Many people assume that aircraft propellers are simple components consisting entirely or mostly of blades. When the engine runs, the blades spin. The blades’ curved shape pushes the air backward, which propels the airplane forward. Some aircraft propellers, however, feature counterweights as well.
The Basic of Counterweights
In aviation, counterweights are weights that help control the pitch of an airplane’s propeller blades. They leverage the centrifugal force of a spinning propeller to adjust the blade angle.
Some counterweights are heavier than others. Small airplanes typically have light counterweights weighing just 1 to 5 pounds. Larger airplanes, on the other hand, have heavier counterweights weighing 10 to 20 pounds. Regardless, they all share the same basic purpose of pitch control.
What Type of Propellers Use Counterweights?
Not all aircraft propellers use counterweights. Rather, they are limited to adjustable-pitch propellers. There are two primary types of aircraft propellers: fixed-pitch and adjustable-pitch. As the name suggests, fixed-pitch propellers have a static blade pitch that’s determined by the manufacturer.
Adjustable-pitch propellers, on the other hand, can change their blade pitch. Depending on the specific type, the blade pitch can be adjusted on the ground or in-flight via cockpit controls. Counterweights are used exclusively with adjustable-pitch propellers. Fixed-pitch propellers don’t support pitch adjustments, so there’s no reason for them to have counterweights.
How Propeller Counterweights Work
Propeller counterweights work by using the spinning blades’ own force to make pitch adjustments. When the blades spin, they produce a centrifugal force. Counterweights are placed relative to the pitch axis, so this centrifugal force creates a torque. This torque will help to move the blades to the desired pitch position.
RPMs and Counterweights
The rate at which the propeller blades spin will affect the counterweights’ centrifugal force. RPM and centrifugal force are directly correlated. When RPM increases, so will the centrifugal force acting on the counterweights. When RPM decreases, the centrifugal force will decrease as well.
Feathering and Counterweights
Counterweights are commonly used for feathering. Feathering is the process of rotating a propeller to a very high pitch. It’s designed to reduce drag and improve efficiency.
Some aircraft propellers use counterweights as part of their feathering system. The counterweights help to move the blades toward the “feathered” position. With that said, there are different types of feathering systems, some of which use other methods to achieve this blade position.
