Ice and Rain Protection — Page 261, Lesson 329

We're starting a new topic now: propeller protection systems, under ice and rain protection. Let me walk you through it. First, why do we even need this? Ice forming on a propeller blade distorts the aerofoil section. That distortion causes a loss in efficiency, and worse, it can produce unbalance and destructive vibration. So the build-up of ice must be prevented. There are two systems in use: an anti-icing fluid system, or an electrically powered thermal de-icing system. Let's take the fluid system first. The fluid system provides a film of freezing point depressant fluid to the propeller blade surfaces during flight. That fluid mixes with the water or ice and reduces the freezing point of the mixture. So instead of ice forming, you get a slushy mixture that won't freeze solid. The fluid is distributed to each propeller blade from a slinger ring, which is mounted on the back of the propeller hub. The fluid is pumped into this ring through a delivery pipe from a supply tank. Now, some propellers have rubber overshoes fitted to the blades to assist the distribution of the fluid. On that type of installation, fluid is fed from the slinger ring to a small trough, which is part of the overshoe. Then it's forced by centrifugal action along longitudinal grooves in the overshoes. On propellers without overshoes, fluid is fed from the slinger ring through a pipe to the root of the blade, and then distributed by centrifugal action. The fluid may be pumped to the slinger ring from the supply tank by an independent electrically driven pump, but air pressure is sometimes used instead. The electric pump may be controlled by a switch, and in some installations the pump speed may be varied by means of a rheostat. Check valves are sometimes provided to prevent loss of fluid when the pump is not operating. The supply pressure is typically 10 psi. Where air pressure is used to supply fluid, a relief valve is usually fitted to the air supply line, and a control valve is provided to regulate the fluid flow. So to recap the key parts: slinger ring on the back of the hub, delivery pipe from the supply tank, overshoes with troughs and longitudinal grooves, centrifugal action doing the distribution, an electric pump or air pressure for delivery, check valves to stop fluid loss, 10 psi typical supply pressure, and a relief valve plus control valve on the air supply line. That's the complete fluid anti-icing system.