Takeoff
Takeoff is the phase of flight in which an aerospace vehicle goes from the ground to flying in the air.
For aircraft that take off horizontally, this usually involves starting with a transition from moving along the ground (taxiing) on a runway. For balloons, helicopters and some specialized fixed-wing aircraft (VTOL aircraft such as the Harrier), no runway is needed. Takeoff is the opposite of landing.
Horizontal takeoff
Power settings
For light aircraft, usually full power is used during takeoff. Large transport category (airliner) aircraft may use a reduced power for takeoff, where less than full power is applied in order to prolong engine life, reduce maintenance costs and reduce noise emissions. In some emergency cases, the power used can then be increased to increase the aircraft's performance. Before takeoff, the engines, particularly piston engines, are routinely run up at high power to check for engine-related problems. The aircraft is permitted to accelerate to rotation speed (often referred to as Vr). The term rotation is used because the aircraft pivots around the axis of its main landing gear while still on the ground, usually because of manipulation of the flight controls to make this change in aircraft attitude.
The nose is raised to a nominal 5°–15° nose up pitch attitude to increase lift from the wings and effect liftoff. For most aircraft, attempting a takeoff without a pitch-up would require cruise speeds while still on the runway.
Fixed-wing aircraft designed for high-speed operation (such as commercial jet aircraft) have difficulty generating enough lift at the low speeds encountered during takeoff. These are therefore fitted with high-lift devices, often including slats and usually flaps, which increase the camber and often area of the wing, making it more effective at low speed, thus creating more lift. These are deployed from the wing before takeoff, and retracted during the climb. They can also be deployed at other times, such as before landing.
The speeds needed for takeoff are relative to the motion of the air (indicated airspeed). A headwind will reduce the ground speed needed for takeoff, as there is a greater flow of air over the wings. Typical takeoff air speeds for jetliners are in the 130–155 knot range (150–180 mph, 240–285 km/h). Light aircraft, such as a Cessna 150, take off at around 55 knots (63 mph, 100 km/h). Ultralights have even lower takeoff speeds. For a given aircraft, the takeoff speed is usually dependant on the aircraft weight; the heavier the weight, the greater the speed needed[citation needed]. Some aircraft are specifically designed for short takeoff and landing (STOL), which they achieve by becoming airborne at very low speeds.
Speed required
The takeoff speed required varies with air density, aircraft gross weight, lift coefficient, and aircraft configuration (flap or slat position, as applicable). Air density is affected by factors such as field elevation and air temperature. This relationship between temperature, altitude, and air density can be expressed as a density altitude, or the altitude in the International Standard Atmosphere at which the air density would be equal to the actual air density.
Operations with transport category aircraft employ the concept of the takeoff V-Speeds, V1, VR and V2. These speeds are determined not only by the above factors affecting takeoff performance, but also by the length and slope of the runway and any peculiar conditions, such as obstacles off the end of the runway. Below V1, in case of critical failures, the takeoff should be aborted; above V1 the pilot continues the takeoff and returns for landing. After the co-pilot calls V1, he/she will call VR or "rotate," marking speed at which to rotate the aircraft. The VR for transport category aircraft is calculated such as to allow the aircraft to reach the regulatory screen height at V2 with one engine failed. Then, V2 (the safe takeoff speed) is called. This speed must be maintained after an engine failure to meet performance targets for rate of climb and angle of climb.
In a single-engine or light twin-engine aircraft, the pilot calculates the length of runway required to take off and clear any obstacles, to ensure sufficient runway to use for takeoff. A safety margin can be added to provide the option to stop on the runway in case of a rejected takeoff. In most such aircraft, any engine failure results in a rejected takeoff as a matter of course, since even overrunning the end of the runway is preferable to lifting off with insufficient power to maintain flight.
If an obstacle needs to be cleared, the pilot climbs at the speed for maximum climb angle (Vx), which results in the greatest altitude gain per unit of horizontal distance travelled. If no obstacle needs to be cleared, or after an obstacle is cleared, the pilot can accelerate to the best rate of climb speed (Vy), where the aircraft will gain the most altitude in the least amount of time. Generally speaking, Vx is a lower speed than Vy, and requires a higher pitch attitude to achieve.
Assisted takeoff
Assisted takeoff is any system for helping aircraft into the air (as opposed to strictly under its own power). The reason it might be needed is due to the aircraft's weight exceeding the normal maximum takeoff weight, insufficient power, or the available runway length may be insufficient, or a combination of all three factors. Assisted takeoff is also required for gliders, which do not have an engine and so are unable to take off by themselves.
Vertical takeoff
Vertical takeoff refers to aircraft or rockets that take off in a vertical trajectory. Vertical takeoff eliminates the need for airfields. Most vertical take off aircraft are also able to land vertically, but there were certain rocket-powered aircraft of the Luftwaffe that only took off vertically, landing in other ways. The Bachem Ba 349 Natter landed under a parachute after having taken off vertically. Other late Third Reich projects, such as the Heinkel P.1077 Julia or the Focke-Wulf Volksjäger 2 climbed to their ceiling at a nearly vertical angle and landed later on a skid.[1]
VTOL
Vertical take-off and landing (VTOL) aircraft include fixed-wing aircraft that can hover, take off and land vertically as well as helicopters and other aircraft with powered rotors, such as tiltrotors.[2][3][4][5] Some VTOL aircraft can operate in other modes as well, such as CTOL (conventional take-off and landing), STOL (short take-off and landing), and/or STOVL (short take-off and vertical landing). Others, such as some helicopters, can only operate by VTOL, due to the aircraft lacking landing gear that can handle horizontal motion. VTOL is a subset of V/STOL (vertical and/or short take-off and landing).
Besides the ubiquitous helicopter, there are currently two types of VTOL aircraft in military service: craft using a tiltrotor, such as the Bell Boeing V-22 Osprey, and some aircraft using directed jet thrust such as the Harrier family.
Rocket launch
A rocket launch is the takeoff phase of the flight of a rocket. Launches for orbital spaceflights, or launches into interplanetary space, are usually from a fixed location on the ground, but may also be from a floating platform such as the San Marco platform, or the Sea Launch launch vessel.
See also
- Balanced field takeoff
- V speeds
- Climb
- Cruise
- Descent
- Landing
- Space launch, the spaceflight equivalent
References
- ↑ Ulrich Albrecht: Artefakte des Fanatismus; Technik und nationalsozialistische Ideologie in der Endphase des Dritten Reiches (in German)
- ↑ "Vertical Takeoff & Landing Aircraft," John P. Campbell, The MacMillan Company, New York, 1962.
- ↑ Rogers 1989.
- ↑ Laskowitz, I.B. "Vertical Take-Off and Landing (VTOL) Aircraft." Annals of the New York Academy of Sciences, Vol. 107, Art.1, 25 March 1963.
- ↑ "Straight Up - A History of Vertical Flight," Steve Markman and Bill Holder, Schiffer Publishing, 2000.
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