LANDING PERFORMANCE
The primary consideration in landing is dissipation of the airplane’s kinetic energy. Any factor affecting velocity must be considered when trying to reduce the landing distance. Final approach is flown at the lowest velocity feasible. In the landing distance equation the net accelerating forces are reversed. Drag and rolling friction are now desirable and of course, thrust is not.
•An increase in weight will increase landing distance since a greater airspeed is required to support the airplane.It also takes more break energy to slow a heavier aircraft, lengthening stopping distance
•An increase in elevation, temperature or humidity will increase landing distance since the reduced density results in a higher landing velocity.
•A tailwind increases landing distance since it increases ground speed.
•High lift devices decrease landing distance because they reduce the ground speed during the landing.
•A headwind reduces landing distance because it reduces ground speed.
The net decelerating force can be increased by use of three different techniques:
Aerodynamic braking is accomplished by increasing the parasite drag on the airplane by holding a constant pitch attitude after touchdown and exposing more of the airplane’s surface to the relative wind. This method of braking helps to reduce wear on the brakes. Drag chutes, spoilers, and speed brakes are also considered aerodynamic braking. Aerodynamic braking is used at the beginning of the landing roll. Aerodynamic breaking can also be used in flight to reduce airspeed or increase descent rates when necessary.
Mechanical braking (also called frictional or wheel braking) is effective only after enough weight is transferred to the wheels and the airplane has slowed sufficiently. A common procedure is to raise flaps or use spoilers to decrease lift and transfer the airplane’s weight to the wheels when transitioning from aerodynamic to mechanical braking. Mechanical braking is used toward the end of the landing roll.
Some airplanes use reverse thrust or reverse pitch propellers (called beta) to shorten the landing roll. Thrust is usually negligible after touchdown, but in the case of reverse thrust or “beta” equipped airplanes, thrust increases the net decelerating force.
CROSSWINDS
Since winds do not always blow directly down the runway, the possibility of a crosswind takeoff or landing exists. The rudder is the primary means of maintaining directional control in order to compensate for the crosswind during takeoff or landing. The pilot must also place the ailerons into the wind during a crosswind takeoff or landing. The ailerons are not used to maintain directional control, but to overcome the lateral stability that is trying to roll the airplane away from the sideslip relative wind (crosswind).
Crosswind control inputs increase drag and will require additional power to compensate for this drag.
GROUND EFFECT
A phenomenon, known as ground effect, significantly reduces induced drag and increases effective lift when the airplane is within one wingspan of the ground. Because takeoffs and landings are conducted at low airspeeds, induced drag makes up a large portion of the total drag on the airplane. As an airplane nears the ground, the downwash at the trailing edge of the wing is unable to flow downward. The decrease in downwash allows the total lift vector to rotate forward, increasing effective lift and decreasing induced drag. When the aircraft is one wingspan above the ground induced drag is reduced by only 1.4%, at one-fourth the wingspan, induced drag is reduced by 23.5%, and a maximum reduction of 60% occurs just prior to touchdown or after liftoff
Because of the increased lift, it is possible to get airborne at an airspeed below normal flying speed. As an airplane takes off and leaves ground effect, induced drag increases and lift decreases, which could cause an altitude loss, possibly resulting in an unintentional gear-up landing.
Entering ground effect (during landing) increases effective lift and decreases induced drag by preventing the aft inclination of the lift vector. When the plane enters ground effect it will float down the runway if the pilot does not reduce thrust.
