Ask anyone what sound a helicopter makes and he or she will usually reply by making a characteristic "whup, whup, whup" sound. Now a team of NASA and industry engineers working at the agency's Ames Research Center is looking into methods of quieting that sound and damping vibrations. They hope to reduce noise pollution, increase military stealth, and provide a smoother ride.
The Large Rotor Test Apparatus (LRTA) helps researchers simulate hovering and low speed flight in the world's largest wind tunnel.
The source of the noise in question is blade-vortex interaction. This takes place on the right side of the helicopter, where airflow effects from a blade rotating forward combine with the forward velocity of the vehicle itself. The swirling blade-tip vortex (generated by high-pressure air on the lower surface curling around to the low-pressure area on top) from one blade gets slammed into the next blade coming around, causing the familiar "popping" noise and some cabin vibration. The effect is mitigated on the left side because the motion of the blade is opposite the freestream flow.
Here's the pitch. To alleviate this noise and vibration, engineers have turned to motion-control technology. The idea is simply to input appropriate low-frequency variations of rotor blade pitch-angle (analogous to a screw, it's the "bite" the blade makes with the air, roughly the blade angle of attack) into the control system to avoid noise production or cancel vibration. To test out the technique, engineers are using the Large Rotor Test Apparatus (LRTA) mounted within the world's largest wind tunnel—the 80- × 120-foot cross section tunnel at Ames. Here, researchers simulate hovering and low speed flight up to 115 mph. Mounted on balance supports in the tunnel, the LRTA is a large, generic helicopter body that can accommodate full-size rotor systems. A Sikorsky UH-60 Blackhawk rotor is being used for the noise alleviation tests now underway. According to Ames Aero and Acoustics Group Leader Tom Norman, the LRTA will be moved into the 40- × 80-ft test section in the closed-loop portion of the tunnel this summer for four months of testing at speeds up to 230 mph.
Engineers from Allied Aerospace Industries (formerly Dynamic Engineering, Newport News, VA), which does systems design and integration of aerospace test equipment, developed the LRTA. To induce the controlled-pitch cycling of the rotor blades, they replaced each of the three direct-acting servoactuators used to set blade pitch angles with a 30-inch-long, symmetric pivoting rocker arm. Individual electric ball screw actuators from Duff Norton (Charlotte, NC) act on the arms to move the swash plate, which controls blade pitch (see sidebar). For the rotor currently under test, actuator max speed is limited to 0.4 inches/sec for safety, says Norman, to preclude a "runaway" situation. For the periodic inputs, each rocker's central pivot is actually a camshaft, which when rotated back and forth by a hydraulic rotary actuator made by MTS Systems (Eden Prairie, MN), induces pitch-angle changes at up to 35 Hz into the blades.
Long and winding road. Getting to this point has taken some time due to limited budgets. Ron Gold, electronics manager with Allied Aerospace, says that in the early 1990s the company was asked to build an all-analog control console to be used with the smaller RTA. "This was all-analog without microprocessors in the loop due to the lack of confidence in digital control systems being built at the time," he notes.
"It was decided to go digital when the LRTA control console contract was awarded around 1997," he says. "The concerns were tight for redundancy and safety, and there were still doubts about digital computers used in a closed-loop system," Gold adds. These issues resulted in a ten-sided, "potato-shaped" pitch-control boundary outside of which the rig is prevented from operating (see figure). There are also manual, operator-discretion stop areas for some of the "odd portions," as Gold calls them, within the potato because of insufficient computer processing power to achieve the reaction time necessary for safe operation. The automatic-stop potato-shaped boundary can be edited for system or structural changes to a helicopter or the LRTA rig, he adds. The control-console system was delivered in 1999 and checked out with software starting in mid-2000.
Wind up. When an operator "flies" the LRTA in the wind tunnel, three pitch control commands (vertical collective and integrated longitudinal and lateral cyclic) in the form of joystick voltage outputs are sent to a mixer in the console's controller. Here a DMC-1738 three-channel circuit card (a motion controller from Galil Motion Control (Rocklin, CA) loaded with the appropriate kinematic equations of the rotor and rig, and defined potato-boundary limits) converts the inputs into integrated output voltages. These in turn go to the three amplifiers driving each of the electric ball-screw actuators that position the swash plate. A Measurement Specialties (formerly Schaevitz; Hampton, VA) linear variable differential transformer (LVDT) sensor mounted on each of the linear actuators provides actuator position feedback. The company also furnishes the RVDTs to measure rocker-arm rotary actuator positions. Pairs of DMC-1738 cards, LVDTs, and RVDTs are used for redundancy—in the event a card or feedback device fails, an output control relay transparently switches control to the other loop. This circuitry is duplicated in the backup console that can take over from the primary one.
In the upcoming tests, the program will also evaluate a rotating direct-drive system for varying blade pitch. This system, built by ZF Luftfahrttechnik (Kassel, Germany), features individual hydraulic actuators on each blade changing pitch angle by ±6° at upwards of 30 Hz. A hydraulic slip ring provides the hydraulic pressure to the rotating assembly.
For safety, and due to budget limits, running an LRTA test in the wind tunnel is done manually, according to Farid Haddad, a Raytheon sensor controls system engineer who is contracted to the test site. A wind-tunnel operator, the pilot, and the LRTA motor-generator-driven rotor operator work as a team. "It's a conservative approach, requiring coordination," says Haddad. "The three operator stations are not linked since that effort would require debugging software," he adds, which undoubtedly would impact the limited funding.
So hopefully soon, this shoestring-budget effort will give the helicopter world technology to quiet the choppers and make for smoother flying.