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 and produces propeller unbalance and destructive vibration and increases the weight of the blades.
A typical fluid system includes a tank to hold a supply of anti-icing fluid. [Figure 7-27] 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. The fluid under pressure of centrifugal force 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 not used on modern aircraft, giving way to the electric deicing systems.
An electric propeller-icing control system consists of an electrical energy source, a resistance heating element, system controls, and necessary wiring. [Figure 7-28] 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 a sequence or all on order.
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 7-29. 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 7-30] 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 check list or system inspection.
Propeller Synchronization and Synchrophasing
Most multi-engine aircraft are equipped with propeller synchronization 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.
A typical synchrophasing system is an electronic system. [Figure 7-31] 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 pickup 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.
An autofeather system is used normally 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 prop to feather) if two torque switches sense low torque from the engine. This system has a test-off-arm switch that is used to arm the system.