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A fine pitch would be used during take-off and landing, whereas a coarser pitch is used for high-speed cruise flight. This is because the effective angle of attack of the propeller blade decreases as airspeed increases. To maintain the optimum effective angle of attack, the pitch must be increased.
The requirement for pitch variation is shown by the propeller performance during the Schneider Trophy competition in 1931. The Fairey Aviation Company fixed-pitch propeller used was partially stalled on take-off and up to 160 mph (260 km/h) on its way up to a top speed of 407.5 mph (655.8 km/h). [36]
Since the lift is very much greater than the drag, the phugoid is at best lightly damped. A propeller with fixed speed would help. Heavy damping of the pitch rotation or a large rotational inertia increase the coupling between short period and phugoid modes, so that these will modify the phugoid.
Variable-pitch propellers have significant advantages over the fixed-pitch variety, namely: the ability to select the most effective blade angle for any given speed; when motorsailing , the ability to coarsen the blade angle to attain the optimum drive from wind and engines;
Lightly loaded propellers for light aircraft and human-powered boats mostly have two blades, motor boats mostly have three blades. The blades are attached to a boss (hub), and while it can be a big boss, it should be as small as the needs of strength allow – with fixed-pitch propellers the blades and boss are usually a single casting.
In adjustable-pitch propeller control system, the pilot has to adjust the propeller pitch angle and thus angle of attack of the propeller blades (typically with a lever) to achieve the desired propeller rotational speed. The increased pitch (blade angle of attack) increases the load on the engine and therefore slows it down, and vice versa.
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