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What is the total reaction of a propellor comprised of?
- Torque (rotational drag) - in line with the plane of rotation
- Thrust - corresponds to lift as the productive force
What is the effect of speed on a fixed pitch prop?
- 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
What is the principle of operation of a constant speed propellor?
- 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.
What are the general design requirements of a propellor?
- 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
What are the requirements for efficiency?
- Large diameter forsmall slipstream velocity
- High RPM to reduce RAF helix angle at high speeds
- Efficient blade section and AoA
What is the diameter of a prop limited by?
- Undercarriage and spacing
- Compressibility effects with high tip velocities. Compressibility increases torque and decreases thrust
What is the formula for prop solidity?
Solidity= No. of blades x Chord at radius r/ Circumference at radius r
How can an increase in solidity be achieved?
- Increasing blade chord (penalty of drag losses and increased Centrifugal Twisting Moment)
- Increasing the number of blades ( more than 5 is inefficient)
Why are the root sections of the prop more cambered than the tips?
High lift blade sections suffer most from compressibility effects therefore blade tips need more of a high speed design
A compromise between RPM and diameter must be reached to avoid what?
What is Centrifugal Twisting Moment?
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.
What is Aerodynamic Twisting Moment?
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
What causes a windmilling propellor?
- 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