As in the case of landing, the specific techniques necessary may vary greatly between various types of airplanes and various operations but certain fundamental principles will be common to all airplanes and all operations. The specific procedures recommended for each airplane type must be followed exactly to insure a consistent, safe takeoff flying technique.
TAKEOFF SPEED AND DISTANCE. The takeoff speed of any airplane is some minimum practical airspeed which allows sufficient margin above stall and provides satisfactory control and initial rate of climb.. Depending on the airplane characteristics, the takeoff speed will be some value 5 to 25 percent above the stall or minimum control speed. As such, the takeoff will be accomplished at a certain value of lift coefficient and angle of attack specific to each airplane configuration. As a result, the takeoff airspeed (EAS or CAS’) of any specific airplane configuration is a function of the gross weight at takeoff. Too low an airspeed at takeoff may cause stall, lack of adequate control, or poor initial climb performance. An excess of speed at takeoff may provide better control and initial rate of climb but the higher speed requires additional distance and may provide critical conditions for the tires.
The takeoff distance of an airplane is affected by many different factors other than technique and, prior to takeoff, the takeoff distance must be determined and compared with the runway length available. The principal factors affecting the takeoff distance are as follows:
(1) The gross weight of the airplane has a considerable effect on takeoff distance because it affects both takeoff speed and acceleration during takeoff roll.
(2) The surface winds must be considered because of the powerful effect of a headwind or tailwind on the takeoff distance. In the case of the cross wind, the component of wind along the runway will be the effective headwind or tailwind velocity. In addition, the component of wind across the runway will define certain requirements of lateral control power and the limiting component wind must not be exceeded.
(3) Pressure altitude and temperature can cause a large effect on takeoff distance, especially in the case of the turbine powered airplane. Density altitude will determine the true airspeed at takeoff and can affect the takeoff acceleration by altering the powerplant thrust. The effect of temperature alone is important in the case of the turbine powered aircraft since inlet air temperature will affect powerplant thrust. It should be noted that a typical turbojet airplane may be approximately twice as sensitive to density altitude and five to ten times as sensitive to temperature as a representative reciprocating engine powered airplane.
(4) Specific humidity must be accounted for in the case of the reciprocating engine powered airplane. A high water vapor content in the air will cause a definite reduction in takeoff power and takeoff acceleration.
(5) The runway condition will deserve consideration when the takeoff acceleration is basically low. The runway slope must be compared carefully with the surface winds because ordinary values of runway slope will usually favor choice of the runway with headwind and upslope rather than down – slope and tailwind. The surface condition of the runway has little bearing on takeoff distance as long as the runway is a hard surface.
Each, of these factors must be accounted for and the takeoff distance properly computed for the existing conditions. Since obstacle clearance distance is generally a function of the same factors which affect takeoff distance, the obstacle clearance distance is usually related as some proportion of the takeoff distance. Of course, the takeoff and obstacle clearance distances related by the handbook data will be obtained by the techniques and procedures outlined In the handbook.
TYPICAL ERRORS. The takeoff distance of an airplane should be computed for each takeoff. A most inexcusable error would be to attempt takeoff from a runway of insufficient length. Familiarity with the airplane handbook performance data and proper accounting of weight, wind, altitude, temperature, etc., are necessary parts of flying. Conditions of high gross weight, high pressure altitude and temperature, and unfavorable winds create the extreme requirements of runway length, especially for the turbine powered airplane. Under these conditions, use of the handbook data is mandatory and no guesswork can be tolerated.
One typical, error of takeoff technique is the premature or excess pitch rotation of the airplane. Premature or excess fitch rotation of the airplane may seriously reduce the takeoff acceleration and increase the takeoff distance. In addition, when the airplane is placed at an excessive angle of attack during takeoff, the airplane may become airborne at too low a speed and the result may be a stall, lack of adequate control (especially in a crosswind), or poor initial climb performance. In fact, there are certain low aspect ratio configurations of airplanes which, at an excessive angle of attack, will not fly out of ground effect. Thus, over-rotation of the airplane during takeoff may hinder takeoff acceleration or the. initial climb. It is quite typical for an airplane to be placed at an excess angle of attack and become airborne prematurely then settle back to the runway. When the proper angle of attack is assumed, the airplane simply accelerates to the takeoff speed and becomes airborne with sufficient initial rate of climb. In this sense, the appropriate rotation and takeoff speeds or an angle of attack indicator must be used.
If the airplane is subject to a sudden pull-up or steep turn after becoming airborne, the result may be a stall, spin, or reduction in initial rate of climb. The increased angle of attack may exceed the critical angle of attack or the increase in induced drag may be quite large. For this reason, any clearing turns made immediately after takeoff or deck launch must be slight and well within the capabilities of the airplane.
In order to obviate some of the problems of a deficiency of airspeed at takeoff, usual result can be an excess of airspeed at takeoff. The principal effect of an excess takeoff airspeed is the greater takeoff distance which results. The general effect is that each 1 percent excess takeoff velocity incurs approximately 2 percent additional takeoff distance. Thus, excess speed must be compared with the additional runway required to produce the higher speed. In addition, the aircraft tires may be subject to critical loads when the airplane is at very high rolling speeds and speeds in excess of a basically high takeoff speed may produce damage or failure of the tires.
As with the conditions of landing, excess velocity or deficiency of velocity at takeoff is undesirable. The proper takeoff speeds and angle of attack must be utilized to assure satisfactory takeoff performance.