Fundaments Operations of Aircraft Propellers

This text outlines the fundaments operations and aspects of aircraft propellers. It details the components, forces and workings of a propeller as well as discussing the difference between the different propeller types.

Introduction

Propeller types are defined by blade pitch as being fixed or variable which will be further detailed later in the text. To fully appreciate the differences and understand the advantages of different pitched propellers we must first consider the fundamental characteristics of propellers. Usually propellers have two, three, or four blades; for high-speed or high-powered airplanes, six or more blades are used. In some cases these propellers have an equal number of opposite rotating blades on the same shaft, and are known as dual-rotation propellers. Small single engine aircraft have the propeller mounted on the front as multi-engine aircraft have them set on the wings.

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Pitch

What is pitch? Pitch is important as it is the main differential from propeller type to propeller type. Essentially pitch relates to the angle of the blade in respects to a flat plane. It is the helical blade path or simpler the distance the propeller blade covers during a full rotation and the cut it has on the air. Pitch is referred to in two ways, fine and coarse. A fine pitch propeller has a low blade angle, will try to move forward a small distance through the air with each rotation, and will take a 'small' bite of the air.

It requires relatively low power to rotate, allowing high propeller speed to be developed, but achieving only limited airspeed. This is like having a low gear in your automobile. (Brandon 2008) A coarse pitch propeller has a high blade angle, will try to advance a long distance through the air with each rotation, and will take a big 'bite' of the air. It requires greater power to rotate, limiting the propeller speed that can be developed, but achieving high airspeeds. This is like having a high gear in your automobile. (Brandon 2008)

The Blades

The propeller blades are in fact aerofoils producing lift and drag. As the propeller spins the leading edge of the blade cut through the atmosphere and accelerates a tube of air or relative airflow  the diameter of the propeller moving the aircraft forward. This rotation is able to work because the propeller blades are designed slightly different to wing aerofoils as they have a small twist in them so that the greatest angle is at the blade root and the smallest at the top, due to the different angle and speed that each section of the blade travels.

These blade elements are in place at different angles because the linear velocity increase towards the tip of the blade as it has a greater distance to travel, the angles prevent bending making each section advance through the air at the same rate. The blade angles combined with the forward motion and the circular rotation of the propeller keep constant the best angle of attack (AOA). The twist causes the blade path to follow an approximate helical path easiest seen in a linear form. This action is similar to a screw being turned in a solid surface, except that in the case of the propeller a slippage occurs because air is a fluid.

Forces Acting on the Propeller

Aircraft that are not jet powered use a propeller which converts the rotational power from an aircrafts engine into aerodynamic forces; thrust power moving the aircraft forward through the atmosphere and propeller torque which acts in the plane of rotation. The plane of rotation is perpendicular to the propeller shaft. Propellers are conventionally placed in front of the engine on the engine drive shaft. During cruising flight the propeller torque balances the engine torque and the thrust balances the aircrafts drag force.

The propeller rotates clock wise and when the forces are not balance the torque reaction increases a rolling friction on the aircraft. As the blades produce a thrust force, the thrust force pulls on the thinnest section of the blade attempting to bend the tips. For single engine aircraft with the propeller mounted on the front the clockwise rotation creates a vortex of air or slipstream that flows around and down the fuselage to the rudder which affects the lateral movement of the aircraft or a slight yaw to the left during cruising flight Variable-pitched propellers can have their blade angles/pitch altered and will be further explained.

Two different forces experienced in these propellers can affect the blade angle. Centrifugal twisting moment (CTM) Aerodynamic twisting moment (ATM) CTM causes pulling stress at the base of the blade and a twisting force at the pitch change axis produce a finer pitch angle. The blade will want to align itself with the plane of rotation. The relative airflow over the blades produces a total reaction, an ATM where the total reaction is ahead of the pitch change axis, tempting the blade to twist, increasing the blade angle producing a coarser pitch. Occurs when the propeller drives the engine. Caused by; steep dive with no power, sudden reduction in power, engine failure, causing the blades to twist to a finer pitch.

Propeller Types

As stated earlier pitch is a main component between propeller functions. Under the classification of fixed and variable pitch propellers there are four common types, fixed-pitch, ground-adjustable, variable-pitch and constant-speed propeller. The first two are fixed propellers as the other two are variable.

There are a few versions of variable-pitch propellers that may be seen in the aviation industry, two-position propeller, in flight-adjustable propeller, automatic propeller and the constant-speed propeller. The most commonly used at present will be concentrated on, elaborating on fixed through to the variable propellers and the enhancements of pitch control.

Fixed-Pitch Propeller

The cheapest and crudest propulsion aero-device is the fixed-pitch propeller. Although it has been superseded many a time it is the most common type of propeller used in sport aviation.

The fix-pitch means that the pitch of the propeller is decided by the manufacture, there is only one setting and the performance of the aircraft is confined by the constraint of that one setting. This means to reach the optimum RPM/airspeed the propeller has to function through inefficient speeds. Normally there are two versions, a climb propeller with a fine pitch setting or a cruise propeller with a coarse pitch setting. Ground-adjustable propeller: The pitch for a ground-adjustable propeller is able to be set for the condition of flying the aircraft will be doing but only before the flight.

However it is still a fixed propeller as once the pitch is set in cannot be changed during the operation of the aircraft. These propellers are mainly installed on ultra light and experimental aircraft. More usually they are used as a low cost way to try out various pitches to determine the propeller pitch that best suits an aircraft.

Variable-Pitch Propeller

A variable-pitch propeller is exactly what the name implies; the pitch can be controlled and adjusted in flight to the most efficient setting for a certain phases of flight. Simply during take-off the propeller would be set to a fine pitch allowing the engine to develop reasonable revs and then to a coarser pitch during cruising flight speed. The engine will be ticking over comfortable while the propeller cuts through more air. Combine this with throttle control a wide variety of power settings can be achieved maintaining airspeeds with the limits of the aircrafts engine speeds. This feature of a variable-pitch propeller will provide you with performance advantages, including: Reduced take-off roll and improved climb erformance. Fine pitch allows the engine to reach maximum speed and hence maximum power at low airspeeds. Vital for take-off, climb, and for a go-around on landing. (Brandon, 2008) Improved fuel efficiency and greater range. Coarse pitch allows the desired aircraft speed to be maintained with a lower throttle setting and slower propeller speed, so maintaining efficiency and improving range. (Brandon, 2008) Higher top speed.

Coarse pitch will ensure your engine does not over speed while the propeller absorbs high power, producing a higher top speed. (Brandon, 2008) Steeper descent and shorter landing roll. With a fine pitch and low throttle setting, a slow turning propeller is able to add to the aircraft's drag, so slowing the aircraft quicker on landing. (Brandon, 2008)

Constant-Speed Propeller

The constant-speed propeller is a special case of variable pitch, which is considered in a family of its own, and offers particular operating benefits.

A constant-speed propeller allows the pilot to control the power just by the throttle once the propeller/engine speed has been optimally selected (actually controlling the absolute pressure of the fuel/air mix in the intake manifold MAP which then determines power output). This is controlled by a governor or constant speed unit (CSU) which detects the propeller speed and acts to keep it at the selected engine/propeller speed selected by the pilot and vice versa. If the propeller speed increases then the CSU will increase the pitch a little to bring the speed back within the limits.

Thus creating vastly efficient running components during phases of flight (The governor or constant speed unit CSU may be an electronic device that detects the rotational speed of a slip-ring incorporated in the propeller hub, and controls operation of a servomotor/leadscrew pitch change actuator in the hub assembly. Or, it may be an hydraulic fly-ball governor attached to the engine, using engine oil to operate a hydraulic pitch change piston in the hub assembly. In the first case, the cockpit control device is likely to be knobs and switches. In the hydraulic system, the governor is likely to be cable operated from a cockpit lever — JB.

While allowing the pilot to ignore the propeller for most of the time, the pilot must still choose the most appropriate engine/propeller speed for the different phases of flight. Take-off, go-around and landing. A high speed setting is used when maximum power is needed for a short time such as on take-off. The high speed setting may also be used to keep the propeller pitch low during approach and landing, to provide the desired drag and be ready for a go-around should it be required. (Brandon, 2008) Climb and high speed cruise.

A medium speed setting is used when high power is needed on a continuous basis, such as during an extended climb, or high speed cruise. (Brandon, 2008) Economic cruise. A low speed setting is used for a comfortable cruise with a low engine speed. This operation produces low fuel consumption and longer range, while the advantages of low noise and low engine wear are also enjoyed. (Brandon, 2008) Edge of the airfoil is the cutting edge that slices into the air. As the leading edge cuts the air, air flows over the blade face and the camber side.

Blade Face is the surface of the propeller blade that corresponds to the lower surface of an airfoil or flat side, we called Blade Face. Blade Shank (Root) is the section of the blade nearest the hub. Blade Tip is the outer end of the blade farthest from the hub. Plane of Rotation is an imaginary plane perpendicular to the shaft. It is the plane that contains the circle in which the blades rotate.  Blade Angle is formed between the face of an element and the plane of rotation. The blade angle throughout the length of the blade is not the same.

The reason for placing the blade element sections at different angles is because the various sections of the blade travel at different speed. Each element must be designed as part of the blade to operate at its own best angle of attack to create thrust when revolving at its best design speed. Blade Elements are the airfoil sections joined side by side to form the blade airfoil. These elements are placed at different angles in rotation of the plane of rotation. The reason for placing the blade element sections at different angles is because the various sections of the blade travel at different speeds.

The inner part of the blade section travels slower than the outer part near the tip of the blade. If all the elements along a blade is at the same blade angle, the relative wind will not strike the elements at the same angle of attack. This is because of the different in velocity of the blade element due to distance from the centre of rotation.  Relative Wind is the air that strikes and passes over the airfoil as the airfoil is driven through the air. Angle of Attack is the angle between the chord of the element and the relative wind. The best efficiency of the propeller is obtained at an angle of attack around 2 to 4 degrees.

Blade Path is the path of the direction of the blade element moves. Pitch refers to the distance a spiral threaded object moves forward in one revolution. As a wood screw moves forward when turned in wood, same with the propeller move forward when turn in the air. Geometric Pitch is the theoretical distance a propeller would advance in one revolution. Effective Pitch is the actual distance a propeller advances in one revolution in the air. The effective pitch is always shorter than geometric pitch due to the fact that air is a fluid and always slips.

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