Piston Engines - Propellers — Page 181, Lesson 234

We're in the middle of the propeller chapter, and I want to walk you through what happens when you bring a feathered propeller back to life, and then a clever mechanical safeguard that keeps a piston engine from being damaged on the ground. First, the restart sequence. The oil that feathers the propeller is held in the fine-pitch side of the pitch change cylinder. To unfeather, you release that oil back into the constant speed unit, the CSU, by energizing a solenoid through a button mounted in the cockpit. That oil then forces the piston off the feather stop and drives the blades towards fine pitch. Now, as soon as the blades present an angle of attack to the aircraft's relative airflow, the aerodynamic reaction will start turning the propeller and the engine. At that point, all you need to complete the restart is ignition and fuel, set in accordance with the operating manual. There's a handling point here. You must not place the propeller control lever to its maximum rpm setting during this unfeathering. By keeping it off the maximum rpm, you prevent a violent over-swing in yaw as engine power is restored. That's a real operational caution — the power comes back quickly, and the asymmetric thrust can swing the nose hard if you've already asked for full rpm. Now, the second part of this excerpt is about a device called the centrifugal latch, which acts as a feathering stop. Let me set the scene. When an aircraft with a single acting propeller is stopped on the ground after flight, the propeller sits in fully fine pitch. There's a considerable quantity of pressure oil trapped in the fine-pitch side of the pitch change cylinder, holding the blades in that fully fine position. But that oil is opposed by the force of the feathering spring. After shutdown, the trapped pressure gradually leaks away through the fine clearances of the CSU control valve. So, overnight, the feathering springs gradually push the propeller blades towards the fully feathered position. Now, why does that matter? On a free turbine turboprop, that slow drift to feather is perfectly acceptable. But on a piston engine, it's a problem. If the blades have crept towards feather, the starter motor would face an unacceptably high loading when you try to start the engine. To prevent that, we fit centrifugal latches. These latches are disengaged while the engine is running, but they engage at an rpm below the manufacturer's chosen setting — typically 700 rpm. The latch assembly engages latch pins that are attached to the rear of the pitch change piston, after the piston has moved forward by an amount equivalent to about — and here the text cuts off, but the principle is that the latch physically stops the blades from creeping any further towards feather once the engine is shut down and the rpm decays through that threshold. So the sequence is: engine running, latches disengaged, blades free to move. Engine shut down, rpm falls, and as it drops below that manufacturer's setting, the latches engage and lock the pitch change piston, holding the blades in a position that keeps starter loading acceptable. That's the centrifugal latch doing its job as a feathering stop.