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
The powerplant thrust is the principal force to provide the acceleration and, for minimum takeoff distance, the output thrust should be at a maximum. Lift and drag are produced as soon as the airplane has speed, and the values of lift and drag depend on the angle of attack and dynamic pressure.
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,
(2) at least a 9 percent decrease in rate of acceleration,
(3) at least a 21 percent increase in takeoff distance.
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).
|The effect of wind on landing distance is identical to
the effect on takeoff distance. Figure 17-61 illustrates the general effect
of wind by the percent change in takeoff or landing distance as a function
of the ratio of wind velocity to takeoff or landing speed.
The effect of proper takeoff speed is especially important when runway lengths and takeoff distances are critical. The takeoff speeds specified in the airplane's flight handbook are generally the minimum safe speeds at which the airplane can become airborne. Any attempt to take off below the recommended speed could mean that the airplane may stall, be difficult to control, or have a very low initial rate of climb. In some cases, an excessive angle of attack may not allow the airplane to climb out of ground effect.
On the other hand, an excessive airspeed at takeoff may improve the initial rate of climb and "feel" of the airplane, but will produce an undesirable increase in takeoff distance. Assuming that the acceleration is essentially unaffected, the takeoff distance varies as the square of the takeoff velocity.
Thus, 10 percent excess airspeed would increase the takeoff distance 21 percent. In most critical takeoff conditions, such an increase in takeoff distance would be prohibitive and the pilot must adhere to the recommended takeoff speeds.
The effect of pressure altitude and ambient temperature is to define primarily the density altitude and its effect on takeoff performance. While subsequent corrections are appropriate for the effect of temperature on certain items of powerplant performance, density altitude defines specific effects on takeoff performance. An increase in density altitude can produce a two fold effect on takeoff performance: (1) greater takeoff speed and (2) decreased thrust and reduced net accelerating force. If an airplane of given weight and configuration is operated at greater heights above standard sea level, the airplane will still require the same dynamic pressure to become airborne at the takeoff lift coefficient. Thus, the airplane at altitude will take off at the same indicated airspeed as at sea level, but because of the reduced air density, the true airspeed will be greater.
The effect of density altitude on power plant thrust depends much on the type of powerplant. An increase in altitude above standard sea level will bring an immediate decrease in power output for the unsupercharged reciprocating engine. However, an increase in altitude above standard sea level will not cause a decrease in power output for the supercharged reciprocating engine until the altitude exceeds the critical operating altitude. For those powerplants which experience a decay in thrust with an increase in altitude, the effect on the net accelerating force and acceleration rate can be approximated by assuming a direct variation with density. Actually, this assumed variation would closely approximate the effect on airplanes with high thrust to weight ratios.
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:
(1) Pressure altitude and temperature - to define the effect of density altitude on distance.
(2) Gross weight - a large effect on distance.
(3) Runway slope and condition - the effect of an incline and retarding effect of snow, ice, etc.
(4) Wind - a large effect due to the wind or wind component along the runway.