|M.Sc Thesis||Department of Mechanical Engineering|
|Supervisor:||Assoc. Prof. Gendelman Oleg|
|Full Thesis text|
Undesired vibrations in mechanical structures are a rather common problem in many branches of engineering and can be caused by various mechanisms. Helicopters' rotor systems are especially known to be susceptible to aeromechanical instabilities such as ground and air resonance. Due to the high vibration levels of the helicopter, there are possibilities to cause harmful inﬂuences to passengers and the helicopter's systems, where the vibration of the rotor blades is extremely important as it constitutes a vital part regarding the stability of the aircraft. Periodic external forces acting on the blade are induced by the aerodynamic force, and excites the vibration of the blade at integral multiple frequencies of the rotation. For this reason, helicopter designers have been forced to use many different methods of damping and counter-acting vibrations to an acceptable level.
The following work investigates the behavior of the passive hydraulic dampers installed upon Sikorsky's CH-53 and UH-60 helicopters with special emphasis on the dynamic characteristics of the instrument which is mainly described by hysteretic force-velocity response. This thesis investigates and explains the mechanisms and the operational principles by which the hydraulic damper functions. It introduces an analytical model and compares analytical calculations of the damper’s dynamic behavior taken from the fluid mechanics field to results obtained from a self-built experimental system. The results from this part show a strong match between the experimental and the analytical calculations and indicate the presence of a dynamic invariant expressed in the force-velocity profile of the damper.
In addition, this study also focuses on finding the influences of the hydraulic fluid (damper oil) compressibility on the damping response and thereby tries to confirm wheatear the assumption to refer to the hydraulic fluid as incompressible is correct. In order to understand this, a new set of equations is built to achieve the solution for the compressible case. The results show that as bigger the compressibility of the fluid is the bigger the hysteretic response is, thus confirming that in most cases including ours the assumption is valid.