Eurocopter believes that helicopter technology could develop dramatically over the next decade.
Andrzej Jeziorski/MUNICH
Although ancient toys and drawings show that the basic principle of rotary-wing aircraft dates back centuries, the history of the helicopter as a useful flying machine is generally thought to have begun with Igor Sikorsky's VS-300 in 1939.
The practical helicopter is therefore more than 30 years younger than the practical aeroplane. This is seen as a boon by Helmut Huber, research and technology director of Franco-German manufacturer Eurocopter, who argues that, while the technical development of fixed-wing aircraft has reached maturity, helicopters still have an enormous amount of untapped potential and could develop dramatically in the coming years, given sufficient research efforts.
Eurocopter Deutschland, the German part of the consortium, is now arguing its case for German Government support of a new civil- helicopter research programme.
Bonn has pledged a total of DM600 million ($430 million) to back German civil-aerospace research over the next four years, to be paid on the condition that industry supplies an equal amount from its own funds. Eurocopter is pushing the politicians to dedicate 15% of the research funding - DM60 million - to its programme, dubbed "Helicopter 2010". Huber is not over-optimistic about achieving this, but is hoping for at least a 10% share.
The goal of the Helicopter 2010 programme is to develop technology which Eurocopter thinks could be incorporated into civil designs in 15 years' time - the length of time Huber estimates it will take for the new technologies to mature. The initial four-year Government-support package, is seen as a first tranche of research aid, for Germany's aerospace industry, which has long been demanding Government efforts to help it compete against the formidable US industry.
Eurocopter has identified key technological goals it hopes to achieve in the programme. Among other things, it predicts that tomorrow's helicopter will be 50% cheaper to operate, have all-weather capabilities, and be faster, more comfortable and quieter than today's machines.
NOISE THE KEY
Huber believes that noise reduction is the most important goal for which to strive. He intends to tackle both external and internal noise, hoping to achieve internal cabin-noise levels below 80dBA - a level comparable with present-day fixed-wing aircraft - and external noise levels 10EPNdB (equivalent perceived noise decibels) below International Civil Aviation Organisation (ICAO) limits, which vary according to take-off weight.
Tightened noise regulations, and the fact that some local noise limits set standards even more stringent than those of ICAO, means that extra efforts will have to be put into reducing the noise footprints of future helicopters.
Eurocopter's EC135 light helicopter has already achieved external noise levels 5EPNdB below the ICAO limit, but further reductions will require the application of new technology. The main and tail rotors are the primary sources of external noise, and it is here that Eurocopter is focusing its attention.
One approach is to look at advanced blade-tip designs to reduce noise caused by compressibility effects. The tip of an advancing rotor- blade generally travels at Mach 0.9 to 0.95, generating shock waves. "Here the noise gets very excessive," says Huber.
The solution is either to sweep the blade tip back, or to reduce the rotor revolutions per minute (RPM). Yet manipulating the aerodynamics of the tip introduces complexities into the blade structure which have an effect on the component's life and cost.
Tip-speed reduction, however, is an approach, which has substantial untapped potential, although there are limits to the benefits to be gained.
"The slower you turn [the rotor], the higher you have to load the blade - also, the slower you turn, the higher the torque," says Huber. This requires more work to be done by both the gearbox and the tail rotor, and a heavier main rotor.
Huber believes that a variable-RPM rotor would produce the optimum benefits: the helicopter's main rotor would turn more slowly at low altitude, where noise consideration is more critical and faster at high altitude. Up to now investigations have been carried out with rotors which vary RPM by only "a few per cent". In future, however, RPM variations of at least 15% may be feasible, although this will be at the cost of an increase in empty weight.
Another approach is to use active rotor control to combat the noise caused by blade-vortex interaction (BVI) - the collision of a blade with the high-energy tip vortex generated by the preceding blade. This can be alleviated by active blade-control techniques such as higher harmonic control (HHC).
Wind tunnel testing and flow visualisation over the past few years have taught engineers much about how rotor vortices behave and the flight conditions under which BVI noise is produced. Since blade-tip vortices are swept downwards by rotor thrust, the worst noise is generated in descending flight.
HHC softens this noise by modulating rotor-blade pitch through an angle of 1¡ or 1.5¡ three or four times per revolution, either producing a weaker tip vortex or reducing the violence of the impact, says Huber. This technique has already been tested in wind tunnels. The blade modulation is controlled by a system, which includes an externally mounted microphone to measure the noise and select the optimum phase angle to counteract it.
Tail-rotor noise can be reduced, by uneven blade spacing. On the Fenestron-type shrouded tail-rotor favoured by Eurocopter, which can have six to ten blades, substantial benefits can be achieved by varying the angles between the rotor blades. This produces a different frequency-spectrum of noise, with a reduced pure tone: the frequency peaks are lower, although they are more numerous and represent the same overall noise energy.
QUIETENING THE CABIN
Internal cabin noise is another problem facing researchers: modern helicopters have internal-noise levels of about 90dBA, compared with some 80dBA in fixed-wing passenger aircraft. The problem is a complex one, with the aircraft power plant located right next to the small passenger-compartment, and a complex frequency spectrum to deal with - from the low-frequency drumming of the rotor vortices, to the high-pitched whine generated by the gearbox.
To tackle this, Eurocopter engineers are working on an integrated noise-reduction concept, with tailored passive and active noise-control devices such as Helmholtz resonators and active panels - piezo-ceramic plates mounted in the cabin walls, which are vibrated electrically to damp out incoming noise by destructive interference.
Active absorbers can also be used to isolate the gearbox from the rest of the airframe, preventing noise being transmitted through the structure.
Vibrations also reduce passenger comfort, and Eurocopter is considering fighting them by introducing full, six-axis (three force and three moment), isolation of the main rotor and gearbox from the fuselage, either by passive or active means. The principle is already being applied in modern helicopters, but today's isolation systems only work in three axes, says Huber.
At the moment, helicopter occupants feel vibration accelerations of about 0.1g. Huber believes that levels between 0.02g and 0.05g can be achieved in the Eurocopter programme.
Another key Helicopter 2010 goal is the development of all-weather capability to allow helicopters to be operated in zero-visibility conditions - an important goal, particularly for rescue applications.
The first relevant technology, which could be applied to this goal, is the use of satellite navigation. By using a specially equipped police car as a ground station, a rescue helicopter could use a differential global-positioning capability, giving its position relative to the car with an accuracy of "a few metres". Huber hopes to combine this with a three-dimensional digital-mapping capability giving an accurate picture of the surrounding landscape with the helicopter's position.
Eurocopter is also investigating using a rapidly scanning laser beam to detect obstacles in poor visibility - although this system could not be used in fog or snowfall, when the beam would be scattered.
Possibly the most significant all-weather system under development, however, is the rotor-mounted HeliRadar, a rotating-antenna synthetic-aperture radar (SAR), designed to give the pilot an artificial external view.
SAR generates high-resolution pictures even under the worst weather conditions. Using an extremely high frequency (some 35GHz) radar beam, the system uses the rotation of the rotor, to scan the landscape and produce its picture.
The antennae are fitted in a cruciform mounting on the rotor head, and the radar returns are converted into a picture by a powerful computer, now under development, which will be capable of processing 10 million processing operations a second.
A prototype antenna supporting structure and transmitter-receiver installation, have already, been mounted on a BK.117 and are to be test flown later this year. A development simulator has been constructed to evaluate various pilot-interface systems.
At the moment, Huber envisages that the system will produce an image on a head-down liquid-crystal display. The idea of a head-up display (HUD) is under consideration, but Huber says that to display such a detailed radar picture on a HUD would be "extremely difficult".
He adds that the concept has attracted interest from the German defence ministry. The technology could be ready to use operationally within five to eight years, he estimates.
REDUCING COSTS
Another major goal Eurocopter hopes to achieve is the halving of direct operating costs.
"Operating costs are very much influenced by all the maintenance efforts for the dynamic components [of a helicopter]," says Huber. Eurocopter is working on methods of extending component lifetimes by the use of advanced, fatigue-resistant, materials and health and usage monitoring systems, which will provide a far more accurate analysis of when a component needs to be replaced than is now available.
A further strategy, which Eurocopter is following, is the simplification of complex components such as the main rotor. The company is already working on the next generation of main rotors beyond the advanced bearingless main- rotors in use on the EC135 and the new EC120.
At the moment, a typical helicopter is ten times as expensive to operate per passenger kilometre as a fixed-wing aircraft, says Huber, but he repeats, the technology is three decades younger.
Source: Flight International
