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between power and drag. Keeping the aircraft in proper
trim enhances this natural tendency. The static longitudinal
stability of the aircraft tends to return the aircraft to the
original trimmed condition.
An aircraft flying in steady, level flight at point C is in
equilibrium. [Figure 2-10] If the speed were increased
or decreased slightly, the aircraft would tend to remain at
that speed. This is because the curve is relatively flat and
a slight change in speed does not produce any significant
excess or deficiency in power. It has the characteristic of
neutral stability, i.e., the aircraft’s tendency is to remain at
the new speed.
Reversed Command
The characteristics of flight in the region of reversed command
are illustrated at point B on the curve in Figure 2-10. If the
aircraft is established in steady, level flight at point B, lift is
equal to weight, and the power available is set equal to the
power required. When the airspeed is increased greater than
point B, an excess of power exists. This causes the aircraft
to accelerate to an even higher speed. When the aircraft is
slowed to some airspeed lower than point B, a deficiency
of power exists. The natural tendency of the aircraft is to
continue to slow to an even lower airspeed.
This tendency toward instability happens because the
variation of excess power to either side of point B magnifies
the original change in speed. Although the static longitudinal
stability of the aircraft tries to maintain the original trimmed
condition, this instability is more of an influence because of
the increased induced drag due to the higher angles of attack
in slow-speed flight.
Trim
The term trim refers to employing adjustable aerodynamic
devices on the aircraft to adjust forces so the pilot does not
have to manually hold pressure on the controls. One means is
to employ trim tabs. A trim tab is a small, adjustable hinged
surface, located on the trailing edge of the elevator, aileron,
or rudder control surfaces. (Some aircraft use adjustable
stabilizers instead of trim tabs for pitch trim.) Trimming is
accomplished by deflecting the tab in the direction opposite
to that in which the primary control surface must be held.
The force of the airflow striking the tab causes the main
control surface to be deflected to a position that corrects the
unbalanced condition of the aircraft.
Because the trim tabs use airflow to function, trim is a
function of speed. Any change in speed results in the need
to re-trim the aircraft. An aircraft properly trimmed in pitch
seeks to return to the original speed before the change. It is
very important for instrument pilots to keep the aircraft in
constant trim. This reduces the pilot’s workload significantly,
allowing attention to other duties without compromising
aircraft control.
Slow-Speed Flight
Anytime an aircraft is flying near the stalling speed or the
region of reversed command, such as in final approach for a
normal landing, the initial part of a go around, or maneuvering
in slow flight, it is operating in what is called slow-speed
flight. If the aircraft weighs 4,000 pounds, the lift produced
by the aircraft must be 4,000 pounds. When lift is less
than 4,000 pounds, the aircraft is no longer able to sustain
level flight, and consequently descends. During intentional
descents, this is an important factor and is used in the total
control of the aircraft.
However, because lift is required during low speed flight
and is characterized by high angles of attack, flaps or other
high lift devices are needed to either change the camber of
the airfoil, or delay the boundary level separation. Plain
and split flaps [Figure 2-11] are most commonly used to
change the camber of an airfoil. It should be noted that with
the application of flaps, the aircraft will stall at a lower
angle of attack. The basic wing stalls at 18° without flaps
but with the application of the flaps extended (to CL-MAX
position) the new angle of attack at which point the aircraft
will stall is 15°. However, the value of lift (flaps extended
to the CL-MAX position) produces more lift than lift at 18°
on the basic wing.
Delaying the boundary layer separation is another way to
increase CL-Max . Several methods are employed (such as
suction and use of a blowing boundary layer control), but the
2-9
Figure 2-11. Various Types of Flaps. Figure 2-12. Vortex Generators.
If allowed to slow several knots, the airplane could enter
the region of reversed command. At this point, the airplane
could develop an unsafe sink rate and continue to lose speed
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Instrument Flying Handbook仪表飞行手册上(33)
