Figure 11.14 Underspeed condition
BETA RANGE OPERATION
Some turboprop engines are provided with a system of control defined by “Alpha” and “Beta”
ranges of propeller operation. The Alpha range is used at high speed during the take-off run, in flight and during the initial, high speed part of the landing roll-out. The Beta range, however, is
Most propellers make this selection via a lever on the central control console, sometimes the throttle lever. Warning lights then illuminate to indicate that all propellers have carried out the selection, which is merely to move to a much finer pitch setting termed “Ground Fine Pitch”.
There will be a significant aerodynamic braking effect as the propeller goes into ground fine pitch. Power control is normal while taxiing and during the initial part of the take-off run. Later in the take-off run, however, the normal process of pitch coarsening with increasing TAS will cause the “Flight Fine Pitch” stop (inside the pitch control unit) to re-engage automatically.
Later propellers may be equipped with a much greater range of blade movement in the Beta range. Extending from around +8° to -3° pitch (full reverse), it is similarly selected at the same time as the older system, i.e. during the high-speed, initial part of the landing roll-out. In this case however, the braking effect from reverse pitch is much better than would result from merely ground fine.
When the flight fine pitch stop is withdrawn, the power lever can be moved rearward, through the gate into the beta range. Weight-on-wheels switches ensure that this can only happen on the ground. With the propeller (RPM) lever left at fully fine (max. RPM), the Beta range is controlled by rearward movement of the power lever. Pitch is increasingly made more negative as power is increased. RPM varies with PCU governor control being over-ridden as the power levers are so arranged as to raise and lower the PCU control valve to obtain the pitch changes required. A mechanical feed-back system resets the control valve to neutral once the required pitch angle has been obtained.
While the propeller blades are transiting into the reverse position, the PCU speeder spring is pushed downwards to give a downward selection of the control valve. This simulates an underspeed, ensuring that any pressure oil will be sent to the fine pitch side of the pitch change piston. The follow up cam on the blade root via a yoke, cam and beam linkage will remove the control valve selection when the desired blade angle has been achieved.
SYNCHRONISING
In order to reduce tiring noise and vibration on propeller driven aircraft, the Engine/Propeller assemblies are often provide with a means to equalise the RPM. A Synchronisation system will reduce the annoying “beat frequency” and lower noise levels significantly.
The aircraft will have a designated “Master Engine” whose PCU can generate an RPM signal to a control unit also receiving RPM signals from the other “slave” engines. When the synchronising system is engaged, any RPM differences between the master and slave engines will be sensed by the control unit. This generates proportional, positive or negative current output to torque motors mounted on the slave PCUs; such that lower RPM will cause the torque motor to turn one way, while higher rpm will cause a rotation of the torque motor in the opposite direction.
The torque motor rotation will reset the speeder spring to ensure a correction to slave RPM.
When no difference in RPM exists between “master” and “slave”, no output is sent to the slave torque motors. Many aircraft are provided with a visual indication (synchroscope) of slave engine RPM differences in the form of miniature propellers which only rotate when an RPM difference exists.
Figure 11.21 Woodward synchronization system for a light twinFigure 12.19 Woodward synchronization system for a light twin
Figure 11.22 The master engine arrangement of a transport aircraftFigure 12.20 The master engine arrangement of a transport aircraft
SYNCHROPHASING
A further significant improvement in noise levels can be obtained by ensuring that adjacent propeller tips are separated by some optimum angle to prevent noisy interference. Some aircraft provide the pilot with a means of manually “fine tuning” this angle to obtain the quietest result.
Figure 11.23 Synchrophasing Positions
Figure 12.21 Synchrophasing Positions REDUCTION GEARINGPurpose
Where a powerful aero engine needs a large propeller to convert its power into thrust, too large a diameter would bring the risk of sonic compressibility and blade flutter if the propeller were rotated too fast.
In order to be able to use a large diameter propeller, the engine, turning at its maximum RPM, cannot be directly connected to the propeller; so the drive speed must be reduced to a more suitable level by a reduction gear placed in the driveline between engine and propshaft.
Reduction Gear Types Parallel Spur Gear
¾
Figure 11.25 Two types of spur type reduction gear arrangement Figure 12.22 Two types of spur type reduction gear arrangement
This type of reduction gear, while mechanically simple and relatively cheap to produce, takes up a lot of room at the front of the engine as the axes of the gears are parallel. It has been used mostly on Vee type, in line, water-cooled engines. e.g. Rolls Royce Merlin and Griffon.
Epicyclic Reduction Gear
Figure 11.26 Spur and Bevel Planetary Gears
Figure 12.23 Spur and Bevel Planetary GearsThis layout is quite compact and has the advantage of concentric layout. Everything rotating about the same centre-line. The gears may be straight cut, bevelled, or helically cut to impart a degree of end-thrust which, being proportional to the torque passing through to the propeller, may be used to provide a torque indication system in the engine’s instrumentation.
TORQUE METER