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

Abstract

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. .