Helicopter Engineering and The Control Of Vibration

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Helicopter engineering


Application of new technology to the ANGLO-ITALIAN EH101

 

The application of modern technology has brought lots of benefits to products these days, which effects much of our confidence in the future as well as the new challenges and chances which we may expect

 

The EH 101, designed by the partnership of Agusta and GKN Westland employs new technology extensively. The principal new product technologies employed are:

 

Advanced aerodynamic composite blades; A composite elastomeric hub; An active vibration control system (ACSR); Composite structural components and aluminium lithium structural components; Health and Usage Monitoring systems; An all digital AFCS automatic flight control systems; Electronic Instrument Systems; and helicopter Integrated Avionic Systems employing Dual Data Buses.

 

To highlight the benefits of just one of these items, as an example, let us consider the main rotor. If we had retained the aerodynamic technology of the previous generation of helicopters, the aircraft would have needed to employ two extra blades to match the performance of the new rotor. These extra blades would have had a significant consequential impact on increased installed power requirements, and increased empty weight and overall weight for the missions to be performed. This improvement is a direct consequence of the 37% improvement in cruise blade loading available from the advanced aerodynamics employed.

 

Helicopter vibration control


Helicopter vibration control methodology is based on the following three issues

 

• rotor dynamics
• fuselage dynamics
• choice of appropriate antivibration device.

 

Helicopter rotor dynamics


Helicopter vibration from the main rotor is the primary source of problems. Minimization starts with appropriate tuning of main rotor characteristics. The dynamic response of a rotor blade to the aerodynamic excitation depends on blade natural frequencies, generalized masses and modal damping which gives the amplification or reduction of blade roots, vibratory loads to the aerodynamic excitation.

 

The aerodynamic parameters are selected mainly to improve helicopter performance in hover and in forward flight. The main parameters are

 

• induced velocities
• planform shape : rectangular or tapered
• tip shape : swept, anhedral
• twist.

 

Induced velocities due to the fuselage or the blade vortex interactions are a significant parameter. Fuselage optimization to reduce aerodynamic drag leads to designing compact rotor heads. In these conditions, the blades are close to the body, which amplifies the interactions in terms of fuselage induced velocities exciting the blade and gives high rotor head vibratory loads.

 

The number of blades is thus a highly significant factor as far as vibrations are concerned. A general argument in the helicopter community is the higher the number of blades the lower the dynamic loads at the rotor head. The choice of the number of blades is strongly influenced by other criteria like performance, price and autorotation capability.

 

The latest aerodynamic studies are producing new blades which are no longer rectangular but tapered with evolving tips. Their twist can be modified and an anhedral added to improve their performances in hover or at high speed. The planform influences the spanwise distribution of the aerodynamic loads as well as the dynamic properties of the blades. Tapering leads, for example, to low generalized masses for those modal shapes where dynamic response and vibration level are increased.

 

High twist is favourable for hover and low speed performance. The linear aerodynamic theory shows that higher harmonics blade flatwise loads are proportional to twist. The current blade design methodology is an optimization of aerodynamic performances as well as a change in internal structure to improve dynamic behaviour. The simplest methodology involves retaining a margin between blade modal frequencies and hub excitation frequencies. It is possible to increase the generalized mass or shift the frequency of the modes most critical for vibrations with tuning masses. Optimization techniques involve local stiffness and mass adjustments to globally reduce aerodynamic excitations and blade response to obtain low N-per-rev hub loads (moment, vertical and lateral shears).

 

Helicopter fuselage dynamics


The fuselage response to rotor excitations must carefully be considered to enable high comfort aircraft to be obtained. The fuselage response varies extensively with the excitation frequency.

 

The structure design must be supported by finite element airframe analysis. In the design phase, every main architecture choice like implementation of frames, installation of heavy parts (engines, gearbox, …) and interface between mechanical parts and fuselage must be validated by dynamics considerations. As far as new structures are concerned the effects of composites make the prediction of natural frequencies and mode shapes more difficult.

 

The difficulty comes from different new elastic coupling terms and the structural design concept of the helicopter composite fuselages is different from that of metals. Another problem is the structure identification methodology to ensure proper fuselage mode placement : finite element analysis as well as correlative ground shake tests are needed. The global optimization of the structural models is impractical. This is why every company is looking for simplified models which are much easier to use for parametric studies and optimization techniques.

 

Antivibration devices.


Upgrading of performance, mission duration and versatibility looking into increased level of comfort, imperfect control of forced vibration dynamic and aerodynamic problems at design level, require the necessity for developing antivibration devices. The problem proves difficult since the vibration technology has to meet the following requirements

 

• system with an unlimited service life

 

• reliability
• reduced maintenance
• minimum weight
• minimum dimensions.

 

The antivibration devices are broken down into 3 classes

 

• at the rotor hub
• at the rotor-to-fuselage interface – upper deck
• in the fuselage.

 

In these three classes, there are three categories of antivibration devices : passive, semi-active and active systems.