Propeller auxiliary systems improve the safety, reliability, and operational efficiency of aircraft propellers in various flight conditions. These systems include ice control, synchronization and synchrophasing, and autofeathering mechanisms that help maintain propeller performance, reduce vibration, and enhance aircraft controllability. This article explains the purpose and operation of these essential propeller auxiliary systems.
Ice Control Systems
Ice formation on a propeller blade, in effect, produces a distorted blade airfoil section that causes a loss in propeller efficiency. Generally, ice collects asymmetrically on a propeller blade, producing propeller unbalance, destructive vibration, and increased blade weight.
Anti-Icing Systems
A typical fluid system includes a tank to hold a supply of anti-icing fluid. [Figure 1]
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| Figure 1. Typical propeller fluid anti-icing system |
This fluid is forced to each propeller by a pump. The control system permits variation in the pumping rate so that the quantity of fluid delivered to a propeller can be varied, depending on the severity of icing.
Fluid is transferred from a stationary nozzle on the engine nose case into a circular U-shaped channel (slinger ring) mounted on the rear of the propeller assembly. Under centrifugal force, the fluid is transferred through nozzles to each blade shank.
Because airflow around a blade shank tends to disperse anti-icing fluids to areas where ice does not collect in large quantities, feed shoes, or boots, are installed on the blade leading edge. These feed shoes are a narrow strip of rubber extending from the blade shank to a blade station that is approximately 75 percent of the propeller radius. The feed shoes are molded with several parallel open channels in which fluid flows from the blade shank toward the blade tip by centrifugal force. The fluid flows laterally from the channels over the leading edge of the blade.
Isopropyl alcohol is used in some anti-icing systems because of its availability and low cost. Phosphate compounds are comparable to isopropyl alcohol in anti-icing performance and have the advantage of reduced flammability. However, phosphate compounds are comparatively expensive and, consequently, are not widely used. This system has disadvantages in that it requires several components that add weight to the aircraft, and the time of anti-ice available is limited to the amount of fluid on board. This system is no longer used on most modern aircraft, having been replaced by electric deicing systems.
Deicing Systems
An electric propeller-icing control system consists of an electrical power source, a resistance heating element, system controls, and necessary wiring. [Figure 2]
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| Figure 2. Typical electrical deicing system |
The heating elements are mounted internally or externally on the propeller spinner and blades. Electrical power from the aircraft system is transferred to the propeller hub through electrical leads, which terminate in slip rings and brushes. Flexible connectors are used to transfer power from the hub to the blade elements.
A deice system consists of one or more on-off switches. The pilot controls the operation of the deice system by turning on one or more switches. All deice systems have a master switch and may have another toggle switch for each propeller. Some systems may also have a selector switch to adjust for light or heavy icing conditions or automatic switching for icing conditions.
The timer or cycling unit determines the sequence of which blades (or portion thereof) are currently being deiced, and for what length of time. The cycling unit applies power to each deice boot, or boot segment, in sequence or all at once, depending on the system design.
A brush block, which is normally mounted on the engine just behind the propeller, is used to transfer electricity to the slip ring. A slip ring and brush block assembly is shown in Figure 3.
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| Figure 3. Deicing brush block and slip ring assembly |
The slip ring rotates with the propeller and provides a current path to the blade deice boots. A slip ring wire harness is used on some hub installations to electrically connect the slip ring to the terminal strip connection screw. A deice wire harness is used to electrically connect the deice boot to the slip ring assembly.
A deice boot contains internal heating elements or dual elements. [Figure 4]
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| Figure 4. Electric deice boot |
The boot is securely attached to the leading edge of each blade with adhesive.
Icing control is accomplished by converting electrical energy to heat energy in the heating element. Balanced ice removal from all blades must be obtained as nearly as possible if excessive vibration is to be avoided. To obtain balanced ice removal, variation of heating current in the blade elements is controlled so that similar heating effects are obtained in opposite blades.
Electric deicing systems are usually designed for intermittent application of power to the heating elements to remove ice after formation but before excessive accumulation. Proper control of heating intervals aids in preventing runback, since heat is applied just long enough to melt the ice face in contact with the blade. If heat supplied to an icing surface is more than that required for melting just the inner ice face, but insufficient to evaporate all the water formed, water will run back over the unheated surface and freeze. Runback of this nature causes ice formation on uncontrolled icing areas of the blade or surface.
Cycling timers are used to energize the heating element circuits for periods of 15 to 30 seconds, with a complete cycle time of 2 minutes. A cycling timer is an electric motor-driven contactor that controls power contactors in separate sections of the circuit. Controls for propeller electrical deicing systems include on-off switches, ammeters or loadmeters to indicate current in the circuits, and protective devices, such as current limiters or circuit breakers. The ammeters or loadmeters permit monitoring of individual circuit currents and reflect operation of the timer. To prevent element overheating, the propeller deicing system is used only when the propellers are rotating and for short test periods of time during the takeoff checklist or system inspection.
Propeller Synchronization and Synchrophasing
Most multi-engine aircraft are equipped with propeller synchronizing systems. Synchronization systems provide a means of controlling and synchronizing engine rpm. Synchronization reduces vibration and eliminates the unpleasant beat produced by unsynchronized propeller operation. The synchrophasing system is designed to maintain a preset angular relationship between the designated master propeller and the slave propellers.
A typical synchrophasing system is an electronic system. [Figure 5]
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| Figure 5. Synchrophasing system |
It functions to match the rpm of both engines and establish a blade phase relationship between the left and right propellers to reduce cabin noise. The system is controlled by a two-position switch located forward of the throttle quadrant. Turning the control switch on supplies direct current (DC) power to the electronic control box. Input signals representing propeller rpm are received from magnetic pickups on each propeller. The computed input signals are corrected to a command signal and sent to an rpm-trimming coil located on the propeller governor of the slow engine. Its rpm is adjusted to that of the other propeller.
Autofeathering System
An autofeather system is normally used only during takeoff, approach, and landing. It is used to feather the propeller automatically if power is lost from either engine. The system uses a solenoid valve to dump oil pressure from the propeller cylinder (this allows the propeller to feather) if two torque switches sense low torque from the engine. This system uses a TEST–OFF–ARM switch to arm the system.
The autofeather system automatically energizes the holding coil (pulling in the feather button) when a loss of engine power results in a propeller thrust drop to a preset value. This system is switch-armed for use during takeoff and can function only when the power lever is near or in the “takeoff” position.
The NTS device mechanically moves the NTS plunger, which actuates a linkage in the propeller control when a predetermined negative torque value is sensed (when the propeller drives the engine). This plunger, working through control linkage, shifts the feather valve plunger, sending the blades toward feather.
As the blade angle increases, negative torque decreases until the NTS signal is removed, closing the feather valve. If the predetermined negative torque value is again exceeded, the NTS plunger again causes the feather valve plunger to shift. The normal effect of the NTS is a cycling of rpm slightly below the rpm at which the negative torque was sensed.
Unfeathering is initiated by pulling the feather button to the “unfeather” position. This action supplies voltage to the auxiliary motor to drive the auxiliary pump. Because the propeller governor is in an underspeed position with the propeller feathered, the blades will move in a decreased pitch direction under auxiliary pump pressure.
The pitch lock operates in the event of a loss of propeller oil pressure or an overspeed. The ratchets of the assembly become engaged when the oil pressure, which keeps them apart, is dissipated through a flyweight-actuated valve, which operates at an rpm slightly above 100% rpm. The ratchets become disengaged when high pressure and rpm settings are restored.
At the “flight idle” power lever position, the control beta follow-up low-pitch stop on the beta set cam (on the alpha shaft) is set about 2° below the flight low-pitch stop setting, acting as a secondary low-pitch stop. At the “takeoff” power lever position, this secondary low-pitch stop sets a higher blade angle stop than the mechanical flight low-pitch stop. This provides for control of overspeed after rapid power lever advance, as well as a secondary low-pitch stop.