Doing the Twist

NIA method may one day reduce excessive rotorcraft racket and vibration

Heard from a distance, the sheer noise generated by helicopter blades is unmistakable. For combat troops on the ground, or civilians peacefully asleep in their beds, such hubbub can be troublesome, even if for different reasons.  Getting rid of it would be welcome news for military planners and noise-disturbed suburbanites alike.
 
Quieting rotorcraft clamor is, however, a complicated proposition.  It remains the long-term goal of ongoing NIA research, part of a collaboration with NASA and the U.S. Army Research Laboratory that focuses on such helicopter-specific parameters as pressure and moment distributions, lift, drag, and location and strength of the blade-tip vortex.  
 
In particular, NIA research scientist Dr. David Fogarty is leading work aimed at understanding noise made by helicopter blade-vortex interaction, known by its acronym BVI. The effort, part of a larger initiative known as the Active Twist Rotor Project, examines the effects demonstrated by a small electric voltage applied to actuators embedded within the structure of rotorcraft blades.
 
“Sometimes when you look at an airplane there are vortices coming off the wings,” Fogarty says. “It’s the same principle in a helicopter.  When the helicopter blades rotate, each tip creates a pressure wave that radiates outwards. Basically, we’re trying to use active twist to reduce the BVI noise, as well as helicopter vibrations.”
 
The actuators, made from a ceramic-like composite material, can be modified literally on the fly. But describing their function mathematically --- and how the blade tips should be optimally configured --- is complex.
 
That’s where CAMRAD comes in.
 
Generating a New Database

Short for Comprehensive Analytical Model of Rotorcraft Aerodynamics and Dynamics, CAMRAD is an involved computer code that, when combined with other software-based approaches, can be used to predict BVI acoustic impact.  One of Fogarty’s immediate goals is to generate a database describing noise and vibration under a broad range of flight conditions, such as different speeds, the angle at which the helicopter flies and the effect of variable weather conditions.
 
“Our code is the start of the analysis,” Fogarty says.  “We input the specifics of the helicopter flight.  Is it descending?  Ascending?  In level flight?”
 
Fogarty says future work will focus on alternative rotor designs and active-control concepts, including active flaps and high-lift rotors. Such study will focus on noise and vibration-reduction potential, and whether or not both can be reduced simultaneously.
 
If the project succeeds, one long-term result could be a stealthy helicopter that would have the element of surprise during military operations.  Likewise, civilian craft would provide less jarring rides for their paying passengers --- not to mention more soothing passage for airborne and terrestrial listeners alike.
 
“In an ideal world, once the wind tunnel tests are done, you could implement the results into an actual helicopter.  A new design would be a possibility,” Fogarty says.  “But that’s in an ideal world. If we can get a simultaneous reduction in vibration and noise --- and that’s the goal at the end of the day --- that would be fantastic.”
 
Experimental evaluation of the Active Twist Rotor computational approach will be conducted primarily in NASA Langley Research Center’s Rotorcraft Hover Test Facility and its Transonic Dynamics Tunnel.  Other Langley wind tunnels may also be used to validate analytical findings.

David E. Fogarty, Ph.D. Research Scientist II