The Effect of a Propeller Pitch Angle on an Airplane Thrust

Written by keith evans Google
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The Effect of a Propeller Pitch Angle on an Airplane Thrust
A propeller's pitch controls how much air it moves. (propeller image by Artem Lapenko from Fotolia.com)

On an aeroplane, the propeller or, on multi-engine planes, propellers provide the thrust needed to move the aeroplane forward. Acting as giant and powerful fans, propellers force air movement by slicing through the atmosphere at an angle. Adjusting this angle, known as the propeller pitch, can have a number of effects on the aeroplane's thrust, engines and even stopping power.

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Forward Thrust

As an aeroplane's propeller revolves, the pitch of the propeller causes it to scoop air and push the captured air backward. In response to this backward pressure on the air, the aeroplane moves forward. According to Recreational Aviation Australia, Incorporated, increasing the pitch of the propeller blade allows the blade to scoop more air; this increase in air volume provides a greater backward force that pulls the aeroplane forward with more power. The amount of air pushed toward the rear of the plane is tied to forward thrust so strongly, according to Recreational Aviation Australia, that the performance of an aeroplane depends largely on the propeller's pitch angle.

Free-Stream Velocity

As an aeroplane moves through the air, airflow over the plane's components creates considerable stress on the entire vehicle. If an engine becomes disabled, or if a pilot chooses to shut off the engine, air flowing over the propeller blades may force the blades to turn; this revolution of the blades can, in turn, create harmful pressure on the attached engine. According to the US Government's Centennial of Flight website, pilots of planes with adjustable propellers may choose to place the blades at an angle that allows them to move freely through the air without turning, a configuration known as a feathered propeller. By creating an angle that responds neutrally to passing air, an angle known as free-stream velocity, the plane's pilot can avert damage and prevent expensive repairs.

Negative Thrust

Though the pitch of an aeroplane's propeller can help move it through the air more quickly, some pilots adjust the propeller pitch to achieve the opposite effect. According to Recreational Aviation Australia, the pilot of a landing plane may alter the propeller's pitch angle to direct air toward the front of the plane rather than the rear. As a result of this change, the propeller creates a negative amount of thrust and the engine acts as a powerful brake to slow the plane after landing.

Variables

A number of factors work alongside pitch to affect an aeroplane's thrust. According to Recreational Aviation Australia, aeroplane manufacturers express propeller dimensions in terms of the number of blades, the diameter of the blades' revolution and the pitch. Just as increasing pitch can create additional power, using a longer blade or adding propellers can result in additional thrust.

Considerations

Although adjusting a propeller's pitch allows a pilot great flexibility in controlling the aeroplane, not all planes allow the pilot to alter pitch. On some aeroplanes with fixed-pitch propellers, according to the Centennial of Flight website, pilots must control a plane's thrust using only the engine speed. In addition, pilots of planes with adjustable-pitch propellers must maintain familiarity with operation of the propeller, as improperly adjusting pitch can create undesirable effects that include unexpected amounts of thrust and complete power loss.

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