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Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
Basic Propeller Principles

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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




Basic Propeller Principles

The aircraft propeller consists of two or more blades and a
central hub to which the blades are attached. Each blade of
an aircraft propeller is essentially a rotating wing. As a result
of their construction, the propeller blades are like airfoils
and produce forces that create the thrust to pull, or push,
the aircraft through the air. The engine furnishes the power
needed to rotate the propeller blades through the air at high
speeds, and the propeller transforms the rotary power of the
engine into forward thrust.

A cross-section of a typical propeller blade is shown in
Figure 4-35. This section or blade element is an airfoil
comparable to a cross-section of an aircraft wing. One
surface of the blade is cambered or curved, similar to the
upper surface of an aircraft wing, while the other surface is
.at like the bottom surface of a wing. The chord line is an
imaginary line drawn through the blade from its leading edge
to its trailing edge. As in a wing, the leading edge is the thick
edge of the blade that meets the air as the propeller rotates.

Airfoil sections of propeller blade.
Figure 4-35. Airfoil sections of propeller blade.

Blade angle, usually measured in degrees, is the angle
between the chord of the blade and the plane of rotation
and is measured at a specific point along the length of the
blade. [Figure 4-36] Because most propellers have a .at
blade "face," the chord line is often drawn along the face
of the propeller blade. Pitch is not blade angle, but because
pitch is largely determined by blade angle, the two terms are
often used interchangeably. An increase or decrease in one is
usually associated with an increase or decrease in the other.

The pitch of a propeller may be designated in inches. A
propeller designated as a "74-48" would be 74 inches in
length and have an effective pitch of 48 inches. The pitch
is the distance in inches, which the propeller would screw
through the air in one revolution if there were no slippage.

When specifying a fixed-pitch propeller for a new type of
aircraft, the manufacturer usually selects one with a pitch
that operates efficiently at the expected cruising speed of the
aircraft. Every fixed-pitch propeller must be a compromise
because it can be efficient at only a given combination of
airspeed and revolutions per minute (rpm). Pilots cannot
change this combination in flight.

When the aircraft is at rest on the ground with the engine
operating, or moving slowly at the beginning of takeoff,
the propeller efficiency is very low because the propeller is
restrained from advancing with sufficient speed to permit
its fixed-pitch blades to reach their full efficiency. In this
situation, each propeller blade is turning through the air at
an AOA that produces relatively little thrust for the amount
of power required to turn it.

Propeller blade angle.
Figure 4-36. Propeller blade angle.

To understand the action of a propeller, consider first its
motion, which is both rotational and forward. As shown by
the vectors of propeller forces in Figure 4-36, each section of
a propeller blade moves downward and forward. The angle
at which this air (relative wind) strikes the propeller blade is
its AOA. The air deflection produced by this angle causes the
dynamic pressure at the engine side of the propeller blade to
be greater than atmospheric pressure, thus creating thrust.

The shape of the blade also creates thrust because it is
cambered like the airfoil shape of a wing. As the airflows
past the propeller, the pressure on one side is less than that
on the other. As in a wing, a reaction force is produced in the
direction of the lesser pressure. The airflow over the wing
has less pressure, and the force (lift) is upward. In the case
of the propeller, which is mounted in a vertical instead of a
horizontal plane, the area of decreased pressure is in front of
the propeller, and the force (thrust) is in a forward direction.
Aerodynamically, thrust is the result of the propeller shape
and the AOA of the blade.

Thrust can be considered also in terms of the mass of air
handled by the propeller. In these terms, thrust equals mass
of air handled multiplied by slipstream velocity minus
velocity of the aircraft. The power expended in producing
thrust depends on the rate of air mass movement. On average,
thrust constitutes approximately 80 percent of the torque
(total horsepower absorbed by the propeller). The other
20 percent is lost in friction and slippage. For any speed of
rotation, the horsepower absorbed by the propeller balances
the horsepower delivered by the engine. For any single
revolution of the propeller, the amount of air handled depends
on the blade angle, which determines how big a "bite" of
air the propeller takes. Thus, the blade angle is an excellent
means of adjusting the load on the propeller to control the
engine rpm.

The blade angle is also an excellent method of adjusting the
AOA of the propeller. On constant-speed propellers, the blade
angle must be adjusted to provide the most efficient AOA at
all engine and aircraft speeds. Lift versus drag curves, which
are drawn for propellers, as well as wings, indicate that the
most efficient AOA is small, varying from +2° to +4°. The
actual blade angle necessary to maintain this small AOA
varies with the forward speed of the aircraft.