TITLE ANTI RESONANCE ISOLATION SYSTEM FOR ADVANCED HELICOPTERS
FIELD
The present invention relates to the design of anti resonance isolation system for reducing the transmissibility of 4/rev contents of all rotor hub forces and moments from the rotor/gearbox * unit to the fuselage. In case of the total failure of the spring of ARIS the system ensures transfer of static loads to the fuselage. It is functional in manoeuvres at typical load factors. It significantly reduces the vibration in the continuous power-on at an rpm range of 98-104%.
BACKGROUND
The Anti Resonance Isolation System (ARIS) is a six-degree of freedom vibration isolation system. ARIS isolates the fuselage from the rotor-induced vibrations. Four units of ARIS are installed between the main gearbox (MGB) and fuselage. It is placed at ± 45° position to the fuselage centerline. This results in each unit being subjected to reaction forces generated by main rotor forces
DESCRIPTION
The diagram is a simplified, one-degree of freedom ARIS-idealization. The static and dynamic rotor loads are transferred to the fuselage through the spring due to which the spring deflects; as a result, the pendulum, which is supported by two elastomertc bearings, is displaced angularly. The elastomeric bearings are positioned at the center of the spring and the center of the diaphragm.
Static loads are transmitted to the fuselage through the spring. At a special frequency (4/rev) for which the ARIS unit is tuned, the induced inertia forces at the pendulum mass will cancel out with the dynamic force of the spring resulting in no dynamic force transmitted to fuselage. The center of the ARIS unit is 150 mm above the transmission deck.

An ARIS unit consists of a spring mass system and it transmits low frequency loads due to flight conditions and absorbs high frequency loads due to vibration caused by rotation of the main rotor. The ARIS units, each of 2 degree of freedom, which are interposed between rotor and fuselage system, isolate vibratory loads pertaining to 3 forces and 3 moments arising from the rotor and hence ARIS is effective in all 6 degree of freedom.
As ALH is four bladed main rotor, the most significant main rotor induced cabin vibration has a frequency of 4/rev=21 Hz*.
The speed of the main rotor is 314.06 R.P.M
The frequency is =4/rev=4x314.06/60=21 Hz.
The ARIS spring, has 7 individual elements bonded together to get the desired radial stiffness. Each spring elements are fabricated out of R-Glass prepreg cloth layer with built thickness of 6.25 mm.
3. CONCLUSION:
- It reduces the transmissibility of 4/rev contents of all rotor hub forces and moments from the rotor/gearbox unit to the fuselage.
- In case of the total failure of the spring of ARIS the system ensures transfer of static loads to the fuselage.
- It is functional in manoeuvres at typical load factors.
- It significantly reduces the vibration in the continuous power-on at an rpm range of 98-104%.