A controllable pitch propeller (CPP) or variable pitch propeller is a type of propeller

Propeller

A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...

 with blades that can be rotated around their long axis to change their pitch

Blade pitch

Blade pitch or simply pitch refers to turning the angle of attack of the blades of a propeller or helicopter rotor into or out of the wind to control the production or absorption of power. Wind turbines use this to adjust the rotation speed and the generated power...

. If the pitch can be set to negative values, the reversible propeller can also create reverse thrust for braking or going backwards without the need of changing the direction of shaft revolutions.
The earliest known attempt towards creating a variable-pitch propeller of any type for an aircraft, though only adjustable on the ground before or after flight, was on the IdFlieg

Idflieg

The Idflieg was the bureau of the German War Office that oversaw German military aviation prior to and during World War I....

-numbered "R.30/16" example of the Zeppelin-Staaken R.VI four-engined World War I-era Riesenflugzeug heavy bomber in 1918.

The French aircraft firm Levasseur displayed a variable pitch propeller at the 1921 Paris Airshow which it claimed had been tested by the French government in a ten-hour run and could change pitch at any engine rpm.

Dr Henry Selby Hele-Shaw

Henry Selby Hele-Shaw

Henry Selby Hele-Shaw FRS was an English mechanical and automobile engineer. He was the inventor of the variable-pitch propeller, which contributed to British success in the Battle of Britain in 1940, and he experimented with flows through thin cells. Flows through such configurations are named in...

 and T E Beacham patented an hydraulically-operated variable-pitch propeller (based on a variable stroke pump) in 1924 and presented a paper on the subject before the Royal Aeronautical Society in 1928, though it was received with scepticism as to its utility.