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Mechanical Aircraft Carrier.docx (Size: 25.82 KB / Downloads: 109)
An aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers thus allow a naval force to project and implimentation air power worldwide without having to depend on local bases for staging aircraft operations. They have evolved from wooden vessels, used to deploy balloons, into nuclear powered warships that carry dozens of fixed and rotary wing aircraft.
Types of aircraft carriers based on configuration
There are three main configurations of aircraft carrier in service in the world's navies:
• Catapult Assisted Take-Off But Arrested Recovery (CATOBAR)
• Short Take-Off But Arrested Recovery (STOBAR)
• Short Take-Off Vertical Landing (STOVL)
CATOBAR (Catapult Assisted Take Off But Arrested Recovery) is a system used for the launch and recovery of aircraft from the deck of an aircraft carrier. Under this technique, aircraft launch using a catapult assisted take off and land on the ship (the recovery phase) using arrestor wires. Although this system is more costly than alternative methods, it provides greater flexibility in carrier operations, since it allows the vessel to support conventional aircraft. Alternate methods of launch and recovery can only use aircraft with STOVL or STOBAR capability. Only three countries still operate carriers that use the CATOBAR system; the U.S. Nimitz class, and USS Enterprise (CVN-65), France's Charles De Gaulle, and Brazil's NAe São Paulo. The Future French aircraft carrier is planned to be built as CATOBAR. In order to save money, the British Queen Elizabeth class aircraft carriers will be built as STOVL carriers but will be able to be converted to CATOBAR or STOBAR if desired at a later date. India's second aircraft carrier of Vikrant class is planned to be of 65,000 tons and to utilize steam catapults.
STOBAR (Short Take Off But Arrested Recovery) is a system used for the launch and recovery of aircraft from the deck of an aircraft carrier, combining elements of both STOVL and CATOBAR. Aircraft launch under their own power using a ski-jump to assist take-off (rather than using a catapult like most carriers). However, these are conventional, rather than STOVL aircraft, and thus require arrestor wires to land on the ship. The Russian Navy aircraft carrier Admiral Kuznetsov is the only current example of a STOBAR carrier, another will be the Indian Vikramaditya and the future Vikrant class aircraft carrier. The STOBAR system is simpler to build than CATOBAR — but it works only with light, and lightly armed, fighter aircraft that have a high thrust to weight ratio.
When the Eurofighter was proposed for the "Future Carrier Borne Aircraft" it was envisaged that it would operate in a STOBAR configuration. The FCBA is to be deployed on the British Royal Navy's next generation carriers, CVF. Instead, the Lockheed Martin Lightning II, operating in a STOVL configuration, will be the FCBA.
STOVL is an acronym for Short Take Off and Vertical Landing.
This is the ability of some aircraft to take off from a short runway or take off vertically if it does not have a very heavy payload and land vertically (i.e. with no runway). The formal NATO definition (since 1991) is:
A Short Take-Off and Vertical Landing aircraft is a fixed-wing aircraft capable of clearing a 15 m (50 ft) obstacle within 450 m (1500 ft) of commencing take-off run, and capable of landing vertically.
This is often accomplished on aircraft carriers through the use of "ski-jump" runways, instead of the conventional catapult system. STOVL use tends to allow aircraft to carry a larger payload as compared to during VTOL use, while still only requiring a short runway. The most famous example is probably the Hawker Siddeley Harrier Jump Jet, which though technically a VTOL aircraft, is operationally a STOVL aircraft due to the extra weight it carries at take off for fuel and armaments. The same is true of the F-35B Lightning II, which demonstrated VTOL capability in test flights but is operationally STOVL.
As "runways at sea," modern aircraft carriers have a flat-top deck design that serves as a flight deck for take-off and landing of aircraft. Aircraft take off to the front, into the wind, and land from the rear. Carriers steam at speed, for example up to 35 knots (65 km/h), into the wind during take-off in order to increase the apparent wind speed, thereby reducing the speed of the aircraft relative to the ship. On some ships, a steam-powered catapult is used to propel the aircraft forward assisting the power of its engines and allowing it to take off in a shorter distance than would otherwise be required, even with the headwind effect of the ship's course. On other carriers, aircraft do not require assistance for take off—the requirement for assistance relates to aircraft design and performance. Conversely, when landing on a carrier, conventional aircraft rely upon a tailhook that catches on arrestor wires stretched across the deck to bring them to a stop in a shorter distance than normal. Other aircraft—helicopters and V/STOL (Vertical/Short Take-Off and Landing) designs—utilize their hover capability to land vertically and so require no assistance in speed reduction upon landing.
Conventional ("tailhook") aircraft rely upon a landing signal officer (LSO) to control the plane's landing approach, visually gauging altitude, attitude, and speed, and transmitting that data to the pilot. Before the angled deck emerged in the 1950s, LSOs used colored paddles to signal corrections to the pilot. From the late 1950s onward, visual landing aids such as mirrors provided information on proper glide slope, but LSOs still transmit voice calls to landing pilots by radio.
Since the early 1950s, it has been common to direct the landing recovery area off to port at an angle to the line of the ship. The primary function of the angled deck landing area is to allow aircraft who miss the arresting wires, referred to as a "bolter," to become airborne again without the risk of hitting aircraft parked on the forward parts of the deck. The angled deck also allows launching of aircraft at the same time as others land.
The above deck areas of the warship (the bridge, flight control tower, and so on) are concentrated to the starboard side of the deck in a relatively small area called an "island." Very few carriers have been designed or built without an island and such a configuration has not been seen in a fleet-sized carrier. The "flush deck" configuration proved to have very significant drawbacks, complicating navigation, air traffic control and numerous other factors.
A more recent configuration, used by the British Royal Navy, has a "ski-jump" ramp at the forward end of the flight deck. This was developed to help launch VTOL (or STOVL) aircraft (aircraft that are able to take off and land with little or no forward movement) such as the Sea Harrier. Although the aircraft are capable of flying vertically off the deck, using the ramp is more fuel efficient. As catapults and arrestor cables are unnecessary, carriers with this arrangement reduce weight, complexity, and space needed for equipment. The disadvantage of the ski jump—and hence, the reason this configuration has not appeared on American supercarriers—is the penalty that it exacts on aircraft size, payload, and fuel load (and hence, range): Large, slow planes such as the E-2 Hawkeye and heavily-laden strike fighters like the F/A-18E/F Super Hornet cannot use a ski jump because their high weight requires either a longer takeoff roll than is possible on a carrier deck, or catapult assistance.
Early flight decks: The first flight decks were inclined wooden ramps built over the forecastle of naval warships. Eugene Ely made the first fixed-wing aircraft take-off from a warship from USS Birmingham on 14 November 1910. Two months later, on 18 January 1911, Ely landed his Curtiss pusher plane on a platform on Pennsylvania anchored in San Francisco Bay, using the first ever tailhook system, designed and built by circus performer & aviator Hugh Robinson. Ely told a reporter: "It was easy enough. I think the trick could be successfully turned nine times out of ten." On 4 May 1912, Commander Charles Samson became the first man to take off from a ship which was underway when he flew his Short S27 off of HMS Hibernia, which was steaming at 10.5 kn (12.1 mph; 19.4 km/h). Because the take-off speed of early aircraft was so low, it was possible for an aircraft to make a very short take off when the launching ship was steaming into the wind. Later, removable "flying-off platforms" appeared on the gun turrets of battleships and battle cruisers, allowing aircraft to be flown off for scouting purposes, although there was no chance of recovery.
On 2 August 1917, while performing trials, Squadron Commander Edwin Dunning landed a Sopwith Pup successfully on board the flying-off platform of HMS Furious, becoming the first person to land an aircraft on a moving ship. However, on his third attempt, a tire burst as he attempted to land, causing the aircraft to go over the side, killing him; thus Dunning also has the dubious distinction of being the first person to die in an aircraft carrier landing accident. The landing arrangements on Furious were highly unsatisfactory. In order to land, aircraft had to manoeuvre around the superstructure. Furious was therefore returned to dockyard hands to have a 300 ft (91 m) deck added aft for landing, on top of a new hangar. The central superstructure remained, however, and turbulence caused by it badly affected the landing deck.
Full length decks: The first aircraft carrier that began to show the configuration of the modern vessel was the converted liner HMS Argus, which had a large flat wooden deck added over the entire length of the hull, giving a combined landing and take-off deck unobstructed by superstructure turbulence. Because of her unobstructed flight deck, Argus had no fixed conning tower and no funnel. Rather, exhaust gases were trunked down the side of the ship and ejected under the fantail of the flight deck (which, despite arrangements to disperse the gases, gave an unwelcome "lift" to aircraft immediately prior to landing). The lack of a command position and funnel was unsatisfactory, and Argus was used to experiment with various ideas to remedy the solution. A photograph in 1917 shows her with a canvas mock-up of a starboard "island" superstructure and funnel. This was placed on the starboard side because the rotary engines of some early aircraft created torque which pulled the nose left, meaning an aircraft naturally yawed to port on take-off; therefore, it was desirable that they turned away from the fixed superstructure. This became the typical aircraft carrier arrangement and was used in the next British carriers, HMS Hermes and Eagle.
After World War I, battle cruisers that otherwise would have had to have been discarded under the Washington Naval Treaty - such as the British Furious and Glorious-class and the American USS Lexington and Saratoga- were converted to carriers along the above lines. Being large and fast they were perfectly suited to this role; the heavy armoring and scantlings and low speed of the converted battleship Eagle served to be something of a handicap in practice. Because the military effectiveness of aircraft carriers was then unknown, early ships were typically equipped with cruiser-caliber guns to aid in their defense if surprised by enemy warships. These guns were generally removed during World War II and replaced with anti-aircraft guns, as carrier doctrine developed the "task force" (later called "battle group") model, where the carrier's defense against surface ships would be a combination of escorting warships and its own aircraft.
In ships of this configuration, the hangar deck was the strength deck, and an integral part of the hull, and the hangar and wooden flight deck were considered to be part of the superstructure. Such ships were still being built into the late '40s, classic examples being the U.S. Navy's Essex and Ticonderoga-class carriers. However, in 1936, the Royal Navy began construction of the Illustrious-class. In these ships, the flight deck was now the strength deck, an integral part of the hull, and was heavily armored to protect the ship and her air complement. Although the armored carrier concept in this form remained something of a dead end, the flight deck as the strength deck was adopted for later construction. This was necessitated by the ever-increasing size of the ships, from the 13,000 ton USS Langley in 1922 to over 100,000 tons in the latest Nimitz-class carriers.
Landing on flight decks
Landing arrangements were originally primitive, with aircraft simply being "caught" by a team of deck-hands who would run out from the wings of the flight deck and grab a part of the aircraft to slow it down. This dangerous procedure was only possible with early aircraft of low weight and landing speed. Arrangements of nets served to catch the aircraft should the latter fail, although this was likely to cause structural damage.
Landing larger and faster aircraft on a flight deck was made possible through the use of arresting cables installed on the flight deck and a tailhook installed on the aircraft. Early carriers had a very large number of arrestor cables or "wires". Current U.S. Navy carriers have three or four steel cables stretched across the deck at 20 ft (6.1 m) intervals which bring a plane, traveling at 150 mph (240 km/h), to a complete stop in about 320 ft (98 m). The cables are set to stop each aircraft at the same place on the deck, regardless of the size or weight of the plane. During World War II, large net barriers would be erected across the flight deck in order that aircraft could be parked on the forward part of the deck and recovered on the after part. This allowed increased complements, but resulted in lengthened turn-around times as aircraft were shuffled around the carrier to allow take-off or landing operations.
A barricade is an emergency system used if a normal arrestment cannot be made. Barricade webbing engages the wings of the landing aircraft, and momentum is transferred to the arresting engine.
Angled flight deck
The angled flight deck was invented by Royal Navy Captain (later Rear Admiral) Dennis Campbell. With this type of deck, (also referred to as a "skewed deck" or "canted deck" or the "angle"), the aft part of the deck is widened and a separate runway is positioned at an angle from the centerline. The angled flight deck was designed with the higher landing speeds of jet aircraft in mind, which would have required the entire length of a centerline flight deck to stop. The design also allowed for concurrent launch and recovery operations, and allowed aircraft failing to connect with the arrestor cables to abort the landing, accelerate, and relaunch (or "bolter") without risk to other parked or launching aircraft. The redesign allowed for several other design and operational modifications, including the mounting of a larger island (improving both ship-handling and flight control), drastically simplified aircraft recovery and deck movement (aircraft now launched from the bow and re-embarked on the angle, leaving a large open area amidships for arming and fuelling), and damage control. Because of its utility in flight operations, the angled deck is now a defining feature of STOBAR and CATOBAR equipped aircraft carriers.
The angled flight deck was first tested on HMS Triumph, by painting angled deck markings onto the centerline of the flight deck for touch and go landings. This was also tested on the USS Midway the same year. It should be noted that in both tests, the arresting gear and barriers remained oriented to the original axis deck. From September-December 1952, the USS Antietam had a rudimentary sponson installed for true angle deck tests, allowing for full arrested landings, which proved during trials to be superior. In 1953, Antietam trained with both US and British naval units, proving the worth of the angle deck concept. HMS Centaur was modified with overhanging angled flight deck in 1954. The U.S. Navy installed the decks as part of the SCB-125 upgrade for the Essex-class and SCB-110/110A for the Midway-class. In February 1955, HMS Ark Royal became the first carrier to be constructed and launched with the deck, followed in the same year by the lead ships of the British Majestic-class (HMAS Melbourne) and the American Forrestal-class (USS Forrestal).
Another British innovation is the ski-jump ramp, which came about as a means of improving take off for the VSTOL BAE Sea Harrier "jump-jet" on the small Invincible class aircraft carriers. They are most common on aircraft carriers supporting STOVL aircraft such as the Harrier, but the Russians also used them with conventional MiG-29s.
The ski jump is a ramp which is curved upwards at its forward end. For STOVL aircraft the aircraft starts by making a conventional rolling takeoff with the jet exhausts set to provide maximum forward thrust. As the plane nears the end of the ramp (the ski jump portion) the jet exhausts are rotated to provide upward lift as well as forward thrust. Rolling over the ski ramp launches the plane both upwards and forwards. As the plane leaves the ski jump ramp it continues to accelerate horizontally until the wings can provide the needed lift.
For conventional aircraft such as the MiG-29 the aircraft just rolls down the runway in the obvious manner. Again, rolling over the ski ramp launches the plane both upwards and forwards.
Such takeoffs allow a larger takeoff weight than a straight vertical launch because the wings provide some lift even at low speeds, and the ski jump ramp provides a vertical impetus when most needed, right at takeoff at the slowest takeoff speed.
These takeoffs use less runway than a takeoff over a flat surface because the plane takes off at a lower speed, using both the ski jump ramp's vertical impetus and the deflected jet engines to generate lift.
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.
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 Principe de Asturias, but 750 ft (230 m) without the ski jump ramp.
For a MiG-29 launching over the ski jump ramp on the Tbilisi, takeoff speed is reduced from about 140 kn (160 mph; 260 km/h) to about 70 kn (81 mph; 130 km/h) (depending on many factors such a gross weight).
Carriers using STOVL aircraft and a ski jump ramp do not need catapults nor arresting gear.
With the exception of the United States and France, every navy in the world that operates STOVL naval aircraft uses ski jump ramps
Take off from deck
An aircraft catapult is a device used to launch aircraft from ships—in particular aircraft carriers—as a form of assisted take off. It consists of a track built into the flight deck, below which is a large piston or shuttle that is attached through the track to the nose gear of the aircraft.
Older aircraft did not have a launch bar integrated in the nose gear; instead, a wire rope called a catapult bridle was attached to the aircraft and the catapult shuttle. The ramps at the catapult ends on older carriers were used to catch these ropes so they could be reused; bridles have not been used on aircraft since the end of the Cold War and all carriers commissioned since then have not had the ramps. The last carrier commissioned with a bridle catcher was USS Carl Vinson; starting with USS Theodore Roosevelt the ramps were deleted. During Refueling and Comprehensive Overhaul refits in the late 1990s–early 2000s, the bridle catchers were removed from the first three Nimitz-class aircraft carriers. USS Enterprise is the last operational carrier with the ramps still attached.
At launch, a release bar holds the aircraft in place as steam pressure builds up, then breaks (or "releases"; older models used a pin that sheared), freeing the piston to pull the aircraft along the deck at high speed. Within about two to four seconds, aircraft velocity due to the action of the catapult plus apparent wind speed (ship's speed plus or minus "natural" wind) will be sufficient to allow an aircraft to fly away, even after losing one engine.
Although German Lufthansa had used seaplane tenders that used engine steam to launch their Dornier transatlantic mailplanes as early as 1933, the modern steam catapult was a British invention. The use of steam to launch aircraft was suggested by Commander Colin C. Mitchell RNVR, and trials on HMS Perseus from 1950 showed its effectiveness. Navies introduced steam catapults, capable of launching the heavier jet fighters, in the mid-1950s. Powder-driven catapults were also contemplated, and would have been powerful enough, but would also have introduced far greater stresses on the airframes and may have been unsuitable for long use.
Nations that have retained large aircraft carriers and high performance CATOBAR (Catapult Assisted Take Off But Arrested Recovery) (the United States Navy, Brazilian Navy, and French Navy) are still, out of necessity, using catapults. Other navies operate STOVL aircraft, such the Sea Harrier or AV-8B Harrier II, which do not require catapult assistance, from smaller and less costly ships. The Russian Su-33 "Flanker-D" can take off from aircraft carriers without a catapult, albeit at a reduced fuel and armament load. U.S. Navy tactical aircraft use catapults to launch with a heavier warload than would otherwise be possible. Larger planes, such as the E-2 Hawkeye and S-3 Viking, require a catapult shot, inasmuch as their thrust-to-weight ratio is too low for a conventional rolling takeoff on a carrier deck.
The commonly-used steam catapult relies on the availability of large quantities of high-pressure steam- found in the vast majority of 20th century capital ships. The steam charges a steam accumulator so that it may be released faster than it can be produced by the ship.
The steam catapult consists of two slotted cylinders similar in principle to those used by the Clegg & Samuda atmospheric railway. The cylinders—typically 18 inches in diameter—contain free pistons connected to a shuttle which protrudes through a slot in the flight deck. The nosewheel of the aircraft to be launched is attached to the shuttle by a launch bar.
On completion of the launch the piston is traveling at high speed and would cause damage if not stopped in a controlled fashion. This is done by a water brake, which is a horizontal dashpot into which sea water is pumped with a swirling action as fast as it can flow out of the open end. The combination of the slight compressibility of the aerated water, the restriction as the water is expelled from the dashpot and the force produced by the expelled water hitting the front of the piston assembly itself serves to absorb the energy of the piston without damage. At that point a return mechanism readies the piston and shuttle for the next launch.
Electromagnetic Aircraft Launch System
Electromagnetic Aircraft Launch System (EMALS) is a system under development by the United States Navy to launch aircraft from carriers using a linear motor drive instead of steam pistons used in conventional aircraft catapults. This technology reduces stress on airframes because they can be accelerated more gradually to takeoff speed than steam-powered catapults. EMALS also uses less fresh water, reducing the need for energy-intensive desalinization.
The EMALS is being developed by General Atomics for the U.S. Navy's newest Ford-class aircraft carriers. It was also considered for the Royal Navy's new Queen Elizabeth-class aircraft carriers (CVF), but the Ministry of Defense and the Royal Navy opted for a Vertical/Short Takeoff and Landing (VSTOL) configuration instead with the option of installing steam generators for steam powered catapults, or according to some sources EMALS, at a later date. In August 2009, speculation mounted that the UK may drop the STOVL F-35B for the CTOL F-35C model, which would mean the carriers being built to operate conventional (CV) take off and landing aircraft utilizing the US-designed non-steam EMALS catapults. In June 2010, it was reported that the land-based prototype of the system had passed initial tests with the first aircraft launch from the system expected by the end of 2010.
The EMALS uses a linear induction motor (LIM), which uses electric currents to generate magnetic fields that propel a carriage down a track to launch the aircraft. The EMALS consists of four main elements
Compared to steam catapults, EMALS weighs less, occupies less space, requires less maintenance and manpower, is more reliable, and uses less energy. Steam catapults, which use about 614 kilograms of steam per launch, have extensive mechanical, pneumatic, and hydraulic subsystems. EMALS uses no steam, which makes it suitable for the Navy's planned all-electric ships. The EMALS could be more easily incorporated into a ramp, which would reduce the aircraft’s takeoff speed and consequently the launch energy.
Compared to steam catapults, EMALS can control the launch performance with greater precision, allowing it to launch more kinds of aircraft, from heavy fighter jets to light unmanned aircraft. EMALS can also deliver 122 megajoules of energy, 29 percent more than steam's approximately 95 megajoules. The EMALS will be more efficient than the 5-percent efficient steam catapults.
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