Piston Engines - Propellers — Page 174, Lesson 221

Let’s pick up right where the propeller blade angles leave off, because this is where the real engineering starts. I want to walk you through the Alpha and Beta ranges first, because everything about variable pitch propellers hangs off these two definitions. Look at Figure 12.8. Imagine a propeller blade that can rotate through its full range of angles. At one extreme you have “feathered” — that’s as coarse as it is possible to go. At the other extreme you have “reverse pitch” — that’s as fine as it is possible to go in normal propeller control. So the whole sweep of blade angles runs from feathered, the coarsest, all the way down to reverse, the finest. Now we split that sweep into two named ranges. The “Alpha” range — and note, Alpha is the flight range — runs from feathered down to “flight-fine” pitch. The “Beta” range — the ground range — runs from flight-fine pitch down to reverse pitch. So the dividing line between Alpha and Beta is that flight-fine pitch setting. Alpha is everything coarser than flight-fine, Beta is everything finer. The method of control within these two ranges is described later in the chapter, so for now just hold the distinction: Alpha is flight, Beta is ground. Now, the core problem with varying pitch is twofold — one of actuation and one of control. Let’s take actuation first, because that’s what this excerpt is about. Theoretically you could design either pneumatic or electrical actuation of the pitch change mechanism. Pneumatic is unknown in practice, and electrical is quite rare. The preferred method has turned out to be hydraulic, and here’s the clever part — it uses the engine’s lubrication system as the source of hydraulic power. The oil pressure is boosted where necessary by a small, additional oil pump mounted in the CSU or PCU. So CSU is the constant speed unit, PCU is the propeller control unit — either way, that’s where the extra boost pump lives. Now let’s look at the single acting propeller, because that’s the classic hydraulic design. It’s constructed basically like any other propeller — the blades are arranged around a central, engine-driven hub, and mounted to the front is the cylindrical hydraulic pitch-change mechanism. Inside that pitch change cylinder is a moveable piston, and it’s pushed rearwards by boosted engine oil pressure. Now, although it’s possible to arrange things otherwise, usually this rearward movement of the piston turns the blades towards fine pitch. How? Through a mechanical linkage behind the piston that operates an actuating pin on the butt of each blade. That pin is off-set, so it imparts the correct range of angular motion as the piston moves. So that’s the fine pitch direction — hydraulic pressure pushing the piston rearwards. Now, what moves the blades the other way, towards coarse pitch? That’s provided by either a spring, or centrifugally actuated counterweights. Most propellers of this type contain both. Some propellers replace the spring with compressed gas, and that requires a reversal of the hydraulic direction — so the hydraulics would push the other way. Now, the springs have a dual function, and I want you to hold onto both. First, they assist the centrifugal counterweights in operating the blades to coarse pitch. Second — and this is the safety-critical one — where the facility is provided, the springs actuate the blades into the feathered position when rpm is low, with the consequent loss of centrifugal action. Think about that: at low rpm, the centrifugal counterweights lose their effectiveness because there’s less centrifugal force. The spring takes over and drives the blades to feather. That’s the fail-safe behaviour that protects the engine. So to summarise the single acting principle: boosted oil pressure pushes the piston rearwards to fine pitch, and either a spring, centrifugal counterweights, or both, drive the blades to coarse pitch and, at low rpm, to feather. The double acting propeller, which you’ll see in Figure 12.10, works differently — but that’s the next step.