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Figure 12.1 Blade Nomenclature

Blade Terminology

Most of the terms in the diagram below are explained fully in the principles of flight book and are repeated here as a reminder. Those that are important from the mechanical point of view we will discuss further.

Figure 11.2 Terminology Figure 12.2 Terminology

Pitch, or Blade Angle

The propeller blade is set into its hub so that its chord line forms an angle with the plane of rotation of the whole propeller. This is called pitch, or blade angle.

Figure 12.3 Blade angle Angle of Attack

The path of the propeller blade through the air, a helix, determines the direction from which it will receive its relative airflow. This path is the resultant of blade rotational velocity and aircraft forward velocity. The blade angle is chosen so that the leading edge is pointing into the relative airflow at a small angle of attack. (Ideally 2-4 degrees).

Figure 12.4 Angle of Attack

Blade Twist

As the rotational speed of any point on a propeller blade increases with its radius from the centre of the hub, then the magnitude of the total reaction generated along the blade will also increase with increase of radius.

This would lead to a marked increase in thrust developed at the outer part of the blade when compared with the root area, which would exaggerate the bending forces along the blade.

To even out the thrust developed along the blade, the angle of attack is maintained by reducing the blade angle from root to tip.

Figure 11.5 Figure 12.5

FIXED PITCH PROPELLERS Disadvantages

A fixed pitch propeller receives its relative airflow from a direction governed by the aircraft’s true airspeed (TAS) in the direction of flight and its own RPM in the plane of rotation. The operating angle of attack will be the angle between the relative airflow and the chord line of the propeller blade. This chord line will be set at an angle to the plane of rotation; the “blade angle”

or propeller “pitch angle”.

Referring to Figure 12.6, it can be seen that an increase in TAS will reduce the angle of attack, whereas an increase in RPM will increase it.

Figure 12.6 Reduced TAS and increased RPM Propeller Efficiency

At high forward speed/low RPM (power off dive) it is possible to reduce the angle of attack to zero, while at low TAS/high RPM (climb) it is possible to stall the propeller blade. Both extremes are obviously inefficient and therefore undesirable. The conclusion that must be drawn is that for a given fixed pitch, a propeller will only work efficiently at one combination of TAS and RPM. The efficiency achieved will usually be in the range 80-90% and is properly rendered as:

Propeller efficiency % = Thrust power x 100 Engine power

Figure 11.7 Propeller Efficiency CurvesFigure 12.7 Propeller Efficiency Curves

With a fixed pitch propeller being driven by a Piston Engine, the RPM is dependent on the power setting (throttle position) selected by the pilot and the TAS of the aircraft. It would be possible to overspeed the engine in a dive if the throttle were not backed off (closed). Conversely, with the aircraft stationary on the ground it may not be possible to achieve rated RPM with the throttle fully open.

VARIABLE PITCH (CONSTANT SPEED) PROPELLERS Advantages over Fixed Pitch Propellers

The power setting of a piston engine is defined by a combination of manifold pressure (boost) and RPM. Where separate Power Lever and RPM Lever control is provided, it is possible to vary one while leaving the other constant, so optimising the operation of the engine/propeller combination to give best efficiency/fuel economy and least engine wear and tear.

In order to achieve this a “Variable Pitch” propeller must be used; enabling the pilot to select a propeller pitch and thus to vary RPM independently of manifold pressure, provided that the propeller is operating between its internal fine and coarse pitch stops.

Once an RPM has been selected, a control unit (CSU - Constant Speed Unit or PCU - Propeller Control Unit) will automatically vary the propeller pitch angle and therefore its angle of attack to the prevailing relative airflow in order to maintain the selected RPM despite airspeed and manifold pressure variations.

Variable pitch propellers can also incorporate a “Feathering” feature, the advantages of which will be discussed later in this chapter.

ALPHA AND BETA RANGE Definitions

Referring to Figure 12.8 below, it can be seen that it is possible to provide a range of propeller blade angles ranging from “feathered”, as coarse as it is possible to go, all the way to “reverse pitch”, as fine as it is possible to go in normal propeller control.

The “Alpha” (flight) range of pitch angles ranges from “feathered” to “flight-fine” pitch, while the “Beta”(ground) range of angles is from “flight fine” pitch to “reverse” pitch.

The method of control within alpha and beta ranges will be described later in this chapter

Figure 11.8 Alpha and Beta RangeFigure 12.8 Alpha and Beta Range

VARIABLE PITCH PROPELLERS

The basic problem with varying pitch is twofold; one of actuation and one of control. The problem(s) and normal methods of solution will be examined in turn.

Actuation

Theoretically it should be perfectly possible to design either pneumatic or electrical actuation of a propeller’s pitch change mechanism, the former is unknown and the latter quite rare. The preferred method of pitch change actuation has turned out to be hydraulic, utilising the engine’s lubrication system as the source of hydraulic power. The pressure boosted where necessary by a small, additional oil pump mounted in the CSU or PCU.

SINGLE ACTING PROPELLER - PRINCIPLE OF OPERATION

A single acting propeller is constructed basically like any other, in that the blades are arranged around a central, engine-driven hub with the cylindrical hydraulic pitch-change mechanism mounted to the front.

The pitch change cylinder contains a moveable piston which is pushed rearwards by boosted engine oil pressure. Although it is possible to arrange things otherwise, usually this rearward movement of the piston will turn the propeller blades towards fine pitch. This is accomplished by a mechanical linkage behind the piston operating an actuating pin on the butt of each blade;

off-set so as to impart the correct range of angular motion.

Figure 11.9 The Single Acting PropellerFigure 12.9 The Single Acting Propeller

Blade rotation towards coarse pitch is provided by either a spring, or centrifugally actuated counter weights. Most propellers of this type, however, will contain both. Some propellers replace the spring with compressed gas, requiring a reversal of the hydraulic direction.

The springs have a dual function, they assist the centrifugal counterweights in operating the propeller blades to coarse pitch and, where this facility is provided, actuate the blades into the feathered position when RPM is low with consequent loss of centrifugal action.

CSU/PCU Functions

The function of the control unit in controlling RPM at the pilot’s command is to control the oil flow in three

modes:-Oil supply to fine pitch. (RPM increases)

¾

DOUBLE ACTING PROPELLER - PRINCIPLE OF OPERATION

The double acting propeller may be similar in mechanical operation to the single acting unit, or may achieve pitch angle change via a cam-slot operated, rotating bevel gear actuating bevel gear segments at the base of each blade .

The link operated mechanism will be used as the generic type for study purposes.

This type of propeller has a similar, if rather larger pitch change cylinder mounted to the front of the hub. It also contains an hydraulic piston, but this is now isolated from the centre of the hub and the fore-and-aft links provided with pressure seals. This allows hydraulic pressure to be directed to either side of the piston. Fine-pitch oil to one side and coarse-pitch oil to the other.

Assistance from springs or centrifugal counter-weights is therefore not required.

Figure 11.10 Double Acting PropellerFigure 12.10 Double Acting Propeller

CSU/PCU Functions

As with the single acting propeller’s controller, there are three control modes for the CSU\

PCU:

Deliver fine-pitch oil (increase RPM). Allow drain of coarse-pitch oil.

¾

Oil shut-off/hydraulic lock. (Constant RPM)

¾

Deliver coarse-pitch oil. (Decrease RPM) Allow drain of fine-pitch oil.

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THE CONSTANT SPEED PROPELLER - OPERATION

A constant speed propeller must be capable of all the pitch change operations mentioned above, as selected by operation of the RPM lever in the aircraft cockpit. It must also be capable of maintaining a selected RPM, within its own operational limits, through changes in airspeed, altitude and power setting.

When the CSU senses that RPM is as selected, no action ensues. However, changes in any of the above mentioned external conditions will result in a tendency to either increase RPM above, or decrease RPM below that selected.

A tendency for RPM to increase, an overspeed condition, must be met with a supply of oil to the coarse pitch side of the pitch change unit’s piston. The pitch will then coarsen and propeller torque will rise as a result of the increase in the blade angle of attack. Propeller torque now exceeds engine torque and will cause RPM to decrease back to the selected setting. As RPM drops back to where it should be, the valve selection in the CSU which caused the oil flow in the first place must be removed progressively.

A tendency for the propeller to underspeed must be met with the opposite reaction. A supply of oil must be sent to the fine pitch side of the operating piston to decrease the propeller’s pitch angle. This will decrease the propeller’s torque. Engine torque now exceeds propeller torque, so RPM will tend to rise to regain the pilot’s selection. When propeller torque equals engine torque, RPM remains constant.

Figure 11.11 Propeller with various pitch angles Figure 12.11 Propeller with various pitch angles

THE SIMPLE CONSTANT SPEED UNIT

Propeller pitch change and thus RPM are controlled by the Constant Speed Unit (CSU). This is engine driven and thus detects any changes to engine RPM so as to correct it via propeller pitch changes. Coarse pitch to correct an overspeed and fine pitch to correct an underspeed.

Figure 11.12 A Simple Constant Speed UnitFigure 12.12 A Simple Constant Speed Unit

A CSU is engine driven from a convenient gear, usually at the front of the engine, just behind the propeller itself. The drive shaft usually also drives a small oil pressure boosting pump to raise the pressure of the engine’s own lubrication supply to a more useful figure. (120-200 psi would be satisfactory)

The drive also rotates a centrifugal flyweight assembly in which the weights are “L” shaped and arranged to provide the upward movement of a double-landed hydraulic control valve. This upward force is opposed by a coil spring (speeder spring) acting downward on the control valve.

This spring is arranged such that its compressive downward force may be adjusted through the up and down movement of a rack and pinion. The pinion is rotated by pilot operation of the RPM lever. Pushing the RPM lever forward will rotate the pinion so that the rack is pushed down, compressing the spring and tending to push down the control valve. Pulling the RPM lever to the rear will result in spring compressive force being reduced.

The “On Speed” condition

The control valve receives pressure oil from the engine and the CSU booster pump and is arranged so that the oil is trapped and prevented from passing to the pitch change cylinder while the engine is “on speed” with no change of RPM selected. This is because the selected spring pressure downwards is exactly balanced by the flyweight force upwards as in Figure 12.12

The “Overspeed” condition

Should the engine’s torque exceed the torque generated by the propeller during flight, RPM would tend to rise. This will lead to a rise in centrifugally generated flyweight force and lift up the control valve against the spring force.

The rise of the control valve will expose the coarse pitch line to the pitch change cylinder so that pressure oil may flow to the coarse pitch side of the piston. At the same time, the fine pitch line is exposed and connected to drain.

The propeller blades will move towards coarse pitch, increasing their angle of attack to the relative airflow, generating more total reaction and thrust and raising the propeller’s torque.

When the propeller’s higher torque matches the engine’s torque, the rise in RPM will be arrested, the RPM returning to the selected setting. When this is achieved, the flyweights will fall back to their previous, balanced position with regard to spring force, the coarse and fine oil ports will close and the CSU resumes the “on speed” condition.

The “ Underspeed” condition

In this condition the propeller’s torque exceeds the engine’storque, causing RPM to decrease.

Centrifugal flyweight force will decline and the CSU’s spring force will now exceed that produced by the fly weight assembly. The flyweights will collapseinwards. This will cause the control valve to be pushed down by the spring force, exposing the fine pitch oil port to pressure, while connecting the coarse pitch oil port to drain.

Pressure oil will now flow to the fine pitch side of the pitch change piston, moving the propeller blades to a smaller angle of attack to the relative airflow. This will, in turn, cause a decrease in total reaction, thrust and propeller torque.