly half.

Ships/classes with ski-jumps

 * Anadolu (Turkey)
 * Canberra-class landing helicopter docks (Australia)
 * Cavour (Italy)
 * Chakri Naruebet (Thailand)
 * Giuseppe Garibaldi (Italy)
 * Hermes/Viraat (UK/India)
 * Invincible-class aircraft carriers (UK)
 * Juan Carlos I (Spain)
 * Kuznetsov-class aircraft carriers (Soviet Union/Russia/China)
 * Príncipe de Asturias (Spain)
 * Queen Elizabeth-class aircraft carrier (UK)
 * Liaoning (China)
 * Shandong (China)
 * Trieste (Italy)
 * Vikramaditya (India)
 * Vikrant (R11) (India)