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Aerodynamics

I FOR PILOTS

Reprinted 3 Apri11967 With Change A Included

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

EXHIBIT I l-f i:-f
reversed 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 and friction forces, so the airplane

slows down. To develop this net decelerating

force, the drag and friction 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 can be increased

on some airplanes through the use of aerodynamic 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 done with the

wheel brakes. Caution should be used in

applying the brakes too early. During the

initial landing roll, the major portion of the

airplane's weight is supported by the wings.

The use of brakes at this time is not effective because the normal force on the 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 Wings and thereby transfer

the weight to the tires and allow the brakes

to be applied earlier in the landing run.

This weight shift can be aided by retracting

the flaps, as this also reduces the lift developed by the wings.

The condition of the runway surface can

play an important part upon the required

stopping distance of the airplane. A wet runway may have stopping characteristics similar to the icy runway. The coefficientoffriction can drop to almost nothing. The airplane

tire can be traveling at such a velocity that it

will "hydroplane." This means that the airplane is riding on a film ofwater and the tire

has no contact with the runway surface. The

friction force is negligible in this condition

and the landing distance is increased greatly.

As the airplane slows down, the tire comes

in contact with the runway and normal braking

can then be applied.

LANDINGVELOCITY. The landing velocity of an airplane is about 1. 3 times its stalling speed; therefore, any high lift device that

decreases the stalling speed also decreases

the landing speed. Adecreased landing velocity decreases both the kinetic energy and the

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 (ground speed). This, of course, results

in reduced kinetic energy (with respect to the

ground) which decreases the landing distance.

LANDINGWEIGHT. The weight ofthe airplane 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 difficult to predict; therefore,

landing distance charts are supplied in each

handbook and show the results of actual

flight tests conducted under differing conditions. These charts are based on equations

13.7 and 13.8 and, therefore, take into consideration the factors affecting the landing

distance.

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