Propellors Flashcards
Describe the forces acting on a propellor blade
When airflow flows over the propeller blade, a change in pressure generates an aerodynamic force on the aerofoil.
A propeller TR is resolved into, one component perpendicular to plane of rotation (thrust) and one component in the plane of rotation called propeller torque (resistance to motion in the plane of rotation).
Describe the RPM/airspeed relationship.
If propeller RPM is constant, forward velocity determines the direction of the relative airflow and angle of attack.
AKA airspeed increase = AoA decrease
For a given RPM there’ll only be one forward velocity (true airspeed) at which the fixed-pitch propeller will operate at its most efficient AoA.
What are the most effective blade sections?
Most effective in producing thrust lie between 60 - 90% of the blade radius.
The greatest useful thrust being at approximately 75% of the blade radius.
Describe the forces on effective blade sections and why these sections are less effective. (Propeller hub, inner propellor section, outer section and propellor tip).
Near the hub propeller sections must be thick for structural strength. Which affects effective aerodynamic design.
the inner propeller sections are less effective because they must have a high blade angle. Effectively tilting the local TR more toward plane of rotation thus less of TR is resolved as forward thrust.
Smaller blade angle of the outer section the TR is tilted more forward providing more useful thrust.
Near propeller tip, vortices are formed and increase the aerodynamic drag resulting in a tilting back of the TR, thus near the tips of the propeller less useful thrust is produced.
The propeller tip is the fastest travelling part of the aircraft at tip. speeds approaching the local speed of sound theres a significant increase in drag which tilts the TR more rearward.
Explain the purpose of a Constant-speed propeller.
Constant speed propeller has a blade angle that automatically adjusts to any position between two in-flight limits at the fine and coarse ends of its range.
Varying the pitch of the blade will allow peak efficiency to be maintained over a wider range of speeds.
Describe the operation of a CSU
CSU governs the propeller speed at an RPM set by the pilot.
Operates by adjusting the pitch angle of the blade so that the propeller torque remains equal and opposite to engine torque regardless of airspeed or power setting.
Describe the operation of a CSU with changes in power setting.
At a given RPM, with throttle open (MAP increased), engine torque will increase. RPM will therefore want to increase so CSU will sense this and coarsen the pitch blade (blade angle) so propeller and engine torque are the same.
The selected RPM is maintained because the increase in engine power has been absorbed by the increase In AoA of the blades.
At this higher AoA the propeller will produce more thrust and there will be a gain in performance.
What happens if the throttle is closed?
CSU will decrease the blade angle (fine the pitch) so the selected RPM is maintained. Decrease in the AoA of the blade less thrust will be produced.
Describe the operation of the CSU with changes in airspeed.
If airspeed is reduced, propeller torque will increase because the propeller blades have a higher AoA at the lower airspeed, as a result rpm will want to drop
the CSU will fine off the pitch to keep propeller torque matched to engine torque thus maintaining the selected RPM.
If the airspeed is increased without adjusting the throttle, the CUS will coarsen the pitch to maintain the RPM.
Describe the correct procedure for handling manifold pressure and propeller controls.
Increase power:
- Increase mixture (Rich)
- increase RPM (Pitch)
- increase MP to desired value (Power)
Decrease power:
- Decrease MP to desired value (Power)
- decrease RPM (Pitch)
- increase mixture (Rich)
Describe the forces acting on a propeller during windmilling.
If there’s a loss in engine torque to the propeller, the CSU will fine off the pitch. A point is reached where RAF approaches the blade at a negative AoA which is large enough to produce a TR in the reverse direction to the normal.
torque component of this reversed TR now acts in the same direction as engine torque would normally act and will(windmill) the engine even though no power is being produced.
The reversed thrust component (windmilling drag) now also acts in the same direction as aircraft drag thus adding significant drag to the aircraft.
Describe the forces acting on a propeller when feathered.
Feathering involves turning the blades to the AoA with the oncoming airflow at which no net propeller torque is produced.
In this position, the blade sections nearer the hub will have a positive AoA, while those nearer the tips will have a negative AoA.
The two will cancel out and no turning moment on the propeller will be generated.
When the propeller is in the feather position minimal drag is produced.
Describe the forces acting on reverse thrust
If the propellers blades are turned through the fine pitch stop to a blade angle of about -20 degrees and power is applied reverse thrust is obtained.
Explain Centrifugal twisting moments
CTM - when the rotation of the propeller creates a centrifugal force on both sides of the blade wanting to make it fine pitch
Explain aerodynamic twisting moments.
The ATM occurs whenever the aerodynamic total reaction force (TR) does not act on a line through the pitch change axis.
In normal operation, the CoP of the blade is usually forward of the pitch change axis resulting in an ATM.
That tends to coarsen the blade angle but because ATM is much weaker it only partially offsets the CTM.
