Сборник технических текстов для домашнего чтения по английскому языку. Морозова М.А. - 18 стр.

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is ready, the brakes are released and the airplane accelerates down the runway. At a
pre-determined speed, the pilot pulls back on the stick to pitch the airplane upward
about five degrees. Although the nose wheel is off the ground, the main gear remains
on the runway because there is not yet enough airflow over the wings to create suffi-
cient lift to raise the aircraft. After a little while, the airplane reaches the speed (90
knots) at which its wings produce lift slightly greater than its weight and it takes off.
While the airplane climbs away from the runway the pilot must raise the land-
ing gear (this decreases the drag) and the flaps, then let it accelerate to the desired
climb speed. Once this speed
is reached, it is maintained by raising the nose slightly
and "trimming" off all control stick pressures.
Straight and Level Flight
If an airplane maintains a given altitude, airspeed, and heading, it is said to be
in "straight and level flight." This condition is achieved and maintained by equalizing
all opposing forces. Lift must equal weight so the airplane does not climb or descend.
Thrust must equal drag so the airplane does not speed up or slow down. The wings
are kept level so the airplane does not turn. Any imbalance will result in a change in
altitude or airspeed. It is the pilot's responsibility to prevent or correct for such an im-
balance.
Proper trim is essential for maintaining this balance. If the pilot, by being "out
of trim," is forced to maintain a given amount of stick pressure, the arm holding the
stick will eventually tire. But in the short term the pilot must very precisely hold that
pressure -- any change will result in a change in attitude. If the airplane is properly
trimmed, the correct stick position is held automatically, and no pressure need be ex-
erted.
Obviously, an airplane cannot remain indefinitely in this ideal condition. Due
to mission, airspace, and fuel requirements, the pilot must change the airspeed, alti-
tude, and heading from time to time
.
Speed
Speeding up and slowing down is not simply a matter of changing the throttle
setting (changing the force produced by the engines). Airspeed can also be changed
by changing the drag. Many aircraft are equipped with a "speed brake" for this pur-
pose --a large metal plate that can be extended out into the wind stream, increasing
parasite drag and slowing the airplane.
As an airplane speeds up or slows down, the amount of air passing over the
wing follows suit. For instance, to maintain a constant altitude as the airspeed is de-
creasing, the pilot must compensate for this decreased airflow by changing the AOA
(pulling back on the stick) to equalize the amount of lift to the weight of the airplane.
All this works nicely until stall speed is reached, when an increase in AOA is met
with a decrease in lift, and the airplane, its weight not completely countered by lift,
begins to dramatically lose altitude. Conversely, an increase in airspeed must be met
with a decrease in the AOA (moving the stick forward) to maintain a constant alti-
tude. As airspeed increases or decreases, trim must be changed as well.
Mach number is the most influential parameter in the determination of range
for most jet-powered aircraft. The most efficient cruise conditions occur at a high al-

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