As aircraft use became more vital in the movement of cargo and passengers, the necessity to fly in nearly all weather conditions became more impor- tant. However, before flight in bad weather could be considered safe, auxiliary systems had to be devised that could either prevent or remove ice formations from an aircraft. Like the aircraft structure, pro- pellers are susceptible to ice buildups and must be equipped with a system to remove ice accumula- tions. If allowed to accumulate, ice formations can distort a propeller blade's airfoil shape causing a loss in propeller efficiency and thrust. Furthermore, ice usually forms unevenly on a propeller blade and produces propeller unbalance and destructive vibration.
Currently, aircraft propellers may use either an anti-icing or a de-icing system. The difference between the two is that an anti-icing system prevents the
Figure 12-62. The synchrophasing control panel is mounted in the cockpit so cockpit crew members can adjust pro- peller blade phase angles in flight.
formation of ice whereas a de-icing system removes ice after it has accumulated.
FLUID ANTI-ICING
A typical fluid anti-icing system consists of a con- trol unit, a tank that holds a quantity of anti-icing fluid, a pump to deliver the fluid to the propeller, and nozzles. The control unit contains a rheostat which is adjusted to control the pump output. Fluid is pumped from the tank to a stationary nozzle
Figure 12-63. A typical propeller anti-icing system consists of a fluid tank, a rheostat control, a slinger ring for each propeller, and a fluid pump.
installed just behind the propeller on the engine nose case. As fluid passes through the nozzle, it enters a circular U-shaped channel called a slinger ring. A typical slinger ring is designed with a deliv- ery tube for each propeller blade and is mounted on the rear of the propeller assembly. Once the fluid is in the slinger ring, centrifugal force sends the anti-icing fluid out through the delivery tubes to each blade shank. [Figure 12-63]
In order to disperse the fluid to areas which are more prone to ice buildups, feed shoes are typically installed on the leading edge of each propeller blade. Each feed shoe consists of a narrow strip of rubber that extends from the blade shank out to a blade sta- tion that is approximately 75 percent of the propeller radius. Feed shoes are molded with several parallel open channels that allow centrifugal force to direct fluid from the blade shank toward the blade tip. As anti-icing fluid flows along the channels, the relative wind also carries the fluid laterally from the chan- nels over the leading edge of each blade.
The most commonly used anti-icing fluid is iso-propyl alcohol because of its availability and low
cost. Some other anti-icing fluids are made from phosphate compounds and are comparable to iso-propyl alcohol in anti-icing performance. Anti-icing fluids made from phosphate compounds also have the advantage of reduced flammability, however, they are comparatively expensive.
ELECTRIC DE-ICE
A typical propeller de-icing system is electrically operated. An electrical propeller de-icing system consists of a power source, power relay, resistance heating elements, system controls, and a timer or cycling unit. The resistance heating elements may be mounted either internally or externally on each pro- peller blade. Externally mounted heating elements are known as de-icing boots and are attached to each blade with an approved bonding agent. System con- trols include an on/off switch, loadmeter, and pro- tective devices such as current limiters or circuit breakers. The loadmeter is an ammeter which per- mits monitoring of individual circuit currents and visual verification of proper timer operation.
A typical electric propeller de-ice system supplies aircraft electrical system power to the propeller hub
Figure 12-64. Aircraft electrical power is used to operate this propeller de-icing system. When the timer closes the relay, electrical current flows to the carbon brushes which, in turn, pass the current to the rotating slip rings on the propeller hub. Flexible con- nectors carry the current from the slip rings to each heating element.
through a set of brush blocks and slip rings. The brush blocks are mounted on the engine case just behind the propeller while the slip rings are mounted on the back of the propeller hub assembly. Flexible connectors on the propeller hub transfer power from the slip rings to each heating element. [Figure 12-64]
Electrical de-icing systems are usually designed for intermittent application of power to the heating ele- ments for removal of small ice accumulations. De-icing effectiveness diminishes if ice accumulations are allowed to become excessive. Furthermore, proper control of heating intervals is critical in the prevention of runback. Runback refers to a condition where melted ice reforms behind a blade's leading edge. Heat should be applied just long enough to melt the ice face in contact with the blade. If the applied heat is more than that required to loosen the ice, but insufficient to evaporate all the resulting water, water can run back over the unheated blade surface and freeze again. Runback may result in a
dangerous build-up of ice on blade areas which have no de-icing protection.
hi addition to preventing runback, heating intervals are also carefully controlled to avoid excessive pro- peller vibrations. Ice accumulations must be removed from propeller blades evenly and in a balanced fash- ion from opposite blades to prevent excessive vibra- tion. This is accomplished through the use of timing circuits that cycle power in a predetermined sequence to the blade heating elements. Cycling timers energize the heating elements for periods of 15 to 30 seconds with a complete cycle time of two minutes.
The de-icing boots may be checked for proper warming sequences during pre-flight inspections by turning the system on and feeling the boots. However, exercise caution and limit ground testing to prevent element overheating. Electric propeller de-icing systems are designed for use when the pro- pellers are rotating, and for short periods of time during ground runup.