It is one of the most famous soundtracks in film history: the end of Francis Ford Coppola’s Vietnam drama, “Apocalypse now”, features a song by Doors in which the sound of a helicopter flying over the heads of the audience is simulated. But outside the cinema, what served the former cult band as a legendary stylistic device in its classic “The End” is something much more mundane: noise. To cut this back, scientists from Eurocopter and the EADS Corporate Research Centre (CRC) invented the adaptive rotor blade.
The helicopter’s characteristic noise, regular beat, has nothing to do with its engine – it is entirely attributable to the rotary wings, or rotor blades. Developments in the aerodynamic properties of rotor blades since the era of the Vietnam War have drastically reduced the amount of noise emitted while cruising. For one, this is a consequence of the noise limits for helicopters set by the International Civil Aviation Organisation (ICAO), the first of which were imposed back in the early 1980s. But especially during the descent, when the helicopter inevitably comes within human hearing range, additional noise is generated by a different phenomenon. As the blades rotate, air vortices are generated at the blade tips. During descent, each rotor blade passes through the air vortex created by the previous one. This abrupt change of pressure is like the crack of a whip, and it sounds like one as well. On the ground this sound translates into the characteristic beating noise.
At a cruise speed of 250 km/h the rotor of a helicopter the size of the EC 145 rotates at a speed of about 400 rpm, causing the blade tips to pass through the air at a speed of over 750 km/h. When added to the speed of the helicopter itself, the net effect is that when the blade advances it passes through the air at about the speed of sound, whereas when it retreats its speed is halved to about 500 km/h. This constant change of free stream velocities causes additional cyclic flight loads which also manifest themselves on board, primarily in the form of vibration transmitted by the rotor blades.
“To avoid the beating noise and the vibrations, the rotor blade must be able to avoid the air vortex,” explains Michael Grünewald of the EADS CRC in Munich. In search of a solution, the scientists have joined forces with engineers at Eurocopter led by Valentin Klöppel to investigate two possibilities. The first of these is a hydraulic system at the blade root which causes the entire rotary wing to move. Preliminary research results were encouraging, but the hydraulic components called for by this solution required a lot of energy and were expensive to build. So then the researchers turned to the point at which the noise arises – on the blade itself.
They developed movable control flaps which are integrated into the trailing edge of each rotor blade and are able to induce the blade to make an evasive manoeuvre. When these are actuated, the rotor blade executes a vertical evasive manoeuvre which significantly reduces the impact of the vortex and hence the noise. What sounds simple in principle can only be accomplished in practice with the aid of advanced technology, considering the enormous centrifugal forces which prevail at the blade tips when the rotor is running at maximum speed. Expressed in numbers, it is up to 800 times the acceleration due to gravity. Every gram too much quickly translates into almost a kilogram of weight, which not only has to be securely anchored but must also function reliably and not disturb the rotor balance.
To this end, the EADS researchers developed a lightweight, compact piezoceramic actuator. The length of these bar-shaped devices changes upon exposure to a potential difference. But as the change is only very small, the engineers designed a mechanical frame which translates the expansion of the piezo elements into the desired deflection. The actuators are still able to move a 25 cm wide flap by 10 degrees. At the same time, actuator and mechanical frame are so thin that they fit into the flat rotor blades. Depending on the test set-up, two or three such flap modules are employed on each rotor blade.
What is so clever about this solution is not just the mechanics – considerable development effort has also gone into the controls. Microphones positioned on the helicopter skids record the airborne sound level of the rotor. Special software is able to detect the collision of blade and vortex from the signals. Instead of microphones, another possibility is the use of a sensor which measures the sound-generating pressure fluctuations on the rotor blade with a piezofilm integrated into the blade. A sensor of this type is so responsive to pressure fluctuations that it can be incorporated into the rotor blade structure below the blade surface itself.
The central computer in the helicopter airframe determines when and how the flaps need to be actuated on the basis of these values. It sends the relevant commands by optical fibre through the rotor mast to a power electronics unit which, at the experimental stage, still sits proudly on the rotor head rather like a large beehive. This converts the commands to electrical impulses which go to the piezoceramic actuators. The actuators respond so quickly that the flaps open and close within one revolution of the rotor, i.e. between 15 and 40 times per second. As a result of these upward- and downward-acting forces, the rotor blades twist by an angle of one to two degrees on top of their normal movement. “In this way we are using the aerodynamic forces and making significant savings in additional energy compared with the hydraulic solution,” explains Michael Grünewald.
“Moreover, we can also use the rotor flaps to reduce vibration,” he adds. The vibration problem arises primarily as a result of the air flow into the rotor, which is typically asymmetrical in a helicopter. The solution is to use the trailing-edge flaps to create additional vibration in such a way that this exactly cancels out the original vibration. Here, instead of the microphone, the sensors used are strain gauges on the blade root and rotor mast or acceleration sensors in the cabin.
Before the scientists could prove that the adaptive rotor blade does in practice what theory suggests, the functional performance and safety of the design had to be demonstrated at aerospace test centres, i.e. the wind tunnel, the whirl rig, bending rig and whirl tower. In autumn 2005 the system was tried out in flight for the first time on an EC 145 helicopter at the Eurocopter site at Donauwörth, Bavaria. The first test flights were very promising: both noise and vibration were perceptibly lower. However, the functional and operational safety of the system were the foremost considerations in the test series. Thus the pilots established that actuation of the flap control on its own had only a marginal effect on the helicopter flight attitude.
One of the central aims of further development is to reduce the size of the power electronics unit, which currently still weighs about 65 kilograms. The engineers believe a realistic goal is to reduce this to about 10 kilograms and integrate it into the airframe or rotor head cover.
Once this has been accomplished, Eurocopter specialist Valentin Klöppel believes it will be possible to completely eliminate the harsh beating sound and reduce the noise level by about six dBA. This would cut the subjective noise level by about one-third. And the occupants of the helicopter would be subjected to only 10 percent of the vibration severity experienced today.
The rotor beat would then be history, a memory in the soundtrack of Coppola’s Vietnam epic.
