Aircraft ski-jump

A view of the Russian Admiral Kuznetsov showing a prominent ski-jump ramp

A ski-jump is a curved ramp that allows aircraft to take off from a runway that is shorter than the aircraft's required takeoff roll. By launching the aircraft at a slight upward angle, the ski-jump gives the aircraft additional time to build up velocity and reach stable flight. Ski-jumps are commonly used to launch airplanes from aircraft carriers that lack aircraft catapults.

Principles

A conventional aircraft must build up forward speed during a takeoff roll. As the forward velocity increases, the wings produce lift that counteracts the force of gravity. At a certain speed, when lift exceeds aircraft weight, the aircraft will have reached a regimen of stable flight. A long runway is required to allow the plane to reach flight speed using only its own engines for power.

On an aircraft carrier, some heavy aircraft cannot attain stable flight before reaching the end of the flight deck during an unassisted takeoff. The problem is exacerbated by the fact that the full length of the deck is not available for the takeoff roll. If lift is less than gravity, the aircraft will lose altitude after the wheels leave the flight deck and possibly fall into the sea. An aircraft catapult can be used to accelerate the aircraft to takeoff speed before reaching the end of the deck. A NACA study from 1952 proposed the use of a ski-jump after the aircraft catapult to assist aircraft with low lift-curve slopes.[1] On aircraft carriers without catapults, aircraft must take off using under their own power.

A ski-jump at the end of the flight deck will redirect the aircraft to a slightly positive angle from the horizontal. As a result, part of its forward velocity is converted into upward motion, and the aircraft is launched from the deck with a positive rate of climb. Although the aircraft still has insufficient lift to counteract gravity, this will only reduce its rate of climb. Meanwhile, the ski-jump launch has given the aircraft additional time to accelerate.[2] By the time its upward velocity has decayed to zero, the aircraft will be going fast enough for its wings to produce enough lift to balance out gravity. 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.[1]

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.[3]

Aircraft carrier operation

STOBAR

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

On STOBAR aircraft carriers, 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 drops 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.[4]

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).[5]

With the exception of the United States, France and Brazil, every navy in the world that currently operates naval fixed-wing aircraft from carriers uses ski-jump ramps.[6]

STOVL

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

STOVL aircraft 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. The use of ski-jumps for STOVL aircraft was suggested by Lt Cdr D.R. Taylor, RN, in 1973,[2] and initial testing was carried out at RAE Bedford.

Ski-jump ramp takeoffs are considered safer than takeoffs over a flat-top carrier. When a Harrier launches from an American LHA (Landing Helicopter Assault) it might 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, the plane 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.[6]

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 a 12° ski-jump ramp designed as on the Spanish aircraft carrier Principe de Asturias, but 750 ft (230 m) without the ski-jump ramp.[6]

The United States is the only country which currently operates STOVL aircraft from carriers without ski-jump ramp.[6]

Land operation

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.[7]

References

  1. 1 2 Reed III, Wilmer H. (1952-11-05). "An Analysis of the Effect of a Curved Ramp on the Take-off Performance of Catapult-Launched Airplanes" (PDF). National Advisory Committee on Aeronautics.
  2. 1 2 Fozard, John (4 December 1976). ""Ski Jump" Harrier". Flight. 110 (3534): 1630–1635. Retrieved 9 November 2015.
  3. ↑ "The Problem with Ski-Jump Aircraft Carriers". Strike Fighter Consulting Inc. October 4, 2013.
  4. ↑ Gordon, Yefim (2006). Sukhoi Su-27 Flanker - WarbirdTech Vol 42. Specialty Press. p. 69. ISBN 9781580071963.
  5. ↑ Gordon, Yefim (2006), Mikoyan MiG-29, Hinckley, UK: Midland Publications, p. 84, ISBN 9781857802313
  6. 1 2 3 4 History.navy.mil
  7. ↑ Turner, Elijah W. (May 1991). "Aircraft Operations from Runways with Inclined Ramps (Ski-jump)" (PDF).
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