Reducing vibration and noise is key to the goal of gaining public acceptance of the rotorcraft as a routine mode of transport. A Boeing team has now successfully made use of advanced materials to develop and test a new helicopter rotor that could go a long way toward unlocking the vehicle’s potential.
The smart material actuated rotor technology (Smart) system, developed jointly by Boeing and the Defense Advanced Research Projects Agency (DARPA), is claimed to reduce vibration by up to 80 percent–delivering an “almost jet-smooth ride”–and cut noise by a full 10 decibels during takeoff and landing. This is achieved by using electrically controlled, on-blade trailing-edge flaps that react to sensor signals and thus improve the rotor system’s vibratory, acoustic and aerodynamic performance.
According to William Warmbrodt, chief of the aeromechanics branch at NASA Ames, “This technology-development program evaluates the potential of dynamically morphing blade structures to achieve revolutionary improvements in rotorcraft performance and mission capability.”
Whirl-tower testing has confirmed that the trailing-edge flaps work, and flight tests are the next goal. The tests have been completed at Boeing’s Mesa, Ariz. facility using a modified five-blade bearingless rotor from an MD 900 Explorer. The $10 million joint program teams Boeing with DARPA, NASA, the U.S. Army and three universities (UCLA, MIT and the University of Maryland).
Microwave Oven Technology
In helicopters, high vibration levels occur whenever unsteady aerodynamic loads act on the rotor blades. By introducing smart material devices in the blade, the behavior of the blade during its rotation can be controlled to reduce vibration. Moreover, the airfoil shape can be adjusted to the periodically changing flow conditions to increase aerodynamic efficiency.
Friedrich Straub, Boeing’s principal investigator on the project, explains that Smart is “part of a larger program by DARPA to apply smart materials and structures to enhance the effectiveness of a variety of military vehicles. We got started in 1998 with the aim of showing that we could apply new structures to modulate the aerodynamic forces acting on a rotor blade. Once we could demonstrate that, we used those structures to work toward more specific goals, such as that of reducing noise or vibration.”
The project relies on piezoelectric devices, a technology that has been used for several decades in alarm clocks, microwave ovens and sonar systems. Piezoelectric plates behave like small actuators, changing shape when charged by electric currents. In this system, scores of plates are formed into stacks and built into actuators that are installed in each blade. Together they produce a force and stroke output that can be coupled, via a single link, to a three-foot-long trailing-edge flap on each blade (the MD 900’s blades are 17 feet long). The flap moves, and that modulates the aerodynamic forces and cancels the vibrations.
“We focused on piezoelectric material because it gave us the bandwidth or frequency response we needed,” said Straub. The weight penalty added to each blade is around five pounds.
“The whirl tower is a stand fixed to the ground, onto which you connect the rotor at a certain height above ground. This setup is used for system checkout and hover testing. We ran our rotor on the stand for about 13 hours, with the new flaps in operation for about half that time. Given the unsteady environment and axial flow conditions, you cannot really measure any noise or vibration reduction on the whirl tower, but it does show that all components of the system are working properly and helps us to confirm our computerized predictions.
System Works in Full Scale
“Our tests demonstrated that the active flap system works in full scale, thus confirming it met the requirement,” said Straub. “It achieves unprecedented aerodynamic flow control. We calculate that this system can deliver an 80-percent reduction in airframe vibration, as well as up to a 10-decibel reduction in the noise that is perceived on the ground as a helicopter passes overhead.”
The next phase is flight testing the system. This could start as early as next year but further progress rather depends on funding issues since the group is currently looking for some form of industry or government sponsor. If money becomes available, the team should be able to deliver a production version of Smart in about five years. New production rotorcraft would benefit from the new technology, but even a retrofit may be a practical, if expensive, proposition. The system would have to be tuned to the vibration characteristics of each individual airframe.
The goal of DARPA’s Smart materials and structures demonstration program is to create a shift in the design of not just helicopter rotor blades, but also undersea vehicles and torpedoes, aircraft wings and engine inlets. Boeing’s rotor-blade work could be translated into vibration reduction elsewhere on an airframe. Straub said that, years ago, engineers tested a system that used the same principle to make significant improvements.
“You could use this material to actively reduce airframe vibration, in the same way the EH-101’s active vibration control system uses airframe-mounted actuators [see sidebar on page 94], but using far less weight and less power to achieve the same goal. You need a sensor, computer and control law. Algorithms then take the input from the sensor signal and compute what the actuator needs to be doing to minimize, or ideally eliminate, that signal.
“Although we call it Smart technology, the piezoelectric plates still require accelerometers on the airframe to tell them how to move, so the plates themselves aren’t that smart.”
So far the program is viewed as a success by Boeing and DARPA, which have contributed the bulk of the initial $10 million in research and development costs. However, forward flight tests are needed to demonstrate that the system can deliver the expected dramatic noise and vibration reductions. The application to the U.S. Army’s helicopter fleet–notably in the UH-60 Black Hawk and CH-47 Chinook–can then be considered. But delivering comparable reductions to the noise that those beasts produce would be quite an achievement.
How the EH-101 Does It
Conventional vibration control uses passive techniques that not only lack performance, but can also result in a weight penalty exceeding 1 percent of the gross aircraft weight. Active systems have therefore been developed to give improved performance with less weight penalty. As these systems are generally more complex, involving electronic and mechanical systems, demonstration of reliability is an issue. However, since they are self-tuning, unlike most passive devices, maintenance requirements might be lower.
Active systems can range from relatively simple self-tuning absorbers to extremely complex individual blade-control (IBC) systems. The benefits and risks associated with the different types of system have a wide range. IBC, for example, aims also to improve rotor performance and reduce noise, but such systems, historically at least, have posed major implications for the airworthiness of the flight-control system.
With their Smart design, Boeing and DARPA hope to overcome this limitation.
The airframe active-vibration-control system developed by GKN-Westland for the EH-101 is relatively low in risk but has high-authority actuators, so airworthiness implications still have to be considered.
The basic control philosophy behind the three-engine helicopter’s active control of structural response (ACSR) system is the same principle of superposition. A controlled secondary excitation is applied to a structure that is excited by a primary source, to minimize the response of the structure. In helicopters the primary source is the aerodynamic rotor hub force. To achieve the secondary excitation, four active gearbox struts–in reality, hydraulic-powered actuators–are distributed around the rotorhead and apply the controlled forces to the helicopter fuselage through a total of 10 piezoelectric accelerometers. A multivariable controller measures the vibration at several locations in the structure that are remote from the actuators and determines the secondary (control) forces that need to be applied.
Results for the cockpit and cabin average levels are about 0.1g–most positions are generally within the 0.15g target level throughout the speed range. At 140 knots the system is claimed to reduce vibration at the 10 control positions by an impressive 87 percent.