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approximate power settings necessary to maintain various
airspeeds in straight-and-level flight are known in advance.
However, to change airspeed by any appreciable amount, the
common procedure is to underpower or overpower on initial
power changes to accelerate the rate of airspeed change.
(For small speed changes, or in airplanes that decelerate or
accelerate rapidly, overpowering or underpowering is not
necessary.)
Consider the example of an airplane that requires 23"
mercury (Hg) of manifold pressure to maintain a normal
cruising airspeed of 120 knots, and 18" Hg of manifold
pressure to maintain an airspeed of 100 knots. The reduction
in airspeed from 120 knots to 100 knots while maintaining
straight-and-level flight is discussed below and illustrated in
Figures 5-22, 5-23, and 5-24.
Instrument indications, prior to the power reduction, are
shown in Figure 5-22. The basic attitude is established and
maintained on the attitude indicator. The specific pitch,
bank, and power control requirements are detected on these
primary instruments:
Altimeter—Primary Pitch
Heading Indicator—Primary Bank
Airspeed Indicator—Primary Power
Supporting pitch-and-bank instruments are shown in
Figure 5-23. Note that the supporting power instrument is
the manifold pressure gauge (or tachometer if the propeller
is fixed pitch). However, when a smooth power reduction to
approximately 15" Hg (underpower) is made, the manifold
pressure gauge becomes the primary power instrument.
[Figure 5-23] With practice, power setting can be changed
with only a brief glance at the power instrument, by sensing
5-10
Figure 5-22. Straight-and-Level Flight (Normal Cruising Speed).
Figure 5-23. Straight-and-Level Flight (Airspeed Decreasing).
5-11
Figure 5-24. Straight-and-Level Flight (Reduced Airspeed Stabilized).
the movement of the throttle, the change in sound, and the
changes in the feel of control pressures.
As thrust decreases, increase the speed of the cross-check
and be ready to apply left rudder, back-elevator, and aileron
control pressure the instant the pitch-and-bank instruments
show a deviation from altitude and heading. As proficiency
is obtained, a pilot learns to cross-check, interpret, and
control the changes with no deviation of heading and altitude.
Assuming smooth air and ideal control technique, as airspeed
decreases, a proportionate increase in airplane pitch attitude
is required to maintain altitude. Similarly, effective torque
control means counteracting yaw with rudder pressure.
As the power is reduced, the altimeter is primary for pitch,
the heading indicator is primary for bank, and the manifold
pressure gauge is momentarily primary for power (at 15"
Hg in this example). Control pressures should be trimmed
off as the airplane decelerates. As the airspeed approaches
the desired airspeed of 100 knots, the manifold pressure
is adjusted to approximately 18" Hg and becomes the
supporting power instrument. The ASI again becomes
primary for power. [Figure 5-24]
Airspeed Changes in Straight-and-Level Flight
Practice of airspeed changes in straight-and-level flight provides
an excellent means of developing increased proficiency in all
three basic instrument skills, and brings out some common
errors to be expected during training in straight-and-level flight.
Having learned to control the airplane in a clean configuration
(minimum drag conditions), increase proficiency in crosscheck
and control by practicing speed changes while extending
or retracting the flaps and landing gear. While practicing, be
sure to comply with the airspeed limitations specified in the
POH/AFM for gear and flap operation.
Sudden and exaggerated attitude changes may be necessary
in order to maintain straight-and-level flight as the landing
gear is extended and the flaps are lowered in some airplanes.
The nose tends to pitch down with gear extension, and when
flaps are lowered, lift increases momentarily (at partial flap
settings) followed by a marked increase in drag as the flaps
near maximum extension.
Control technique varies according to the lift and drag
characteristics of each airplane. Accordingly, knowledge of
the power settings and trim changes associated with different
combinations of airspeed, gear and flap configurations will
reduce instrument cross-check and interpretation problems.
For example, assume that in straight-and-level flight
instruments indicate 120 knots with power at 23" Hg/2,300
revolutions per minute (rpm), gear and flaps up. After
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Instrument Flying Handbook仪表飞行手册上(83)
