Propeller

Question 1

Blade section

Answer 1

Is a cambered aerofoil shape

Question 2

Blade back

Answer 2

The cambered/lifting side of the blade

Question 3

Blade face

Answer 3

Flatter side of the blade

Question 4

Blade angle

Answer 4

The angle the chord line makes with the plane of rotation

Question 5

Helix / pitch angle

Answer 5

The angle between the RAF / resultant velocity vector and the plane of rotation

Question 6

Blade angle = ___ + ____

Answer 6

AoA + helix angle

Question 7

Helical twist / blade twist

Answer 7

Refers to the propeller blades having a reduction in blade angle from hub to tip (this enables the propeller as a whole to operate efficiently as its designed to have the most efficient AoA along its whole length)

Question 8

Forces acting on a propeller blade

Answer 8

• Propeller torque force (component of TR in the plane of rotation)
• Thrust (component of TR perpendicular to the plane of rotation)

Question 9

Fixed pitch propeller - RPM / airspeed relationship

Answer 9

As airspeed increases the AoA of a fixed pitch blade at a constant RPM will decrease, so for a fixed pitch propeller there will be only one TAS at which the propeller will operate at its most efficient AoA

Question 10

Effective blade sections

Answer 10

The most effective blade sections of the propeller in producing thrust are those that lie between 60 - 90% of the radius with the greatest useful thrust being produced at approx 75% of the blade radius.

Question 11

Why are inner propeller sections less effective

Answer 11

Near the hub propeller sections must be thick for structural strength, this can interfere with with effective aerodynamic design, there is also interference to the airflow from the engine + associated structures and of necessity they must have a relatively high blade angle

Question 12

Why are outer propeller / tip sections less effective

Answer 12

Aerodynamic drag is inceased due to:
- vortices forming at prop tips
- the tip is the fastest travelling part so at the tip speeds may approach the speed of sound

Question 13

Constant speed propeller (variable pitch)

Answer 13

Propeller pitch can be adjusted to obtain optimum AoA over a wider range of airspeeds

Question 14

CSU

Answer 14

Constant speed unit - it contains a governor which controls the rpm to that selected by the pilot by adjusting the pitch angle of the blades so that propeller torque remains equal and opposite to engine torque regardless of changes to airspeed and / or throttle setting

Question 15

Constant speed propeller - Throttle opened

Answer 15

MP increases so engine torque increases, as a result the rpm will want to increase but as soon as the CSU senses this it will increase the blade angel (coarsen the pitch) so the prop torque is increased to match the increase in engine torque. The selected rpm will be maintained because the increase in engine power has been absorbed by the increased AoA of the blades. At this higher AoA the propeller will produce more thrust and there will be a gain in performance

Question 16

Constant speed propeller - Throttle closed

Question 17

Propeller efficiency

Question 18

Windmilling

Answer 18

If there is a loss in engine torque to the prop the CSU will fine off the pitch in an attempt to maintain the rpm to that selected by the pilot, a point is reached where the RAF approaches the blade at a negative AoA that is large enough to produce TR in the opposite direction.