Description:1 Title of the Invention
Serviceability Check Procedure for Vibration Monitoring System of a Helicopter
2 Field of the Invention
Procedure of serviceability check of Vibration Monitoring System of a Helicopter is an invention in the field of vibration measurement and control of a helicopter.
3 Background of the Invention
Helicopters by way of their working are subjected to high levels of vibration. The main rotor is the primary source of vibration in a helicopter. Each main rotor blade is an aerofoil section, which produces lift. Aerodynamic conditions that change as the blade rotates generate dynamic loads. The frequencies of these loads relate to the number of blades and the speed of rotation (blade passing frequency). Further Tail rotors are the same as main rotors except that they operate at the higher rotational speed and contributes towards vibration.
There are other components such as drive shafts and gears in gearboxes, engine shafts and gears, component drive shafts.
Vibration monitoring system installed on a helicopter continuously monitors the vibration data by means of input from several accelerometers, analyses and process the inputs and provide warnings and rotor track and balance solutions. It also provides prognostic and diagnostics of various drive train components for predictive health monitoring and optimized maintenance.
Many times it was observed that vibration monitoring and warning generation wasn’t happening despite vibration values being high while flying. Further many diagnostic solution provided by vibration monitoring system were observed inadequate directly pointing to malfunction of the system. As the same Vibration monitoring system was being used for different configuration of helicopter, because of software settings mismatch vibration monitoring and annunciation was not happening.
Hence, the requirement was felt to check the functionality of vibration monitoring system on ground by simulating different vibration situation and thereby confirming the serviceability of vibration monitoring system LRUs and their interconnection in a helicopter.
3.1 Prior Art
Vibration monitoring system does have a built in test facility to test various parameters like memory test, vibration channel test, vibration sensor test, power test and communication channel like MIL 1553 and ARINC 429 test. However the test performed doesn’t ensure that vibration acquired by the system is correctly getting transferred and annunciated through Integrated Architecture Display system and Intercom system. For complete analysis recorded data in the system needs to be recorded in Ground based system (laptop) and data needs to be analyzed. This is time consuming activity and high skilled activity, hence a simple solution was sought so that system functioning is ensured and pilots are aware of the vibration data throughout the flight.
4 Brief Summary of the Invention
This invention is about development of means by which working of vibration monitoring system of a helicopter is simulated on ground thereby establishing its satisfactory performance on ground. After testing satisfactory functioning on ground helicopter is given a go ahead for flying.
Vibrations are a significant problem which must be taken into account very carefully as it may cause alternate stresses or loads on components which may lead to premature fatigue failure. Also it may cause severe discomfort to crew and passengers. Rotors are the main cause of vibration and Vibration monitoring system monitors main rotor and tail vibrations in axial and radial directions. It provides vibration warning to crew through annunciation in Multi-function Displays and audio warning through intercom system of helicopter if threshold vibration in radial or axial direction is breached for main rotor and tail rotor.
It is imperative that satisfactory functioning of vibration monitoring system is ensured before helicopter is allowed for flying. By this invention a procedure has been made where by means of a test box externally vibration signals are fed to vibration monitoring system and annunciation of vibration warning on cockpit displays and headset (Intercom system) are artificially generated for declaring the system serviceable.
5 Detail Description of the Drawings
Figure 1 shows the position of different components of vibration monitoring system of Advanced Light Helicopter. For the sake of clarity only 17 accelerometers, intelligent blade tracker (1), Main rotor magnetic pick-up (4) and Tail rotor photocell (19) is shown in Figure 1.
For this invention only Accelerometers (3,5,17,18), Main rotor magnetic pick-up (4) and Tail rotor photocell (19) are used. Further Figure 1 shows the interconnection between Signal Generator (SG), Test Box (TK) and helicopter system to be performed for testing.
Figure 2 shows the Test Box (TK) which is having Switch (S1) to switching ON the box, switch (S2) for selecting MR (Main Rotor) or TR (Tail Rotor), switch (S3) for selecting accelerometer either Axial/Lateral or Radial/ Vertical. It also has connection (G1) for case ground. It has two channel connection (CH1) and (CH2) where signal generator input can be provided. These inputs are carried through connectors (P1, P2, P3) and used to connect to vibration monitoring system of helicopter. (P1) and (P2) are for connecting to Main rotor magnetic pick-up (4) and Tail rotor photocell (19) connectors.
Figure 3 shows the simple graph of relation between displacement, velocity and acceleration of a vibrating signal.
6 Detail Description of the Invention
Helicopter Vibration monitoring system provides vibration warning in the cockpit based on different regimes of flying. It incorporates Rotor track and balance, vibration warning and health monitoring of dynamic components. It has 17 accelerometers as per Figure 1 positioned in different places of helicopter, feeding vibration data to its computer called as Modern signal processing unit controlled by Cockpit Control head for generation of different modes of operation.
Out of 17 accelerometers, 5 are for Main/Tail Rotor Track and Balance/Ride Quality Monitoring, 5 are for Gearbox Monitoring, 4 are for Shakti Engine Monitoring and 3 are for Tail Rotor Drive System Monitoring as explained in Figure 1.
Accelerometers (2,3) are for cockpit lateral and vertical. Accelerometer (5) is for MR lateral. They are used for indicating vibrations which are transferred from Main rotor to floor board of the helicopter. Accelerometers (17, 18) are for tail rotor axial and radial. These 5 accelerometers are used for Main rotor Track and balance/ ride quality monitoring.
Accelerometers (6,7,8) are for health monitoring of Main gear box, Accelerometers (9,10,11,12) are for health monitoring of engine, Accelerometer (13,20) are for health monitoring of two other gear box and Accelerometer (14,15,16) are for Tail drive system.
Modern signal processing unit receives the data from accelerometer and other sensors and by filtering, FFT analysis and complex processing provides the frequency domain. Based on different flight regimes Modern signal processing unit produces the data. VMS incorporates two monitor modes to measure, record and trigger warnings to pilot on the cockpit if vibrations are above the threshold value. The first monitor mode continuously monitors the Tail Rotor 1/rev unbalance in Axial and Radial direction. The second monitor mode monitors the Main Rotor 1/rev unbalance in Lateral and vertical direction and Engine overall vibration in every 5 minutes time interval. Warnings are triggered by VMS to pilot on the IADS Systems page in case the vibrations are recorded above the set thresholds. Table A and B represents the thresholds set for the warnings for Advanced Light Helicopter (ALH) in monitor mode. The thresholds set for tail rotor unbalance gets reduced automatically whenever measurement for Rotor Track and Balance (RTB) or Initial Tail Rotor (ITR) is initiated. The rationale for setting a lower threshold for RTB is to ensure that imbalance of a newly adjusted or newly installed rotor is detected at a lower level and hence facilitates corrections earlier for optimized balancing. After correction, the higher monitor-mode threshold caters for vibration levels for all flight regimes and if the vibration crosses the higher threshold, it is indicative of sudden in-flight deterioration of the rotor. Once measurement is completed (in about 25 sec.) the thresholds resets back to the monitor mode limits.

RTB Mode
Tail Rotor Axial Radial
HIGH EXCESS HIGH EXCESS
0.7 ips 1.0 ips 0.7 ips 1.5 ips
Table A

RTB Mode
Main Rotor Axial or Radial
HIGH EXCESS
0.5 ips 0.7 ips
Table B

The invention uses the operation in rotor track and balance (RTB) and monitor mode to ensure the serviceability of Vibration monitoring system. For checking that for a corresponding vibrating signal experienced by accelerometer Vibration monitoring system is able to generate and produce vibration warning signal is the aim of the invention.
Vibrations are harmonic in nature and A body is said to vibrate when it oscillates about a reference point. It is directly a byproduct of different rotating arrangements in a helicopter. It can be expressed in terms of displacement, velocity or acceleration. In simplified manner relation between displacement, velocity and acceleration can be understood as per Figure 3.
D = X sin(????)
where D is the displacement caused due to vibration having Peak amplitude of X and angular frequency of ?? in terms of rad/s.
Velocity V corresponding to this V= X ?? (????),
Acceleration A corresponding to this A= - X ??2 sin(????), ------- (A)

Accelerometer upon experiencing acceleration converts this acceleration into voltage in mV (milli Volt) based on their sensitivity which is expressed either in mV/g or mV/m/s2.
Based on these calculation vibration is determined.
Vibration usually in a helicopter is measured in inches per second (ips) i.e. in the form of displacement.
Any helicopter which rotates with main rotor frequency of X1M rpm (revolution per minute) and if it has tail rotor frequency of Y1M rpm. Frequency level where vibrations are more severe and all logics of vibration monitoring system are defined to continuously monitor corresponding to these frequencies.
Frequency of main rotor f1M = X1M /60 Hz
Frequency of tail rotor f1T = Y1M /60 Hz
First accelerometer is disconnected from the helicopter and mating connector is installed with connector of Test Box. Corresponding voltage signal as per Table A and B of frequency either f1M Hz or f1T Hz is fed through Test Box and warning annunciation is monitored.
Detailed procedure is as follows-
• Firstly Tail Rotor – Photocell , Radial & Axial Accelerometer and Main Rotor – Magnetic Pick up (4), MR Lateral (5) & Cockpit Vertical (3) accelerometer are disconnected.
• Testing is done sequentially, firstly Main rotor circuit is tested than tail rotor testing is performed. Connection is performed as per Figure 1.
Main Rotor-
Switch (S1) of Test Box (TK) is made ON. Main Rotor connection is selected through Switch (S2). Lateral accelerometer connection is selected by operating switch (S3). Plug (P1) of Test Box (TK) is connected to Main rotor magnetic pick up (4) connections and Plug (P3) is connected to Lateral accelerometer connection (After removal of accelerometer and magnetic pickup). For Channel (CH1) and (CH2) of Signal Generator (SG) correct voltage as per Table C and D is selected and fed to Channels (CH1) and (CH2) of Test Box (TK). After testing Lateral accelerometer, testing for vertical cockpit accelerometer is performed by operating switch (S3) to correct position.
Then in similar manner testing is performed.
As per vibration level defined in table A and Table B and frequency of main rotor and tail rotor (5.23 Hz or 26.05 Hz respectively) as voltage signal from signal generator is fed to vibration monitoring system of Advanced light Helicopter as per Table C and D.
System Channel Frequency Voltage (Vrms)
Tail Rotor System Ch-1 : Photocell f1T Hz (sin) 5 V
Ch-2: Axial/Radial Accelerometer As per calculated value
Main Rotor System Ch-1 : Magnetic Pick up f1M Hz (sin) 5 V
Ch-2 : Lateral/Cockpit Vertical accelerometer As per calculated value
Table C

Ch-2Axial/ Radial Accelerometer output is calculated for high and excess values of vibration values. These values are fed by signal generator SG to the Test Box (TK) .

• Warnings appeared on Multi-function displays of Helicopter is monitored. Appearance of Proper warning proves out the serviceability of the system. Voice Warnings also appear in the headset and this also is monitored. , Claims:1. A Test Box (TK) for doing serviceability check procedure of vibration monitoring system comprising of:
- Plurality of switches (S1, S2, S3) for changing the connections like powering up, change of Main Rotor to Tail rotor and selection of accelerometer connections;
- ground connections (G1) for providing ground connection to the Box;
- Channels (CH1, CH2) for providing inputs through signal Generator (SG);
- Plurality of Connectors (P1, P2, P3) for connecting the Box to the helicopter.
2. A serviceability check procedure of testing Vibration monitoring system by using Test Box (TK) as claimed in Claim 1 by feeding requisite value through the use of Signal Generator (SG) and interconnections provided in Figure 1.