Piston Engines - Propellers — Page 174, Lesson 228

We're now looking at the two conditions that make the constant speed unit, or CSU, actually do its job: the overspeed condition and the underspeed condition. These are the heart of how a variable pitch propeller maintains a selected rpm. Let's start with the overspeed condition. Imagine you're in flight and the engine suddenly produces more torque than the propeller is absorbing. Torque, remember, is the twisting force. If the engine's torque exceeds the propeller's torque, the rpm will tend to rise. That's the overspeed condition. Now, here's where the CSU's cleverness comes in. The CSU has flyweights — those are small rotating weights that are spun by the engine. As rpm rises, the centrifugal force on those flyweights increases. Centrifugal force is the outward pull you feel when something spins. So, with higher rpm, the flyweights swing outward with more force. That increased flyweight force lifts the control valve up against the spring force. The spring is trying to push the valve down, and the flyweights are trying to push it up. In overspeed, the flyweights win. When the control valve lifts, it exposes the coarse pitch line to the pitch change cylinder. That means pressure oil can now flow to the coarse pitch side of the piston. At the same time, the fine pitch line is exposed and connected to drain — so oil can leave the fine pitch side. This is a hydraulic push: pressure on one side, release on the other. The propeller blades then move towards coarse pitch. Coarse pitch means the blades are at a larger angle of attack to the relative airflow. A larger angle of attack generates more total reaction — that's the overall aerodynamic force on the blade — and more thrust. And critically, more thrust means the propeller's torque rises. So the propeller starts absorbing more of the engine's power. Here's the key feedback loop: as the propeller torque rises, it eventually matches the engine's torque. At that point, the rise in rpm is arrested — stopped. The rpm returns to the selected setting. When that happens, the flyweights fall back to their previous balanced position against the spring force. The coarse and fine oil ports close, and the CSU resumes what we call the "on speed" condition. That's the state where everything is balanced and rpm is exactly where you set it. Now let's look at the opposite case: the underspeed condition. Here, the propeller's torque exceeds the engine's torque. That causes rpm to decrease. As rpm drops, the centrifugal flyweight force declines — the flyweights don't swing out as hard. Now the spring force exceeds the flyweight force. So the flyweights collapse inwards, and the spring pushes the control valve down. This exposes the fine pitch oil port to pressure, while connecting the coarse pitch oil port to drain. So pressure oil flows to the fine pitch side of the pitch change piston. The blades move to a smaller angle of attack to the relative airflow. A smaller angle of attack decreases total reaction, thrust, and propeller torque. So the propeller now absorbs less power. With less propeller torque, the engine's torque now exceeds the propeller's, and rpm tends to rise again. That rise in rpm increases propeller torque until it once again matches the engine's. And as rpm rises, flyweight force increases until it exactly balances the selected spring force. The control valve returns to neutral, both fine and coarse pitch ports close off, and the CSU and propeller are back "on speed." One important detail to note: during normal operation, the movement of the control valve is very small. The change in propeller rpm is smooth and progressive — not jerky. That's what makes the system feel seamless in flight. So the whole system is a balance of forces: flyweight centrifugal force versus spring force. Overspeed pushes the valve up, coarse pitch, more propeller torque. Underspeed lets the spring push the valve down, fine pitch, less propeller torque. And the system constantly hunts for that balance point where rpm stays exactly where you've selected it.