90
100 miles per hour to close to the speed of sound. A look at the wing's secondary
control surfaces -- flaps and slats -- will explain how the highly adaptable wing
accommodates this range of speed.
Flaps are located on the aft or "trailing edge" of the wing and increase both lift
and drag. Because of the increased drag, flaps are only partially extended on takeoff,
and fully extended on landing. The added drag created by full flaps on landing allows
the aircraft to approach at a steep angle without excessive air speed. Think of a car as
it rolls downhill. If the hill is too steep, the car's speed becomes excessive. By using
the drag of the brakes, the speed stays under control.
The greater the wing's camber, or curvature, the better the wing will perform at
slow air speeds. Flaps extend downward and rearward, thereby increasing both
camber and wing area. The flaps extend in several segments, creating gaps, known as
slots, in the wing. The slots allow high energy air from below the wing to vent over
the top of the wing. This high energy air "tricks the wing into thinking" it is traveling
faster than it actually is, enhancing the low speed performance. On the leading edge
of the wing are the slats. As the slats extend forward another slot is opened on the
leading edge of the wing. The combination of the slats and flaps creates an airfoil of
significant camber for low speed operations.
The wing is said to be "dirty" when the flaps and slats are extended, referring
to the high amount of drag the wing experiences. In the dirty configuration the wing
is capable of low speed flight; the high drag would, however, prohibit high speed
flight. After takeoff the wing is "cleaned up." After climbing through 400 feet the air
speed is allowed to increase, reducing the need for the flaps and slats. After the
completion of the flap and slat retraction, the aircraft can accelerate to cruise speed.
During the approach for landing, the flaps and slats are again extended. The flaps are
set to an initial setting, usually one degree. The leading edge slats are later deployed.
When initiating the final descent for landing, the flaps are fully extended. Now that
the wing is again in its low speed configuration, the aircraft can be slowed to its final
approach speed.
Aircraft designers know that no wing is efficient at all speeds. By using flaps,
slats and slots, a wing can take on beneficial low speed characteristics for takeoff and
landing, and characteristics beneficial for high speed flight while at cruise. If it were
not for flaps and slats, the takeoff and landing speeds would have to be much greater
and runways would be prohibitively long.
17.4. EXERCISES
1. Ответьте на вопросы.
1.
How are wings designed in modern jet transport aircraft?
2.
What is the range of speed the transport aircraft fly at ?
3.
Where are flaps located and what are they for?
4.
When are the flaps partially and fully extended?
5.
Why does the aircraft approach at a steep angle without excessive air speed
?
6.
What does the better perform the wing depend on at slow air speeds?
