Takeoff Performance
The minimum takeoff distance is of primary interest in the operation of any airplane because it defines the runway requirements. The minimum takeoff distance is obtained by taking off at some minimum safe speed which allows sufficient margin above stall and provides satisfactory control and initial rate of climb. Generally, the liftoff speed is some fixed percentage of the stall speed or minimum control speed for the airplane in the takeoff configuration. As such, the liftoff will be accomplished at some particular value of lift coefficient and angle of attack. Depending on the airplane characteristics, the liftoff speed will be anywhere from 1.05 to 1.25 times the stall speed or minimum control speed. To obtain minimum takeoff distance at the specific liftoff
speed, the forces which act on the airplane must provide the maximum
acceleration during the takeoff roll. The various forces acting on the airplane
may or may not be under the control of the pilot, and various techniques may be
necessary in certain airplanes to maintain takeoff acceleration at the highest
value. In addition to the important factors of proper technique, many other variables affect the takeoff performance of an airplane. Any item which alters the takeoff speed or acceleration rate during the takeoff roll will affect the takeoff distance. For example, the effect of gross weight on takeoff distance is significant and proper consideration of this item must be made in predicting the airplane's takeoff distance. Increased gross weight can be considered to produce a threefold effect on takeoff performance: (1) higher liftoff speed, (2) greater mass to accelerate, and (3) increased retarding force (drag and ground friction). If the gross weight increases, a greater speed is necessary to produce the greater lift necessary to get the airplane airborne at the takeoff lift coefficient. As an example of the effect of a change in gross weight, a 21 percent increase in takeoff weight will require a 10 percent increase in liftoff speed to support the greater weight. A change in gross weight will change the net accelerating force, and change the mass which is being accelerated. If the airplane has a relatively high thrust to weight ratio, the change in the net accelerating force is slight and the principal effect on acceleration is due to the change in mass. The takeoff distance will vary at least as the square of the gross weight. For example, a 10 percent increase in takeoff gross weight would cause: (1) a 5 percent increase in takeoff velocity,
For the airplane with a high thrust to weight ratio, the increase in takeoff distance might be approximately 21 to 22 percent, but for the airplane with a relatively low thrust to weight ratio, the increase in takeoff distance would be approximately 25 to 30 percent. Such a powerful effect requires proper consideration of gross weight in predicting takeoff distance. The effect of wind on takeoff distance is large, and proper consideration also must be provided when predicting takeoff distance. The effect of a headwind is to allow the airplane to reach the liftoff speed at a lower groundspeed while the effect of a tailwind is to require the airplane to achieve a greater groundspeed to attain the liftoff speed. A headwind which is 10 percent of the takeoff airspeed will
reduce the takeoff distance approximately 19 percent (Fig. 17-63a and b).
However, a tailwind which is 10 percent of the takeoff airspeed will increase
the takeoff distance approximately 21 percent. In the case where the headwind
speed is 50 percent of the takeoff speed, the takeoff distance would be
approximately 25 percent of the zero wind takeoff distance (75 percent
reduction).
Proper accounting of pressure altitude (field elevation is a poor substitute) and temperature, is mandatory for accurate prediction of takeoff roll distance. The most critical conditions of takeoff performance are the result of some combination of high gross weight, altitude, temperature, and unfavorable wind. In all cases, it behooves the pilot to make an accurate prediction of takeoff distance from the performance data of the Airplane's Flight Handbook, regardless of the runway available, and to strive for a polished, professional takeoff technique. In the prediction of takeoff distance from the handbook data,
the following primary considerations must be given:
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