 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Assisted take-off" – news ·newspapers ·books ·scholar ·JSTOR(December 2007) (Learn how and when to remove this message) |
Inaviation,assisted takeoff is any external system for helpingaircraft (more specificallyfixed-wing aircraft) to become airborne (take off) forflight, as opposed to doing soconventionally using strictly their ownpropulsion. The reasons assistance might be needed include: the combinedweight of the aircraft and itspayloads exceeding themaximum takeoff weight; insufficientenginepower by design or malfunction, insufficient availablerunway length (such as on theflight deck of anaviation warship), or a combination of the above. Assisted takeoff is also required forgliders, which do not have any engine and thus are unable to propel themselves for takeoffs.
The most well-known type of assisted takeoff is via anaircraft catapult, which is used on modernaircraft carriers to launch fixed-wingcarrier-based aircraft from the shortrunway distances available on theflight deck. The catapult is built into the flight deck as a slot track, with asliding piece known as ashuttle, which protrudes above the deck. The aircraft is attached to the shuttle using a tow bar or launch bar mounted to the nose landing gear (an older system used a steel cable called a catapult bridle; the forward ramps on older carrier bows were used to catch these cables), and is flung off the deck at about 15 knots above minimum flying speed, achieved by the catapult in a four-second run.
The current mainstay catapult system issteam catapult, first invented by theBritish in the 1950s. It is essentially asingle-actionsteam engine, in which the shuttle is attached to apiston andpropelled down the track under the high pressure of thesuperheated steam within thecylinder under the catapult track. Thesteam can be generated bysuperheaterboilers or, in the case ofnuclear-poweredsupercarriers, by heating fromnuclear reactors. TheUnited States Navy andFrench Navy are the only current users of steam catapults, with the latter using the same C-13-3 catapult imported from theUnited States.
Into the21st century, the United States have planned to replace steam catapults with anelectromagnetic catapult system based on the working principles oflinear induction motor, which instead ofpneumatics usesLaplace force generated by the trackarmatures to propel the catapult shuttle, allowing shorter launch intervals, more variable, precise control over the launch power, lessimpulsive wear on the aircraft's airframe, and reduced maintenance againstrusting andleakages. The new EM catapult system, called theElectromagnetic Aircraft Launch System (EMALS), was developed byGeneral Atomics based on anAC motor-flywheel construct and is currently installed on thelead ship of theFord-class aircraft carriers, theUSS Gerald R. Ford (CVN-78). A separate EM system based on aDC motor-supercapacitor construct has also been developed inChina jointly by theChina Shipbuilding Industry Corporation and theChinese Academy of Engineering, and is currently installed on the newly commissionedPeople's Liberation Army Navy aircraft carrierFujian (18), as well as the currentlyfitting-outType 076 amphibious assault shipSichuan (51) and the upcomingType 004 aircraft carriers.
In jet-assisted takeoff (JATO) and the similar rocket-assisted takeoff (RATO) systems, additional engines are mounted on theairframe which are used only during takeoff. After that these attached engines are usually jettisoned, or else they just become parasitic deadweight that adddrag to the aircraft. However, some aircraft such as theAvro Shackleton MR.3 Phase 2, had permanently attached JATO engines. The four J-47 turbojet engines on the B-36 were not considered JATO systems; they were an integral part of the aircraft's propulsion, and were used during takeoff, climb, and cruise at altitude. TheHercules LC-130 can be equipped with a JATO rocket system to shorten takeoff as used in the LC-130 Skibird for polar missions.[1]
During WW2 the GermanArado Ar 234 and theMesserschmitt Me 323 "Gigant" used rocket units beneath the wings for assisted takeoff. Such systems were popular during the 1950s, when heavy bombers started to require two or more miles of runway to take off fully laden. This was exacerbated by the relatively low power available fromjet engines at the time—for example theBoeing B-52 Stratofortress required eightturbojet engines to yield the required performance, and still needed RATO for very heavy payloads (a proposed update of the B-52 replaces these with half the number of much more powerful engines). In aCold War context, RATO and JATO bottles were seen as a way for fighter aircraft to use the undamaged sections of runways of airfields which had been attacked.
Glider aircraft which do not have an engine also require an assisted takeoff. Apart from self launching gliders,EASA recognizes four other launch methods:winch launches,aerotows,bungee launches andcar tows.[2]
Early pioneers in powered and unpowered flight used gravity to accelerate their aircraft to a speed which allowed its wings to generate enough lift to achieve independent flight. These included attempts to achieve flight from towers, city walls and cliffs. Generally more successful were attempts in which speed was built up by accelerating down hills and mountain slopes, sometimes on rails or ramps.
Another form of gravity assistance is when an aircraft is released from a largermother ship or mother craft. This may be because the daughter craft is incapable of taking off normally e.g. the atmospheric flight tests of theSpace Shuttle.
Usually the rationale for such a system is to free the daughter craft from the need to climb to its release height under its own power. This allows the daughter craft to be designed with fewer weight and aerodynamic restrictions allowing for exotic configurations to be used or tested, for example the recentSpaceShipOne, and previously theBell X-1 and otherX-planes.
In the interwar years, in order to achieve long ranges with the technology of the time, trials were undertaken with floatplanes piggy-backed atop flying boats. With the floatplane carried part of the way to its destination and freed from having to use any of its own fuel in the initial climb, these combinations could deliver light but time-critical cargos faster and farther than a single individual aircraft (for example theShort Mayo Composite).
Hot air balloons have acted as "motherships" tohang gliders andpara gliders in altitude and distance record attempts.