Aircraft Performance

As discussed in Chapter 6, engine pressure ratio (EPR) is the ratio, the increase in takeoff distance would be approximately ratio between exhaust pressure (jet blast) and inlet (static) 25 to 30 percent. Such a powerful effect requires proper pressure on a turbo jet or turbo fan engine. An EPR gauge consideration of gross weight in predicting takeoff distance. tells the pilot how much power the engines are generating. The higher the EPR, the higher the engine thrust. EPR is The effect of wind on takeoff distance is large, and proper used to avoid over-boosting an engine and to set takeoff and consideration must also be provided when predicting takeoff go around power if needed. This information is important to distance. The effect of a headwind is to allow the aircraft to know before taking off as it helps determine the performance reach the lift-off speed at a lower groundspeed, while the of the aircraft. effect of a tailwind is to require the aircraft to achieve a greater groundspeed to attain the lift-off speed. In addition to the important factors of proper procedures, many other variables affect the takeoff performance of an A headwind that is 10 percent of the takeoff airspeed reduces aircraft. Any item that alters the takeoff speed or acceleration the takeoff distance approximately 19 percent. However, a rate during the takeoff roll affects the takeoff distance. tailwind that is 10 percent of the takeoff airspeed increases the takeoff distance approximately 21 percent. In the case For example, the effect of gross weight on takeoff distance where the headwind speed is 50 percent of the takeoff speed, is significant, and proper consideration of this item must be the takeoff distance would be approximately 25 percent of made in predicting the aircraft’s takeoff distance. Increased the zero wind takeoff distance (75 percent reduction). gross weight can be considered to produce a threefold effect on takeoff performance: The effect of wind on landing distance is identical to its effect on takeoff distance. Figure 11-19 illustrates the general 1. Higher lift-off speed effect of wind by the percent change in takeoff or landing 2. Greater mass to accelerate distance as a function of the ratio of wind velocity to takeoff 3. Increased retarding force (drag and ground friction) or landing speed. If the gross weight increases, a greater speed is necessary to The effect of proper takeoff speed is especially important produce the greater lift necessary to get the aircraft airborne when runway lengths and takeoff distances are critical. The at the takeoff lift coefficient. As an example of the effect of takeoff speeds specified in the AFM/POH are generally a change in gross weight, a 21 percent increase in takeoff the minimum safe speeds at which the aircraft can become weight requires a 10 percent increase in lift-off speed to airborne. Any attempt to take off below the recommended support the greater weight. speed means that the aircraft could stall, be difficult to control, or have a very low initial ROC. In some cases, an A change in gross weight changes the net accelerating force and changes the mass that is being accelerated. If the aircraft 80 has a relatively high thrust-to-weight ratio, the change in the n e 70 e li net accelerating force is slight and the principal effect on nc e acceleration is due to the change in mass. Percent increase 60 ef er in takeoff or R landing distance 50 For example, a 10 percent increase in takeoff gross weight 40 would cause: 30 Ratio of wind • A 5 percent increase in takeoff velocity velocity to takeoff 20 or landing speed • At least a 9 percent decrease in rate of acceleration 10 Tailwind 30% 20% 10% • At least a 21 percent increase in takeoff distance 10% 20% 30% Headwind 10 With ISA conditions, increasing the takeoff weight of the Ratio of wind 20 velocity to takeoff average Cessna 182 from 2,400 pounds to 2,700 pounds (11 or landing speed 30 percent increase) results in an increased takeoff distance from 440 feet to 575 feet (23 percent increase). 40 Percent decrease 50 in takeoff or For the aircraft with a high thrust-to-weight ratio, the increase 60 landing distance in takeoff distance might be approximately 21 to 22 percent, but for the aircraft with a relatively low thrust-to-weight Figure 11-19. Effect of wind on takeoff and landing. 11-15