Variable pitch propeller[PDF]

Introduction

The propeller consists of a hub centered and fixed on the engine output shaft, blades, sometimes a de-icing system and a variable pitch propeller system. The fixing assembly is protected by the propeller cone. Its function is to guide the air for the cooling of the engine.

There are at least 2 blades on a propeller. As the number of blades increases, the signature sound decreases.

A fixed pitch propeller is very simple to use (speed linked to engine RPM); nevertheless, a compromise between the engine RPM and the engine torque is found to avoid damaging it. The variable pitch propeller allows you to choose the right pitch for all flight phases, even during an engine failure.

FUNCTION

The variable pitch propeller is equipped with a mechanism for varying the pitch of the blades, and a regulation system. With the propeller control (blue control) you can change the blade angle (see the following chapter).

Blade Angle - Variable Pitch Propeller Article
Propeller Blade Angle – FAA Pilot’s Handbook of Aeronautical Knowledge


When the engine settings are selected, the propeller governor automatically adjusts the blade angle to maintain the desired RPM. The engine lubricating system provides the oil needed to operate the governor. A pump is usually integrated in the governor. This allows for a faster blade angle change (thanks to the increased pressure). By a system of rack and pinion, the pilot modifies the angle of attack of the blades by means of the propeller control.

Variable Pitch Propeller Mechanism
Variable Pitch Propeller Mechanism – FAA Pilot’s Handbook of Aeronautical Knowledge


The blades are mounted on the hub spider for angular adjustment. The centrifugal force of the blades is transmitted to the hub spider through blade clamps and then through ball bearings. The propeller thrust and engine torque is transmitted from the blades to the hub spider through a bushing inside the blade shank. In order to control the pitch of the blades, a hydraulic piston-cylinder element is mounted on the front of the hub spider. The piston is attached to the blade clamps by means of a sliding rod. The piston is actuated in the forward direction by means of oil pressure supplied by a governor, which overcomes the opposing force created by the counterweights.


VARIABLE PITCH PROPELLER OPERATION

Before starting the engine, the propeller control must be set in the low pitch position (high RPM). When starting, this position reduces the propeller drag, facilitates starting and gradually warms the engine. The variable pitch propeller control is typically blue (between the throttle and mixture control).

Seneca III Cockpit - Propeller Control
Seneca III Cockpit


During preheating, the blade variation mechanism should be operated gently through one complete cycle. A complete cycle corresponds to :

  • initial position, low pitch,
  • position high pitch,
  • engine speed stabilization,
  • waiting position, low pitch (takeoff position).

This is to determine if the system is working properly and to circulate the oil through the regulation system.

An aircraft equipped with a variable pitch propeller will have better takeoff performance than one equipped with a fixed pitch propeller.

During a flight, to avoid detonations damaging to the engine (high manifold pressure and low engine RPM), it is preferable to follow the sequence of actions proposed bellow :

  • to increase the power, you must first use the propeller control to select a low pitch, then increase the manifold pressure,
  • to reduce the power, first reduce the manifold pressure, then increase the propeller pitch.
Variable Pitch Propeller BE58 Control
BE58 Cockpit - X Plane 11


Excessive manifold pressure raises the cylinder combustion pressures; this can lead to mechanical damage to the engine. Excessive pressure also produces high-engine temperatures.

For take-off, it is recommended to adopt (see your flight manual) a “low pitch” to have the maximum power. The regulation system acts continuously to avoid overspeeding during acceleration phases (RPM limitation). When cruising, you can adopt a “high pitch” to increase the incidence of the blades. The aircraft flight manual indicates the RPM number according to your cruising altitude.

Throughout the approach and landing phases, for safety reasons, it is advisable to come back in “low pitch”, to have all the available power in case of a go-around.


Propeller Pitch Angle – FAA Airplane Flying Handbook
Propeller Pitch Angle – FAA Airplane Flying Handbook


Choosing the right propeller pitch reduces fuel consumption during flight.


CRUISE PARAMETER MANAGEMENT

The engines parameters while cruising can be modified (manifold pressure and engine RPM) according to the chosen consumption. The flight manual of your aircraft details these parameters in the "Normal Procedures" section.

Beechcraft Baron 58 - Recommended Cruise Power
Recommended Cruise Power - BE58


Depending on the cruising altitude and the chosen engine parameters, the speed varies. In the "Performance" chapter, a graph combines altitude and speed according to the engine parameters chosen.

Cruise Speeds - Pressure Altitude - BE58 Performance Section
True Airspeed / Pressure Altitude - BE58


In the example above, at 7500ft (standard day ISA) several speeds are adjustable:

  • 171 kt with 20.5 IN. HG. and 2100 RPM (economy cruise power),
  • 185 kt with 21.0 IN. HG. and 2300 RPM (recommended cruise power),
  • 191 kt with 24.0 IN. HG. and 2300 RPM (recommended cruise power),
  • 198 kt with 24.5 IN. HG. and 2500 RPM (maximum cruise power).
The engine parameters and its management detailed in chapter Cruise Parameter Management are taken from the flight manual of a Beechcraft Baron 58. They should not be applied for all aircraft.

See also

Reference

  • Aircraft Pilot’s Manual – CépaduèsEdition – Chapter 1 – Driving the engine
  • SKYbrary library – Variable Pitch Propeller SKYbrary library – Constant Speed Propeller
  • Pilot’s Handbook of Aeronautical Knowledge – Chapter 7: Aircraft Systems
  • Pilot’s Handbook of Aeronautical Knowledge – Chapter 5: Aerodynamics of Flight
  • Airplane Flying Handbook - Chapter 12: Transition to Complex Airplanes

Author

  • VID 529524 - Creation

DATE OF SUBMISSION

  • 04:57, 28 January 2022

COPYRIGHT

  • This documentation is copyrighted as part of the intellectual property of the International Virtual Aviation Organisation.

DISCLAIMER

  • The content of this documentation is intended for aviation simulation only and must not be used for real aviation operations.