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Instrument Flying Handbook
Aerodynamic Factors
Drag Curves

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Instrument Flying
Handbook

Preface

Table of Contents

Chapter 1. Human Factors
Chapter 2. Aerodynamic Factors
Chapter 3. Flight Instruments
Chapter 4. Section I
Airplane Attitude Instrument
Flying
Using Analog Instrumentation
Chapter 4. Section II
Airplane Attitude Instrument
Flying
Using an Electronic Flight
Display

Chapter 5. Section I
Airplane Basic
Flight Maneuvers
Using Analog Instrumentation
Chapter 5. Section II
Airplane Basic
Flight Maneuvers
Using an Electronic Flight
Display

Chapter 6. Helicopter
Attitude Instrument Flying

Chapter 7. Navigation Systems
Chapter 8. The National
Airspace System

Chapter 9. The Air Traffic
Control System

Chapter 10. IFR Flight
Chapter 11. Emergency
Operations

If a chart is not available the density altitude can he estimated
by adding 120 feet for every degree Celsius above the ISA. For
example, at 3000 feet pressure altitude (PA), the ISA prediction
is 9°C (15°C - lapse rate of 2° C per l,000 feet x 3 = 6°C).
However, if the actual temperature is 20° C (11° C more than
that predicted by ISA) then the difference of 11° C is multiplied
by 120 feet equaling 1,320. Adding this figure to the original
3,000 feet provides a density altitude of 4,320 feet (3,000 feet
+ 1,320 feet).

Lift

Lift always acts in a direction perpendicular to the relative
wind and to the lateral axis of die aircraft. The fact that lift is
referenced to the wing, not to the Earth's surface, is die source
of many errors in learning flight control Lift is not always
"up." Its direction relative to the Earth's surface changes as
die pilot maneuvers the aircraft.

The magnitude of the force of lift is directly proportional to
the density of the air, the area of the wings, and the airspeed. It
also depends upon the type of wing and the angle of attack. Lift
increases with an increase in angle of attack up to the stalling
angle. at which point it decreases with any further increase
in angle of attack. In conventional aircraft, lift is therefore
controlled by varying the angle of attack and speed.

Relationship of Lift to Angle of Attack.
Figure 2-7. Relationship of Lift to Angle of Attack.

Pitch Power Relationship
An examination of Figure 2-7 illustrates the relationship
between pitch and power while controlling flight path and
airspeed. In order to maintain a constant lift, as airspeed is
reduced, pitch must he increased. The pilot controls pitch
through the elevators, which control the angle of attack.
When back pressure is applied on the elevator control, the tail
lowers and the nose rises, thus increasing the wing's angle of
at1aek and lift. Under most conditions the elevator is placing
downward pressure on the tail. This pressure requires energy
that is taken from aircraft performance (speed). Therefore,
when the CO is closer to the aft portion of the aircraft the
elevator downward Forces are less, this results in less energy
used for downward forces, in torn resulting in more energy
applied to aircraft performance.

Thrust is controlled by using the throttle to establish or
maintain desired airspeeds. The most precise method
of controlling flight path is to use pitch control while
simultaneously using power (thrust) to control airspeed. In
order to maintain a constant lift., a change in pitch requires a
change in power, and vice versa.

If the pilot wants the aircraft to accelerate while maintaining
altitude, thrust must be increased to overcome drag. As
the aircraft speeds up, lift is increased. To prevent gaining
altitude, the pitch angle must be lowered to reduce the
angle of attack and maintain altitude. To decelerate while
maintaining altitude, thrust must be decreased to less than die
value of drag. As the aircraft slows down, lift is reduced. To
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 arc plotted on a graph,
die 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 8
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 die 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.

 

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