FLUG REVUE November 2005: Piezo flaps reduce helicopter noise

November 2005

PIEZO FLAPS COUNTER NOISE

By Renate Strecker

There cannot and never will be a helicopter that is whisper quiet – that is, not audible to an observer on the ground – because physical laws simply cannot be surmounted. Whenever air meets rapidly rotating rotor blades, vortices are formed. These in turn inevitably produce a loud background noise on the landing approach as the rotor blades keep passing through their own vortices and producing new ones. Even with the most unusual inventions, this “disturbance” cannot be completely eliminated. All one can do is try to reduce it.

After working on the problem for several years, Eurocopter has now demonstrated that advanced, ingenious technology can cut the noise drastically: piezoelectric regulators (actuators) on the rotor blades can reduce not only the noise but also unpleasant vibration in the helicopter cabin.

The first flight in the world of a helicopter equipped with this new technology took place on 8 September at the Eurocopter site in Donauwörth. The test vehicle was a modified BK 117. Test pilot Manfred Kuck and flight test engineer Manfred Hausner performed a series of approaches with different flight profiles, during which the vortex noise, which otherwise would have been a characteristic drumming sound, was neutralised.

The extremely elaborate technology has a quite simple effect: the blade is controlled so that it avoids the vortices. It receives instructions to briefly alter its blade angle in such a way that it passes either under or over the vortex – without the pilot having to do anything.

The flap modules that are integrated into every blade contain the piezoceramic control elements consisting of a series of thin (0.15 mm each) ceramic discs one on top of the other. They rest on a frame that magnifies the relatively small movement of the control element by a factor of 10. Two straps are used to move the flap at the blade trailing edge upwards or downwards by up to five degrees. The blade responds to this by altering the blade angle by up to two degrees.

The control elements are powered by the on-board battery. Cables run from the battery between the fuselage and the rotor mast. The power is conveyed from the rigid fuselage into the rotating mast via an inductive slip ring and from there in turn by cable through the rotor blades to the various control elements.

A computer in the rear of the helicopter is used to predict the location of disturbing vortices. It responds to a microphone mounted on the skid and passes the information by optical fibre to the electronics installed on the rotor head within a fraction of the second. The amplifiers located there assign the necessary power to the relevant control elements. All that the pilot notices as regards this influencing of the blades is a slight change in the control pressure on the stick.

In this area Eurocopter Germany has a technological lead over the competition from the USA and Japan. Under the direction of Dr. Valentin Klöppel, Aeromechanic and Flight Control Programme Manager, the engineers believe that the controlling elements can reduce the noise emissions on the landing approach by up to 6 dB(A). 10 dB(A) would be equivalent to halving the noise level. One of the pleasing side-effects is that influencing the blades in this way also has the effect of significantly reducing the vibration in the airframe. In this area improvements of up to 90 percent can be achieved.

The programme is being run in partnership with EADS, DaimlerChrysler and the German Aerospace Centre (DLR) and is part-funded by the Federal Ministry for Economic Affairs.

After the successful first flight, Dr. Klöppel stressed that “the demonstration system still has to be miniaturised and optimised for commercial use.” Eurocopter is apparently already working on improved flaps and a bearingless rotor. The technology could be generally implemented on all large helicopters from around 2012.

If the system can then be used for primary control of the helicopter, it would be possible to dispense entirely with control rods, hydraulics and swash plate.

From page 94 of FLUG REVUE 11/2005

Last updated 12 October 2005

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