University of Twente Student Theses


Towards the design of a vibration monitoring system for a rotor blade system in operation : an experimental investigation

Madhar, M.C. (2016) Towards the design of a vibration monitoring system for a rotor blade system in operation : an experimental investigation.

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Abstract:In order for critical mechanical systems to maintain its functionality, their condition must be periodically evaluated to decide upon the required maintenance activities to be performed. Vibration monitoring is recognized as a valuable tool for providing information about the current state of the component. Developments of integrated sensors, embedded processing and wireless communication provide possibilities for on-blade monitoring, where the actual loading is taking place. However, the obtained vibration signals can be decomposed in many different ways. Finding the parameters that actually describe the system and defining their critical values that indicate a substantial amount of damage are not trivial problems. This research then contributes to the experimental investigations of vibrations in a rotor blade system by means of a demonstrator. The investigations were done for the forward flight case of the helicopter. With this flight condition the helicopter rotor blades are exposed to a cyclic changing loading of the airflow. Phenomena found in rotating blades will then be different due to this non-axisymmetric airflow. Most interesting phenomenas to explore were the influences of lagging moments on blade flapping, the dissymmetry of lift and finally the harmonic response of flexible blades during forward flight. The demonstrator consists of a hanging, non-rotating blade that allows flapping, lagging and pitching motions. The phenomenas in the forward flight were translated into appropriate requirements and measurement strategies for the RBS demonstrator to represent these effects. The experiments on the demonstrator showed how sensitive the blade reacted to a series of different excitations, therefore proving that vibration monitoring on the rotor blade is a complex task for the different phenomena encountered in forward flight. From the measurements, it could be seen that lagging influences the flapping motion by reducing its baseline frequency amplitude and increasing harmonics in the blade. With these higher harmonics less rigid behavior is found in the blade's motion, leaving the blade more vulnerable to aeroelastic effects. As rotor blade coning can be seen as variation of the rotational stiffness in the blade's hinges, this determines the stiffness distribution throughout the blade. Cyclic changes of the loading in forward flight can result in variations of this stiffness distribution. In the demonstrator this was represented with three different torsion springs in the blade's hinges. The analyses showed then that when the blade had a structural mode at a harmonic of the excitation frequency, the blade responses at that frequency amplified. The eigenfrequencies of a rotating blade are a combination of structural properties and stress stiffening effects. The influences of lagging on flapping in the demonstrator showed then that the flapping modes can shift. As damage was introduced in the system, reduction of the baseline excitations in the response could be seen with an increase of higher frequency vibrations. Due to the sensitivity of the system to higher harmonics, a clear boundary between the operations of the pristine and damaged blade was hard to realize. Therefore future investigations should account for these occurrences, so that further analyses is possible to find the deviations in operations between these two cases.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
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