Make It Stop
A version of this article appears in the June 9 edition of Aviation Week & Space Technology.
Vibration has been an enemy of rotorcraft designers since before Igorbegan building helicopters. Not only is it debilitating to passengers and crews, vibration is also detrimental to avionics and components, reducing reliability and life.
The unsteady aerodynamics of rotor blades in forward flight produce vibratory loads that enter the hub, travel down the mast to the gearbox and from there into the airframe. Traditionally, vibration has been reduced using absorbers placed in key locations—initially passive and tuned to a single rotor rpm, but more recently active and able to adjust to variable speeds.
Now Sikorsky plans to demonstrate that vibration can virtually be eliminated in a helicopter by using a combination of rotor and gearbox suppressors to create a “choke point” that blocks unsteady loads from entering the airframe.
The “zero-vibe” demonstration late this year will follow flight tests of a hub-mounted vibration suppressor (HMVS), completed in March, that achieved a substantial reduction in main-rotor vibration. Anti-vibration actuators will be added to the main gearbox to create the choke point and achieve a global reduction in airframe vibration, says Bill Welsh, Sikorsky dynamics technical fellow.
Current helicopter active vibration control systems can reduce forces in specific areas of the fuselage, such as the cockpit, but can increase the problem in other parts of the airframe. Sikorsky’s goal is to “eliminate all vibration from the main rotor, through the main gearbox to the airframe,” he says. “We want to stop the gearbox moving so we create a choke point that stops vibration entering the airframe.”
Developed by Lord and Sikorsky, under the Army Aviation Applied Technology Directorate’s (AATD) Active Rotor Component Demonstration program, the HMVS was flight-tested on aBlack Hawk. The HMVS comprises four brushless electric ring motors, each with an eccentric tungsten mass, in a housing atop the main rotor hub. One pair turns in the same direction as the rotor and the other pair in the opposite direction, but at four times the speed of the rotor.
In the hover, no vibration suppression is required, so each pair of eccentric masses is 180 deg. apart. As the helicopter moves into forward flight, and the blades start to produce vibratory loads in the plane of the rotor, the masses are moved closer together to produce an unbalanced load.
“We arrange the phase of the unbalanced load to counter what the rotor is producing,” says Welsh. “If the rotor is producing a 1,000-lb. load at 140 kt., then the device produces 1,000 lb. to counter. We need masses spinning in both directions to get complete suppression of rotor in-plane loads.”
Flight test results were “as expected . . . we put the device on a UH-60A and got significantly lower vibration than the current bifilar [vibration absorber],” Welsh explains. Mounted atop the hub, the bifilar (left in photo) comprises dynamic masses on the ends of four arms that are tuned to absorb rotor vibrations. The HMVS (right) is 50 lb. lighter than the Black Hawk bifilar, and electrical power is supplied via the slip ring already used to power the rotor anti-icing system.
Through the 1980s, higher harmonic control (HHC) of blade pitch was pursued as a way to reduce rotor vibration, but it was limited to certain frequencies. More recently, there has been research into individual blade control, with actuators on the rotor; but both involve placing increased demand on the flight-critical rotor control system. “HHC never promised to nullify every load. This does, and we are not wearing out the main rotor servos,” notes Welsh.
The zero-vibe flight tests will be performed under an AATD program known as the Combat Tempered Platform Demonstration, which aims to increase the durability and survivability of Army helicopters. In addition to the HMVS, Sikorsky will install four Lord active vibration-control (AVC) spinning-mass actuators at the four corners of the main gearbox. “This will give us a total of six knobs to turn to stop the motion of the gearbox. The key is positioning them very near the choke point, which first-generation AVC didn’t do,” Welsh explains.
To reduce risk, the first generation of AVC systems “put the active suppressors where the old absorbers used to be . . . and allowed vibration to leak through.” Forces entering the fuselage are sensed by 14 accelerometers distributed around the airframe. “They feed information into a control computer that tells the system how much force to put out in what places to minimize vibration,” Welsh says.
While the goal of zero-vibe is to achieve “very low” global 4/rev vibration levels, “there is a little bit of higher harmonic vibration—8/rev, 12/rev—but low enough that the crew will not notice,” he adds. “It is close enough to zero to be good enough.”