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prevent losing altitude, the pitch angle must be increased in
order to increase the angle of attack and maintain altitude.
Drag Curves
When induced drag and parasite drag are plotted on a graph,
the total drag on the aircraft appears in the form of a “drag
curve.” Graph A of Figure 2-8 shows a curve based on thrust
versus drag, which is primarily used for jet aircraft. Graph B
of Figure 2-8 is based on power versus drag, and it is used
for propeller-driven aircraft. This chapter focuses on power
versus drag charts for propeller-driven aircraft.
Understanding the drag curve can provide valuable insight
into the various performance parameters and limitations of
the aircraft. Because power must equal drag to maintain a
steady airspeed, the curve can be either a drag curve or a
power required curve. The power required curve represents
the amount of power needed to overcome drag in order to
maintain a steady speed in level flight.
The propellers used on most reciprocating engines achieve
peak propeller efficiencies in the range of 80 to 88 percent.
As airspeed increases, the propeller efficiency increases until
it reaches its maximum. Any airspeed above this maximum
point causes a reduction in propeller efficiency. An engine
that produces 160 horsepower will have only about 80
percent of that power converted into available horsepower,
2-7
Figure 2-8. Thrust and Power Required Curves.
Figure 2-9. Regions of Command.
approximately 128 horsepower. The remainder is lost energy.
This is the reason the thrust and power available curves
change with speed.
Regions of Command
The drag curve also illustrates the two regions of command:
the region of normal command, and the region of reversed
command. The term “region of command” refers to the
relationship between speed and the power required to
maintain or change that speed. “Command” refers to the input
the pilot must give in terms of power or thrust to maintain a
new speed once reached.
The “region of normal command” occurs where power must
be added to increase speed. This region exists at speeds higher
than the minimum drag point primarily as a result of parasite
drag. The “region of reversed command” occurs where
additional power is needed to maintain a slower airspeed.
This region exists at speeds slower than the minimum drag
point (L/DMAX on the thrust required curve, Figure 2-8) and
is primarily due to induced drag. Figure 2-9 shows how one
power setting can yield two speeds, points 1 and 2. This is
because at point 1 there is high induced drag and low parasite
drag, while at point 2 there is high parasite drag and low
induced drag.
Control Characteristics
Most flying is conducted in the region of normal command:
for example, cruise, climb, and maneuvers. The region of
reversed command may be encountered in the slow-speed
phases of flight during takeoff and landing; however, for
most general aviation aircraft, this region is very small and
is below normal approach speeds.
Flight in the region of normal command is characterized
by a relatively strong tendency of the aircraft to maintain
the trim speed. Flight in the region of reversed command is
characterized by a relatively weak tendency of the aircraft to
maintain the trim speed. In fact, it is likely the aircraft exhibits
no inherent tendency to maintain the trim speed in this area.
For this reason, the pilot must give particular attention to
precise control of airspeed when operating in the slow-speed
phases of the region of reversed command.
Operation in the region of reversed command does not imply
that great control difficulty and dangerous conditions exist.
However, it does amplify errors of basic flying technique—
making proper flying technique and precise control of the
aircraft very important.
Speed Stability
Normal Command
The characteristics of flight in the region of normal command
are illustrated at point A on the curve in Figure 2-10. If the
aircraft is established in steady, level flight at point A, lift is
equal to weight, and the power available is set equal to the
power required. If the airspeed is increased with no changes
2-8
Figure 2-10. Region of Speed Stability.
to the power setting, a power deficiency exists. The aircraft
has a natural tendency to return to the initial speed to balance
power and drag. If the airspeed is reduced with no changes
to the power setting, an excess of power exists. The aircraft
has a natural tendency to speed up to regain the balance
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Instrument Flying Handbook仪表飞行手册上(32)
