PTC B1.1 Notes - Sub Module 17.5 (Propeller Ice Protection)

PIATrainingCentre(PTC) Module17PROPELLERCategory A/B1 Sub Module 17.5 Propeller Ice Protection

ISO 9001 - 2008 Certified For Training Purpose Only

PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.5 Mar 2014

MODULE17

SubModule17.5

PROPELLERICEPROTECTION



PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.5 - i Mar 2014

Contents

INTRODUCTION1

FLUID(LIQUID)ICEPROTECTIONSYSTEMS2

ELECTRICALICEPROTECTIONSYSTEMS4



PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.5 - ii Mar 2014

Page Intentionally Left Blank



PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.5 - 1 Mar 2014

INTRODUCTION Propellers and spinners are exposed to an environment that under certain climatic conditions can lead to ice on the surface rapidly impairing their efficiency, leading to a loss of thrust and an increase in weight. Another problem with ice formation on a propeller is that if unevenly distributed, it can lead to an imbalance that will cause excessive vibration. Ice build up on a propeller can also lead to ice throw, where chunks of ice are thrown off the propeller at high speed due to centrifugal force. These lumps of ice can cause considerable damage.

Ice protection systems fall into two major categories depending upon the purpose for which the ice protection system is used. They are:

Anti-Icing De-icing




FLUID (LIQUID) ICE PROTECTION SYSTEMS Liquid ice protection systems can be used as either anti-ice or de-ice systems. The system is designed to project a film or fluid over the surface of the blade which when mixed with water will reduce its freezing point. If ice is already present the fluid will penetrate below the ice layer and reduce its surface tension sufficiently to enable it to be thrown off by centrifugal force. A typical fluid ice protection system is shown in Figure 17.5.1. The fluid is stored in a tank and passes via a filter to an electric supply pump. The pump is controlled by a switch on the instrument panel. In some installations the speed of the pump and thus the quantity of fluid supplied to the propeller can be varied by the use of a rheostat. The output fluid from the pump goes through pipelines which terminate at the rear of the propeller hub (Figure 17.5.2). Attached to the propeller hub is a U shaped channel called a slinger ring and from points around the slinger ring delivery nozzles are arranged to apply the fluid along the leading edge root section of each blade. Centrifugal force will then disperse the fluid along the blades leading edge and the airflow over the blades will allow a film of fluid to be deposited on the face and camber sides of the blades.

The airflow around the blade root however is fairly disturbed and does not always disperse the fluid where it is most required, that is, where ice build-up is greatest. Propellers with this type of ice protection system usually have boots or feed shoes installed along their leading edges. An overshoe consists of a strip of rubber or plastic material set into the leading edge of the blade, from the delivery nozzle at the root end along the blades length. The shoe extends approximately 2/3 of the length of the blade, and has several open parallel channels in which the fluid can flow under the influence of centrifugal force. The overflow of the channels along the length of the overshoe will evenly disperse the fluid over the blade.




Figure 17.5.01




Figure 17.5.02




ELECTRICAL ICE PROTECTION SYSTEMS Electrical ice protection systems are used on most turbo-props. Resistance wire heater elements are embedded in rubber and cemented from the root to about 2/3rds of the blade's length along the leading edge. The rubber is usually protected by wire gauze to withstand light stone damage and erosion. Often the aerodynamic spinner and engine intake lip are also protected from ice formation using this method.




Figure 17.5.03




This type of ice protection system works on the cyclic principle (Figures 17.5.04, 17.5.05 and 17.5.06). The current is fed to the propeller blades, spinner, and the engine intake lip by an automatic time switch. Part of the intake lip is continuously heated. This method ensures that the areas that have de-iced do not turn to water and then flow backwards to freeze again on the unheated trailing edge. The cyclic method also conserves electrical power so a smaller alternator can be installed. The cyclic timers have two speeds to use under differing ambient temperature conditions. Fast is used at temperatures from 6C to +10C when icing conditions are prevalent, e.g. in rain or clouds. Slow is used at temperatures of -6C and below.




Figure 17.5.04




Figure 17.5.05

Figure 17.5.06




The operation of the cyclic de-icing system is usually indicated by flashing lights (usually green or blue) or an ammeter showing the current consumed by the elements. Some aircraft have a phase test switch which enables the operator to check the current drawn from each phase of the A.C. supply. A typical control and test panel is shown in figure 17.5.07.

Figure 17.5.07




System Operation During each cycle rapid heating and cooling takes place. A thin layer of ice is allowed to form on the leading edges of the propeller blades. This thin layer of ice acts as an insulator so that when the current is switched on by the cyclic timer the temperature rises more rapidly than it would on an unprotected surface. The ice layer next to the heating element melts and the thin layer of ice is easily dispersed by centrifugal and aerodynamic forces. The cyclic timer now transfers the power from the blade to the engine intake, and the leading edge of the blade rapidly cools allowing another thin layer of ice to form and the cycle is repeated. Electrical power is carried to the propeller blades and spinner by a brush box, which contains several carbon brushes which are spring loaded to contact slip rings on the rear plate of the propeller's hub. The current is then carried to the blades by cables to the blade roots.




Figure 17.5.08




Manual Override Relay When the manual-override relays are not energized, the current flows through brushes riding on slip rings mounted on the propeller spinner bulkhead and into the heating elements bonded to the propeller blades. The slip rings are connected to the heater elements through flexible conductors that allow the blades to change their pitch angle. The timer sends current through the right propeller for about 90 seconds, then switches over and sends current through the left propeller for 90 seconds. Some propeller de-icing systems have two separate heating elements on each blade. Current flows through the right propeller outboard element for about 30 seconds, then through the right propeller inboard element for the same length of time. After the right propeller is de-iced, the timer shifts over and sends current through the left propeller outboard elements and then the left propeller inboard elements. Current cycles of the two propellers are controlled by the timer as long as the propeller Auto Prop De-ice switch is on. When the Manual Prop De-icer switch is held in its momentary on position, the two manual-override relays are energized and current flows directly from the bus to the blades without going through the timer.

The pilot can easily tell whether or not the de-icing system is operating correctly in the Automatic mode by watching the propeller ammeter. It will indicate a flow of current each time one of the heater elements draws current.




Figure 17.5.14

PTC B1.1 Notes - Sub Module 17.5 (Propeller Ice Protection)

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