| Home | Privacy | Contact |

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

| First | Previous | Next | Last |

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




Gyroscopic Action
Before the gyroscopic effects of the propeller can be
understood, it is necessary to understand the basic principle
of a gyroscope. All practical applications of the gyroscope
are based upon two fundamental properties of gyroscopic
action: rigidity in space and precession. The one of interest
for this discussion is precession.

Precession is the resultant action, or deflection, of a spinning
rotor when a deflecting force is applied to its rim. As can be
seen in Figure 4-41, when a force is applied, the resulting
force takes effect 90° ahead of and in the direction of

Gyroscopic precession.
Figure 4-41. Gyroscopic precession.

The rotating propeller of an airplane makes a very good
gyroscope and thus has similar properties. Any time a force
is applied to deflect the propeller out of its plane of rotation,
the resulting force is 90° ahead of and in the direction of
rotation and in the direction of application, causing a pitching
moment, a yawing moment, or a combination of the two
depending upon the point at which the force was applied.

This element of torque effect has always been associated with
and considered more prominent in tailwheel-type aircraft,
and most often occurs when the tail is being raised during
the takeoff roll. [Figure 4-42] This change in pitch attitude
has the same effect as applying a force to the top of the
propeller's plane of rotation. The resultant force acting 90°
ahead causes a yawing moment to the left around the vertical
axis. The magnitude of this moment depends on several
variables, one of which is the abruptness with which the tail
is raised (amount of force applied). However, precession,
or gyroscopic action, occurs when a force is applied to any
point on the rim of the propeller's plane of rotation; the
resultant force will still be 90° from the point of application
in the direction of rotation. Depending on where the force is
applied, the airplane is caused to yaw left or right, to pitch
up or down, or a combination of pitching and yawing.

It can be said that, as a result of gyroscopic action, any yawing
around the vertical axis results in a pitching moment, and any
pitching around the lateral axis results in a yawing moment.
To correct for the effect of gyroscopic action, it is necessary
for the pilot to properly use elevator and rudder to prevent
undesired pitching and yawing.

Asymmetric Loading (P-Factor)
When an aircraft is flying with a high AOA, the "bite" of
the downward moving blade is greater than the "bite" of the
upward moving blade. This moves the center of thrust to the
right of the prop disc area, causing a yawing moment toward
the left around the vertical axis. To prove this explanation is
complex because it would be necessary to work wind vector
problems on each blade while considering both the AOA of
the aircraft and the AOA of each blade.

Raising tail produces gyroscopic precession.
Figure 4-42. Raising tail produces gyroscopic precession.

This asymmetric loading is caused by the resultant velocity,
which is generated by the combination of the velocity of the
propeller blade in its plane of rotation and the velocity of the
air passing horizontally through the propeller disc. With the
aircraft being flown at positive AOAs, the right (viewed from
the rear) or downswinging blade, is passing through an area of
resultant velocity which is greater than that affecting the left
or upswinging blade. Since the propeller blade is an airfoil,
increased velocity means increased lift. The downswinging
blade has more lift and tends to pull (yaw) the aircraft's nose
to the left.

When the aircraft is flying at a high AOA, the downward
moving blade has a higher resultant velocity, creating more
lift than the upward moving blade. [Figure 4-43] This might
be easier to visualize if the propeller shaft was mounted
perpendicular to the ground (like a helicopter). If there
were no air movement at all, except that generated by the
propeller itself, identical sections of each blade would have
the same airspeed. With air moving horizontally across this
vertically mounted propeller, the blade proceeding forward
into the flow of air has a higher airspeed than the blade
retreating with the airflow Thus, the blade proceeding into
the horizontal airflow is creating more lift, or thrust, moving
the center of thrust toward that blade. Visualize rotating the
vertically mounted propeller shaft to shallower angles relative
to the moving air (as on an aircraft). This unbalanced thrust
then becomes proportionately smaller and continues getting
smaller until it reaches the value of zero when the propeller
shaft is exactly horizontal in relation to the moving air.

The effects of each of these four elements of torque vary in
value with changes in flight situations. In one phase of flight,
one of these elements may be more prominent than another.
In another phase of flight, another element may be more
prominent. The relationship of these values to each other
varies with different aircraft—depending on the airframe,
engine, and propeller combinations, as well as other design
features. To maintain positive control of the aircraft in all
flight conditions, the pilot must apply the flight controls as
necessary to compensate for these varying values.

Asymmetrical loading of propeller (P-factor).
Figure 4-43. Asymmetrical loading of propeller (P-factor).