Helicopter Engineering and Vibration Control
Application of new technology to the ANGLO-ITALIAN EH101
Our confidence in the future and the new opportunities and challenges that we may look forward to are very much based on the benefits that the application of new technology has brought to today's products.
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 aluminum 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 method is based on the following three issues
o rotor dynamics
o fuselage dynamics
o 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
o induced velocities
o planform shape: rectangular or tapered
o tip shape: swept, anhedral
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 anterior added to improve their performance in hover or at high speed. The planform effects 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 favorable 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 behavior. The simplest method involves retaining a margin between blade model 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 correlation 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.
Upgrading of performance, mission duration and versatility 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
o system with an unlimited service life
o reduced maintenance
o minimum weight
o minimum dimensions.
The antivibration devices are broken down into 3 classes
o at the rotor hub
o at the rotor-to-fuselage interface – upper deck
o in the fuselage.
In these three classes, there are three categories of antivibration devices: passive, semi-active and active systems.