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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




Torque and P-Factor
To the pilot, "torque" (the left turning tendency of the
airplane) is made up of four elements which cause or produce
a twisting or rotating motion around at least one of the
airplane's three axes. These four elements are:
1. Torque reaction from engine and propeller,
2. Corkscrewing effect of the slipstream,
3. Gyroscopic action of the propeller, and
4. Asymmetric loading of the propeller (P-factor).

Torque Reaction
Torque reaction involves Newton's Third Law of Physics—
for every action, there is an equal and opposite reaction. As
applied to the aircraft, this means that as the internal engine
parts and propeller are revolving in one direction, an equal
force is trying to rotate the aircraft in the opposite direction.
[Figure 4-39]

Torque reaction.
Figure 4-39. Torque reaction.

When the aircraft is airborne, this force is acting around
the longitudinal axis, tending to make the aircraft roll. To
compensate for roll tendency, some of the older aircraft are
rigged in a manner to create more lift on the wing that is being
forced downward. The more modern aircraft are designed
with the engine offset to counteract this effect of torque.

NOTE: Most United States built aircraft engines rotate the
propeller clockwise, as viewed from the pilot's seat. The
discussion here is with reference to those engines.

Generally, the compensating factors are permanently set
so that they compensate for this force at cruising speed,
since most of the aircraft's operating lift is at that speed.
However, aileron trim tabs permit further adjustment for
other speeds.

When the aircraft's wheels are on the ground during the
takeoff roll, an additional turning moment around the vertical
axis is induced by torque reaction. As the left side of the
aircraft is being forced down by torque reaction, more weight
is being placed on the left main landing gear. This results in
more ground friction, or drag, on the left tire than on the right,
causing a further turning moment to the left. The magnitude
of this moment is dependent on many variables. Some of
these variables are:
1. Size and horsepower of engine,
2. Size of propeller and the rpm,
3. Size of the aircraft, and
4. Condition of the ground surface.

This yawing moment on the takeoff roll is corrected by the
pilot's proper use of the rudder or rudder trim.

Corkscrew Effect
The high-speed rotation of an aircraft propeller gives a
corkscrew or spiraling rotation to the slipstream. At high
propeller speeds and low forward speed (as in the takeoffs
and approaches to power-on stalls), this spiraling rotation
is very compact and exerts a strong sideward force on the
aircraft's vertical tail surface. [Figure 4-40]

Corkscrewing slipstream.
Figure 4-40. Corkscrewing slipstream.

When this spiraling slipstream strikes the vertical fin it causes
a turning moment about the aircraft's vertical axis. The more
compact the spiral, the more prominent this force is. As the
forward speed increases, however, the spiral elongates and
becomes less effective. The corkscrew .ow of the slipstream
also causes a rolling moment around the longitudinal axis.

Note that this rolling moment caused by the corkscrew .ow
of the slipstream is to the right, while the rolling moment
caused by torque reaction is to the left—in effect one may
be counteracting the other. However, these forces vary
greatly and it is the pilot's responsibility to apply proper
corrective action by use of the flight controls at all times.
These forces must be counteracted regardless of which is
the most prominent at the time.