NFAC team puts novel helicopter rotor system to the test
By Philip Lorenz III , AEDC/PA
/ Published April 07, 2008
ARNOLD AIR FORCE BASE, Tenn. -- Testing on a full-scale, state-of-the-art helicopter rotor system for a Department of Defense customer is underway at the U.S. Air Force Arnold Engineering Development Center's (AEDC) National Full-scale Aerodynamics Complex (NFAC) at Moffett Field, Calif.
This marks the first military test entry since the wind tunnel complex was reactivated to full operational capability under AEDC's operational control and management.
The testing is a two-part collaborative effort between the Defense Advanced Research Projects Agency (DARPA) and NASA and the U.S. Army. DARPA is sponsoring the initial phase of the project with NASA and the Army funding the second phase.
The test article, a Boeing Corp., Smart Material Actuated Rotor Technology (SMART) helicopter rotor, is being tested in the NFAC's 40-by-80-Foot Wind Tunnel to study the system's forward flight characteristics and to collect data to validate cutting-edge aero-acoustic analysis codes.
"DARPA has a program called the helicopter quieting program," explained Jeffrey Johnson, Arnold's test engineering group lead at NFAC. "Their overall goal is to try and develop codes for predicting the acoustics of helicopters. One component of the program is to take this active flap rotor system, and run it at several specific test conditions to get acoustic measurements so they can have that as a database to compare their acoustic prediction codes against."
He pointed out that the acoustic prediction codes currently exist, but they are empirical and apply only to conventional designs and are not very accurate. The new tools are physics-based, so should provide predictions of higher quality and for novel rotor designs.
Johnson said to fully appreciate the test's significance; it helps to know some of the background information on the test article and what it takes to execute testing on it.
"It's a helicopter rotor that has blades that have been modified with special flaps near the outboard section of the blade," he said. "These flaps are deflectable by way of an internal actuator that is also imbedded in the blade. The production helicopter blade has been modified during its fabrication process to allow for these cutouts for the flaps and also a pocket in the blade for this compact piezo-ceramic actuator system which uses high voltage direct current to deflect the flap up and down at high frequencies."
Johnson said testing the active flap system has two primary goals for the NASA/Army portion of the testing - to reduce vibrations and noise. He emphasized that the test is complex and requires a well-coordinated team effort to be successful.
"The biggest challenge is getting the test article installed and then hooking up and verifying all of the systems required to safely operate the motor and rotor system," he said. "A significant part of that challenge is checking out all the instrumentation and data systems associated with making measurements of the rotors' health, the research instrumentation and the actuator power associated with making it all work."
Dr. William Warmbrodt, NASA Ames Research Center's Aeromechanics Branch Chief, said the NFAC is uniquely qualified to conduct testing of the SMART rotor. He is the acting program manager for DARPA on the test and helps to execute the technical aspects of NASA's and the Army's part of the project.
"This is the only facility in the world that can test full-scale helicopter rotor systems," he said. "We are testing rotor systems that have never been operated within simulated forward flight conditions and to do this safely and still get the kind of quality data out of them is a unique challenge."
Dr. Warmbrodt described helicopters as a unique flying platform requiring all the tools available to properly test to improve system designs and capabilities.
"Helicopters are the most complex mechanical systems flying today," he said. "In the wind tunnel you can only simulate steady operating conditions and as such the rotors get tested in a certain way and then you rely on your analysis to take it beyond what you have demonstrated in the tunnels, understanding dynamic behavior in a real fight environment."
Johnson said the baseline SMART rotor without a flap was originally designed by McDonnell Douglas (MD) about 15 years ago for the MD-900, a twin-engine, approximately 7,000 pound helicopter. The advanced rotor has provided an excellent system to test the prediction codes against.
"We previously tested the baseline rotor to 200 knots," he said. "For this test we will simulate level flight in the test section to a velocity of 155 knots. This is the specific test condition DARPA is interested in for their part of the test."
Dr. Warmbrodt put the two phases of the testing into perspective.
"The DARPA part of this testing is to acquire the scientific quality data to validate the next generation of helicopter design and analysis tools," he said. "The NASA and the Army part is to quantify the benefit of on-blade controls to improve the aeromechanical behavior of an edge-wise flying rotor."
Dan Newman, the program manager in DARPA's Tactical Technology Office responsible for the Helicopter Quieting Program (HQP), said having the SMART rotor and the NFAC available for the testing has been a winning combination.
"The SMART rotor is a DARPA, NASA, Army asset that had been developed and fabricated in the past to develop SMART materials as a mechanism for on-blade control," he said. "The NFAC is a national asset. It's critical - we are nowhere near eliminating wind tunnels to ensure our systems are safe before we operate them. Our computational tools allow us to investigate the design state rapidly, effectively and efficiently. The wind tunnel ensures that those predictions are grounded to an absolute performance baseline."
Dr. Warmbrodt said the benefits from the current round of testing will be especially useful in the long run.
"The dividend is that these design tools developed by DARPA will then be used for next-generation Department of Defense service platforms," he said. "Those platforms will have superior mission performance capabilities that would include greater range, potentially improved speed, vibratory characteristics, as well as improved survivability characteristics - a quieter rotor that would be less detectable and have a higher likelihood of achieving mission success."
However, he dispelled the idea that DARPA was trying to design a helicopter with a "stealth capability."
"In fact, we're actually going to be testing known conditions where we have every expectation we're going to make the rotor really noisy," he said. "What we want to show is that the analysis can predict that. Just as it needs to be able to show nominal operations, it also needs to show that when we operate the flaps. We want to know if these codes will actually give us the right results when the rotor is quiet, or when it gets noisy, or when we put it in a condition that a normal rotor would never see because it would never experience that type of dynamic blade motion."
Mr. Newman said NFAC's staff and crew are motivated and up to the test's many challenges.
"There have been significant challenges in bringing this facility back on line, such as the data system," he said. "AEDC has put safety of flight and the protecting the resources as its highest priority, which is appropriate. For the Helicopter Quieting Program test, the indoor test allows us to run a range of conditions that we couldn't do efficiently, effectively, safely or affordably in flight. A challenge for this test is measuring in plane acoustics. This is the noise radiated from the rotor toward the horizon, as opposed to that radiated down below the aircraft. The significant investment in acoustics in the 1990s has not been vetted for these types of measurements."
He expressed confidence in the team's ability to overcome these challenges and sees a promising future for the NFAC.
"We're the first of hopefully a series of large-scale acoustic tests here," he said.