Piston Engines - Propellers — Page 181, Lesson 230

I want to walk you through the Propeller Control Unit — the PCU. This is the next step up from the basic CSU, the Constant Speed Unit, which you already know controls propeller pitch to hold a set rpm. The PCU does the same job, but it's built for turboprop engines, and it adds a whole set of extra functions that a basic CSU simply doesn't have. Let me start with what makes the PCU different in its most basic role. It's generally similar to the basic CSU and controls a propeller in the same way. But it's used with turboprop engines, particularly those controlled by a single flight deck lever instead of the more usual double presentation of separate power and rpm levers. So instead of the pilot moving one lever for power and another for rpm, there's just one lever. That single lever is connected to both the PCU and the engine's Fuel Control Unit — the FCU. Because of that connection, rpm and fuel flow are altered together. That's the key point: they change simultaneously, in a co-ordinated fashion. Why does that matter? Because it lets the engine overcome the combined inertia of the propeller and the compressor/turbine assembly together. Think about what that means. When you demand rapid acceleration, you've got a heavy propeller and a heavy turbine spool that both resist speeding up. If fuel and rpm moved separately, you could easily over-stress things. But because the PCU and FCU work together, you get rapid acceleration without the danger of over-stressing the turbine and other "hot end" components. The hot end is the turbine section — the part that sees the highest temperatures — and it's the most vulnerable to thermal and mechanical stress during rapid power changes. Now, if you look at the PCU itself, you'll see it contains a number of additional components when compared with a basic CSU. Let me walk you through those one by one, because each one exists to handle a specific emergency or special condition. First, there's the Mechanical Feathering Lever. This lever is located in the PCU, above the standard components. Its job is to mechanically lift the control valve upward into the feather position when the engine high pressure — HP — cock is closed in flight. Let me unpack that. The HP cock is the fuel shut-off valve. When you close it in flight, you're cutting fuel to the engine. But you don't want the propeller just windmilling uselessly, creating drag. So this mechanical lever physically lifts the control valve into the feather position. Feathering means rotating the blades edge-on to the airflow so they generate zero aerodynamic force — I'll come back to that exact definition in a moment. The point here is that this is a mechanical backup: it doesn't rely on electrical power or oil pressure, it's a direct mechanical lift of the valve. Next component: the Valve Lift Solenoid and Piston, which handles autofeathering. Let me set the scene. In the event of a low torque signal in the engine's torque meter system, coupled with a high power selection, a turboprop's propeller is usually furnished with the means to feather itself. That's called "Autofeather." The scenario is critical: you're on take-off or go-around, close to the ground, you've selected high power, and the engine isn't producing the torque it should. The propeller feathers itself automatically. This leaves the pilot free to concentrate on controlling the aircraft, which may be close to the ground. That's the whole purpose — hands-off feathering in a critical phase of flight. Here's how it works mechanically. The PCU has a Valve Lift Solenoid which is energized at the same time as a separate feathering pump's electric motor is energized. So two things happen together: the solenoid gets power, and a dedicated feathering pump starts running. The separate feathering oil supply is now able to go to the valve lift piston, raising the control valve into an exaggerated coarse pitch position — that's the feather position. Then the feathering oil supply can go to the coarse pitch side of the pitch change piston, pushing it onto the feathering stop as the fine pitch oil drains away. Now, the feathering stop — this is an important definition. It's an internal stop, within the pitch change mechanism, which coincides with that blade position, edge-on to the aircraft's airflow, which will generate zero aerodynamic force in either direction. Let me make sure that's clear. When the blade is edge-on to the airflow, the air can't push it forward or backward — zero force either way. The propeller will stop, unless some drive force is applied. So the feathering stop is the mechanical limit that holds the blade in exactly that neutral, edge-on position. The propeller stops turning because there's no aerodynamic force to keep it spinning, and no engine drive. The third additional component is the Pitch Lock Solenoid, which handles ground fine and reverse pitch. Many turboprops and a few high-powered piston engined aircraft are provided with a means to aerodynamically reverse the pitch of their propellers, or to select a super-fine pitch — called ground fine — several degrees finer than the finest pitch available in flight, which is called flight fine. Let me be careful with those two terms. Flight fine is the finest pitch you can use in the air. Ground fine is even finer — several degrees finer — and it's only used on the ground. And this ground fine capability is confined to those turboprops whose gas generator spool and propeller drive are physically connected. That's a specific design condition — the gas generator spool and the propeller drive must be mechanically linked for ground fine to be available. The mechanical details within the pitch change mechanism will be discussed later, but the PCU contains a Pitch Lock Solenoid which, when energized, will allow — and that's where the excerpt cuts off. So I'll leave that mechanism for when we continue. Let me pull this together, because there's a clear logic to the PCU. The basic CSU holds rpm. The PCU does that too, but it adds three protective functions, each triggered by a specific condition. The Mechanical Feathering Lever handles the deliberate case — you close the HP cock, you get feathering, mechanically. The Valve Lift Solenoid handles the automatic case — low torque plus high power selection, and the propeller feathers itself to save the aircraft during take-off or go-around. And the Pitch Lock Solenoid handles the ground case — allowing reverse pitch or ground fine, which is finer than flight fine, and only on turboprops with a physically connected gas generator spool and propeller drive. Each of these is a solenoid or a mechanical lever acting on the same control valve, lifting it to a different position — feather, or ground fine, or reverse. That's the elegant part: one control valve, multiple ways to move it, each serving a different emergency or ground condition.