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Wednesday, February 1, 2012

Leading Edge: Quiet Please

By Frank Lombardi

The beginning of a new year seems like an appropriate time to look at what lies ahead with evolving helicopter systems and technologies. One area that always welcomes improvement is that of helicopter noise and vibration. A quieter, smoother helicopter makes those who fly in them smile bigger, and those who are on the ground complain less; a win-win for all. There are many oscillating forces and moments at work creating sources of noise and vibration in flight. One of the most well known sources is due to blade-vortex interaction (BVI), otherwise known as “blade slap.”

Just like an airplane wing, higher-pressure air will spill out from under a rotor blade at the tip, get sucked toward its upper surface, and create a strong corkscrewing swirl of air, or tip vortex. Under normal conditions in level flight, these vortices tend to sink and lose energy as they get blown below the rotor. In autorotation, they pass above the helicopter as the aircraft descends quickly. However, in a partial-power descent, or when the helicopter is rolled into a turn, an approaching blade will often come into the vicinity of a tip vortex left by a preceding blade. When the high-energy swirling air strikes the rotor blade, it causes a sudden change in angle of attack and an associated change in pressure on the surface of the blade. This is what creates the loud impulsive noise that can be annoying to those on the ground. There can be increased vibrations in the cockpit due to BVI, especially as the aircraft is slowed on approach. As you begin to “ride the burble,” the tip vortices impart high vibratory loads on the blades that pass down the rotor shaft, into the fuselage, and into the seat of your pants until the helicopter slows to a hover and the vortices are once again blown down beneath the rotor.

There is more than one way to lessen BVI. In recent months, Eurocopter has publicized its latest efforts to make friendlier-sounding, smoother-riding helicopters, developing both passive and active systems to combat blade-vortex interaction. The manufacturer’s passive system, which is called Blue Edge, involves redesigning the conventional straight rotor blade from the root outward, to include forward sweep, aft sweep, and anhedral (droop) at the tip. The intensity of the BVI is governed by the distance between the vortex center and the plane of the blade, the strength of the vortex at the time it meets the blade, and how parallel the vortex is to the blade edge when they meet. Blue Edge combats all three of these. The forward and aft sweep of the blade sit at an oblique angle to the vortex during contact, while the anhedral, or drooped tip weakens and deposits the vortex lower, giving the approaching blade more clearance over the top. This has produced a reduction of noise levels by 3-4 decibels, according to the company. Blue Edge has been said to increase hover performance as well, since reducing the strength of tip vortices will increase rotor efficiency.

Eurocopter’s Blue Pulse technology is an active noise/vibration canceling system that uses piezoelectric actuators to control trailing edge flaps on each rotor blade. Piezoelectric actuators can change their shape when an electrical voltage is applied, and actuate those flaps at 15-40 times per second, effectively flying it up and over or down and under the approaching vortex with every revolution, greatly reducing transmitted noise and vibration. Eurocopter has reported a 5-decibel decrease with the system.

These methods of BVI reduction have been around for a while, but only now are they coming into their own. The constant development of materials technology is what has allowed this to be so. The unconventional shape of the Blue Edge rotor would plague engineers with difficult bending and twisting loads that try to distort the shape of the blade in flight, were it not for the advent of composite materials that can resist such aeroelastic effects. With Blue Pulse, piezoelectric actuators made largely of ceramic light-weight materials and virtually no mechanical parts have proven to be very robust while living in the high-g, oscillatory environment of the spinning rotor; something that has long-troubled earlier projects exploring active-blade control.

Until these newer systems are fully integrated into the helicopter production line, it falls upon us to keep the negative public perception of our flying to a minimum. Knowing what causes BVI and the regimes of flight it is commonly encountered in allows us to do our best to avoid it, and we should continue to be good neighbors and use “fly friendly” techniques until technology gives us a quiet helping hand.

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