The lobbying association for the French helicopter industry, the Union Française de l’Hélicoptère (UFH), is raising concerns about gestating noise rules that could practically ban commercial flights from urban areas. The association fears legislators are writing such a rule with input only from heli- port neighbors, some of whom are members of anti-helicopter associations. Meanwhile, German aerospace research center DLR claims to have found simple ways to reduce noise by optimizing flight procedures.
The UFH has sent a letter to the French minister of the environment and transport warning that excessively restrictive rules might force operators out of densely populated areas. The new regulation could cap helicopter movements, setting limits for hours of operation on given days, months, seasons and years. The legislation does not include EMS, rescue, state and emergency preparedness flights.
The implementation of such caps could prevent an operator from maintaining a base near a major city. Yet, moving to a more rural area would not be economical and might simply run operators out of business, according to the UFH.
In the letter, the association pointed out that helicopter operators rely on commercial flights such as executive air charter, tourism and instruction to stay in the black. Moreover, they are the same companies that operate highly regarded EMS and firefighting helicopters.
“Some pilots fly 80 hours per year in EMS, which is not enough to maintain proficiency and make a living,” UFH CEO Thierry Couderc told AIN. These pilots and their employers need other kinds of mission to keep afloat. In other words, excessively restrictive rules might have an indirect–but serious–effect on critical health and public-safety missions.
UFH officials have expressed concern that they were initially excluded from the rulemaking process. “We managed to attend a meeting of the national noise council, which was about to hear from anti-noise lobbies but not from the helicopter industry. Initially, we were simply not invited. An attendee even said he did not understand why we were asked for our opinion,” Couderc told AIN. The president of the council, Eric Diard, is also a member of parliament, representing a district that includes Marignane, where Eurocopter has its headquarters.
For the mid-term, improved approach flight paths might ameliorate some of the noise issues, according to the DLR. As part of a European research project called Friendcopter (which focused, in part, on external noise), researchers determined that applying simple changes to certain flight procedures ‘‘can be of enormous value and reduce noise by up to 10 decibels.”
During descents, noise comes mostly from blade-vortex interaction. This is the interaction of vortices in the air with the main rotor blades. Blade tips shed vortices and the following blades cut through them. This causes the “chopping” sound helicopters make in flight, the DLR explained. Using flight tests and simulation, researchers determined that modified descent procedures would significantly reduce the chopping noise.
When a helicopter makes a conventional landing it descends along a uniform path. The tweaked procedure developed by DLR scientists requires the helicopter to climb once more before landing. After that it lands in a steep, delayed descending trajectory. Climbing is quieter, because the tip vortices are driven downwards under the rotor and the following blades do not encounter them. Similarly, the delayed, steeply descending trajectory enables the vortices to dissipate– this time, above the rotor.
The DLR procedure eliminates the loud chopping sound of the blade-vortex interaction for much of the approach. Instead of the minutes-long noise heard on a typical descent, the new procedure yields noise at most during the one- to two-second transition between climbing and descending, according to the DLR.
The calculated flight procedure was tested with the DLR’s Eurocopter EC135 research helicopter. To assist the pilot in flying the optimized flight path, the researchers installed a “tunnel in the sky” display. Special attention has to be given to the attitude, because it contributes greatly to noise reduction, the DLR added.
Pilots and researchers also found a way to counter noise from the shrouded tail rotor. The counter-torque device contributes to noise because, in a steep descent, the main rotor approaches autorotation. Therefore, the engines exert little torque on the main rotor and there is no need for anti-torque force from the tail.
To keep the helicopter straight, the pilot reverses the flow through the tail rotor, using the pedals. The wake of the fixed blades in the shrouded tail rotor is then driven through the rotor. This results in noisy pressure variations through the rotor. The Friendcopter research project successfully tested the possibility of not reversing the flow. “While this leads to a high angle of yaw, the additional noise of the fenestron is eliminated,” the DLR said.