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Pilot's Handbook of Aeronautical Knowledge
Aircraft Performance

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




The effect of altitude on the range of a propeller-driven aircraft
is illustrated in Figure 10-12. A flight conducted at high altitude
has a greater true airspeed (TAS), and the power required is
proportionately greater than when conducted at sea level. The
drag of the aircraft at altitude is the same as the drag at sea level,
but the higher TAS causes a proportionately greater power
required. NOTE: The straight line that is tangent to the sea level
power curve is also tangent to the altitude power curve.

Effect of altitude on range.
Figure 10-12. Effect of altitude on range.

The effect of altitude on specific range also can be appreciated
from the previous relationships. If a change in altitude causes
identical changes in speed and power required, the proportion
of speed to power required would be unchanged. The fact
implies that the specific range of a propeller-driven aircraft
would be unaffected by altitude. Actually, this is true to the
extent that specific fuel consumption and propeller efficiency
are the principal factors that could cause a variation of
specific range with altitude. If compressibility effects are
negligible, any variation of specific range with altitude is
strictly a function of engine/propeller performance.

An aircraft equipped with a reciprocating engine will
experience very little, if any, variation of specific range up
to its absolute altitude. There is negligible variation of brake
specific fuel consumption for values of brake horsepower
below the maximum cruise power rating of the engine that
is the lean range of engine operation. Thus, an increase in
altitude will produce a decrease in specific range only when
the increased power requirement exceeds the maximum cruise
power rating of the engine. One advantage of supercharging
is that the cruise power may be maintained at high altitude,
and the aircraft may achieve the range at high altitude with
the corresponding increase in TAS. The principal differences
in the high altitude cruise and low altitude cruise are the TAS
and climb fuel requirements.

Region of Reversed Command
The aerodynamic properties of an aircraft generally determine
the power requirements at various conditions of flight, while
the powerplant capabilities generally determine the power
available at various conditions of flight. When an aircraft
is in steady, level flight, a condition of equilibrium must
prevail. An unaccelerated condition of flight is achieved
when lift equals weight, and the powerplant is set for thrust
equal to drag. The power required to achieve equilibrium in
constant-altitude flight at various airspeeds is depicted on a
power required curve. The power required curve illustrates
the fact that at low airspeeds near the stall or minimum
controllable airspeed, the power setting required for steady,
level flight is quite high.

Flight in the region of normal command means that while
holding a constant altitude, a higher airspeed requires a higher
power setting and a lower airspeed requires a lower power
setting. The majority of aircraft flying (climb, cruise, and
maneuvers) is conducted in the region of normal command.
Flight in the region of reversed command means flight in
which a higher airspeed requires a lower power setting and
a lower airspeed requires a higher power setting to hold
altitude. It does not imply that a decrease in power will
produce lower airspeed. The region of reversed command is
encountered in the low speed phases of flight Flight speeds
below the speed for maximum endurance (lowest point
on the power curve) require higher power settings with a
decrease in airspeed. Since the need to increase the required
power setting with decreased speed is contrary to the normal
command of flight, the regime of flight speeds between the
speed for minimum required power setting and the stall speed
(or minimum control speed) is termed the region of reversed
command. In the region of reversed command, a decrease in
airspeed must be accompanied by an increased power setting
in order to maintain steady flight.

Figure 10-13 shows the maximum power available as a
curved line. Lower power settings, such as cruise power,
would also appear in a similar curve. The lowest point on
the power required curve represents the speed at which the
lowest brake horsepower will sustain level flight. This is
termed the best endurance airspeed.

An airplane performing a low airspeed, high pitch attitude
power approach for a short-fleld landing is an example
of operating in the region of reversed command. If an
unacceptably high sink rate should develop, it may be
possible for the pilot to reduce or stop the descent by applying
power. But without further use of power, the airplane would
probably stall or be incapable of flaring for the landing.