Aviation Technical Training Propeller Systems PDF

Aviation Technical Training Aircraft Technician Air Force (TMP No. 213409) Student Resource Module 4: Propeller Systems Topic 1: Propeller Types and Terminology This document is FOR TRAINING PURPOSES ONLY and is not subject to amendment. For current information, the reader should consult...

Aviation Technical Training Aircraft Technician Air Force (TMP No. 213409) Student Resource Module 4: Propeller Systems Topic 1: Propeller Types and Terminology This document is FOR TRAINING PURPOSES ONLY and is not subject to amendment. For current information, the reader should consult the appropriate Defence Instruction or Technical Manual. This document is not to be used as an authority in the working environment.  © BAE Systems. This work is copyright. Apart from any use as permitted under the Copyright Act 1968 and under Contract No. AFTG 01/13, no part may be reproduced by any process without prior written permission from BAE Systems. Aviation Technical 213409-SR-04.01_RevE Page: 2 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Table of Contents LEARNING OUTCOMES AND ASSESSMENT CRITERIA........................................................................ 4 INTRODUCTION..................................................................................................................... 5 PROPELLER TYPES.................................................................................................................. 6 FIXED PITCH............................................................................................................................... 6 GROUND ADJUSTABLE.................................................................................................................. 7 TRACTOR PROPELLER.................................................................................................................... 8 PUSHER PROPELLER..................................................................................................................... 8 CONTRA-ROTATING PROPELLER...................................................................................................... 9 COUNTER-ROTATING PROPELLER.................................................................................................. 10 CONTROLLABLE PITCH................................................................................................................ 11 SCIMITAR................................................................................................................................. 15 PROPELLER TERMS................................................................................................................16 BLADE ANGLE........................................................................................................................... 16 RELATIVE AIRFLOW.................................................................................................................... 17 ANGLE OF ATTACK..................................................................................................................... 18 PITCH DISTRIBUTION.................................................................................................................. 19 PROPELLER PITCH...................................................................................................................... 21 DIRECTION OF ROTATION............................................................................................................ 22 SELF-CHECK YOUR TOPIC KNOWLEDGE.......................................................................................23 Aviation Technical 213409-SR-04.01_RevE Page: 3 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Learning Outcomes and Assessment Criteria Module learning outcomes 4.1 Describe propeller system layout and operation. Assessment criteria 4.1.1. Identify common types of propellers used on aircraft. 4.1.2. Describe propeller terminology. Aviation Technical 213409-SR-04.01_RevE Page: 4 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Introduction Throughout the development of controlled flight, every aircraft required some kind of device to convert engine power to a useable form of thrust. Nearly all of the early aircraft designs used propellers to create thrust. Many unusual designs made their debut in flying machines during the latter part of the 19th century. These ranged from simple wood frame and fabric paddles to elaborate multi-bladed wire-braced designs. As the science of aeronautics progressed propeller designs improved from flat boards, which merely pushed air backward, to aerofoil shapes that produce an aerodynamic reaction as well as displacing air rearwards. By the time the Wright brothers began their first powered flight, propeller designs had evolved into the standard two-bladed style similar in appearance to those used in modern light aircraft. World War I brought about an increase in aircraft speed and engine size, requiring further improvements in propeller design. The most widely used propeller was a four-bladed wooden propeller. Other design improvements developed during the war included an aluminium fixed pitch propeller and the two-position pitch propeller. These designs did not come into wide use until the mid-1920s. As aircraft designs improved, propellers were developed which featured aluminium blades with thinner aerofoil sections. These improvements gave structural strength to the propeller. The advantage of being able to change the pitch of the propeller led to wide acceptance of the two-position propeller and, later, the development of the constant speed propeller system. This same constant speed propeller system is still in use on various modern aircraft. Further refinements of the propeller included a featherable propeller where the entire engine/propeller powerplant could be purposely shutdown after a malfunction had occurred. Another feature of this design was a reversing system that enabled the blades to move into a negative pitch angle, which caused the propeller to push air forward. This enabled shorter landing runs and improved ground manoeuvrability. The advanced propeller included other systems such as ice elimination, automatic feathering and engine synchronising/syncrophasing systems. Propeller designs continue to be improved by the use of new aerofoil shapes, composite materials and multi-bladed configurations. Recent improvements include the use of laminar and symmetrical aerofoils, composite materials, plus gull wing and scimitar propeller designs. Aviation Technical 213409-SR-04.01_RevE Page: 5 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Propeller Types A propeller consists of two or more blades attached to a central hub that is then rotated by an engine. The purpose of the propeller is to convert engine power to useful thrust. The types of propellers used in modern aircraft include:  fixed pitch  ground adjustable  tractor  pusher  contra-rotating  counter-rotating  controllable pitch  constant speed  feathering type  reversing pitch  scimitar. Fixed pitch Fixed pitch is a term used to describe a propeller where the blade angle cannot be changed. Fixed pitch propellers are usually found on light single engine aircraft. A fixed pitch propeller, as shown in Figure 1, can be made of wood, aluminium or steel and is designed for a specific purpose, i.e. cruise or climb. Propeller performance will drop off rapidly if a fixed pitch propeller is operated outside of its designed purpose. Figure 1: Fixed Pitch Propeller Aviation Technical 213409-SR-04.01_RevE Page: 6 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Ground adjustable A ground adjustable propeller is essentially a fixed pitch propeller. However, the propeller pitch can be adjusted to a given angle, but only on the ground when the propeller is not rotating, to give the desired performance for conditions such as low blade angle for short take-offs or high blade angle for increased cruse speed. Ground adjustable propellers are typically fitted to aircraft with low power, low speed, short range and limited altitude to allow one propeller design to be used on differing aircraft designs with the same engine. Blade angle changes are achieved by loosening the retaining clamps around the blades and adjusting blade to the required angle. Figure 2 shows the retaining clamps on a ground adjustable propeller. Blade angle adjustments can be made with the propeller either on a propeller bench or fitted to an aircraft. The adjustment is made by loosening the blade clamps and turning the blades to the required blade angle. Blade angles are then checked by using a propeller protractor and the hub clamp bolts are tightened, blade angles are re- checked to ensure the blade has not changed during the clamping process. Figure 2: Ground Adjustable Propeller Aviation Technical 213409-SR-04.01_RevE Page: 7 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Tractor propeller Tractor propellers are conventionally mounted in front of the engine powerplant. Tractor propellers pull the aircraft through the air and are the most commonly used propeller type. A tractor propeller is shown in Figure 3. Figure 3: Tractor Propellers Pusher propeller Pusher propellers are mounted on a drive shaft from the rear of the engine producing thrust to push the aircraft forward. In some aircraft designs, efficiencies can be gained by mounting the propeller behind the fuselage. Pusher propellers are often used in seaplanes and amphibious aircraft installations. Figure 4 shows a pusher propeller configuration. Figure 4: Pusher Propellers Aviation Technical 213409-SR-04.01_RevE Page: 8 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Contra-rotating propeller The system uses two separate propellers mounted in line on two concentric shafts that rotate in opposite directions, as shown in Figure 5. Figure 5: Contra-rotating Propeller Contra-rotating propellers absorb, and therefore efficiently use, the output of high-powered engines. An advantage of this type of propeller is the cancellation of torque reaction and a reduction of the spiralling slipstream, i.e. a much straighter airflow. Aviation Technical 213409-SR-04.01_RevE Page: 9 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Counter-rotating propeller Found on twin and multi-engine propeller aircraft where the propellers spin in the opposite direction to the other generally clockwise on the left wing and counter-clockwise on the right on a twin engine aircraft. A more recent design is fitted to the A400 M where the individual propellers on each wing spin in the opposite direction to each other. Notice the different direction of rotation on Number 1 and 2 propellers shown in Figure 6. Figure 6: Airbus A400M Aviation Technical 213409-SR-04.01_RevE Page: 10 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Controllable pitch A controllable pitch propeller is shown in Figure 7. Controllable pitch propellers are designed so that the pilot can select any blade angle, within the propellers range, regardless of the aircrafts operational conditions. Figure 7: Controllable Pitch Propeller A controllable pitch propeller permits the propeller blade angle or pitch to be adjusted to give the best performance for particular flight conditions while the propeller is rotating. Controllable pitch propellers can be:  constant speed  feathering  reversing. Aviation Technical 213409-SR-04.01_RevE Page: 11 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Constant speed Aircraft fitted with constant speed propellers allow a selected engine speed to be maintained automatically. If the engine RPM varies, a speed sensitive governor brings the RPM back to the selected speed by altering the propeller blade angle. The constant speed system reduces pilot workload and protects the engine from large RPM fluctuations. A constant speed propeller is shown in Figure 8. Figure 8: Constant Speed Propeller Feathering Feathering is a capability common to many multi-engine aircraft. Feathering is the rotation of a blade to a 90 degree angle, presenting the smallest blade profile to the oncoming airstream. The feathering function ensures that a faulty powerplant can be shut down quickly to minimise further damage and prevent windmilling through aerodynamic forces. Failure to reduce drag or to prevent engine damage could result in loss of aircraft and life. Controllable pitch propellers have mechanisms to change the blade angle to such a position that propeller rotation stops, i.e. the blade chord (at a set distance from the hub) is parallel to the direction of flight. The leading edge of the propeller faces in the same direction that the aircraft is flying as shown in Figure 9. 90 degrees is a high angle (Coarse) Aviation Technical 213409-SR-04.01_RevE Page: 12 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Figure 9: Feathering Propeller Aviation Technical 213409-SR-04.01_RevE Page: 13 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Reversing Moving the blade angle towards a negative angle permits an aircraft to reduce landing runs, which also reduces brake and tyre wear. Reversing also assists in ground handling by allowing the aircraft to be taxied backwards. When reverse has been selected, the propeller blades rotate from a positive angle, which pushes air rearward, to a negative angle, as shown in Figure 10. A negative blade angle pushes air forward, which pushes the aircraft rearward. Note The propeller does not change its direction of rotation when reverse is selected. Only the blade angle changes. Trailling Edge Leading Edge Feather High blade angle Low blade angle Reverse Figure 10: Reversing Propeller Aviation Technical 213409-SR-04.01_RevE Page: 14 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Scimitar A scimitar propeller has an increasing sweep along the leading edge. Typically scimitar propellers are constructed of lightweight or composite materials. The combination of lightweight and efficient aerodynamics results in more power and reduced noise. Figure 11 shows an example of a six-bladed scimitar propeller in the feathered position, as used on the RAAF Hercules C-130J. Figure 11: Scimitar Propeller – Feathered Aviation Technical 213409-SR-04.01_RevE Page: 15 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Propeller Terms The following propeller terms enable a better understanding to be developed of the relationship between propeller rotational velocity and aircraft velocity:  blade angle  angle of attack  pitch distribution  propeller pitch  direction of rotation. Blade angle The blade angle is the angle between the chord line of the blade and the plane of rotation as shown in Figure 12. Blade angle is measured in degrees. Figure 12: Blade Angle Aviation Technical 213409-SR-04.01_RevE Page: 16 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Relative airflow Direction of relative airflow is exactly opposite a blade’s plane of rotation when the propeller is operating on a stationary aircraft. Figure 13 depicts relative airflow when the aircraft is moving forward. The combination of the rotating and forward motion produces a resultant relative airflow that is no longer exactly opposite the blade’s plane of rotation. Increased aircraft velocity with stable rotational velocity will decrease angle of attack. Increased rotational velocity with constant aircraft velocity will increase angle of attack. These concepts are described in greater detail later in this module. Figure 13: Relative Airflow Aviation Technical 213409-SR-04.01_RevE Page: 17 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Angle of attack The angle of attack is the angle between the chord line and the angle of relative airflow, as shown in Figure 14. An angle of attack between two to four degrees produces the highest efficiency. At this angle of attack the incoming air is compressed and then expands when passing the trailing edge, producing the required thrust. Figure 14: Angle of Attack Aviation Technical 213409-SR-04.01_RevE Page: 18 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Pitch distribution Pitch distribution refers to the gradual twist in the propeller blade from shank to tip. Propeller blades are marked off in six-inch increments known as blade stations. Blade twist angle near the shank of the blade will be greater than the angle at the blade tip. Pitch variations are progressive between the shank and the blade tip. Blade stations provide a means of determining propeller performance, locating blade markings and measuring blade angle in relation to other blades on the assembly. A cross-section of each blade station, as depicted in Figure 15, shows the low speed aerofoil near the hub and high speed aerofoil near the tip. The pitch distribution and the change in aerofoil shape along the length of the blade are necessary because each section is moving at a different velocity with the slowest speeds near the hub and the highest speeds near the tip. Maintaining the gradual blade twist also ensures that the correct angle of attack is maintained at two to four degrees along the length of the blade at any given moment of propeller operation, thereby maintaining maximum efficiency. Figure 15: Blade Pitch Distribution Aviation Technical 213409-SR-04.01_RevE Page: 19 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  To show the speed differential of the aerofoil sections on a propeller at a given RPM refer to Figure 16 below. Consider the three sections shown, for this example the propeller is rotating at a constant 1800 RPM, at the 10 inch station the blade travels at 1.6 metres per revolution (172 kmh) the twenty inch blade station at 3.2 m per revolution (344 kmh) and the thirty inch station 4.8 m per revolution (516 kmh). The aerofoil that is suited to give the best performance at 172 kmh would be inefficient at 516 kmh illustrating the need for the gradual twist along the length of the blade. 4.8m Figure 16: Speed of Blades at Three Blade Stations Aviation Technical 213409-SR-04.01_RevE Page: 20 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Propeller pitch Propeller pitch is the distance moved by the propeller in a forward direction in one revolution. The propeller pitch varies with different blade angles. Figure 17 illustrates the propeller pitch of two different blade angles. Figure 17: Propeller pitches of two different blade angles Pitch is not the same as blade angle, however, pitch is largely determined by blade angle. The two terms are often used interchangeably. An increase or decrease in one is associated with an increase or decrease in the other. Aviation Technical 213409-SR-04.01_RevE Page: 21 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Direction of rotation The term direction of rotation (DOR) is a general term used to describe a propeller’s direction of rotation, when viewed from the pilot’s seat or from the aft of the aircraft looking forward. Component location is also viewed this way to ensure that there is no confusion. For example port is left, and starboard is right. Figure 18 shows a propeller with a clockwise DOR. Figure 18: Propeller Direction of Rotation Aviation Technical 213409-SR-04.01_RevE Page: 22 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20  Self-check your Topic Knowledge Activity 1. What type of aircraft are fixed pitch propellers usually found on? 2. Describe the term fixed pitch propeller 3. Describe how to perform a blade angle change on a ground adjustable propeller 4. The type of propeller that is located forward of a powerplant is termed a? 5. List the advantages of a contra-rotating propeller 6. List the advantages of a controllable pitch propeller 7. Define the term feather 8. List the advantages that a reversing propeller offers over a non-reversing propeller 9. What happens to the direction of rotation of a propeller when reverse operation is selected? 10. Describe the following propeller terms: a) blade angle b) helix angle c) angle of attack d) pitch distribution e) propeller pitch 11. How is the direction of rotation of a propeller determined? Aviation Technical 213409-SR-04.01_RevE Page: 23 Template Version No: 1.4 Training © BAE Systems Document Updated: 28/04/20