Piston Engines - Propellers — Page 174, Lesson 223

Let’s pick this up right where the propeller’s pitch-change hardware leaves off, because now we’re looking at the brain that drives it — the control unit, and then the full operating logic of a constant speed propeller. First, the control unit. In your syllabus it’s called the CSU or PCU — Constant Speed Unit, or Propeller Control Unit. Its whole job is to take the pilot’s rpm command and turn it into oil flow, and it does that in exactly three modes. Mode one: oil supply to fine pitch. That makes rpm increase. Mode two: oil shut off, or hydraulic lock. That holds rpm steady. Mode three: drain of fine-pitch oil back to scavenge. That makes rpm decrease. So the control unit is literally a valve that either pushes oil toward fine pitch, seals it in place, or lets it drain away. Now, the double acting propeller. Mechanically it can be built like the single acting unit, or it can change pitch angle through a cam-slot operated, rotating bevel gear that actuates bevel gear segments at the base of each blade. But for study purposes we use the link operated mechanism as the generic type. Here’s the key difference from the single acting propeller. The double acting unit has a pitch change cylinder mounted to the front of the hub, similar but rather larger. Inside it there’s a hydraulic piston, but now that piston is isolated from the centre of the hub, and the fore-and-aft links are fitted with pressure seals. That isolation is what makes it double acting — hydraulic pressure can be directed to either side of the piston. Fine-pitch oil to one side, coarse-pitch oil to the other. Because you can push from both sides, you don’t need springs or centrifugal counter-weights for assistance. The hydraulic pressure does all the work in both directions. And the control unit for the double acting propeller has its own three modes, mirroring the single acting logic. Mode one: deliver fine-pitch oil, and allow drain of coarse-pitch oil — that increases rpm. Mode two: oil shut off, hydraulic lock — constant rpm. Mode three: deliver coarse-pitch oil, and allow drain of fine-pitch oil — that decreases rpm. Now the constant speed propeller as a whole. It must be capable of all the pitch change operations we just listed, selected by the rpm lever in the cockpit. But it must also hold a selected rpm, within its own operational limits, through changes in airspeed, altitude, and power setting. That’s the real meaning of “constant speed” — not just that you can change it, but that it maintains itself. When the CSU senses that rpm is exactly as selected, no action ensues. Nothing happens. But any change in those external conditions — airspeed, altitude, power — creates a tendency for rpm to drift either above or below the selected value. Let’s take the overspeed case first. A tendency for rpm to increase is an overspeed condition. The CSU must respond by supplying oil to the coarse pitch side of the pitch change unit’s piston. The pitch coarsens, and here’s the physics: as the blade angle of attack increases, propeller torque rises. Now propeller torque exceeds engine torque, and that imbalance forces rpm to decrease back down to the selected setting. As rpm drops back to where it should be, the valve selection in the CSU that caused the oil flow in the first place must be removed progressively — you don’t slam it off, you ease it out so you don’t overshoot into an underspeed. Now the opposite case. A tendency for the propeller to underspeed must be met with the opposite reaction. Oil is sent to the fine pitch side of the operating piston, which decreases the propeller’s pitch angle. That decreases propeller torque. Now engine torque exceeds propeller torque, so rpm tends to rise and regain the selected setting. So the whole system is a torque balance. Propeller torque versus engine torque. When they’re equal, rpm is steady. When the CSU shifts pitch, it shifts that balance, and rpm moves until the balance is restored. That’s the constant speed propeller’s operating principle in a nutshell.