Jet Aircraft
Airspeed
Thrust Required
4,000
3,000
2,000
1,000
0 100 200 300 400 500
Induced drag
Parasite
dragTotal drag
Minimum
drag or
L/DMAX
Propeller-Driven Aircraft
Airspeed
0 100 200 300 400 500
Power Required Inches Hg
A B
Parasite
power
required
Total power
required
Minimum
power
required
L/DMAX
Induced drag
Region of
reversed
command
Region of
normal
command
Airspeed
Power Required
1 2
Figure 4-9. Thrust and power required curves.
Figure 4-10. Regions of command.
of Figure 4-9 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,
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 4-9) and is
primarily due to induced drag. Figure 4-10 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
Region of
normal
command
Region of
reversed
command
Power
deficit
Excess
power
Airspeed
Little or no excess
power or power deficit
Power Required
Excess
power
B C
Power
deficit
Figure 4-11. Region of speed stability.
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 4-11 . 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 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 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 4-11] 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 4-10. 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 AOA 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.
Uncontrolled Turbulence
Controlled Vortices
Figure 4-13. Vortex generators.
Plain
Split
Fowler
Slotted
Figure 4-12. Plain and split flaps.
accept minor speed changes knowing that when the pitch is
returned to the initial setting, the speed returns to the original
setting. This reduces the pilot’s workload.
Aircraft are usually slowed to a normal landing speed when
on the final approach just prior to landing. When slowed to
65 knots, (1.3 V SO), the airplane will be close to point C.
[Figure 4-14] At this point, precise control of the pitch and
power becomes more crucial for maintaining the correct speed.
Pitch and power coordination is necessary because the speed
stability is relatively neutral since the speed tends to remain
at the new value and not return to the original setting. In
addition to the need for more precise airspeed control, the pilot
normally changes the aircraft’s configuration by extending
landing flaps. This configuration change means the pilot must
be alert to unwanted pitch changes at a low altitude.
However, because lift is required during low speed flight
and is characterized by high AOA, 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 4-12] 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 AOA. For example,
if the basic wing stalls at 18° without flaps, then with the
addition of flaps to the CL-MAX position, the new AOA that
the wing 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 C L-MAX. Several methods are employed (such as
suction and use of a blowing boundary layer control), but the
most common device used on general aviation light aircraft
is the vortex generator. Small strips of metal placed along
the wing (usually in front of the control surfaces) create
turbulence. The turbulence in turn mixes high energy air from
outside the boundary layer with boundary layer air. The effect
is similar to other boundary layer devices. [Figure 4-13]
Small Airplanes
Most small airplanes maintain a speed well in excess of 1.3
times VSO on an instrument approach. An airplane with a
stall speed of 50 knots (V SO) has a normal approach speed
of 65 knots. However, this same airplane may maintain 90
knots (1.8 VSO) while on the final segment of an instrument
approach. The landing gear will most likely be extended at
the beginning of the descent to the minimum descent altitude,
or upon intercepting the glideslope of the instrument landing
system. The pilot may also select an intermediate flap setting
for this phase of the approach. The airplane at this speed has
good positive speed stability, as represented by point A on
Figure 4-11. Flying in this regime permits the pilot to make
slight pitch changes without changing power settings, and
Horizontal
component of lift
Resultant lift
Vertical
component of lift
Weight
Figure 4-14. Forces in a turn.
Excess power is the available power over and above that
required to maintain horizontal flight at a given speed.
Although the terms power and thrust are sometimes
used interchangeably (erroneously implying they are
synonymous), distinguishing between the two is important
when considering climb performance. Work is the product of
a force moving through a distance and is usually independent
of time. Power implies work rate or units of work per unit
of time, and as such is a function of the speed at which the
force is developed. Thrust, also a function of work, means
the force which imparts a change in the velocity of a mass.
During takeoff, the aircraft does not stall even though it
may be in a climb near the stall speed. The reason is that
excess power (used to produce thrust) is used during this
flight regime. Therefore, it is important if an engine fails
after takeoff, to compensate the loss of thrust with pitch
and airspeed.
For a given weight of the aircraft, the angle of climb depends
on the difference between thrust and drag, or the excess
thrust. When the excess thrust is zero, the inclination of the
flightpath is zero, and the aircraft is in steady, level flight.
When thrust is greater than drag, the excess thrust allows a
climb angle depending on the amount of excess thrust. When
thrust is less than drag, the deficiency of thrust induces an
angle of descent.
Acceleration in Cruise Flight
Aircraft accelerate in level flight because of an excess of
power over what is required to maintain a steady speed. This
is the same excess power used to climb. Upon reaching the
desired altitude with pitch being lowered to maintain that
altitude, the excess power now accelerates the aircraft to its
cruise speed. However, reducing power too soon after level
off results in a longer period of time to accelerate.
Turns
Like any moving object, an aircraft requires a sideward force
to make it turn. In a normal turn, this force is supplied by
banking the aircraft in order to exert lift inward, as well as
upward. The force of lift is separated into two components
at right angles to each other. [Figure 4-14] The upward
acting lift together with the opposing weight becomes the
vertical lift component. The horizontally acting lift and its
opposing centrifugal force are the horizontal lift component,
or centripetal force. This horizontal lift component is the
sideward force that causes an aircraft to turn. The equal and
opposite reaction to this sideward force is centrifugal force,
which is merely an apparent force as a result of inertia.
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
unless the pilot takes a prompt corrective action. Proper pitch
and power coordination is critical in this region due to speed
instability and the tendency of increased divergence from
the desired speed.
Large Airplanes
Pilots of larger airplanes with higher stall speeds may find the
speed they maintain on the instrument approach is near 1.3
VSO, putting them near point C [Figure 4-11] the entire time
the airplane is on the final approach segment. In this case,
precise speed control is necessary throughout the approach. It
may be necessary to temporarily select excessive, or deficient
thrust in relation to the target thrust setting in order to quickly
correct for airspeed deviations.
For example, a pilot is on an instrument approach at 1.3
VSO, a speed near L/DMAX, and knows that a certain power
setting maintains that speed. The airplane slows several knots
below the desired speed because of a slight reduction in the
power setting. The pilot increases the power slightly, and the
airplane begins to accelerate, but at a slow rate. Because the
airplane is still in the “flat part” of the drag curve, this slight
increase in power will not cause a rapid return to the desired
speed. The pilot may need to increase the power higher
than normally needed to maintain the new speed, allow the
airplane to accelerate, then reduce the power to the setting
that maintains the desired speed.
Climbs
The ability for an aircraft to climb depends upon an excess
power or thrust over what it takes to maintain equilibrium.
Radius≈1,500 Radius≈3,500 Radius≈6,500 Radius≈8,000 Radius≈3,500 Radius≈2,000
Figure 4-15. Turns.
The relationship between the aircraft’s speed and bank angle
to the rate and radius of turns is important for instrument
pilots to understand. The pilot can use this knowledge to
properly estimate bank angles needed for certain rates of turn,
or to determine how much to lead when intercepting a course.
Rate of Turn
The rate of turn, normally measured in degrees per second,
is based upon a set bank angle at a set speed. If either one
of these elements changes, the rate of turn changes. If the
aircraft increases its speed without changing the bank angle,
the rate of turn decreases. Likewise, if the speed decreases
without changing the bank angle, the rate of turn increases.
Changing the bank angle without changing speed also causes
the rate of turn to change. Increasing the bank angle without
changing speed increases the rate of turn, while decreasing
the bank angle reduces the rate of turn.
The standard rate of turn, 3° per second, is used as the main
reference for bank angle. Therefore, the pilot must understand
how the angle of bank varies with speed changes, such
as slowing down for holding or an instrument approach.
Figure 4-15 shows the turn relationship with reference to a
constant bank angle or a constant airspeed, and the effects on
rate of turn and radius of turn. A rule of thumb for determining
the standard rate turn is to divide the airspeed by ten and
add 7. An aircraft with an airspeed of 90 knots takes a bank
angle of 16° to maintain a standard rate turn (90 divided by
10 plus 7 equals 16°).
Radius of Turn
The radius of turn varies with changes in either speed or
bank. If the speed is increased without changing the bank
angle, the radius of turn increases, and vice versa. If the speed
is constant, increasing the bank angle reduces the radius of
turn, while decreasing the bank angle increases the radius of
turn. This means that intercepting a course at a higher speed
requires more distance, and therefore, requires a longer lead.
If the speed is slowed considerably in preparation for holding
or an approach, a shorter lead is needed than that required for
cruise flight.
Coordination of Rudder and Aileron Controls
Any time ailerons are used, adverse yaw is produced. Adverse
yaw is caused when the ailerons are deflected as a roll motion
(as in turn) is initiated. In a right turn, the right aileron is
deflected upward while the left is deflected downward. Lift is
increased on the left side and reduced on the right, resulting
in a bank to the right. However, as a result of producing lift
on the left, induced drag is also increased on the left side.
The drag causes the left wing to slow down, in turn causing
the nose of the aircraft to initially move (left) in the direction
opposite of the turn. Correcting for this yaw with rudder, when
entering and exiting turns, is necessary for precise control of
the airplane when flying on instruments. The pilot can tell if
the turn is coordinated by checking the ball in the turn-and-
slip indicator or the turn coordinator. [Figure 4-16]
As the aircraft banks to enter a turn, a portion of the wing’s
vertical lift becomes the horizontal component; therefore,
without an increase in back pressure, the aircraft loses altitude
during the turn. The loss of vertical lift can be offset by
increasing the pitch in one-half bar width increments. Trim
may be used to relieve the control pressures; however, if used,
it has to be removed once the turn is complete.
In a slipping turn, the aircraft is not turning at the rate
appropriate to the bank being used, and the aircraft falls to
the inside of the turn. The aircraft is banked too much for the
rate of turn, so the horizontal lift component is greater than
the centrifugal force. A skidding turn results from excess of
2 MIN TURN
DC ELEC
L R
TURN COORDINATOR
2 MIN.
D.C.
ELEC.
L R
NO PITCH
INFORMATION
2 MIN TURN
DC ELEC
L R
TURN COORDINATOR
2 MIN.
D.C.
ELEC.
L R
NO PITCH
INFORMATION
2 MIN TURN
DC ELEC
L R
TURN COORDINATOR
2 MIN.
D.C.
ELEC.
L R
NO PITCH
INFORMATION
Skidding Turn
Skidding Turn
Slipping Turn
Slipping Turn
Coordinated Turn
rudder into turn
Coordinated Turn
Note the slight differences in rudder placement.
Figure 4-16. Adverse yaw.
centrifugal force over the horizontal lift component, pulling
the aircraft toward the outside of the turn. The rate of turn
is too great for the angle of bank, so the horizontal lift
component is less than the centrifugal force.
An inclinometer, located in the turn coordinator, or turn and
bank indicator indicates the quality of the turn, and should
be centered when the wings are banked. If the ball is off of
center on the side toward the turn, the aircraft is slipping and
rudder pressure should be added on that side to increase the
rate of turn or the bank angle should be reduced. If the ball
is off of center on the side away from the turn, the aircraft
is skidding and rudder pressure toward the turn should be
relaxed or the bank angle should be increased. If the aircraft
is properly rigged, the ball should be in the center when the
wings are level; use rudder and/or aileron trim if available.
The increase in induced drag (caused by the increase in
AOA necessary to maintain altitude) results in a minor loss
of airspeed if the power setting is not changed.
Load Factor
Any force applied to an aircraft to deflect its flight from a
straight line produces a stress on its structure; the amount of
this force is termed load factor. A load factor is the ratio of
the aerodynamic force on the aircraft to the gross weight of
the aircraft (e.g., lift/weight). For example, a load factor of 3
means the total load on an aircraft’s structure is three times
its gross weight. When designing an aircraft, it is necessary
to determine the highest load factors that can be expected in
normal operation under various operational situations. These
“highest” load factors are called “limit load factors.”
Aircraft are placed in various categories (i.e., normal, utility,
and acrobatic) depending upon the load factors they are
designed to take. For reasons of safety, the aircraft must be
designed to withstand certain maximum load factors without
any structural damage.