F002 TTN AEO Vibration PDF

School of Army Aeronautical Engineering AEO /ARTIFICER F002 VIBRATION TTN INTENTIONALLY BLANK RELEASE CONDITIONS 1. The information given in this document is not to be communicated either directly or indirectly to the Press or to any person not authorised to receive it. 2. This information is released by the UK Government to the recipient Government for defence purposes only. 3. This information must be accorded the same degree of security protection as that accorded thereto by the UK Government. 4. This information may be disclosed only within the Defence Departments of the Recipient Government, except as otherwise authorised by the Ministry of Defence. 5. This information may be subject to privately owned rights. 6. This document contains proprietary information made available to MOD subject to the limitations of confidentiality and restricted rights of use. Its existence or contents must not be further disclosed without specific authority. Defence Intellectual Property Rights (DIPR) should be consulted if further use or disclosure is contemplated. 7. This document contains MOD information of significant commercial value. It must not be further disclosed without specific approval of DIPR which will be given for disclosure to persons who have signed relevant confidentiality agreements, licence agreements or contracts. It is made available for release and exploitation through defence Technology Enterprises Ltd to UK firms under confidentiality agreements. Note that this does not affect the right of release by MOD for use on MOD contract work. 8. ITAR Controlled Information. Some of this data may contain ITAR data of which the following statement will apply: WARNING: THIS DOCUMENT CONTAINS TECHNICAL DATA WHOSE EXPORT IS RESTRICTED BY THE ARMS EXPORT CONTROL ACT (TITLE 22.US.C SEC 2751 ET SEQ.) OR THE EXPORT ADMINISTRATION ACT OF 1979, AS AMENDED, TITLE 50 U.S.C.APP.2401 ET SEQ. VIOLATION OF THESE EXPORT LAWS ARE SUBJECT TO SEVERE CRIMINAL PENALTIES. DISSEMINATE IN ACCORDANCE WITH THE PROVISIONS OF DOD DIRECTIVE 5230.25. 9. Copyright. This publication is UK Ministry of Defence © Crown copyright (2023). Material and information contained in this publication may be reproduced, stored in a retrieval system, and transmitted for MOD use only, except where authority for use by other organisations or individuals has been authorised by a Patent Officer of the DIPR. F002 Vibration Prelim Page 1 TTN AUTHORISATION Précis/TTN Title: Subject Title Issue Number 1 Date First Issued May 2024 Date Last Issued May 2024 Date Last Reviewed: Next Review Date Post first delivery instance Reviewed by: Authorised for use by DSAE Course Design Officer: Any comments or suggestions regarding this Précis / TTN should be addressed iaw SAAE MTS. F002 Vibration Prelim Page 2 CONTENTS Release Conditions............................................................................................................ 1 TTN Authorisation.............................................................................................................. 2 Contents............................................................................................................................. 3 Table of Figures................................................................................................................. 5 Glossary of Terms............................................................................................................. 7 Chapter 1 Vibration Analysis............................................................................................ 1 Section 1 Vibration Control.......................................................................................... 1 Section 2 Rationale for Vibration Control..................................................................... 1 Section 3 What is Vibration......................................................................................... 1 Section 4 Vibratory Characteristics............................................................................. 2 Section 5 Phase relationship....................................................................................... 3 Section 6 Phase relation between Vibration Quantities............................................... 5 Section 7 Information Provided by Phase.................................................................... 6 Section 8 Frequency................................................................................................... 6 Section 9 Types of Vibration........................................................................................ 7 Section 10 Frequency Spectrum................................................................................. 9 Section 11 Measurement Parameters....................................................................... 10 Section 12 Possible Causes of Vibration................................................................... 11 Section 13 Modes and Nodes................................................................................... 12 Chapter 2 Helicopter Vibration....................................................................................... 13 Section 1 Introduction................................................................................................ 13 Section 2 Stress and Fatigue.................................................................................... 14 Section 3 Resonance and Damping.......................................................................... 14 Section 4 Aerodynamic sources................................................................................ 15 Section 5 Mechanical Sources.................................................................................. 15 Section 6 Classification of Vibration.......................................................................... 17 Section 7 Describe Frequency (Speed Relationships).............................................. 17 Section 8 Vibration Levels......................................................................................... 18 Chapter 3 Effects of Vibration on Helicopter crews...................................................... 19 Chapter 4 Vibration Analysis.......................................................................................... 21 Section 1 Rationale................................................................................................... 21 Section 2 Vibration Values........................................................................................ 22 Chapter 5 Methods of reducing Vibration...................................................................... 25 Section 1 Introduction................................................................................................ 25 Section 2 Resonant mass.......................................................................................... 25 Section 3 Nodal beam............................................................................................... 26 Chapter 6 Rotor Tracking and Balancing...................................................................... 27 F002 Vibration Prelim Page 3 Section 1 Introduction................................................................................................ 27 Section 2 Helicopter Rotor Balacing and Tracking.................................................... 27 Section 3 Tracking the Main Rotor............................................................................ 29 Section 4 Tail Rotor Tracking and Balancing............................................................. 30 Chapter 7 Vibration Control (VC) Organizational Responsibilities.............................. 31 Section 1 Station / Unit Responsibilities.................................................................... 31 Section 2 Vibration Control Cells (VCC).................................................................... 31 Section 3 Health and Usage Centres (HUCs)........................................................... 32 Section 4 1710 Naval Air Squadron (1710 NAS)....................................................... 32 Section 5 Vibration information in Air System document set..................................... 33 Chapter 8 Health and Usage Monitoring........................................................................ 35 Section 1 Introduction................................................................................................ 35 Section 2 HUM Support............................................................................................. 35 Section 3 Responsibilities.......................................................................................... 36 Chapter 9 Training Requirement.................................................................................... 37 Section 1 Training / and Authorisations for Vibration Control.................................... 37 Section 2 Training of personnel involved in HUM...................................................... 37 Section 3 Deferral of Vibration control techniques..................................................... 37 F002 Vibration Prelim Page 4 TABLE OF FIGURES FIG. 1 Vibratory System (Real Space)................................................................................. 2 FIG. 2 Weight A and Weight B are 180° out of phase.......................................................... 3 FIG. 3 Two Objects Vibrating 90° Out of Phase................................................................... 4 FIG. 4 Two Objects Vibrating in Phase................................................................................ 4 FIG. 5 Phase Relationship between Displacement, Acceleration and Velocity.................... 5 FIG. 6 Vibration represented with respect to time and frequency........................................ 9 FIG. 7 Spectrum of Parameters......................................................................................... 10 FIG. 8 Propagation paths................................................................................................... 11 FIG. 9 Modes and Nodes................................................................................................... 12 FIG. 10 Mounting Axis....................................................................................................... 13 FIG. 11 Typical points of vibration on Helicopters.............................................................. 16 FIG. 12 A type of main rotor damping system.................................................................... 25 FIG. 13 Nodal beam damping............................................................................................ 26 FIG. 14 Blade tip paths...................................................................................................... 28 FIG. 15 Tracking (Rotor tuner display)............................................................................... 29 F002 Vibration Prelim Page 5 INTENTIONALLY BLANK F002 Vibration Prelim Page 6 GLOSSARY OF TERMS Term Meaning A Acceleration The rate of change of velocity per unit of time. This is a vector quantity. Units of measurement are typically feet/sec/sec, metres/sec/sec, and G's (1 G = 32.17 ft./sec/sec = 9.81 m/s/s). Acceleration measurement is usually taken with an accelerometer. Accelerometer A transducer whose output is directly proportional to acceleration. Normally use piezoelectric crystals to produce an output. Alignment A condition whereby the axis of machine components are either coincident, parallel or perpendicular, according to design requirements. Amplification A measure of the susceptibility of a rotor to vibration amplitude Factor when rotational speed is equal to the rotor natural frequency (implies a flexible rotor). For imbalance type excitation, synchronous amplification factor is calculated by dividing the amplitude value at the resonant peak by the amplitude value at a speed well above resonance (as determined from a plot of synchronous response vs. rpm). Amplification Factor is also known as Synchronous. Amplitude The amplitude of a wave is its maximum disturbance from its undisturbed position. Take care, the amplitude is not the distance between the top and bottom of a wave which is called maximum displacement. Or The maximum extent of a vibration or oscillation, measured from the position of equilibrium ASTE Aircraft Servicing Tools and Equipment as defined in: https://www.gov.uk/government/uploads/system/uploads/attachme nt_data/file/227048/acr onyms_and_abbreviations_dec08.pdf Asymmetrical Rotor support system that does not provide uniform restraint in all Support radial directions. This is typical for most heavy industrial machinery where stiffness in one plane may be substantially different than stiffness in the perpendicular plane. Occurs in bearings by design, or from preloads such as gravity or misalignment. F002 Vibration Prelim Page 7 Asynchronous Vibration components that are not related to rotating speed (also referred to as nonsynchronous). AUW All Up Weight. The aircraft gross weight (also known as the all-up weight (AUW)) is the total aircraft weight at any moment during the flight or ground operation. An aircraft's gross weight will decrease during a flight due to fuel and oil consumption. An aircraft's gross weight may also vary during a flight due to payload dropping or in-flight refuelling. At the moment of releasing its brakes, the gross weight of an aircraft is equal to its take-off weight. During flight, an aircraft's gross weight is referred to as the en-route weight or in- flight weight. Axial In the same direction as the shaft centreline. Axial Position The average position, or change in position, of a rotor in the axial direction with respect to some fixed reference position. Ideally the reference is a known position within the thrust bearing axial clearance or float zone, and the measurement is made with a displacement transducer observing the thrust collar. B Balanced For rotating machinery, a condition which exists when the shaft Condition geometric centreline coincides with the mass centreline. Balancing A procedure for adjusting the radial/lateral mass distribution of a rotor so that the mass centreline approaches the rotor geometric centreline. Balancing A rotational speed that corresponds to a natural resonance Resonance frequency. Speed(s) Balanced Condition. For rotating machinery, a condition where the shaft geometric centreline coincides with the mass centreline. Band Pass Filter A filter with a single band extending from lower to upper cut off frequencies. Bandwidth The spacing between frequencies at which a band pass filter attenuates the signal. BIT Built-In-Test. The BIT function is an integral capability of the mission system, or equipment that provides an on-board automated test capability to detect, diagnose or isolate failures. The fault detection, and possible isolation capability, is used for periodic or continuous monitoring of a systems' operational health. The observation and F002 Vibration Prelim Page 8 diagnostic capability can be used as a prelude to maintenance action. Blade Passing A potential vibration frequency on any bladed machine (turbine, Frequency axial compressor, fan, etc.). It is represented by the number of blades times shaft-rotating frequency Brinelling This is the permanent indentation of a hard surface. It is named after the Brinell scale of hardness, in which a small ball is pushed against a hard surface at a pre-set level of force, and the depth and diameter of the mark indicates the Brinell hardness of the surface. Typically caused by external vibration when the shaft is stationary. C C of G The point at which the weight of a body of mass (including a complete aircraft with all items fitted) is assumed to act. Normally, only the fore and aft (longitudinal) location of the centre of gravity is important for symmetrically loaded aircraft. If the lateral, or vertical position of the centre of gravity is likely to have any material effect, reference to it will be made in the Weight and Balance Data of the Air Publication concerned and, when applicable, on the Trim Sheet. Calibration A test during which known values of the measured variable are applied to the transducer or readout instrument, and output readings varied or adjusted. Cause A cause is an event or problem that makes something else happen. A cause is a factor which leads directly to an occurrence. Cavitation A condition which can occur in liquid handling machinery (e.g. centrifugal pumps) where a system pressure decrease in the suction line and pump inlet lowers fluid pressure and vaporization occurs. The result is mixed flow which may produce vibration. Critical Speeds In general, any rotating speed which is associated with high vibration amplitude. Often, the rotor speeds which correspond to natural frequencies of the system. CWP Central Warning Panel. Cycle One complete sequence of values of a periodic quantity. The number of times a vibration frequency occurs over a given period of time. F002 Vibration Prelim Page 9 D Damping The quality of a mechanical system that restrains the amplitude of motion with each successive cycle. Damping of shaft motion is provided by oil in bearings, seals etc. The damping process converts mechanical energy, (kinetic), into other forms, usually heat. Damping, The smallest amount of damping required to return the system to Critical its equilibrium position without oscillation. Degrees Of A phrase used in mechanical vibration to describe the complexity Freedom of the system. The number of degrees of freedom is the number of independent variables describing the state of a vibrating system. Displacement The change in distance or position of an object relative to a reference. DTI. Dial Test Indicator. E Eccentricity, The variation of the outer diameter of a shaft surface when Mechanical referenced to the true geometric centreline of the shaft. Out-of- roundness. F False Brinelling Is damage caused by fretting, with or without corrosion that causes imprints that look similar to Brinelling, but are caused by a different mechanism. Fast Fourier Is a mathematical principle for converting complex sine waves into Transform simple sine waves. The procedure for this is carried out by a computer, or microprocessor. Fault A fault is a problem with a piece of equipment, machine or item that stops it from working correctly. The state of an item characterized by the inability to perform a required function, excluding the inability during preventive maintenance or other planned actions, or due to a lack of external resources (BS 3811). Forced Vibration The oscillation of a system under the action of a forcing function. Typically forced vibration occurs at the frequency of the exciting force. Free Vibration Vibration of a mechanical system following an initial force— typically at one or more natural frequencies. Frequency The repetition rate of a periodic event, usually expressed in cycles per second (Hz), revolutions per minute (rpm), or multiples of a F002 Vibration Prelim Page 10 rotational speed (orders). Orders are commonly referred to as 1x for rotational speed, 2x for twice rotational speed, etc. FRT Forward Repair Team. FTS Flight Test Schedule. The Type Airworthiness Authority (TAA) is the Technical Information (TI) sponsor for an aircraft’s FTS and is responsible for ensuring that, when introducing a new type or mark of aircraft, an FTS is available for use on the first aircraft delivery. OC Handling Squadron (OC HS), or an alternative Subject Matter Expert (SME), may, with the authority of the TAA, and the Release To Service Authority (RTSA) if the Release To Service (RTS) has been signed, arrange to provide an interim FTS if a fully approved FTS cannot be provided by the time the first aircraft is delivered. G g The value of acceleration produced by the force of gravity. Unit is lower case g. Gear Mesh A potential vibration frequency on any machine that contains Frequency gears; equal to the number of teeth multiplied by the rotational frequency of the gear. H Harmonic Frequency component at a frequency that is an integer multiple of the fundamental frequency. Heavy Spot The angular location of the imbalance vector at a specific lateral location on a shaft. The heavy spot typically does not change with rotational speed. Hertz The unit of frequency represented by cycles per second. High Pass Filter A filter with a transmission band starting at a low cut off frequency and extending, intheory, to infinite frequency. High Spot The angular location on the shaft directly under the vibration transducer at the point of closest proximity. The high spot can move with changes in shaft dynamics (e.g., from changes in speed). I IBIT Initiated Built-In-Test. Imbalance Unequal radial weight distribution on a rotor system; a shaft condition such that the mass and shaft geometric centrelines do not coincide. F002 Vibration Prelim Page 11 Impedance, The mechanical properties of a machine system (mass, stiffness, Mechanical damping) that determine the response to periodic forcing functions. IPS Inches Per Second (IPS). Unit of measurement used when displaying velocity vibration. ISA The International Standard Atmosphere (ISA) is an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes or elevations. It has been established to provide a common reference for temperature and pressure and consists of tables of values at various altitudes, plus some formulas by which those values were derived. The International Organization for Standardization (ISO) publishes the ISA as an international standard, ISO 2533:1975.Other standards organizations, such as the International Civil Aviation Organization (ICAO) and the United States Government, publish extensions or subsets of the same atmospheric model under their own standards-making authority. At Sea Level: 15.0°C / 1013.25 mb. L Lateral Vibration Shaft dynamic motion or casing vibration, which is in a direction perpendicular to the shaft centreline. Normally referred to as Radial Vibration. Low Pass Filter A filter whose transmission band extends from zero to an upper cut-off frequency. M Mass In physics, the property of matter that measures its resistance to acceleration. Roughly, the mass of an object is a measure of the number of atoms in it. The basic unit of measurement for mass is the kilogram MI Electro-Magnetic Indicator. Micrometer One millionth (.000001) of a meter. (1 micron = 1 x E-6 meters = 0.04 mils.) MIL One thousandth (0.001) of an inch. (1 mil = 25.4 microns.) N Natural The frequency of a free vibration of a system. Frequency Or F002 Vibration Prelim Page 12 The frequency at which an undamped system with a single degree of freedom will oscillate upon momentary displacement from its position of rest. Determined by Shape, Material, Design, Mass. Noise Any component of a transducer output signal that does not represent the variable intended to be measured. O Oil Whirl An unstable free vibration whereby a fluid-film bearing has insufficient unit loading. Under this condition, the shaft centreline dynamic motion is usually circular in the direction of rotation. Oil whirl occurs at the oil flow velocity within the bearing, usually 40 to 49% of shaft speed. Oil whip occurs when the whirl frequency coincide with (and becomes locked to) a shaft resonant frequency. (Oil whirl and whip can occur in any case where fluid is between two cylindrical surfaces.) P Period The time required for a complete oscillation or for a single cycle of events. The reciprocal of frequency. Phase A measurement of the timing relationship between two signals, or between a specific vibration event and a key phasor pulse. Piezoelectric Any material which provides a conversion between mechanical and electrical energy. For a piezoelectric crystal, if mechanical stresses are applied on two opposite faces, electrical charges appear on some other pair of faces. Polar Plot. Polar coordinate representation of the 1 x vector at a specific shaft location with the shaft rotational speed as a parameter. R Radial Vibration Shaft dynamic motion or casing vibration which is in a direction perpendicular to the shaft centreline. Can be referred to as lateral vibration. Resonance The condition of vibration amplitude and phase change response caused by a corresponding system sensitivity to a particular forcing frequency. A resonance is typically identified by a substantial amplitude increase, and related phase shift. Resonant The natural frequency of vibration at which each body will vibrate Frequency of its own accord if struck by a disturbing force. S F002 Vibration Prelim Page 13 Signature Term usually applied to the vibration frequency spectrum which is distinctive and special to a machine or component, system or subsystem at a specific point in time, under specific machine operating conditions, etc. Used for historical comparison of mechanical condition over the operating life of the machine. Slow Roll Speed Low rotational speed at which dynamic motion effects from forces such as imbalance are negligible. Subharmonic A frequency that is a submultiple of a fundamental frequency. Symptom Noun, Any phenomenon or circumstance accompanying something and serving as evidence of it. A sign or indication of something. T Transducer A device that converts variations in a physical quantity, such as pressure or brightness, into an electrical signal, or vice versa Or A device for translating the magnitude of one quantity into another quantity. Transient Temporarily sustained vibration of a mechanical system. It may Vibration consist of forced or free vibration or both. Typically this is associated with changes in the operating condition of the machine, such as speed, load etc. Trigger Any event which can be used as a timing reference. In aRT5-JS+, trigger can be used to initiate measurement. U Unbalance This is the uneven distribution of mass around an axis of rotation. A rotating mass is said to be out of balance when its centre of mass (inertia axis) is out of alignment with the centre of rotation (geometric axis). V Vector A quantity that has both magnitude and direction. Represented often as a line on a graph. Velocity The rate of change of position per unit of time. This is a vector quantity F002 Vibration Prelim Page 14 Vibration Oxford English dictionary definition:- To shake with small, quick movements For Vibration Analysis and Theory however the following definition is used:- The physical displacement and return of a body about its place of rest, when acted upon by a disturbing force W WDM Wear Debris Monitoring. Weight The weight of an object is the force of gravity on the object and may be defined as the mass times the acceleration of gravity, w = mg. Since the weight is a force, its SI unit is the newton. F002 Vibration Prelim Page 15 INTENTIONALLY BLANK F002 Vibration Prelim Page 16 CHAPTER 1 VIBRATION ANALYSIS SECTION 1 VIBRATION CONTROL. 1. Vibration Control (VC) is the engineering term given to the suppression of vibration by analysis and rectification. It is used for helicopter Rotor Track and Balancing (RTB) and propeller balancing. 2. In addition, it will assist the health monitoring of aircraft engines and other rotating equipment. 3. Aircraft subject to VC must be monitored for vibration as detailed in the aircraft Topic 5A1 (Master Maintenance Schedule). The operation of Vibration Equipment (VE) and the collection of Vibration Analysis (VA) data must be carried out by suitably trained personnel and the resultant data managed by Subject Matter Experts (SME). 4. Vibration Analysis. Vibration Analysis (VA) is the process of measuring, recording, and interpreting vibration data. This may lead to the introduction of Corrective Maintenance activities to reduce the vibration to acceptable levels. The extent of the Corrective Maintenance will be dependent on the outcome of the VA and will be detailed in the appropriate Air System Technical Information (TI). KLP 04C.0.01 SECTION 2 RATIONALE FOR VIBRATION CONTROL. 5. Vibration induced by aerodynamic loads or structural resonance excited by rotating components, such as gas turbines, propellers and helicopter rotors, may induce high levels of stress in parts of the structure or transmission system. These stresses may lead to premature failure through fatigue, including increased secondary damage, such as wear or higher incidence of electronic component failure. 6. Vibration Control (VC) is the suppression of vibration by analysis and rectification. Expedient analysis from vibration testing will limit the time the platform may have succumbed to stresses caused by vibration; the results will highlight any irregularities with the data and subsequently maintain the aircrafts’ airworthiness. KLP 04C.0.02 SECTION 3 WHAT IS VIBRATION. 7. Vibration may best be described as: “The movement of a body back and forth from its place of rest when acted upon by a disturbing force." 8. Vibration is the response of a component to some form of internal or external stimulus or force that is applied to the component. There are three main parameters that can be used to define the level of vibration present. a. Amplitude - how much vibration. b. Frequency - how many times it occurs per minute or second. c. Phase - which describes how it is vibrating. F002 Vibration Chapter 1 Page 1 9. A typical vibratory system FIG. 1 may be considered as a mass attached to a spring whose other end is fixed. Until a disturbing force is applied to the mass to cause it to move, there is no vibration. This is known as a Simple Spring Mass Model. FIG. 1 Vibratory System (Real Space) 10. If an upward force is applied, the mass will move upwards compressing the spring until its upper limit (UL) of travel is reached. When the force is released, ignoring gravity, the restorative force of the spring causes the mass to travel down to the neutral position (NP). Inertial forces then cause the mass to travel through the neutral position to the lower limit (LL) of travel where the spring stiffness prevents the mass from travelling any further. The restorative force of the spring then causes the mass to return through the neutral to the upper limit of travel, this oscillating motion will continue until dissipated by friction of the mass passing through air, and friction within the spring itself. KLP 04C.01.02 SECTION 4 VIBRATORY CHARACTERISTICS. 11. The characteristics of such a sine wave reflect all the parameters needed to identify the vibration, as follows: a. Displacement. The total distance travelled by the vibrating mass from one limit of travel to the other limit, is known as the 'peak to peak (pp) displacement'. It may be expressed in micrometers (µm, formerly microns) where one µm equals one millionth of a metre (0.000001 m or 0.001 mm); it may also be expressed in thousandths of an inch or mils (1 mil = 0.001 in.). F002 Vibration Chapter 1 Page 2 b. Velocity. Since the oscillating mass is moving, it must be moving at some speed. However, the speed of the mass is constantly changing. At each limit of motion the speed is zero since the mass must come to a stop before it can go in the opposite direction. The speed or velocity is greatest as the mass passes through the neutral position. As the velocity is constantly changing throughout the cycle, the highest, or peak velocity, is selected for measurement. Vibration velocity is normally expressed in terms of millimeters per second peak (mm/s peak), or inches per second peak(in/s peak, or ips peak). c. Acceleration. Acceleration is the rate of change of velocity. As explained in Para 7.2, the velocity of the mass reduces to zero at each limit of travel. Each time the mass comes to a stop, it must accelerate at a high rate to pick up velocity as it starts to travel toward the other limit of travel; as the mass passes through neutral the velocity is constant and there is no acceleration. KLP 04C.01.04 SECTION 5 PHASE RELATIONSHIP 12. Phase in Relation to Vibration. Phase is defined as 'the position of a vibrating part at a given instance with reference to a fixed point or another vibrating part.' 13. Phase is the measurement of vibrating motion at one location relative to the vibration at another location or the timing of a vibration in relation to a stationary or moving part. The phase relationship between displacement, velocity and acceleration. 14. In a practical sense, phase measurements offer a convenient way to compare one vibrating part with another; or to determine how one part is vibrating relative to another. For example, two weights in FIG. 2 are vibrating at the same frequency and displacement; however, weight A is at the upper limit of travel at the same instant weight B' is at the lower limit. We can use phase to express the comparison. By plotting one complete cycle of motion of these two weights, starting at the same given instant, we can see that the points of peak displacement are separated by 180° (one complete cycle = 360°). Therefore, we would say that these two weights are vibrating 180° out of phase. 15. In FIG. 2, weight ‘A’ is at the upper limit of travel at the same instant weight ‘B’ is at the bottom of it travel, moving on the opposite direction, these weights are 180° out of phase. FIG. 2 Weight A and Weight B are 180° out of phase F002 Vibration Chapter 1 Page 3 16. In FIG. 3, weight ‘X’ is at the upper limit of travel at the same instant weight ‘Y’ is at the neutral position, moving toward the lower limit. These two weight as Vibrating 90° out of phase. FIG. 3 Two Objects Vibrating 90° Out of Phase. 17. In FIG. 4, weights ‘C' and 'D' reach the upper limit of travel at the same instant. These weights are 'in phase.' FIG. 4 Two Objects Vibrating in Phase F002 Vibration Chapter 1 Page 4 SECTION 6 PHASE RELATION BETWEEN VIBRATION QUANTITIES. 18. FIG. 5 shows the relationship between three vibration measurement parameters defined as: a. Amplitude / Time domain. b. Acceleration / Time domain. c. Velocity / Time domain. FIG. 5 Phase Relationship between Displacement, Acceleration and Velocity. F002 Vibration Chapter 1 Page 5 SECTION 7 INFORMATION PROVIDED BY PHASE. 19. Phase measurements are essential in vibration analysis to diagnose specific component vibrations. Comparative phase measurements are used as follows: a. Balancing. Phase is used to determine the type of unbalance, static or dynamic, and to calculate the amount and angular location of correcting weights. b. Misalignment. Comparative phase measurements reveal the misalignment (angular or offset) and the location. c. Looseness. Phase is used to detect relative movement in components that is due to poor mounting or cracked or broken mounts. KLP 04C.01.05 SECTION 8 FREQUENCY. 20. The number of complete cycles (c) in one second (s) is the frequency (f) and is measured in Hertz (Hz) where: a. 1 Hz = 1 c/s. 21. Also, frequency (f) may be obtained from the formula f = 1/t where t is the time in seconds to complete one cycle. The result is, of course, expressed in Hertz. 22. Resonant Frequency. All bodies, by virtue of their material, design and shapes, have their own natural frequency of vibration at which each body will vibrate of its own accord if struck by a disturbing force. This natural frequency of vibration is known as the resonant frequency. Complex bodies may have many resonant frequencies and consequently may vibrate in a complex manner with each resonating frequency producing its own signal amplitude. 23. Information provided by Frequency. When analysing an aircraft's vibration to pinpoint a particular problem, it is essential to know the vibration frequency. Knowing the frequency helps to identify the component which is at fault. 24. The forces causing vibration are generated by the rotating motion of an aircraft's components. Because these forces change, amplitude varies according to the rotational speed of the parts, it follows that many vibration problems will have frequencies that are closely related to the rotational speeds. It is then possible to identify a defective part by noting the frequency of its vibration and associating that frequency with the rotational speed of the various components. 25. Suppose you are working with a gearbox that is running at 4800 RPM. This gearbox drives a pump that runs at 3200 RPM. If the system is vibrating excessively at 3200 CPM, the gearbox can immediately be eliminated as having nothing to do with the problem. The pump assembly is clearly at fault. F002 Vibration Chapter 1 Page 6 26. Not all problems show vibration frequencies that are exactly equal to the rotational speeds of component parts. It is important to realise that different component problems cause different frequencies of vibration, and that most component vibration consists of many different frequencies. Such complex vibration signals often include frequency harmonics. 27. Although all of the frequencies in a complex vibration signal can be important for analysing component problems, two frequencies are of special importance: the fundamental frequency and the dominant frequency. 28. The Fundamental frequency. The fundamental frequency is equal to the speed of rotation of the rotating component or 1 x RPM. 29. The Dominant frequency. The dominant frequency has the largest amplitude. 30. It is important to realise that the fundamental and dominant frequencies are not always the same. Where the dominant frequency differs from 1 x RPM (the fundamental frequency) the dominant frequency is more indicative of the problem. KLP 04C.01.06 SECTION 9 TYPES OF VIBRATION. 31. Transient Vibration. When a body is briefly struck by a force, it vibrates at its resonant frequency. The vibrations are not sustained, but decrease in amplitude, at the same frequency, to zero, in a time dependent on the material and friction. These are known as transient vibrations. 32. Forced Vibration. If the disturbing force is continuously applied to a body, then the vibration of the body will be maintained until the force is removed. Such a vibration is known as a forced (or driven) vibration. 33. Resonance. A body subject to a forced vibration will vibrate continuously at the frequency of the driving force and, as long as the forcing frequency is very different from the resonant frequency of the system, the amplitudes of the forcing frequency and the resulting vibration remain at their respective levels. As the forced vibration frequency approaches the resonant frequency of the system, the vibration parameters add to overcome the inherent damping of the system and the amplitude of vibration increases sharply. When the forcing and the resonant frequencies coincide, then a state of resonance exists and the amplitude of vibration suddenly becomes many times that of the forcing vibration, often with destructive results as a forced (or driven) vibration. 34. Any object has a natural resonant frequency, that is, a frequency at which it will naturally vibrate when stimulated by an outside force, depending on how it is mounted. For example, if a thin plastic ruler overhanging a desk is struck at the end, it will vibrate at a particular frequency, which depends on how much the ruler is overhanging the desk. If a weight is fixed to the end of the ruler, the natural frequency will be lowered. The force with which the end of the ruler is struck will not alter the frequency of vibration but will alter the amplitude. In the case of the ruler, the vibration decays as the external force is removed. F002 Vibration Chapter 1 Page 7 35. However, if the external force continues and is in phase with the natural vibration, the amplitude of the vibrations will increase to the point where the strength of the ruler is exceeded, and it will break. Where such a situation could exist in a helicopter, a means of damping is used to control the amplitude at the resonant frequency. Ground resonance is an example of vibration that can reach a destructive level if, for example, the undercarriage oleos are unserviceable. 36. The absorption of vibration in a helicopter is a difficult problem, since the structure is not a single uniform mass. Rotor speed varies, and the main rotor is only one of several sources of vibration; it is impossible to eliminate all vibration from a helicopter. Basic vibration, resulting from the design, contributes towards establishing the operating life for primary components, although individual helicopters will vary because of engineering tolerances. However, vibration resulting from an unbalance, or from worn parts, which will either increase the normal levels of vibration above acceptable limits, or produce vibration at abnormal frequencies, must be reduced by maintenance procedures to ensure optimum fatigue life of components. 37. An increase in vibration beyond acceptable levels subjects all components to much higher repetitive stress levels than those for which they were designed, resulting in a greater likelihood of fatigue failure. In the same context, the likelihood of failure is increased many times if the component has already suffered an overload or damage in the form of a stress raiser, such as a nick or a scratch, at which there will be a concentration of stress. Some materials are more sensitive than others to this type of failure. Self-locking nuts may not remain secure at increased vibration levels, leading to fretting of the parts they attach. Pipes, cables, hoses, and controls are more likely to chafe and suffer failure through fatigue. Sensitive instruments and avionics equipment are likely to experience reduced time between failures in the presence of excessive vibration. KLP 04C.01.07 F002 Vibration Chapter 1 Page 8 SECTION 10 FREQUENCY SPECTRUM. 38. Machinery vibrations will inevitably be more complex than the vibration of a simple sine wave. Most-moving components will produce forcing vibrations which will be at differing rates and most components will have one or more resonant frequencies. 39. If any of the parameters are examined with respect to time, the result will appear to be a random arrangement of frequencies and amplitudes; but no matter how apparently confusing the result, it will be composed of a number of pure sine waves of different frequencies and amplitudes,- which can be plotted against frequency to produce the spectrum at, where each point on the curve represents a particular sine wave, (Error! R eference source not found.). FIG. 6 Vibration represented with respect to time and frequency. KLP 04C.01.08 F002 Vibration Chapter 1 Page 9 SECTION 11 MEASUREMENT PARAMETERS. 40. Referencing FIG. 7. If the frequency spectrum of a vibrating machine is plotted in respect of displacement, velocity and acceleration, the following features may be seen: a. Displacement measurement is most sensitive at low frequencies. b. Velocity measurement gives measurable values over all the frequency range, although the results do decrease as the frequency increases. c. Acceleration measurement is most sensitive at higher frequencies. 41. 17 It may be deduced from the Para above, that velocity measurement will give a good general indication of the overall vibration level for all frequencies. For low frequencies (up to about 50 Hz), displacement measurement is most sensitive between 50 Hz and 1 kHz, velocity measurement is most sensitive, and, for higher frequencies (above 1 kHz), acceleration measurement gives the best indication of the level of vibration severity. FIG. 7 Spectrum of Parameters KLP 04C.01.09 F002 Vibration Chapter 1 Page 10 SECTION 12 POSSIBLE CAUSES OF VIBRATION. 42. By the time equipment is put into use, all vibration sources due to design defects should have been eliminated. Any vibrations which occur in use may be due to a defect. The most common causes are as follows: a. Unbalance of rotating parts and rotors. b. Helicopter rotor blades flying out of track. c. Defective dampers. d. Misalignment of couplings and bearings. e. Bent shafts. f. Worn, eccentric or damaged gears. g. Damaged rolling elements of bearings. h. Aerodynamic forces. i. Lack of lubrication in rotating elements. j. Looseness in component attachments. 43. The list is not exhaustive, but, whatever the cause, the vibration will be produced by a force which is changing in either its direction or its strength. The resulting vibration characteristics will be determined by the manner in which the originating forces are generated together with their propagation paths (FIG. 8). FIG. 8 Propagation paths. KLP 04C.01.10 F002 Vibration Chapter 1 Page 11 SECTION 13 MODES AND NODES. 44. It may be observed that the metal top of a workbench vibrates violently when the grinder mounted on it passes through certain frequencies as it runs up to speed or runs down to stop. The bench top is vibrating in one of its characteristic resonant modes, and each similar component has a number of modes which occur at multiples of its lower frequency and depends on how it is fixed. The nodes occur where the vibration amplitude is least, as the component is swivelling about that point; at the antinodes, vibration is maximum, and the deflection is maximum (FIG. 9). 45. If a horizontal panel is vibrating, the position of the nodes may be observed by noting the points at which applied sugar grains collect. The resonant frequencies at which the characteristic flap modes of gas turbine blades occur are observed in a similar manner. 46. Where it is possible to add some stiffness to eliminate resonant frequency, vibration the node points must be avoided, i.e. additional fixings to support a vibrating pipeline, or the workbench mentioned in in the Para above, must be placed away from nodes (or antinodes) to disturb their mathematical relationship. 47. The observed vibration level due to the grinder itself is probably due to unbalanced wear on the grindstone, which can be corrected by dressing the grindstone to restore its original shape. 48. A rotating shaft also has a natural frequency of vibration and may whip violently (or ‘whirl’) in a characteristic mode if it turns at a speed which coincides with this resonant frequency or a multiple of it. Great care is taken in design to ensure that a rotor does not operate at a resonant ‘critical’ speed. Power station turbo-alternator rotors are relatively long with small diameters and run through a number of critical speeds when accelerating to operating speed or stopping; due to this behaviour they are termed 'flexible' rotors and special balancing techniques are required to minimise harmful effects. FIG. 9 Modes and Nodes KLP 04C.01.11 F002 Vibration Chapter 1 Page 12 CHAPTER 2 HELICOPTER VIBRATION. SECTION 1 INTRODUCTION. 1. The majority of helicopter vibrations are caused by out-of-balance rotating elements and rotor generated aerodynamic forces. These are accentuated by ageing and wear in components such as bearings and dampers. The sources of vibration are the engine and transmission and main and tail rotors, acting in the fore-and-aft, vertical and lateral planes. 2. All rotating components are balanced in manufacture, but it is impossible to produce perfect balance. Therefore, centrifugal force will act upon each component, and this may be sensed by a transducer to give its vibration characteristics, or ‘signature’. When all the components in a system are assembled together, as in a helicopter, transducers mounted in the radial (vertical and lateral) and fore-and-aft planes (FIG. 10) at defined points are used to detect component signatures and make it possible to detect any item which exceeds the manufacturer’s limits. 3. The helicopter will be subject to most of the problems associated with vibration analysis, and the successful achievement of smooth vibration-free flight is the aim in this area. In order to show how these problems, arise, and how they are corrected in practice, this section explains in greater detail how the basic principles in this and earlier chapters are applied to this machine. As vibration damping is of critical importance, methods adopted by various manufacturers are described. FIG. 10 Mounting Axis F002 Vibration Chapter 2 Page 13 SECTION 2 STRESS AND FATIGUE. 4. Cyclic stress occurs when a component is repeatedly overloaded, e.g. by excessive vibration amplitudes and, after a given number of cycles, failure will occur. Reducing vibrations will extend component life. These terms are also used when describing the effects of vibration on human beings. The main human problems are due to the main rotor 1R vibrations caused by aerodynamic and mechanical unbalance, corrected by tracking and balancing. SECTION 3 RESONANCE AND DAMPING. 5. Any object has a natural resonant vibration frequency, that is, a frequency at which it will naturally vibrate when stimulated by an outside force, depending on how it is mounted. For example, if a thin wooden rule overhanging a desk is struck at the end, it will vibrate at a particular frequency, which depends on how much the rule is overhanging the desk. If a small weight is fixed to the end of the rule, the natural frequency will be lowered. The force with which the end of the rule is struck will not alter the frequency of vibration but will alter the amplitude. In the case of the rule, the vibration decays as the stimulation is removed. However, should the outside stimulus continue in phase with the natural vibration, the amplitude of the vibrations will increase to the point where the strength of the rule is exceeded, and it will break. Where such a situation could exist in a helicopter, a means of damping is used to control the amplitude at the resonant frequency. Ground resonance is an example of vibration which can reach a destructive level if, for example, the landing gear dampers are unserviceable. 6. The absorption of vibration in a helicopter is a much more difficult problem, since the structure is not a single homogenous mass. Rotor speed varies, and the main rotor is only one of several sources of vibration; it is impossible to eliminate all vibration from a helicopter. Basic vibration, resulting from the design, contributes towards establishing safe operating lives for various primary components, although individual helicopters will vary because of engineering tolerances. However, vibration resulting from an out-of-ring condition, or from damaged of worn parts, which will either increase the normal levels of vibration above acceptable limits, or produce vibration at abnormal frequencies, must be reduced by maintenance procedures. 7. An increase in vibration beyond acceptable levels subjects all parts to much higher repetitive stress levels than those for which they were designed, resulting in a greater likelihood of fatigue failure. In the same context, the likelihood of failure is increased many times if the component has already suffered an overload or damage in the form of a stress raiser, such as a nick or a scratch, at which there will be a concentration of stress. Some materials are more sensitive than others to this type of failure. Self-locking nuts may not remain secure at the increased vibration levels, leading to fretting of the parts they attach. Pipes, cables, hoses and controls, are more likely to chafe and suffer failure through fatigue. Sensitive instruments and avionics equipment are likely to experience less time between failures in the presence of excessive vibration. F002 Vibration Chapter 2 Page 14 SECTION 4 AERODYNAMIC SOURCES. 8. The primary sources of vibration with an aerodynamic origin, are the main and tail rotor blade assemblies. Each blade is of aerofoil section, providing lift (or thrust). When rotors are rotated, the aerodynamic forces will cause vibrations of a frequency depending on the speed of rotation and the number of blades in the rotor. However, anomalies relating to one blade only, such as change of lift due to slight variations in aerodynamic shape (which may be caused by minor damage or natural ageing) will increase vibration amplitude of 1R (one per revolution). In a fully articulated rotor, variations in blade track spacing (blade phase) will cause similar vibrations and, if only one blade is affected, the vibration frequency will again be 1R. The same is true of a tail rotor, except that since the rotational speed is higher the actual vibration, frequency will be higher. This is 1T. SECTION 5 MECHANICAL SOURCES. 9. Transmission, everything that rotates, other than the engines and the aerodynamic sources. a. Rotor head assembly mechanical unbalance combines with the associated aerodynamic effects (section 4). Any wear in rotor head bearings and control linkages will result in increased vibration amplitudes, which may increase component wear rate. b. Drive shafts are balanced on manufacture to prevent excessive vibration, and special assembly techniques are often used, with shaft couplings, to maintain the balance of the whole assembly. Misalignment of a shaft will cause vibration (1S) as a result of the flexing motion induced into the shaft. Wear in the shaft support bearings and coupling splines are further sources of vibration in the drive shaft system. c. Gearboxes, with components running at different speeds and supported on different kinds of bearings, provide sources of vibration at several frequencies. Due to the accumulation of manufacturing tolerances, gear teeth may vary slightly in position and, as a result, tooth loadings may vary at different points on each gear. Varying loadings resulting from variations in attitude and airspeed are transmitted through the rotor mast to the supporting bearings, resulting in changes in loading and, therefore, in vibration. In addition, many gearboxes provide drives for a variety of accessories, such as generators, hydraulic pumps and tachometers: each of these components produces a characteristic vibration, which may vary under differing operating conditions. FIG. 11 shows the engines, transmissions and gearboxes for two typical helicopters (not to scale), illustrating potential vibration sources. 10. Power Plant. Gas turbine engines are relatively smooth running, but high rotational speeds mean that the effects of any imbalance are magnified. Ingestion damage to compressor blades, creep damage to turbine blades and wear on bearings and seals, are all causes of increased vibration level. Similarly, the engine driven accessories will all produce characteristic vibrations. F002 Vibration Chapter 2 Page 15 FIG. 11 Typical points of vibration on Helicopters. KLP 4C.01.12 F002 Vibration Chapter 2 Page 16 SECTION 6 CLASSIFICATION OF VIBRATION. 11. Vibrations may be classified into three frequency levels, low, medium and high. Low frequency vibrations are associated with the main rotor and are of the order of 1R to 2R in a two-bladed machine and 1R to 5R in a five-bladed machine, where 1R represents the rotation speed of the rotor, expressed in rpm; in terms of actual frequency ranges, these are usually between 3 Hz and 20 Hz, dependent upon the design of the machine. High frequency vibrations may be associated with the speed of the tail rotor (1T) and faster rotating components, whilst medium frequency vibrations will lie somewhere between the two extremes. Different manufacturers suggest different classifications for the medium and high frequency vibrations, the differences generally stemming from the number of blades and the relative rotation speeds of the main and tail rotors. 12. In addition to the frequency and amplitude, the plane of the vibration will also be known. This will narrow the range of possible faults causing a vibration at a specific frequency. In the case of a vibration associated with the speed of the main rotor, a lateral vibration would indicate an out-of-balance rotor, whilst a vertical vibration would indicate that the rotor was out-of-track. KLP 04C.01.013 SECTION 7 DESCRIBE FREQUENCY (SPEED RELATIONSHIPS). 13. To be able to relate the frequency of the measured vibration to the speed of difficult rotating components requires an in-depth knowledge of the particular helicopter. A vibration of the order of 1R may be related to the speed of rotation by multiplying the measured frequency (F in Hertz, by 60, to give the speed in revolutions per minute (rpm). This may be expressed as follows: rpm = F(Hz) x 60 14. A 1R vibration is typical of an out-of-balance condition affecting a rotating component. For example, an out-of-balance main rotor rotating at 203 rpm will produce a vibration frequency of: 𝑅𝑃𝑀 203 𝐹(𝐻𝑧) = = = 3.4𝐻𝑧 60 60 15. However, not all vibrations are 1R. In the case of a condition affecting all five blades in the above rotor the frequency would be: 𝑟𝑝𝑚 𝑋 𝑁𝑜 𝑜𝑓𝑏𝑙𝑎𝑑𝑒𝑠 203 𝑋 5 𝐹(𝐻𝑧) = = = 16.9𝐻𝑧 60 60 16. In the same way, the frequency produced by the meshing of gear teeth in the gearbox may be calculated if the number of teeth on the gear and the nominal speed of the gear are known. In this way, it is possible to produce a table of vibration orders for a specific helicopter. These order tables are contained in the appropriate aircraft Topic 5G1. KLP 04C.01.14 F002 Vibration Chapter 2 Page 17 SECTION 8 VIBRATION LEVELS. 17. Having located the source of the vibration from an analysis of the frequency and plane of vibration, it is necessary to ascertain whether the amplitude of the vibration is normal or excessive. In the case of the main and tail rotors, some manufacturers specify a maximum acceptable vibration amplitude, based on a specific accelerometer/transducer sensing point; however, some manufacturers do not. Generally, maximum permitted vibration levels for components and assemblies are determined by in-service experience and statistical analysis of data gathered over a period of time on a helicopter type. KLP 04C.01.15 18. Correction of Excessive vibration. If a rotor goes out of track or balance on a helicopter which has previously flown satisfactorily, an examination of the mechanical components to determine the primary cause must be carried out and rectification carried out before commencing track or balance adjustments. 19. In the case of vibration which has been traced to the tail rotor, the investigation should include an inspection of all parts for wear and damage. Items requiring special attention include the pitch change bearings, the pitch change links and spider and tail rotor hub and gearbox attachments. Where mounting bolts are found to be slack, it will be necessary to assess whether the slackness is the cause of the vibration, or an effect of it. The follow-up work will not only involve re-tightening the bolts, but also a check of the bolts, nuts, holes and locking devices for wear and cracks. Care must be taken to ensure that the inspection covers a sufficiently wide area to discover symptoms of secondary damage, such as cracking in the structure of the tail pylon. As items are renewed, it is important that fits, clearances and tightening torques specified in the Aircraft Maintenance Manual (AMM) are adhered to, in order that the unit shall perform as designed. KLP 04C.02.05 Notes. F002 Vibration Chapter 2 Page 18 CHAPTER 3 EFFECTS OF VIBRATION ON HELICOPTER CREWS. 1. Introduction Vibration causes varying degrees of discomfort to human beings, depending on its frequency and amplitude. These range from ‘just perceptible’, for instance with a displacement of 0.01 mm at a frequency of 10 Hz, to ‘painful’ at 0.04 mm and 36 Hz; but larger, slower oscillations such as the rolling of a ship are very effective generators of nausea and sickness. This Annex mentions some of the effects on the body, with special regard to helicopters, to stress the importance of minimising airborne vibrations. 2. Sound and noise.The first effect of vibration is that it can be heard, as human hearing covers the frequency range of approximately 18 Hz to 18000 Hz. The intensity of the sound (noise) is important, as the ear has a logarithmic response; human hearing is sufficiently sensitive to detect deterioration in mechanical devices. Because of this sensitivity, ear defenders are used to protect against the effects of excessive intensities, for instance when working close to jet engines on the ground. 3. Aircraft frequency ranges The following sources of vibration are known to affect the human body on board an aircraft: a. Turbulence and gusts 0 - 5 Hz b. Helicopter rotor (1R) 3 - 8 Hz c. Structural distortion 1 - 40 Hz d. Piston engines and exhausts 20 Hz - 10 KHz e. Turbojets 60 Hz - 40 KHz f. High speed aerodynamic noise 150 Hz - 40 kHz 4. Low Frequencies. The lower frequencies cause most discomfort to helicopter aircrew and passengers because they correspond to the natural frequency ranges of the organs of the thorax and abdomen, with a distinct resonance effect occurring in the 3 to 6 Hz range. Because these organs are relatively soft and have little restraint, body resonance will create large effects with increasing vibration amplitude. 5. The vibration is transferred to the body from the points with which it is in contact, through the limbs and into the internal organs. At the same time the body as a whole is responding to the overall up and down and side to side motion of the whole helicopter. 6. Higher Frequencies and Specific Effects High frequency vibrations can actually be felt by fingers, feet and legs, and produce a tingling, or pins and needles, sensation. Some effects on other parts are a. The focus of the eyes is affected by 30-40 Hz movement and at 60-90 Hz the eyeballs resonate in their sockets, making it difficult to see the position of instrument panel indicators which may themselves be vibrating. b. The body automatic balance mechanism associated with the inner ear tends to become disoriented. F002 Vibration Chapter 3 Page 19 c. Various frequencies excite resonance in the inner organs, i.e. 2-10 Hz in the stomach region, 17-25 Hz in the chest and 60 Hz in the chest wall. d. At 11-14 Hz, the spine is affected, causing backache, with the head to shoulder joints aching in the 20-30 Hz range. A further resonance effect in the lower jaw and skull has been found between 100 and 200 Hz. e. At certain higher frequencies, e.g. 350 and 700 Hz, the skull/brain combination is affected, with consequent problems. 7..Combined Effects. All these things continue to give feelings of nausea, headaches, stress, fatigue, pain and overall discomfort, and these feelings become progressively worse as time progresses, with further effects, which are complex and difficult to measure, on digestion and muscular activity. As all the people in a given situation are affected to varying degrees, the way that they continue to work together as a team deteriorates. Trying to get comfortable in itself detracts from the efficiency of a mission. 8. The overall effect is that the safety level of the mission must fall, due to the aircrews progressively reduced concentration and ability to react quickly. The mission effectiveness must also suffer, as the ability to navigate and handle weapon systems and to carry out tasks must have diminished during an extended flight. KLP 04C.01.16 F002 Vibration Chapter 3 Page 20 CHAPTER 4 VIBRATION ANALYSIS. SECTION 1 RATIONALE. 1. Vibration analysis techniques are used to identify sources of mechanical vibration so that suitable corrections may be made in order to reduce the amplitude of vibration to an acceptable level. If a vibration detector (transducer) is placed in physical contact with a vibrating surface, then it generates an electrical signal proportional to the vibration amplitude in terms of displacement, velocity or- acceleration, depending on its type. An accelerometer signal can be processed (integrated) to provide all three of these, therefore, the type of transducer often used is an accelerometer. 2. The nature of mechanical systems is such that appreciable displacements only occur at low frequencies; therefore, displacement measurements are of limited value in the general study of mechanical vibrations. Displacement is often used as an indicator of unbalance in rotating machine parts because relatively large displacements usually occur at the shaft rotation frequency, which is the frequency of primary interest for balancing purposes. Velocity measurements are widely used for vibration “severity” measurements. This is due to the fact that vibratory velocity is simply related to vibratory energy and is therefore a measure of the destructive effect of vibration. A given velocity level also signifies constant stress for geometrically similar constructions vibrating in the same mode. Because acceleration measurements are weighted towards high frequency vibration components, this parameter is preferred where the frequency range of interest covers high frequencies. 3. The frequency range of interest in vibration measurements has been increasing steadily over the past two or three decades. Today, many vibration measurements are carried out up to 10 kHz, and often higher. The increased interest in higher frequencies has been prompted by the development of high speed machinery and the recognition that high frequency vibrations carry valuable information about the condition of rolling element (ball, roller, needle) bearings, gear teeth, turbo machinery blading etc. 4. The vibration associated with fluid flow, jet noise, cavitation, etc., is essentially random in nature and must often be measured either alone or together with periodic vibration components. This again calls for more complicated measurement techniques than was common in earlier days. 5. It is difficult to obtain useful information from the time domain input signal, which is converted into the frequency domain for analysis purposes. This enables the characteristic vibration pattern of a complicated assembly to be examined, and its ‘signature’ to be plotted. The frequencies are generally found to lie within the range of 0-1500 Hz (0-9000 cpm or rpm), and as this range is especially suitable for velocity measurement, the units of amplitude used are millimetres, or inches per second (mm/s, or in/s). Shock pulses, which are due to wear in bearings and gears, are dealt with later in this chapter. F002 Vibration. Chapter 4 Page 21 6...To be able to predict the effects of vibration on mechanical structures and the human body, it is not normally sufficient to measure the overall vibration level over the frequency range of interest. Analysis is necessary to reveal the individual frequency components making up the vibration signal. For this purpose, a filter is contained in, or attached to, the vibration measuring instrument, thus making a frequency analyser. The filter will allow only frequency components to be measured which are contained in a narrow frequency band. This filter may be tuned over the whole frequency range of interest so that a separate vibration level reading can be obtained at each frequency within the range of the analyser. 7. The filter section can consist of a number of individual contiguous fixed frequency filters which are scanned sequentially, or a tuneable filter which can be used to scan the frequency range. A third option is the use of a real-time analyser which presents a continuously updated complete frequency spectrum on a display screen that may be interrogated using a cursor. Another fundamental difference between the various filter and analyser types is in the filter bandwidth, narrow or wide, and whether it is a fixed percentage of the tuned frequency or is a constant number of Hz throughout the bandwidth independent of tuned frequency. 8. Vibration occurs in all complex mechanical systems. In order to be able to track down and identify an individual component having an excessive vibration amplitude, it is first necessary to analyse the system frequency spectrum. The principle used, is to establish the basic speed at which the system runs in order to give a fundamental reference frequency, e.g. a turbine shaft, or helicopter rotor, speed, then apply gear ratio and rotor blade patterns to provide a table of component forcing frequencies. A maximum amplitude is assessed for each major component, so that when frequency analysis is performed any excessive peaks are quickly identified. The likely causes may then be identified from fault location tables. 9. Signals may be analysed and the results drawn on graph paper by hand or they may be recorded on a built-in, or external, X-Y plotter. Alternatively, the signals can be captured by a recorder and analysed and plotted later. This has many advantages, as the information can be passed on for further analysis, if required, and re-used to establish a starting point for further tests. It also helps in condition monitoring and trend analysis, which give a general indication of whether deterioration is occurring, and help to predict when further examination should take place. KLP 04C.02.01 SECTION 2 VIBRATION VALUES. 10. It is not possible to stop all the vibration in an aircraft. This is because the structure is not one constant mass and there are many sources of vibration which cannot be prevented. The best you can do is to decrease vibration to a permitted value. Aircraft are construction with sufficient strength to operate with some vibration. Values of vibration can change in given limits because of engineering tolerances. 11. Vibration which results from an out-of-balance condition, from damaged or worn components, or from unsatisfactory maintenance quality, will have one of two results. One result will be to increase the values of vibration to more than the accepted values. The other result will be to cause vibration at unusual frequencies. All of these frequencies must be controlled. F002 Vibration. Chapter 4 Page 22 12. An increase above these accepted vibration values puts components under much higher stress than that for which they were constructed. This results in a decrease in life and an increased risk of failure. The risk of failure is increased more when the faults which follow have occurred: a. -The component has had too much stress put on it at some time before b. -Damage such as nicks or scratches have made a concentration of high stress in an area. 13. Examples of areas which can become unserviceable early because of high vibration values are: a. -Highly stressed structural components. These can become unserviceable through fatigue b. -Lock nuts. If lock nuts do not stay tight the parts which they attach can move. This can cause fretting c. -Split pins. These can move in their holes and wear, and as a result, become unserviceable. The related components will then not be locked d. -Pipes, hoses, cables and controls. These can chafe and possibly fail. This can result in some loss of a system or control, or the total loss of a system or control e. -Sensitive instruments and avionic equipment. These can malfunction, and as a result, become unserviceable. KLP 04C.02.02 KLP 04C.02.03. F002 Vibration. Chapter 4 Page 23 Notes. F002 Vibration. Chapter 4 Page 24 CHAPTER 5 METHODS OF REDUCING VIBRATION. SECTION 1 INTRODUCTION. 1. Whilst good maintenance practices and attention to detail are the main ways of reducing vibration to an acceptable level, there will always be a degree of vibration inherent in the design of a helicopter. Different manufacturers adopt different methods of damping the basic vibration and these methods are outlined below. SECTION 2 RESONANT MASS. 2. The simplest form of vibration damper uses the same principles as the rule overhanging the edge of the desk, where the frequency of vibration can be varied by adding or subtracting mass at the end of the rule. 3. One helicopter has a resonant mass vibration damper mounted under the cabin floor, where it is known as the 'cabin resonator'. Another application of the same principle is to be found in the cyclic controls of a light helicopter, to prevent vibration being transmitted through the control runs. A third variation of the same principle is to be found in the resonant battery mounting used on the Sea King. In this arrangement the battery itself forms the resonant mass and is supported by three cantilever springs. The spring length is adjustable and is set up during construction using special test equipment. Tuning of the vibration absorber to compensate for variations in battery weight is achieved by adding or subtracting weights, weight being added to reduce the resonant frequency below that of the main rotor head (5R). 4. A further application of the same principle is to be found in a type of damper installed on the main rotor head of a small helicopter. Being mounted on the main rotor head, the vibration damper absorbs much of the excitation at source before it can be transmitted to the rest of the helicopter. In this design, a weight is supported in the rotor head by a ball joint which permits it to move in any direction in the horizontal plane. The weight is restrained by three equally-spaced springs, which controls its movement and forms a tuned system. This system is excited by the vibratory loads developed in the rotor head and responds in opposition to them, thus reducing them (FIG. 12). FIG. 12 A type of main rotor damping system. F002 Vibration Chapter 5 Page 25 SECTION 3 NODAL BEAM 5. The response of a weight tuned by a spring is utilised in other forms of vibration damper systems. 6. In one system, the main rotor and gearbox are coupled to the airframe through a nodal beam, which is used to isolate vertical vibrations FIG. 13. The principle can be demonstrated with a long thin piece of wood. If the wood is held horizontal at its mid-point and shaken vertically, the ends, because of the flexibility of the wood, will move in opposite directions to the mid-point, that is, they will be out of phase; this will occur when the induced vibration is near to the natural vibration frequency of the piece of wood. The locations in the beam where the motion changes from one direction to another are known as nodes, or nodal, points and here there is no movement. If a helicopter fuselage is attached at these points, then it will not be subjected to the vertical vibration induced by the rotor. For a helicopter, the beam consists of flexible members at the ends of which are attached inertia weights. The flexible members forming the beam on some helicopters are made of glass fibre, which has a low Young's Modulus of Elasticity and high allowance stress, and is relatively easy to form into complicated shapes. The mounting to the airframe is via elastomeric bearings, and the response of the system being adjusted by the use of tuning weights mounted on arms. FIG. 13 Nodal beam damping. KLP 04C 02.04 F002 Vibration Chapter 5 Page 26 CHAPTER 6 ROTOR TRACKING AND BALANCING. SECTION 1 INTRODUCTION. 1.. The predominant reason for vibration is unbalance in components which rotate at fixed speeds with regard to each other, generally referred to as rotors. Rotor unbalance causes vibration when centrifugal force acts through a rotor centre of gravity not centred exactly on its axis of rotation; as this force is proportional to the square of the speed of rotation, its magnitude becomes higher with increasing speed. 2. Mechanical unbalance can be analysed and corrected, so that the exact weight added, or subtracted, at a specific point, will reduce the unbalance vibration to an acceptable level. Before balancing, it must be confirmed that the vibrations are due to unbalance and not mechanical wear. There must be a facility to add, or subtract, weights on the item under test. 3. In a similar way, unbalance of rotor blades acting in a fluid, or gaseous, environment due to aerodynamic and similar forces acting on misaligned blades can be minimised. This is known as tracking. 4. Balancing and tracking of helicopter rotor blades is of prime importance, in order to achieve the smoothest possible flight. It is especially beneficial for aircrew, due to the effects of vibration on the human body at the range of frequencies encountered in aircraft. Main rotor balance affects the helicopter lateral axis, while tracking affects the vertical axis; a change in one will marginally affect the other. 5. Final adjustment of a rotor may involve several balance/track corrections. Achievement of correct balance gives the maximum benefit in terms of machinery life, and regular checks postpone the need for removal from service for overhaul. SECTION 2 HELICOPTER ROTOR BALACING AND TRACKING. 6. Predominant vibrations in a helicopter are generated from each revolution of the main rotor hub, at a rotor order of 1R. Reduction of vibration due to 1R and its associated orders, are essential to reduce stress and fatigue to a minimum. While the effect of 1R is mainly upon the crew and passengers, higher orders, i.e. 4R (blade passing frequency of a 4-bladed main rotor) and 1T (tail rotor), may have a detrimental effect on components and structure. 7. When dealing with vibration problems emanating from a rotor, a physical check of the mechanical system is required prior to vibration testing, as there is little point in trying to correct balance and track unless it is known that the head is serviceable and, for instance in the case of a fully articulated head, that the dampers are operating properly. With some rotor blades it has been discovered that quite significant weight differences can exist between one blade and another and corrections will be kept to a minimum if they are first matched in sets by weight. 8. Once the blade track and spacing are known to be correct, the rotor may be dynamically balanced. With some main rotors, and most tail rotors, it is advisable to statically balance the rotor prior to fitting it to the helicopter; this will reduce the amount of work required subsequently when the rotor is dynamically balanced. If either the track or the balance is changed, the other condition should be re-checked. F002 Vibration Chapter 6 Page 27 9. Vibration Levels. Having located the source of the vibration from an analysis of the frequency and the plane of vibration, it is necessary to ascertain whether the amplitude of the vibration is normal or excessive. In the cases of the major components, some manufacturers specify a maximum acceptable vibration amplitude, based on a specific transducer sensing point. Generally, maximum permitted vibration levels for components and assemblies are determined by in-service experience and statistical analysis of data gathered over a period of time on a helicopter type. 10. Dynamic and Aerodynamic Unbalance. Helicopters are prone to vibration at the 1R fundamental frequency in both the horizontal and vertical planes. With each revolution of the main rotor, the effect of any unbalance in the rotating mass of the rotor hub and blades imparts a radial vibration. This can be reduced with single-plane dynamic balancing. 11. Ideally, all the rotor blades should follow the same tip path when rotating, regardless of whether the aircraft is on the ground, in the hover or in forward flight. The tip path itself will change momentarily when translating from one mode to another, but stable conditions should then prevail, (FIG. 14). 12. Therefore, if the position of each blade tip can be ‘frozen’ as it passes a fixed reference point, the corresponding positions of all the blades can be observed for any mode of flight. Observing the departure of any blade tip from the reference position with changes in airspeed and the correction of any aerodynamic unbalance detected is known as tracking. FIG. 14 Blade tip paths. F002 Vibration Chapter 6 Page 28 SECTION 3 TRACKING THE MAIN ROTOR 13. An interrupter fitted on the main rotor causes a magnetic pick-up to trigger a tracker stroboscope or line-scan camera. Where the tracker is used, a retro-reflective numeral, or colour is first fitted to the bottom of the tip of each rotor blade The tracker stroboscope is aimed from the helicopter cabin towards a set position so that as it is triggered, the resulting flash is reflected back from the tip reflector to the operator,(FIG. 14). The numbers observed identify the position of each blade. If the track is good the numbers will be superimposed; de-tuning the tracker causes the tip numbers to apparently spread sideways. This separation enables assessment of vertical and lateral spacing to be made 14. The inset in FIG. 15 also shows the indications given by a Rotor tuner display, supplied by the line scan camera. The tip lead/lag and vertical deflection for each blade is displayed for one flight condition, blade 1 in this example being the master blade, and compares with the tracker indications for the blade tip positions shown above the inset, with relation to the indicated datum points around the rotor disc. 15. Causes of Out of Track A rotor is said to be out-of-track when the blade tip paths do not coincide, due to differences in lift of the individual blades. A rotor which is in track in one part of the speed range may become out-of-track in another, due to small differences in blade pitch angle, unavoidable slight profile errors and twist tolerances in manufacture, and the effects of blade ageing and repair. FIG. 15 Tracking (Rotor tuner display). F002 Vibration Chapter 6 Page 29 SECTION 4 TAIL ROTOR TRACKING AND BALANCING. 16. In the case of vibration which has been traced to the tail rotor, the investigation should include an inspection of all parts for wear and damage. Items requiring special attention include the pitch change bearings, the pitch change links and spider, tail rotor hub and gearbox attachments. Where mounting bolts are found to be slack, it will be necessary to assess whether the slackness is the cause of the vibration, or an effect of it. The follow-up work will not only involve re-tightening the bolts, but also a check of the bolts, nuts, holes and locking devices for wear and cracks. Care must be taken to ensure that the inspection covers a sufficiently wide area to discover symptoms of secondary damage, such as cracking in the structure of the tail pylon. As items are renewed, it is important that the fits, clearances and tightening torques specified in the Aircraft Maintenance manual are adhered to, in order that the unit shall perform as designed. 17. Correction to the tail is carried out in much the same way as for the main rotor, although there may be certain differences, as its duty is to counteract the main rotor torque rather than lift the aircraft, and it is mounted in the vertical axis rather than the horizontal. Track is relatively less important because the rotor blades are shorter, but balance is more important as the rotor speed is much higher. 18. Radial balance is brought to a minimum first then small pitch link adjustments are used to reduce the axial unbalance. On aircraft that have fixed pitch links, the only option to reduce excessive axial unbalance is to replace the suspect blade. 19. Tracking, when practicable, is carried out with the aid of reflective tape on the blade tips. Generally, there is no provision for fitting a magnetic pick-up, so an accelerometer is used to both detect out-of balance and to trigger the stroboscope. For tracking, each blade is marked; balancing is carried out with only one of the tail rotor blades marked as a reference. 20. Rotor Tracking and Balancing sequence. In a rotor tracking and balancing sequence, it is normal to check the track first. When considering main rotor track, it must be ensured that the blades are compatible with one another and are correctly pre-tracked. With some types of helicopter, it is preferable that all blades be of a similar age since this will usually necessitate less correction to individual blades. With some types of rotor blades it has been discovered that quite significant weight differences can exist between one blade and another and corrections will be kept to a minimum if they are first matched in sets by weight. Great care-should-be taken when repainting blades during refurbishment, as a small difference in the thickness of the coating can produce a significant difference in weight. 21. Once the track and blade spacing are known to be correct, the rotor may be dynamically balanced. With some main rotors, and most tail rotors, it is advisable to statically balance the rotor prior to fitting it to the helicopter; this will reduce the amount of work required subsequently when the rotor is dynamically balanced. If either the track or the balance is changed, the other condition should be re-checked. 22. After track and balance are confirmed to be correct on the ground, it will be necessary to fly the helicopter at different speeds, and to adjust the pitch change rods, or the blade tabs, in order to achieve an acceptable track and balanced flight at all airspeeds. On helicopter types that do not have the facility for main rotor blade tabbing, a ground/flight compromise may be necessary. KLP 04C.02.06 F002 Vibration Chapter 6 Page 30 CHAPTER 7 VIBRATION CONTROL (VC) ORGANIZATIONAL RESPONSIBILITIES. SECTION 1 STATION / UNIT RESPONSIBILITIES. 1. Station (Stn) / Unit are responsible for: 2. Gathering and maintaining VC data by applying VA techniques by Air System type. 3. The expedient transmission of VC data to the appropriate VCC/HUC. 4. Obtaining vibration measurements: a. After rectification work to reduce vibration, to confirm results. b. After fitting a major assembly, as detailed in the appropriate work card or Maintenance procedure. c. At any time when VA would be of assistance to the maintainer, eg after a heavy landing, blade strike or crew-reported vibration. d. When recommended by VCCs/HUCs. KLP 04C.02.07 SECTION 2 VIBRATION CONTROL CELLS (VCC). 5. Each VCC, under the direction of an individual holding authority level K, is responsible for: a. Gathering, monitoring, and maintaining vibration data by Air System type, based on operating Unit input. b. Monitoring the effect of vibration-related Maintenance and providing advice and assistance to operating Units and local engineering command on VC issues. c. Providing local training on VC techniques. d. Coordinating/assisting with vibration-related trials and equipment bids. e. Maintaining a register of technical personnel authorized to conduct VC activities. f. Ensuring that VC activity is covered within local Quality Management Systems. g. Providing input to the central vibration database managed by 1710 NAS. 6. If no Stn/Ship/Unit VCC exists, Squadrons/Units are to appoint an individual to fulfil the VCC roles at items 1, 2, 5, 6 and 7, as required, with the other elements being supplied by the most appropriate, remote VCC. KLP 04C.02.08. F002 Vibration Chapter 7 Page 31 SECTION 3 HEALTH AND USAGE CENTRES (HUCS). 7. In addition to the roles for VCCs identified above, HUCs provide additional services specific to Air Systems operating RTB within their HUMS system. The HUCs are additionally responsible for: a. Maintaining HGS database integrity and user accounts through routine database administration. b. Maintaining data flow between all levels of HUMS operation. KLP 04C.02.09. SECTION 4 1710 NAVAL AIR SQUADRON (1710 NAS). 8. 1710 NAS is the focal point for all Air Systems condition monitoring associated activities. It provides depth support for vibration control, measurement and analysis. It is responsible for: a. Providing expert technical advice on all aspects of vibration equipment, techniques and analysis methods in respect of Aircraft vibration problems. b. Conducting Resonant Frequency Response (RFR) testing of Aircraft structures, components and Modifications. c. Sponsoring and supporting Air System vibration trials to evaluate new or improved equipment and techniques. d. Maintaining Helicopter Vibration Control Ground Station (HVCGS) databases for all non-HUMS helicopter types. e. Providing functional guidance to VCCs/HUCs to enhance their effectiveness. f. Developing formal VC techniques and advising Type Airworthiness Authority (TAA) on vibration Regulation, AMC, GM and associated processes, including limit reviews and Topic 5G1or equivalent/Interactive Electronic Technical Publication (IETP) amendments. g. Developing and configuring HVCGS and RADS-AT(RN) script files and Rotortuner 5JS+ data cards. h. Providing VCC support to Engine Test Houses and Depth Maintenance facilities. i. Maintaining VE databases for all Air Systems. j. Providing technical support and training for fixed-wing propeller balancing and engine vibration monitoring. k. Providing technical support and training for RADS-AT(RN), Rotortuner 5JS+, VME and USBF. l. Accessing HUMS databases for the purpose of Air System review and analysis as agreed with appropriate stakeholders. m. Providing technical support for HUMS KLP 04C.02.10

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