Helicopter Transmission Research Articles

The Design Concept Application on RMS for Tail Drive System of Civil Helicopter

To raise the market competition ability of the civil helicopter, it is required to have high technical content in RMS including reliability, maintainability, safety, supportability and testability for the new generation civil helicopter. To meet the requirements of the civil helicopter, it is decided to follow strictly the maintenance matrix from the preliminary design phase of the transmission system. To decrease the maintenance cost, the structure will be optimized during the detail design phase and checked by DMU (Digital Mockup) in VPM (Virtual Product Model) system. And to get the high safety and reliability, some advanced design technical are applied and substantiated by some new test methods. This paper explains design concept application on RMS for tail drive system of the civil helicopter by showing some practice for an example. It can provide the method and reference for the development of civil helicopter transmission system.

Failures in the Development and Service of the Helicopter Transmission System

In this paper the failures which may occur in the development and service of the helicopter transmission system are discussed. The mode, characteristic, cause and corrective action for the failures are summarized. Besides, the measures of the design and manufacture to eliminate the failures are recommended.

Helicopter Transmission System Technology Readiness Assessment

This paper states evaluative methods of technology readiness which are used in the national background projects development. The methods conduct helicopter transmission system technology maturity research and assessment of native significant project according to methods of technology readiness assessment. The conclusions indicate that methods of technology readiness assessment have very important promoting action to native project development and scientific decision-making. The project which adopts mature critical technology implies that guarantee the schedule of project development and reduce risk of project development. Actualizing assess methods of technology readiness can be an important part of project development risk management.

Study on General Principle for Structure Fatigue Substantiation of Helicopter Transmission Systems

On the base of reading up the latest overseas certification specifications and integrating the experience of engineering practice, the requirement for structure fatigue substantiation of helicopter transmission system was expounded, and the related key concepts were explained in detail. Furthermore, the general method for structure fatigue substantiation was presented. Thus, the frame of general principle for structure fatigue substantiation of helicopter transmission systems was basically founded.

Application of Contact Analysis in Strength Analysis of Helicopter Transmission System

Taking the strength analysis of the tail rotor transmission system for one type helicopter as an example, the implicit nonlinear finite element contact analysis was taken to the control arms and flange of the tail rotor pitch control system using MSN/NASTRAN in this paper. It was pointed out that rightly modeling the actual load and boundary conditions of components is essential in the strength analysis of the transmission system.

Lubrication Mode and Selection of the Helicopter Transmission System Roller Bearing

The bearings will directly affect the performance of helicopter transmission system whether they are in normal work or not. Lubrication is the essential element for the normal work of bearings. Under the condition of the bearing design and manufacture technique, material technique is becoming more and more advanced, and lubricant becomes “the fifth main part”, which has been got a wide-ranged agreement. One of the most critical factors to improve bearing life is lubricant. This paper analyzes lubrication type and mode, and the failure of helicopter transmission system roller bearing caused by improper lubrication. It further explains the importance of lubrication to the bearings. And how to select lubrication mode and lubricant to improve the bearing life is also summed up.

Development Solution of Civil Helicopter Transmission Train Vibration Monitoring System

Health & Usage Monitoring System (HUMS) is designed for improving airworthiness, reliability and effective maintenance management of helicopter by analysis of detected / diagnosed operation data internal/external environment data collected from the helicopter. Vibration monitoring system plays as an important part of entireness integrated HUMS, which offers a considerable improvement to conventional monitoring techniques. This paper covers development solution of civil helicopter transmission train Vibration monitoring system. Vibration monitoring system (VMS) constantly checks the performance of safety-critical components, provides warnings in advance of potential equipment failures and collects valuable data for routine maintenance of the helicopters.

Development of a Sliding-Rolling Contact Fatigue Tester

Abstract Contact fatigue failure is a common problem experienced in many applications such as bearings, gears, and railway tracks. In recent years, research companies have developed finishing processes that aim to improve components’ contact fatigue life. Preliminary rolling contact fatigue tests have shown that superfinishing processes could potentially improve a component’s contact fatigue life by 300 %. However, before these technologies can move from the laboratories to industrial platforms, more tests are needed to verify the claim. The objective of the work herein is to discuss the completion and verification of a sliding-rolling contact fatigue (S-RCF) test rig. This project is funded by the U.S. Army to assess the real benefit of superfinishing on the contact fatigue life of gears used in helicopter transmission boxes. The proposed tester design uses three rollers around a specimen, a hydraulic loading mechanism, and two servo motors. This configuration of the S-RCF tester allows for shorter testing time, more flexible testing parameters such as any combination of slide-roll ratio between the surfaces, any operating speed, and dry or lubricated testing conditions. Failure is detected with a state-of-the-art eddy current crack detection system, which can also be used to monitor and investigate crack growth for different materials, levels of superfinish, and operating conditions. Preliminary tests on a common gear material (AISI 8620 steel) were performed to assess the mechanical limits as well as the control software performance. This paper presents the detailed development and validation of the tester. It discusses issues involved with servo controllers, electronic gear ratio, and their ability to provide precise speed and slip ratios.

Analysis of low frequency vibrations on the structural connection elements of the Gazelle helicopter

In this paper the procedure and methodology of vibration testing on a Gazelle helicopter are presented with the aim to define efficiency of vibration damping at the structure-pilot seat connection. A particular attention is paid to the theoretical basis of helicopter vibrations, definition of working frequency, regime and profile of flight during the test. The results of the vibration measurement are shown in the frequency domain. Introduction Nowadays, possibility of defining loads and vibrations is one of the most important requests in the design and modification of processes on helicopters. The loads occuring on the main rotor during flight are the basic source of vibrations on the helicopter. Possibilities to satisfy international standards in the area of structural and human vibrations are based on continuous and adequate measurement and analysis of the vibration levels on all elements of helicopters. This paper shows the procedures of vibration measuring and analyzing in order to define efficiency of vibration damping using a modern acquisition system NetdB 12 and its software dBFA Suite. Theoretical analysis of vibration sources on the helicopter main rotor The helicopter rotor operates in a complex aerodynamic flow field. The aerodynamic loads on the rotor blade vary considerably as it moves around the rotor disc, and in steady flight these loads are periodic. A particular attention is paid to a theoretical basis of vibration sources on a helicopter. A complex aerodynamic field in the plane of the helicopter main rotor during different profiles of flight is shown and explained. This part of the paper gives the equations of force and moments on the main rotor with the aim to define basic working frequencies. Sources of vibrations and the determination of working frequencies on the gazelle helicopter The sources of vibrations on the Gazelle helicopter (main rotor, tail rotor and engine) are described as well as the vibration transfer from the sources to the structure through the helicopter transmission and the structure as a receiver of vibrations. The working frequencies of the Gazelle helicopter are defined and the causes for avoiding the low vibration frequency range are explained. Methodology and procedure of vibration testing on the gazelle helicopter The procedure of vibration testing has a few steps (pre-test, test and post test step), given in Fig. 5. After the determination of working frequencies and the testing point, the next step is selecting test equipment and installment of the accelerometers and other probes. After these preparations, the last step in the pre-test procedure is choosing the flight profiles in which the vibrations are measured. The results of testing and analysis of vibration damping at the structure - pilot seat connecting point The testing results are collected in the test phase. The results of the vibration measurement are shown in the time and frequency domain for the chosen flight conditions of the Gazelle helicopter. The results are given for two profiles of flight (for acceleration and for the left and right turn). The analysis must give an answer about the efficiency of vibration damping at the structure - pilot seat connecting point. The conclusions must not be ambiguous and must give enough elements for making a decision about subsequent actions. Vibration levels of in the acceleration phase For this phase of flight, all helicopter flight parameters are recorded and given as a function of time (velocity, altitude, drift of commands, loads und angular velocity) while the vibrations are given in the frequency domain. The vibrations at the tested connecting point are analyzed in the function of velocity, followed by the conclusions and comments about the level of vibrations in the phase of acceleration. Level of vibrations during the left and the right turn For this profile of flight, the frequency spectrum for both turns is given and the fundamental difference in that spectrum is explained. The analysis of these two turns reveals the increase of the vibration level at the test point at a particular frequency. Conclusion The illustrated methodology and the procedures of measuring and analyzing the low-frequency vibration range on the Gazelle helicopter can be applied for all types of aircraft which are in operational use in the Serbian Air Forces. The shown procedure can be used for measuring and analyzing vibrations on all elements of helicopter systems, aiming at meeting international standards, continuous system developing, modifying technical systems and detecting potential failure elements in the systems in operational service. .

Numerical Analysis on Starved Elastohydrodynamic Lubrication Characteristics of Spiral Bevel Gears

The spiral bevel gears of attack helicopter transmission systems are taken as an object in this paper. The numerical analysis model for starved elastohydrodynamic lubrication (SEHL) of spiral bevel gears is established on the basis of the analysis of load tooth contacts analysis and SEHL in elliptical contacts. The SEHL characteristics of spiral bevel gears are also analyzed. The results are as follows: during the course of a gear-tooth meshing cycle of the spiral bevel gear, the extremum of minimal film thickness and maximal film pressure occurs near midpoint of the path of contact, and leans to dedendum. The minimal value of the minimal film thickness is 0.39 um, and the maximal value of the maximal film pressure is 0.69 GPa. The value of minimal film thickness gradually increases with the increasing of meshing point velocity. When the velocity of meshing point exceeds 110 m/s, the increase became tardiness. As the load of meshing point increases, the value of minimal film thickness gently diminishes.

An improvement on helicopter transmission performance through use of high contact ratio spur gears with suitable profile modification design

This is a design/development study on helicopter transmission that had improved performance in terms of vibration and noise behaviour, in compliance with surface durability requirement, compared with what was achievable with previous designs. Different spur gears of both low and high contact ratios (HCRs) have previously been designed and tested in helicopter transmissions with different profile modifications but with limited performance improvement. The desired performance, however, was achieved through the use of a suitable profile modification for the HCR spur gears in mesh. Profile modification type of ‘double relief’ was applied for the first time to two sets of helicopter transmission spur gears, both with HCRs. Possibility of corner contact was avoided while achieving a relatively low value of peak-to-peak transmission error in ‘full load range of gears operating conditions’, which was not usually satisfied by conventional profile designs. With the new profile modification design, a measured noise level reduction of 7—11 dBA and a casing vibration reduction of two to four times were achieved at teeth meshing frequency. The new design philosophy presented in this article is pending worldwide patent by the authors.

Constrained adaptive lifting and the CAL4 metric for helicopter transmission diagnostics

This paper presents a methodology for detecting and diagnosing gear faults in the planetary stage of a helicopter transmission. This diagnostic technique is based on the constrained adaptive lifting (CAL) algorithm, an adaptive manifestation of the lifting scheme. Lifting is a time domain, prediction-error realization of the wavelet transform that allows for greater flexibility in the construction of wavelet bases. Adaptivity is desirable for gear diagnostics as it allows the technique to tailor itself to a specific transmission by selecting a set of wavelets that best represent vibration signals obtained while the gearbox is operating under healthy-state conditions. However, constraints on certain basis characteristics are necessary to enhance the detection of local wave-form changes caused by certain types of gear damage. The proposed methodology analyzes individual tooth-mesh waveforms from a healthy-state gearbox vibration signal that was generated using the vibration separation synchronous signal-averaging algorithm. Each waveform is separated into analysis domains using zeros of its slope and curvature. The bases selected in each analysis domain are chosen to minimize the prediction error, and constrained to have approximately the same-sign local slope and curvature as the original signal. The resulting set of bases is used to analyze future-state vibration signals and the lifting prediction error is inspected. The constraints allow the transform to effectively adapt to global amplitude changes, yielding small prediction errors. However, local waveform changes associated with certain types of gear damage are poorly adapted, causing a significant change in the prediction error. A diagnostic metric based on the lifting prediction error vector termed CAL4 is developed. The CAL diagnostic algorithm is validated using data collected from the University of Maryland Transmission Test Rig and the CAL4 metric is compared with the classic metric FM4.

Application of the Generalised Likelihood Ratio Algorithm to the Detection of a Bearing Fault in a Helicopter Transmission

A Bell 206B main rotor gearbox was run at high load under test conditions in the Helicopter Transmission Test Facility operated by the Defence Science and Technology Organisation (DSTO) of Australia. The test succeeded in initiating and propagating pitting damage in one of the planet gear support bearings. Vibration acceleration signals were recorded periodically for the duration of the test. The time domain vibration signals were converted to angular domain to minimise the effects of speed variations. Auto-Regressive Moving-Average (ARMA) models were fitted to the vibration data and a change detection problem was formulated in terms of the Generalised Likelihood Ratio (GLR) algorithm. Two different forms of the GLR algorithm in window-limited online form were applied. Both methods succeeded in detecting a change in the vibration signals towards the end of the test. A companion paper submitted by the University of New South Wales outlines the corresponding diagnosis and prognosis algorithms applied to the vibration data.

A review of vibration-based techniques for helicopter transmission diagnostics

Over the past 25 years, much research has been devoted to the development of Health and Usage Monitoring (HUM) systems for rotorcraft gearbox and drivetrain components. The promise of HUM systems is the ability to provide accurate information regarding the condition of various flight critical components. This paper reviews the state of the art in vibration-based helicopter transmission diagnostics. The development of various damage detection techniques is discussed from a historical perspective, and the ability of these techniques to detect damage in helicopter transmissions is reviewed. Emerging research trends suggest that improvements in signal processing, sensor development and individual-tooth mesh waveform modelling could improve the performance of current and future helicopter transmission diagnostics.

Analysis of Triaxial Vibration Data for Health Monitoring of Helicopter Gearboxes

Research on the nature of the vibration data collected from helicopter transmissions during flight experiments has led to several crucial observations believed to be responsible for the high rates of false alarms and missed detections in aircraft vibration monitoring systems. This work focuses on one such finding, namely, the need to consider additional sources of information about system vibrations. In this light, helicopter transmission vibration data, collected using triaxial accelerometers, are explored in three different directions, analyzed for content, and then combined using Principal Components Analysis (PCA) to analyze changes in directionality. The frequency content of the three different directions is compared and analyzed using time-synchronously averaged vibration data. To provide a method for analysis and monitoring purposes, the triaxial data are decorrelated using a mathematical transformation, and compared to the original axes to determine their differences. The benefits of using triaxial data for vibration monitoring and diagnostics are explored by analyzing the changes in the direction of the principal axis of vibration formed using all three axes of vibration. The statistical variation introduced due to the experimental variables is further analyzed using an Analysis of Variance approach to determine the effect of each variable on the overall signature. The results indicate that triaxial accelerometers can provide additional information about the frequency content of helicopter gearbox vibrations, and provide researchers and industry with a novel method of capturing and monitoring triaxial changes in the baseline vibration signatures.

A Deterministic Elastohydrodynamic Lubrication Model of High-Speed Rotorcraft Transmission Components

This paper describes a deterministic approach to analyze elastohydrodynamic lubrication (EHL) of helicopter transmission gear surfaces with asperities. A thermal, non-Newtonian, model, accompanied by three specialized models is used to study the mechanisms of film generation, pressure distribution and temperature distributions within the fluid and on the surface, in an EHL line contact. A two-slope viscosity-pressure model is used to avoid overestimation of the viscosity of the lubricant under heavy loads. The three-dimensional roughness description of an actual gear tooth surface is measured using a non-contact surface profiler. The pressure distribution, film thickness distribution and the temperature rise at the contact surface are predicted as a function of the gear operating conditions including normal load, lubricant temperature and rolling/sliding speed. Presented at the 57th Annual Meeting in Houston, Texas May 19–23, 2002

HELICOPTER TRANSMISSION FAULT DETECTION VIA TIME-FREQUENCY, SCALE AND SPECTRAL METHODS

The vast majority of the powerful and effective algorithms in signal processing start with the assumption of stationarity. In addition, the deterministic portion of the signal is often assumed to be composed of complex exponentials which are the solutions to linear time-invariant (LTI) differential equations. Many signals do not comply with these assumptions, however, resulting in disappointment when conventional techniques are used. We now have at hand time–frequency (t–f) and scale transform analyses which can provide new insights into the nature of non-stationary signals. This paper describes some results using reduced interference distributions (RIDs) and scale transforms in the analysis of signals obtained from accelerometers placed strategically on a Westland helicopter transmission. Fault detection algorithms for several types of faults were compared and the methods based on the scale transform performed best followed by RID results. More conventional spectral-analysis-based methods were the least effective.

TOWARD HELICOPTER GEARBOX DIAGNOSTICS FROM A SMALL NUMBER OF EXAMPLES

The United States Navy funded Westland Helicopters LTD to carry out a series of tests on the CH-46E helicopter transmission for the purpose of generating a database that can be used to evaluate diagnostic tools in general, and neural networks in particular. The database is extensive in the number of accelerometers, load conditions and diversity of faults. However, only 1–4 instances (levels) of each fault were included. Thus, the number of examples that can be extracted to train any classification scheme is limited, and well below the minimum required. We argue that instead the database is best used to develop specific signal generation models for the CH-46E helicopter gearbox. Such models can then be used to suggest meaningful features and derive appropriate signal-processing tools. Specifically, we demonstrate that the meshing vibrations induced by the large collector gear located on the quill shaft are significant and may interact with the vibrations induced by other elements attached to the same shaft. An appropriate model is developed and the effect, termed cross-gear-pair interaction, is studied using different signal-processing tools.

Reliability prediction of helicopter transmission systems using stress-strength interference with underlying damage accumulation

Reliability prediction of helicopter transmission systems using stress-strength interference with underlying damage accumulation

Reliability prediction of helicopter transmission systems using stress–strength interference with underlying damage accumulation

This paper describes research into the development of reliability prediction models for rotor transmission systems in which component failure is caused by underlying aging processes such as fatigue, wear or corrosion. Reliability prediction is based on the stress-strength interference methodology, with stress related to surface damage and strength to the limit of allowable damage. The paper describes how damage accumulation and system failure logic is incorporated within the stress and strength functions. For fatigue-dominated processes, damage grows with number of cycles in response to applied load and environmental conditions. The concept of 'operating state' is used to model statistical damage accumulation. Operating states describe load and lubrication conditions and associated statistical damage accumulation rate parameters. Growth rate variance is related to operating state transitions and can be modelled using stochastic techniques. Fatigue strength is obtained from S-N data. Formulated in this way, reliability can be predicted from design and operational parameters rather than historical failure data.