Piston Engines - Propellers — Page 181, Lesson 233

We're now into the feathering and unfeathering of a single-acting propeller. Let's start with feathering. To feather a single-acting propeller, you move the propeller rpm control lever fully to the rear. Then, depending on the particular linkage, you dog-leg it to one side or push it inward. That allows a further rearward movement into the "feathered" position. Now, what does that do inside the constant-speed unit, the CSU? It raises the rack as far as it will go. This simulates an exaggerated "overspeed" condition. Why? Because it removes all loading from the speeder spring, allowing the flyweights to fly right out—if the engine is running—and lifting the control valve right up. But here's a critical detail: most CSUs also cater for the engine stopped situation. In that case, there's zero flyweight force. So a full feather selection will bypass the speeder spring to physically lift the control valve upwards. Once the control valve is up, any oil in the pitch change cylinder can drain away. Then, the counter weights—if the engine is turning—or the spring—if it's not—push the piston onto the feathering stop. That's the fully feathered position. Now for unfeathering. You give the speeder spring some pressure by moving the propeller lever to a position parallel with the lever of the operating engine. That moves the control valve down, ensuring that any pressure oil will be directed to fine pitch. And here's a common practice with single-acting propellers: they provide a reserve of pressurized oil in an accumulator. That oil is trapped by a non-return valve and released by a solenoid-operated valve. So when you need to unfeather, that stored pressure is what gets the blades moving back to fine pitch. Let me make sure you've got the key terms: the CSU, the speeder spring, the flyweights, the control valve, the pitch change cylinder, the counter weights, the feathering stop, the accumulator, the non-return valve, and the solenoid-operated valve. Each one plays a specific role in this sequence.