In normal flight the governor works the blades between the low-pitch stop and the coarse limit. Beyond the coarse limit lies feather, the blades turned edge-on to the airflow at close to 90°, where a stopped propeller makes the least drag; below the fine limit lies reverse, a negative angle that turns engine power into backward thrust.
The low-pitch stop and the centrifugal latch
On a feathering propeller the springs and counterweights are always pushing toward feather, and at every normal engine shutdown the oil pressure that opposes them drains away. Without protection, every parked aircraft would have feathered blades and every engine start would fight them. The centrifugal latch is the protection: below a threshold rpm, spring-loaded latches drop in and hold the blades at the low-pitch stop, so a normal shutdown, rpm decaying through the threshold, leaves the blades fine for the next start. In flight the same latch never interferes with a deliberate feather, because a failed engine windmills fast enough to hold the latches out by centrifugal force.
Feathering and unfeathering
Feathering is commanded from the cockpit by pulling the propeller lever full aft, through a detent or gate into the feather position: the governor's oil is released, and the springs and counterweights the single-acting section described drive the blades to the feathered angle, stopping the windmilling and cutting the dead engine's drag to its minimum. Auto-feathering does the same without being asked: armed for take-off, it watches each engine's torque, and if the torque collapses at high power it feathers that propeller automatically, removing the windmill drag at exactly the moment the aircraft can least afford it. Unfeathering must supply what the stopped engine cannot, oil pressure with no rotation: an unfeathering accumulator releases stored pressure to push the blades off feather (or an electric pump does the work), the airflow starts the propeller windmilling, and the engine can be restarted.
Synchronising and synchro-phasing
Two propellers at almost the same rpm produce a slow, maddening audible beat. A synchronising system removes it by slaving one engine's governor to the other's, trimming the slave's rpm until both match exactly. Synchro-phasing refines the same idea: with the speeds already matched, it also holds a set angular relationship between the propellers' blade positions, chosen for the lowest cabin noise and vibration.
Reduction gearing
The purpose of reduction gearing is to let engine and propeller each run at the speed that suits it: the engine makes its power at high rpm, but a propeller spun that fast would take its blade tips toward the speed of sound, where efficiency collapses and noise soars. The gear between them lets the engine turn fast while the propeller turns slower, both at their best.
After start: the propeller checks
The checks on a constant-speed propeller after engine start begin with oil pressure, because oil is what the whole system runs on. At run-up power the propeller lever is then exercised, pulled back and returned two or three times: the rpm must drop as the blades coarsen and recover as the lever returns, which proves the governor is in command of the blades, and the cycling also fills the hub with warm engine oil in place of the cold oil that sat there overnight. On feathering types a brief pull into the feather detent, with the rpm starting to fall before the lever is promptly returned, proves that path too.
The governor at work in flight
In cruise the governor answers the flight's changes invisibly: nose down and the TAS rises, the blades meet the air at a shallower angle, their load falls and the rpm starts to rise, so the governor coarsens the blades to hold the rpm; nose up and the reverse happens, the governor fining the blades as the aircraft slows. The pilot sees a steady tachometer through all of it. An overspeeding propeller is what the failure of that service looks like: where oil is what holds the blades coarse, a governor or oil failure lets them run to the low-pitch stop, the unloaded rpm climbs past its limit with a rising howl, and the fine, fast-turning propeller adds enormous drag. The response is to close the throttle, raise the nose and slow down, all three reducing the rpm, and to treat the propeller lever as suspect rather than as a cure.
Reverse pitch
On types equipped for it, reverse pitch is selected on the ground by lifting the power lever over a gate at idle and pulling it into the reverse range: the blades run through fine to a negative angle, and applying power then blows air forward, braking the landing roll and saving wheel brakes. The gate exists because the selection must be impossible to make casually: reverse in flight is a catastrophic drag and control problem, and even on the ground the reversed blast kicks up debris ahead of the aircraft for the engine to swallow.
The levers through a flight
The propeller lever's day, phase by phase:
Take-off — full fine, set before power is applied: maximum rpm available for maximum power.
Climb — rpm reduced to the climb setting after the initial climb is established, quieter and gentler on the engine.
Cruise — cruise rpm, the economy setting, paired with cruise MAP.
Approach and landing — the lever goes back to full fine, so that a go-around finds the propeller already set for maximum power.
Shutdown — on turboprops the propeller is typically feathered; on piston feathering twins it is left to the centrifugal latch, ready for the next start.
The order in which throttle and propeller lever move when power is changed, rpm up before MAP up, MAP down before rpm down, is the next lesson's subject, where it joins the rest of engine handling.