Variable-pitch propeller (aeronautics)

In contrast, a propeller set for good cruise performance may stall at low speeds, because the angle of attack is too high.

[1] A shallower angle of attack requires the least torque, but the highest RPM, because the propeller is not moving very much air with each revolution.

When the motorist reaches cruising speed, they will slow down the engine by shifting into a higher gear, while still producing enough power to keep the vehicle moving.

In a single-engine aircraft, if the engine fails, feathering the propeller will reduce drag and increase glide distance, providing the pilot with more options for the location of a forced landing.

In aircraft without a constant speed unit (CSU), the pilot controls the propeller blade pitch manually, using oil pressure.

Small, modern engines with a constant speed unit (CSU), such as the Rotax 912, may use either the conventional hydraulic method or an electrical pitch control mechanism.

Virtually all high-performance propeller-driven aircraft have constant-speed propellers, as they greatly improve fuel efficiency and performance, especially at high altitude.

If the CSU fails, the propeller will automatically return to fine pitch, allowing the aircraft to be operated at lower speeds.

That way, if the CSU fails, that propeller will automatically feather, reducing drag, while the aircraft continues to be flown on the good engine.

The flow of oil and the pitch are controlled by a governor, consisting of a gear type pump speeder spring, flyweights, and a pilot valve.

This will cause the flyweights to move inward due to a lack in centrifugal force, and tension will be released from the speeder spring, porting oil out of the propeller hub, decreasing pitch and increasing rpm.

A pilot requires some additional training and, in most jurisdictions, a formal sign-off before being allowed to fly aircraft fitted with a CSU.

A number of early aviation pioneers, including A. V. Roe and Louis Breguet, used propellers which could be adjusted while the aircraft was on the ground.

The firm claimed that the French government had tested the device in a ten-hour run and that it could change pitch at any engine RPM.

Beacham patented a 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; it met with scepticism as to its utility.

[8] The French firm Ratier produced variable-pitch propellers of various designs from 1928 onwards, relying on a special ball-bearing helicoidal ramp at the root of the blades for easy operation.

At a suitable airspeed a disk on the front of the spinner would press sufficiently on the bladder's air-release valve to relieve the pressure and allow the spring to drive the propeller to coarse pitch.

This was done by pressurizing the bladder with a bicycle pump, hence the whimsical nickname Gonfleurs d'hélices (prop-inflater boys) given to the aircraft ground-mechanics in France up to this day.

[10] As the Hamilton Standard Division of the United Aircraft Company, engineer Frank W. Caldwell developed a hydraulic design, which led to the award of the Collier Trophy of 1933.