Study on Vibration Adjustment Mechanism of S-76D Helicopter Rotor System Based on HUMS System

In recent years, helicopter maintenance technology has been developing rapidly and is transitioning from time-based periodic maintenance to condition-based maintenance. Helicopter Health and Usage Monitoring System (HUMS system) plays an important role in this process. The excitation forces originating from the helicopter rotor system are the primary cause of airframe vibration, and the HUMS system monitors the main rotor balance and trajectory data during each flight and provides an adjustment solution and predicts the vibration level in case of overruns. This paper summarizes the vibration measurement, cause analysis and vibration adjustment methods by studying the vibration adjustment mechanism of the S-76D helicopter rotor system, and provides solution for the same type of vibration problems of the helicopter.


Introduction
The traditional way of overhauling helicopters has been periodic maintenance, and these time/cyclebased maintenance are designed to correct problems associated with normal wear and tear of the aircraft.However, these maintenance activities are not sensitive to the actual condition of the components involved.This approach is based entirely on time/cycles and fixed what is changing regardless of the actual usage of the aircraft, resulting in increased operating costs.While periodic maintenance is on the decline, condition-based maintenance has entered a period of rapid growth.
Condition-based maintenance is a high-tech method that uses sensors to collect information about aircraft parts and analyze them to get the life expectancy of the parts in order to maintain them before failure or malfunction.On condition maintenance is based on the theory of reliability with a prerequisite of accurately grasping the actual condition of the aircraft and the principle of less disassembly, less unloading and less repair.The goal is to maximize the working time of the components and improve the economic efficiency.With the transition from time/cycle-based maintenance to condition-based maintenance, aircraft equipped with HUMS system can provide longer intervals between periodic overhauls.

HUMS System Overview
The HUMS system is an integrated airborne and ground system designed for diagnostics and health utilization monitoring of helicopter powertrain system, propulsion system, auxiliary and structural components.
The HUMS system consists of two sub-systems: the Onboard Database Acquisition Unit (OBAU) and the Ground Station (GS).The HUMS system is schematically shown in figure 1.The OBAU is the core of the HUMS system.It collects and records information about the helicopter on a per-flight basis, and assists in performing a variety of specialized maintenance tasks.Data collected by the OBAU are transferred from the OBAU to the GS via a USB or network cable connection.The GS is used to record the detailed operational history of the helicopter, analyze the collected data and compile it into valid information to support engineering and maintenance use [1].

Figure 1. The schematic diagram of HUMS system
The HUMS system monitors main rotor, tail rotor, drive train/driveline and engine vibration, detects early warning of impending problems to flight and ground crews, provides balancing solutions for main rotor trajectory, tail rotor and output shaft of engine, and collects utilization, fault/self-test, event, status and parameter data.In view of maintenance and utilization of the helicopter, the HUMS system is designed to reduce operating costs and offer technical support by providing timely and accurate information to helicopter operators, maintenance staff and flight crews.

Overview of Helicopter Vibration
The two most important concepts involved in helicopter vibration signals are amplitude and phase.Amplitude can be described using units of velocity (inches per second or ips) or acceleration (g).Phase is the time or angle of rotation between the tachometer pulse and the maximum value of the vibration signal [2].Phase information is needed when performing vibration adjustment tasks: phase determines where to make adjustments and amplitude determines the amount of adjustment.
The amplitude of the vibration depends on the magnitude of the excitation force and the response characteristics of the vibrating structure.Smaller excitation forces lead to lower vibrations and larger excitation forces lead to higher vibrations under the same conditions.Equally important is the structure that responds to these excitation forces.The relevant characterizing elements are mass, elasticity and damping [3].A helicopter structure can vibrate because of its inherent mass and elasticity.In terms of energy relationships, mass stores kinetic energy and elasticity stores potential energy.When the external work on the system, the mass will absorb kinetic energy and have the speed of movement, and then displacement, so that the elastic element to store the deformation energy, and thus has the ability to make the mass to restore the original state.In this way, the continuous transformation of energy leads to repeated movements of the mass of the system (i.e.vibration).
The significance of reducing vibration is to improve passenger and crew comfort, reduce the impact on crew maneuvering, reduce wear and tear on mechanical and electronic components and prevent premature structural fatigue.
In terms of vibration frequency, low frequency vibrations are mostly from the main rotor.The main source of mid-frequency vibration is the main rotor 4P vibration, which may also come from tail rotor gearbox, tail rotor imbalance, or improper maneuvering and failure of vibration control components.High-frequency vibration are often from the engine, main rotor gearbox and input shaft of main rotor gearbox or main reduction and engine-driven accessories.

Main Causes and Forms of Helicopter Vibration
The causes of helicopter vibration include three aspects: (1) the helicopter structural characteristics, (2) the complex aerodynamic environment, and (3) the structural changes that occur during use.The speed of the blade tip is greater than that of the root when hovering.In forward flight, the airspeed of the helicopter alternately affects the blades as the direction of blade rotation changes.These motions produce periodic blade excitation forces, that is, these excitation forces are repeated once or more each time the rotor rotates one revolution.All of these forces cause the rotor blades to bend and twist, which creates forces on the rotor hub.Fortunately, due to the symmetry of the rotor, most of these forces cancel each other out.However, components of the periodic excitation such as 1P, 2P, 3P, and KP (k is the number of blades) are additive and transferred to the fuselage.
Helicopter cabin vibrations are primarily mainly stimulated by the main rotor, with the excitation forces acting together at the center of the hub and transmitted to the cabin through the fuselage [5].Cabin vibration can generally be categorized into three forms by direction: vertical vibration, transverse vibration and rolling.Vertical vibration is related to cone overrun, transverse vibration is related to mass unbalance caused by transverse overrun and spanwise overrun of blade, and rolling is caused by comprehensive factors.The general form of cabin vibration is shown in figure 2 (the vibration direction is not exactly in accordance with the form in the figure, for reference only).

Fuselage Vibration Suppression Methods
There are three aspects to reduce the vibration level of the fuselage caused by the rotor system: (1) reducing the excitation force of propeller hub, (2) reducing the fuselage response, and (3) adjusting the phase relationship between excitation force and response.For maintenance personnel, this can be accomplished mainly by ( 1) and (2).
To reduce the excitation force of propeller hub can be accomplished by adding a double-tuned pendulum mass damper to the hub and/or adjusting the balance and trajectory of the rotor system.To reduce the fuselage response can be achieved by both active vibration control (AVC) and passive vibration suppression [6].The function of the AVC is to minimize the vibration of the fuselage, which is a linear superposition of the vibration caused by the rotor excitation force and the excitation response generated by the AVC system as shown in figure 3. Passive control is realized by ordinary fuselage dampers such as elastic resonators.

Vibration Adjustment Mechanism of S-76D Helicopter Rotor System Based on HUMS System
A vibration system can be considered to be composed of three aspects: input, output and system model.For the helicopter, as a complex vibration system, various types of faults can be regarded as its inputs and the information collected by the HUMS system as its outputs.The solution to the vibration problem is essentially an inverse process.

Data Collection and Transmission of Rotor System
Data of the HUMS rotor system is collected automatically without input during the flights, but also includes a manual mode that allows the pilot to actively collect data.Once the helicopter has met the requirements for automatic and cued capture boundaries (see table 1), the OBAU begins collecting and processing RTB data from the appropriate sensors.Compulsory capture should be prioritized over automatic or prompted capture and will interrupt mechanical diagnostic measurements already in the queue, see table 2 for compulsory capture boundaries [7].The S-76D helicopter has five predefined states (ground, hover, 125KTS, 145KTS and 155KTS) for data capture.Automated capture of the forward flight states (i.e., 125Kts, 145Kts, and 155Kts) is considered to gain better quality of data and is typically included in vibration assessment and diagnostic solutions.During the flight operation, OBAU stores the collected data which includes raw sensor data as well as operationally computational data into the memory.When the stored data is read, they are transferred to a dedicated computer on the ground via the USB port on the helicopter or via a network cable.Only a small portion of the data transferred to the USB is related to the rotor system, but the majority is related to helicopter usage and mechanical diagnostics.These generated files are called Raw Data Files (RDF).

Vibration Analysis
Rotor system vibration analysis for a specific model, excluding system modeling differences, is actually a study of the relationship between response and excitation forces.The rotor system imbalance creates an excitation force, and the response can be measured by accelerometers and tachometers.In the case of known fuselage response and system parameters, vibration analysis is to screen the system input signal to achieve the purpose of discriminating environmental characteristics [8].

Signal
Processing. [9]The measured vibration of the fuselage is often synthesized from a number of vibration components at different frequencies, with the amplitude of each frequency component reflecting the vibration level at different locations.It should be noted that the frequency response is a steady-state response rather than a transient-state response.For actual vibration signals that usually contain many frequencies, spectral analysis is a great approach to interpret the vibration data.The process is as follows: rotor starting point setup → vibration signal measurement → highpass filtering → signal amplifier → A/D Converter → periodic sampling of the vibration signal in combination with rotational speed information → signal preprocessing (Time Synchronous Averaging to reduce noise) → Fast Fourier Transform (FFT) → spectrogram.This approach displays the vibration as a function of frequency rather than time, that is, it is converted from the time domain to the frequency domain.The source of vibration can be traced for each frequency through the spectrogram, so that the unknown source of vibration of the helicopter can be localized and analyzed.
In the spectral analysis, the main concern is the 1P, 2P, 3P and 4P vibration caused by the main rotor.1P vibration is large in amplitude and is the most dangerous, so the trajectory adjustment and the 1P vibration adjustment are generally taken as the starting point in the vibration adjustment.2P and 3P vibration limitations only provide the crew with better comfort, and are independent of the endurance of the components.It allows the helicopter to be flown under the condition that the 2P and 3P vibration of the main rotor exceeds the limit value.Since the performance of the AVC system may vary slightly from flight to flight, it is possible that individual vibration values of the 4P vibration may exceed the limit.Therefore, the 4P vibration that needs to be determined is whether the measured average vibration level exceeds the limit.

Trajectory and Vibration Limit
Conditions. [10]The main rotor ground adjustment is limited to a cone peak difference value of 12 mm in good weather conditions and 25 mm in bad weather.There is no cone limitation for flight adjustment.The main rotor 1P vibration response measured in the cabin is calculated by the following formula: the 1P vertical vibration of cockpit is equal to half of the vector sum of the 1P vertical amplitudes of the captain and co-pilot; the 1P roll vibration of cockpit is equal to half of the vector difference of the 1P vertical amplitudes of the captain and co-pilot.The 1P, 2P and 3P cabin vibration of main rotor limit values are shown in table 3. The RTB analysis of SGBA allows for checking and diagnosing rotor system health and giving guidance to the crew after an out-of-specification index or before a potential failure.It is also can archive the flight data of each helicopter as well as the monitoring system data.

Process of Vibration Adjustment.
Ground track adjustment: Ground cone adjustment is determined and implemented if the trajectory peak difference of the main rotor exceeds 12 mm.The magnetic tachometer on the main rotor swashplate must be operating correctly for ground track measurements to be made.Blade track data is required to adjust the pitch connecting rod of main rotor.Ground balancing of the main rotor is accomplished immediately after the ground track adjustment is completed.
1P balancing of ground main rotor: Ground balancing of the main rotor consists of measurement of 1P roll vibration of main rotor and transverse vibration of pilot seat in the cockpit at flat pitch and 107% Nr.The counterweight is then adjusted so that both cabin roll and transverse vibration are within limits.
Flight adjustment of the main rotor: After completing the ground track and balance of the main rotor, its flight adjustment (1P, 2P, and 3P) can be executed along with the 4P vibration check.The flight adjustment of the main rotor consists of measuring its vibration harmonics of the cockpit vertical, roll, and transverse vibration at five states: flat pitch, hover, 125 knots, 145 knots and 155 knots.If the vibration is not within limits, it can be adjusted by the main rotor pitch connecting rod, inboard and outboard trim tab and hub counterweights [12].
The helicopter logging is recorded if the 1P, 2P and 3P vibration of the main rotor are within limits.A post-flight ground balance correction is performed if only main rotor 1P cockpit roll and/or ground transverse is out of limits.It is not necessary to perform the entire rotor track and balance process after implementing the post-flight ground balance adjustment.

Summary
The rotor system of helicopters has no redundancy design and can be fatal in case of failure.The vibration characteristics of helicopters determine that the vibration signal contains periodic excitation components such as 1P, 2P, KP (K is the number of blades) of the main rotor.The frequency spectrum analysis of vibration response signals by Fourier transform is helpful for fault diagnosis and localization, and facilitating preventive maintenance.This paper summarizes the study of the HUMS system, vibration characteristics and vibration regulation mechanism of the S-76D helicopter, and provides a reference solution for similar problems solving.

Figure 4 .
Figure 4. Trajectory and vibration adjustment of SGBA-Ground cone adjustment

Figure 7 .
Figure 7.The low frequency vibration adjustment method for the S-76D The rotor system vibration adjustment includes two modes: ground adjustment and flight adjustment.All test conditions were measured at 107% Nr.Ground adjustment includes cone conditioning and 1P vibration conditioning.The ground cone of the main rotor must be completed before performing the ground balancing of the main rotor.The cone adjustment of the main rotor provides the starting point for the flight adjustment.The flight adjustment of the main rotor determines if vibration overruns are present by testing the vibration spectrum of captain vertical, co-pilot vertical,

Table 2 .
HUMS RTB manual capture boundaries

Table 3 .
Rotor 1P, 2P, 3P Cabin Vibration Limits 76D helicopter equipped with 3P damper and two force generators at 145kts, 107%Nr conditions, 4P vibration limitations are shown in table 4. It cannot be adjusted by the pitch connecting rod and the counterweight and needs to be adjusted by checking from the 3P damper and AVC system if 4P exceeds the limitations.

Table 4 .
Rotor 4P Cabin Vibration Limits The latest HUMS software used for RDF analysis and recording in S-76D helicopter is SGBA (Sikorsky Ground Base Application) and the RTB (Rotor Track and Balancing) is an important part of it.During a vibration adjustment, RTB provided a solution for a particular S-76D helicopter as shown in figure4, figure5 and figure6.RTB monitors the balance of main rotor and tail rotor and trajectory data of main rotor during each flight.It provides trajectory and balance adjustments when necessary to reduce overall maintenance and the number of test flights associated with adjusting trajectory and balance.RTB also can predict vibration level of the helicopter after the recommended adjustments have been made.