The variable pitch propellers can be adjusted in flight. That means that you can change the blade angle during operation and thus adjust the propeller perfectly to the operating conditions of your aircraft. The purpose of varying pitch angle with a variable-pitch propeller is to maintain an optimal angle of attack (maximum lift to drag ratio) on the propeller blades as aircraft speed varies. Early pitch control settings were pilot operated, either two-position or manually variable. Following World War I, automatic propellers were developed to maintain an optimum angle of attack. This was done by balancing the centripetal twisting moment on the blades and a set of counterweights against a spring and the aerodynamic forces on the blade. Automatic props had the advantage of being simple, lightweight, and requiring no external control, but a particular propeller’s performance was difficult to match with that of the aircraft’s powerplant. An improvement on the automatic type was the constant-speed propeller. Constant-speed propellers allow the pilot to select a rotational speed for maximum engine power or maximum efficiency, and a propeller governor acts as a closed-loop controller to vary propeller pitch angle as required to maintain the selected engine speed.
On some variable-pitch propellers, the blades can be rotated parallel to the airflow to reduce drag in case of an engine failure. This uses the term feathering, borrowed from rowing. On single-engined aircraft, whether a powered glider or turbine-powered aircraft, the effect is to increase the gliding distance. On a multi-engine aircraft, feathering the propeller on a failed engine helps the aircraft to maintain altitude with the reduced power from the remaining engines.
Most feathering systems for reciprocating engines sense a drop in oil pressure and move the blades toward the feather position, and require the pilot to pull the propeller control back to disengage the high-pitch stop pins before the engine reaches idle RPM. Turboprop control systems usually utilize a negative torque sensor in the reduction gearbox which moves the blades toward feather when the engine is no longer providing power to the propeller. Depending on design, the pilot may have to push a button to override the high-pitch stops and complete the feathering process, or the feathering process may be totally automatic.
In some aircraft, such as the C-130 Hercules, the pilot can manually override the constant-speed mechanism to reverse the blade pitch angle, and thus the thrust of the engine (although the rotation of the engine itself does not reverse). This is used to help slow the plane down after landing in order to save wear on the brakes and tires, but in some cases also allows the aircraft to back up on its own – this is particularly useful for getting floatplanes out of confined docks.
Forces Acting on Propeller
Five forces act on the blades of an aircraft propeller in motion, they are:
- Thrust bending force
- Thrust loads on the blades act to bend them forward.
- Centrifugal twisting force
- Acts to twist the blades to a low, or fine pitch angle.
- Aerodynamic twisting force
- As the centre of pressure of a propeller blade is forward of its centreline the blade is twisted towards a coarse pitch position.
- Centrifugal force
- The force felt by the blades acting to pull them away from the hub when turning.
- Torque bending force
- Air resistance acting against the blades, combined with inertial effects causes propeller blades to bend away from the direction of rotation.
SOURCE : Wikipedia