One angle, two jobs.
A propeller blade is a rotating wing, and like a wing it has one angle of attack where it works best. The catch: the air it meets is the sum of rotation and forward flight. Stand still and the blade meets air only from its own rotation — sweeping edge-on, in the plane of the disc, so a fixed blade angle is a huge angle of attack; cruise at 200 km/h and the incoming flow tilts the relative wind, so the same blade sees air arriving increasingly from ahead. One fixed angle cannot be right for both.
A fixed-pitch propeller is therefore always a compromise. Pitch it fine (flat) and you get superb takeoff acceleration — but in cruise the engine screams against the redline while the blades take tiny, inefficient bites. Pitch it coarse and cruise is relaxed — but on the runway the engine cannot reach its power RPM, and your takeoff roll grows uncomfortably long.
Enter the governor.
A constant speed propeller changes the blade angle in flight. You choose an engine RPM; the governor holds it, whatever the airspeed, by rotating the blades. Accelerate and the blades coarsen to absorb the extra speed; slow down and they flatten again. The engine stays at its happiest RPM while the propeller does the shifting — which is why pilots call it the gearbox of the airplane.
Classically this is a flyweight-and-oil device and a second cockpit lever. An electric governor like the constant speed 57 controller replaces the mechanism with a motor, reads engine RPM from a Hall sensor or an RPM signal, and holds the target you dial — with hardwired manual buttons as the mechanical-failure fallback.
Fixed pitch vs governed.
The grey dashed line is a fixed-pitch propeller: RPM is chained to airspeed, and past the red line you must throttle back — giving up power exactly when you want it. The blue line is the governed propeller: RPM stays where you set it while the blade angle (right stat) does the work.
A flight, seen by the governor.
Takeoff: blades at fine pitch, engine straight to its rated RPM, maximum thrust from the first meter of the roll. Climb and acceleration: as IAS rises the governor coarsens the blades; RPM never moves. Cruise: the engine loafs at its efficient setting while the coarse blades convert every horsepower into distance. Approach: slow down and the blades flatten again — so if you push the throttle for a go-around, full takeoff thrust is already rigged.
That last point is the safety argument hiding inside a performance device: a governed propeller is always ready for the go-around, because readiness is just the fine-pitch end of the map it lives on.
One lever, if the panel helps.
The remaining workload is the second lever — choosing the RPM target as speed changes. That is exactly the job IAS mode automates: paired over CAN with an airspeed source (an iris All-in-one, iris Airspeed or iris EFIS), the controller holds a map from indicated airspeed to RPM. On the ground the map commands takeoff pitch; accelerate into cruise and it walks the propeller over; slow for landing and it returns to go-around-ready. One lever — the throttle — and the map flies the propeller.