Prop Theory

• Torque (rotational drag) - in line with the plane of rotation
• Thrust - corresponds to lift as the productive force

• At no forward speed, the RAF lies virtually in the plane of rotation resulting in a high angle of attack
• As speed increases, the RAF changes to reduce the AOA.
• A speed is reached where the AOA is reduced to 0 therefore no thrust is produced

• If prop torque is excessive RPM will fall, a sensor will then cause the blade pitch to be reduced to reduce prop torque
• If prop torque is less than engine torque,RPM will rise therefore the blade pitch will be increased to increase prop torque
• Therefore the constant speed propellor develops constant torque for a given engine power/ RPM output regardless of speed.

• Must be as efficient as possible to minimise engine size, weight and fuel consumption
• Must be as small as possible to avoid long undercarriage legs and wide engine spacing

• Large diameter forsmall slipstream velocity
• High RPM to reduce RAF helix angle at high speeds
• Efficient blade section and AoA

• Undercarriage and spacing
• Compressibility effects with high tip velocities. Compressibility increases torque and decreases thrust

Solidity= No. of blades x Chord at radius r/ Circumference at radius r

• Increasing blade chord (penalty of drag losses and increased Centrifugal Twisting Moment)
• Increasing the number of blades ( more than 5 is inefficient)

High lift blade sections suffer most from compressibility effects therefore blade tips need more of a high speed design

Compressibility effects

As the propellor rotates, any particle of mass tends to be flung out along its radius and causes a twisting effect that acts to reduce the pitch. Size of the twisting moment depends upon RPM and blade width.

Arises from the airflow forces on the blade tending to coarsen the blade. In a steep dive the RAF may produce a negative AoA causing an opposite ATM

• Results from high speed and/or low RPM
• When the RAF provides a negative AoA, the TR acts to drive rather than drag the propellor.
• CTM and ATM now act to reduce the blade pitch thus increasing windmilling.
• Application of power will increase RPM (overspeed)
• Thrust is negative