Jul 9, 2020
The purpose of the propeller is to provide a method of propulsion so the aircraft is able to move forward through the air. The propeller itself consists of two or more blades connected together by a central hub that attaches the blades to the engine shaft. The propeller blades are shaped much like a wing of an aircraft, using the rotation power of an engine rotates the propeller blades produce lift (this lift is referred to as thrust) which moves the aircraft forward.
How Propellers Work
The fundamentals behind how propellers work are related to the physics theories of motion developed over two hundred years ago by Sir Isaac Newton. More specifically his Third Law, that is for every action, there is an equal and opposite reaction (Sir Isaac Newton, 1687).
Keeping this theory in mind, the propeller of an aircraft is used to transform the rotational power of an engine into forward thrust. The propeller works by displacing the air pulling it behind itself (the action), this movement of air then results in the aircraft being pushed forward from the resulting pressure difference (the opposite reaction). The more air that is pulled behind the propeller the more thrust or forward propulsion is generated.
Propellers can be made up of anywhere from a single blade to six or more blades in line with the efficiency needs of different aircraft. Aircraft performance requirements and engine power are the major determining factors in the number of propeller blades. As engine power increases, additional blades are needed to efficiently utilise the increased level of power. The angle of a propeller’s blades and its overall size and shape (along with the power of the engine) affect the amount of thrust generated.
Propeller Theory – Forces Acting On A Propeller
Propeller blades are constructed in a similar way to a wing, as such they are subject to some of the same aerodynamic forces such as drag and lift (with wings this is lift, with propeller it is called thrust). The difference is that a propeller has the additional forces of rotational speed and forward acting momentum.
- Centrifugal Forces. The centrifugal force is the force experienced by the propeller blades when turning at speed. This force is effectively pulling them away from the aircraft.
- Centrifugal And Aerodynamic Twisting. Any asymmetrical spinning object generates a centrifugal twisting force, the propeller is no different with the force of its spinning action twisting the blades to a fine pitch.
- Vibration. The vibration of the propeller blades is caused by disturbance is the aerodynamics of the propeller as it passes through the air, and close to the wings and fuselage as well as engine variations.
- Torque Bending. The torque bending forces are the natural resistance of the air producing resistance against the blades as they turn and the resulting propensity of the propeller blades to want to bend in the opposite direction of the rotation.
- Thrust Bending. The force of the propeller pushing the air backwards is called the thrust, this places the propeller blades under pressure and bends the blades forward.
Aircraft Propeller Design
The basic engineering behind how propeller blades work hasn’t changed a great deal over the years, however there have been a number of significant modifications in the materials used to build propellers and modifications to their use consistent with gains in engine power and technology which has led to greater efficiency.
- Wood Blades. Wood was the primary material used to make propellers for aircraft prior to World War II. They are now usually only seen in hobby or vintage aircraft in keeping with the restoration era. The wood propeller construction process consists of several layers (5-9) of wood glued together to make it stronger, more resilient and less likely to warp. Common types of wood used are yellow birch, black cherry, sugar maple, and black walnut
- Aluminium Alloy Blades. The introduction of more powerful aircraft engines has rendered use of wooden propellers all but obsolete in larger aircraft. Aluminium alloy blades are stronger, lighter and easier to repair as well as having higher rotation speeds making them a more popular choice.
- Composite Blades. Composite propeller blades are made from carbon fibre, these blades offer reduced weight, less noise and lower vibration, and are more durable and easier to repair than other types of propellers.
Types Of Aircraft Propeller
There are three basic types of an aircraft propeller, each with its own variations – the fixed pitch propeller, constant speed propeller or the ground adjustable propeller.
- Fixed Pitch Propeller. These propellers are made with the angle (pitch) built into the propeller, it cannot be changed. They are designed for optimum operation under optimum conditions, meaning aircraft performance will be affected under varying conditions. Fixed pitch propellers are often seen on single-engine aircraft that fly at low speeds, with limited range, or altitude.
- Constant Speed Propellers. Sometimes called a Controllable-Pitch Propeller, these propellers are designed with a variable pitch (angle) which can be altered in-flight while the propeller is rotating. This means the propeller can be adjusted during flight to better suit the changing conditions.
- Ground-Adjustable Propellers. As the name suggests these propellers are able to be adjusted only while on the ground when the propeller is not in use. The angle or pitch of the blade is manually altered, these propellers are not often seen in today’s modern aircraft.
While there have been many tweaks in propeller design over the last century, the fundamentals of this relatively simple part of an aircraft remain largely unchanged. Here are the basic terms associated with the operation of aircraft propellers.
- Chord Line. The chord line of a propeller is an imaginary line drawn through the centre of the blade from its leading edge (at the hub) to its trailing edge (tip).
- Pitch. The blades of a propeller are not straight, they are on an angle similar to that of a screw. The pitch is effectively a measure of how far the propeller would move forwards in one revolution. The pitch is used to control the speed of the air leaving the back of the propeller. The pitch of a propeller blade changes as you move along its surface from one end to the other. Its steepest or shortest at the central hub and shallowest at the outer tip. The pitch is calculated using the formula: pitch = 2.36 diameter height/width.
- Blade Angle. This is the angle between the chord line and the plane of rotation and is measured (in degrees) at a specific point along the length of the blade. While the terms pitch and angle are often used interchangeably the pitch is technically not the angle of the propeller blade. However the pitch is largely determined by blade angle, the two terms are often used interchangeably. An increase or decrease in one is usually associated with an increase or decrease in the other.
- Angle Of Attack. This is defined as the angle at which the air strikes the propeller blade. In simple terms the angle of attack can be described as the difference between where a wing is pointing and where it is going. Increasing the angle of attack results in an increase in both lift and induced drag, up to the point of a stall. The twist of a propeller blade is used to maintain a more constant angle of attack along the length of the blade to counteract the differences in blade speed at the hub and the tip of the propeller.