Right, let's pick this up — Page 174, Lesson 225

Right, let's pick this up. We've just seen how a change in pitch angle affects the balance of torque. When the pilot selects a coarser pitch, the propeller's torque demand drops, so the engine torque momentarily exceeds it and the rpm tends to rise back toward the pilot's selection. The rpm only stays constant when propeller torque equals engine torque again. Now, the key question is: who actually makes that pitch change happen? That's the job of the Constant Speed Unit, the CSU. Let me walk you through how this simple constant speed unit works, because it's the heart of the whole system. The CSU is engine driven. That's a crucial point. Because it's driven by the engine itself, it can directly sense any change in engine rpm. If the rpm rises, it detects an overspeed; if the rpm falls, it detects an underspeed. And it corrects those by changing the propeller pitch. Coarse pitch corrects an overspeed, fine pitch corrects an underspeed. So the logic is: speed too high, make the propeller bite harder; speed too low, let it bite less. Let's look at the physical layout. The CSU is driven from a convenient gear, usually at the front of the engine, just behind the propeller. That drive shaft does two jobs. First, it drives a small oil pressure boosting pump. This raises the pressure of the engine's own lubrication oil supply to a more useful figure. For this system, something in the region of 120 to 200 psi would be satisfactory. So we're taking the engine's normal oil and boosting it to a pressure high enough to move the propeller blades. Second, the drive rotates a centrifugal flyweight assembly. These weights are L-shaped, and they're arranged so that as they spin, they produce an upward movement on a double-landed hydraulic control valve. So the faster the engine spins, the harder the flyweights push upward on that valve. Opposing that upward force is a coil spring, called the speeder spring, which acts downward on the control valve. So we have a constant tug-of-war: flyweights pushing up, spring pushing down. The spring's compressive force can be adjusted through the up and down movement of a rack and pinion. The pinion is rotated by the pilot's rpm lever. Push the rpm lever forward, the pinion rotates, the rack is pushed down, compressing the spring, which tends to push the control valve down. Pull the rpm lever to the rear, and the spring compressive force is reduced. So the pilot isn't directly moving the blades; he's setting the spring tension, which sets the rpm the system will hold. Now, the "On Speed" condition. This is the steady state. The control valve receives pressure oil from the engine and the CSU booster pump. When the engine is on speed, with no change of rpm selected, the valve is arranged so that the oil is trapped and prevented from passing to the pitch change cylinder. Why? Because the selected spring pressure downwards is exactly balanced by the flyweight force upwards. The forces cancel, the valve sits in its neutral position, and no oil flows to move the blades. The pitch stays exactly where it is, and rpm stays constant. So the whole system is a balance of forces. The pilot sets a spring force; the engine's rpm generates a flyweight force. When they match, nothing happens. When they don't match, the valve moves, oil flows, pitch changes, and rpm is corrected. That's the simple constant speed unit in a nutshell.