Takeoff
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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 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 dependent on the aircraft weight; the heavier the weight, the greater the speed needed.[1] Some aircraft are specifically designed for short takeoff and landing (STOL), which they achieve by becoming airborne at very low speeds.
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Take-off Optimization
A take-off on any transport aircraft is with some amount of 'roll' on the ground during which, the aircraft accelerates to the required speed at which the aircraft can safely leave the ground. Take-off Optimization is a process by which the maximum weight for a safe take off is calculated for a given set of conditions like runway dimensions, slope, elevation, winds, temperature etc., Along with the calculation of this maximum safe weight, an associated set of speeds called take-off speeds is also calculated. The regulations specify that in a twin or multi engine aircraft, the weight for the take off is to be calculated assuming that the critical engine will fail at the most critical instant during the take off. The take off weight thus calculated, usually varies with different factors and this weight is referred to as the take-off performance limited maximum take-off weight. Some of these factors are internal or related to the selections made on the aircraft. The internal factors mostly relate to the design features of the aircraft, the thrust/power output from the engine(s) of the aircraft, utilization of high-lift devices like flaps, slats, boundary layer control and any other such aspects. Even if the runway conditions permit the aircraft to take off at a very high weight, the weight with which take-off is being attempted should be restricted as below the Maximum Take Off Weight (MTOW) as certified for the model of the aircraft. The exact thrust available from the engine(s) or being used from the engines will also have an effect on deciding the Weight of the aircraft with which a safe take-off can be attempted. Some specific malfunctions to some of the systems could be allowed during the take off as per the regulations and the list of such malfunctions is listed in a document called Minimum Equipment List (MEL) and Configuration Deviation List (CDL) which are approved by the regulator. However, some of these MELs or CDLs invoked prior to take-off could have performance implications which could restrict the maximum weight with which a safe take off can be attempted for a given set of conditions. There are also many other speeds and restrictions applicable to the aircraft which would restrict the maximum take off weight. VMCG, VMCA, VMBE, Tire limiting speeds, Minimum Unstick speed (VMU), etc., are some such limits. There are two other factors which have to be definitely considered related to the certified maximum zero fuel weight and the certified maximum landing weight. The maximum landing weight could sometimes get restricted to a lower value due the low runway length or the high density altitude of the destination airport. The actual weight at the time of take off cannot exceed (1) the performance limited MTOW calculated (2) the sum of any restricted landing weight and the trip fuel and (3) actual zero fuel weight plus the total fuel on board. In rare cases, if there are high hills on the route, that could pose an additional restriction. Regulations require that the aircraft should be able to clear the hills (if any) with a safe margin after suffering critical engine failure.
VMCG (minimum controllable speed on ground) is a calibrated airspeed at or below which with the aircraft still on ground, if the critical engine fails with the other engine(s) operating at the take off power, directional control cannot be maintained with the use of aerodynamic controls alone. To maintain directional control in such a scenario, either nose wheel steering or differential brakes or a combination of both will need to be used. Directional control in such a scenario is also possible if the power/thrust on the other live engine(s) is appropriately reduced and in this scenario with lower thrust/power from other engine(s) the value of VMCG will reduce. The speed V1 which is called the take off decision speed, is to be selected in such a way that it is positively above this value.
VMCA (minimum controllable speed in the air) is a calibrated airspeed at or above which it is possible to maintain directional control after getting airborne with the critical engine failed and the other(s) operating at take off power. Regulations allow a bank of 5 degrees into the live engine which is considered as 'no bank' along with the full rudder to the same side, leading to an effectively side-slipping situation to maintain direction. The speed V2 which is referred to as the take off safety speed, is to be selected in such a way that it is positively above this value. In fact, regulations specify that V2 should be more than 1.1 times VMCA and the value of VR should be at least 1.05 times VMCA.
VMBE is a speed that is dependent mainly on the capabilities of aircraft's brake system to absorb energy and the available runway length. After starting a take off roll on the runway with a particular all up weight, if a reject take off (stoppage) is attempted at a speed above this, stoppage within the available runway length is not guaranteed. Of course, the density altitude and the existing winds will have an effect on this value. The value of VMBE will reduce if the weight is reduced or if a brake system of better capability is installed. The value of V1 is to be selected as something below this.
Depending on the external factors like air density, runway length, runway slope, elevation, existing surface winds, Outside Air Temperature, etc., the speeds will need to be varied to achieve the best performance.
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. Normally ground speed for takeoff varies between 250 km/h to 475 km/h.
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.[2]
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.[3][4][5][6] 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 helicopter, there are 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
- ↑ Scott, Jeff (4 August 2002) "Airliner Takeoff Speeds". Aerospace Web. Retrieved 12 August 2015
- ↑ 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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