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# Aerodynamics

I FOR PILOTS

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## A'R T R A' N' N G COM MAN D

EXHIBIT 

# I l-fi:-freversed command are discussed in Chapter



### 9.
NET DECELERATING FORCE. In equation 13.7, the forces that comprise the net acceleraUng force are reversed. The acceleration force is now in the direction of the drag andfriction forces, so the airplane slows down. To develop this netdecelerating force, the drag andfriction must be greater than the thrust force. The thrust force must still be considered, even though the engine is usually at idle. There is a residual thrust force' that must be overcome by the drag and friction if the airplane is to decrease its velocity. Some airplanes are equipped with thrust reversal units or propellers which are capable of reversing their pitch and, thereby, of developing a thrust force in the direction of the retarding forces. If this is the case, the sign of the thrust in equation 13.7 will become positive. This naturally increases the retarding force and decreases the landing distance required.

The net decelerating force canbeincreased

on some airplanes through the use ofaerodynamic braking. This is nothing more than increasing the drag on the airplane during the landing roll. You should remember that the drag force is proportional to the square of the velocity, so aerodynamic braking is only effective during the initial landing roll when the airplane is at its higher velocities.

If a short-field landing is to be made, it is possible to increase the friction force far above that of any aerodynamic force that could be applied to the airplane. The increase in the friction force is donewiththe wheel brakes. Caution should be used in applying the brakes too early. During the initial landing roll, the major portion ofthe airplane's weight is supported bythe wings. The use of brakes at this time is not effective because the normal force onthe wheels is low and the resulting friction force developed-is small. It is also possible that the wheels might lock and the tires blowout if the 'brakes are applied at this time. The velocity must be decreased to allow enough weight to be transferred to the wheels where the brakes will be effective. Some airplanes have wing spoilers that are used to destroy



### 88 the lift on the Wingsand thereby transfer the weight to the tires and allowthe brakes
to be applied earlier in the landing run. This weight shift canbe aided by retracting the flaps, as this also reduces the lift developed bythe wings.

The condition of the runway surface can play an important part upon the required stopping distance ofthe airplane. Awetrunwaymay have stopping characteristics similar to the icy runway. The coefficientoffriction can drop to almost nothing. Theairplane tire can be traveling at such avelocitythat it will "hydroplane." This means that the airplane is riding on a film ofwater andthe tire has no contact withthe runway surface. The friction force is negligible in this condition andthe landing distance isincreased greatly. As the airplane slows down,the tire comes in contact with therunwayandnormal braking

canthen be applied.

LANDINGVELOCITY. Thelanding velocity ofan airplane is about 1.3times its stalling speed; therefore, anyhighlift devicethat decreases the stalling speed also decreases the landingspeed. Adecreased landingvelocity decreases boththe kinetic energy andthe required landing distance. Landings at airfields at high altitudes result in higher final approach true airspeeds which result in longer landing distance.

WIND EFFECT. A head wind results in lower final approach and touchdown velocities (groundspeed). This, ofcourse, results in reduced kinetic energy (withrespect tothe ground) whichdecreases thelandingdistance.

LANDINGWEIGHT. The weight oftheairplane has the same effect upon the landing distance as it does on the takeoff distance. Decreased weight requires less runway. Landing Performance Summary

As with takeoff performance, the landing performance is difficulttopredict; therefore, landing distance charts are supplied in each handbook and show the results of actual flight tests conductedunder differing conditions. These charts are based onequations 13.7 and 13.8 and, therefore, take into consideration the factors affecting the landing distance. As' craft efficie streng chapte

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