In the arcane world of helicopter rotor aerodynamics, two concepts that show promise for enhancing safety and performance in the world of high-density-altitude heavy lift are under development on opposite sides of the U.S. Briefings on both were presented at the American Helicopter Society’s annual forum last month in Phoenix.
Dr. Preston Martin of the Army/NASA Rotorcraft Division at Ames Research Center of Mountain View, Calif., described one concept– a variable-droop leading edge (VDLE) device that has been tested in the Ames wind tunnel. The other, a miniature trailing-edge (MiTE) “effector” (trim tab) for active rotor control, was conceived and mathematically modeled by faculty and students at Penn State University’s aerospace engineering department. It has been tested in the PSU low-speed wind tunnel.
The VDLE concept was studied as part of an Army-sponsored research program to explore dynamic rotor blade stall onset and control. Previous work had shown a dynamically deforming leading-edge airfoil and a slatted rotor blade airfoil to delay stall onset, but both had limitations. The VDLE airfoil, drooped dynamically on the retreating side of the rotor disc, proved to offer a way to modify local adverse flow effects and improve airfoil performance on the retreating side. Test results indicated slight reduction in maximum lift, a large reduction in maximum drag, and significant reduction in peak pitching moment. Martin noted that the hinged model VDLE tested in the Ames wind tunnel is a concept demonstrator and that “application of this concept to a rotor blade requires a completely different design.”
The Penn State trailing-edge moveable tab has likewise demonstrated similar changes in lift and airfoil moment. Compared to the clean airfoil, maximum lift coefficient is increased up to 30 percent by employing a flap on the lower surface at the trailing edge. The idea is simple and decades old. Penn State researchers call their device a Gurney flap after its originator, race car driver, builder and designer Dan Gurney, who used a fixed tab to optimize download force exerted by the aerodynamic “wing” on one of his machines.
Whereas Gurney and chief mechanic Peter Wilkins used trial-and-error in Gurney’s Santa Ana, Calif. shop to find the right setting, the PSU team used sophisticated computerized mathematical prediction and analysis along with wind-tunnel results to validate their concept. Their findings indicate that MiTE technology can enhance rotor performance while controlling vibration and thus reducing certain types of noise. The researchers made preliminary efforts to develop a suitable actuation scheme, and they fabricated and bench tested a piezoelectric bending mechanism.
An advance in rotorcraft design simulation using physically based modeling to derive correct mathematical models of rotorcraft, phenomena and subsystems was described by Dr. R.W. Du Val, president of Advanced Rotorcraft Technology, also in Mountain View. He said the traditional approach has been to create empirical functions without capturing the physics of the internal processes. He added that using physical models greatly enhances the realism of rotorcraft simulation and can reduce development costs through reusability. He noted that a second major simulation breakthrough, called Multi-Body Dynamics, allows a rotorcraft model to be arbitrarily assembled from a predefined library of modeling elements representing generic rotorcraft phenomena and subsystems. He added that a recent real-time implementation of Multi-Body Dynamics has extended this technology from rotorcraft engineering analysis to training simulators.
Also in the simulation field, a pair of Liverpool (UK) University researchers, Gareth Padfield and Mark White, explained how the PC-based simulation technology embodied in the HeliFlight system is providing lower-cost flight simulation with full motion, wide-field-of-view visuals, programmable force feel and a comprehensive modeling environment. They said its first year in use has shown that the system has adapted to a variety of handling qualities, pilot-vehicle technology research and training roles. Illustrative of HeliFlight’s dual-mode capability, a European Union program to develop handling qualities for a civil tiltrotor has selected the system as a development and demonstration tool.
A Briefing from the Brass on Helo War Role
Most of Forum 59’s opening general session spotlighted the war on terrorism, and in particular the Iraq campaign in which helicopters played an unprecedented role. Retired Army Gen. Barry McCaffrey, now an NBC news analyst, joined Lt. Gen. Richard “Commander” Cody, a renowned leader of Army airborne forces, in assessing the achievements of rotary-wing aviation in Iraq and Afghanistan.
McCaffrey cited the need to support both the worldwide war on terrorism and to secure U.S. borders, efforts he said will require many additional helicopters. The former White House drug-enforcement czar predicted that it will be at least five years before the Department of Homeland Security becomes fully functional. Internationally, he analyzed the threat posed by North Korea and the communist nation’s potential capabilities, both nuclear and conventional. Responding to an Egyptian questioner in the audience who asked why the U.S. continues to support oppressive Muslim regimes, including that in Egypt, McCaffrey replied, “The main problem in the Middle East is incompetent and corrupt governments” that foster discontent among their populations. He added, “We will sustain additional terrorist attacks…we must make sure it’s not with weapons of mass destruction.”
Gen. Cody followed with an impressive damage assessment tally of Iraqi equipment destroyed by Boeing AH-64 Apache attack helicopters. He described how the entire 101st Airborne Division’s 2nd Brigade was heli-lifted–vehicles and all–250 miles deep into Iraq, leapfrogging the 3rd Infantry Division, without a single aircraft loss or personnel casualty. Cody noted how the Apaches cleared the way and covered the flanks of the UH-60 Blackhawk fleet on its way to the Karbala area, where the airborne force met and destroyed the Republican Guards Medina Division. In the process, the AH-64s wrecked “a staggering number of air defense artillery pieces,” along with 82 tanks, 142 infantry fighting vehicles and a large amount of other mobile equipment.
Cody illustrated the lethal effect of the Apache on the morale of Saddam Hussein’s forces with a quote from an Iraqi soldier to an Arab journalist. The soldier, interviewed in Baghdad after fleeing from the front, said, “We’re defenseless against the Apaches, so I ran.” Cody added, “The Apache is probably the toughest combat aircraft in the world,” and cited the testimony of Al-Qaeda and Taliban captives in Afghanistan who said there was “no place to hide” from the AH-64.
The general also praised Boeing’s creation of rapid air transportability kits used to move complete AH-64D Longbow Apaches from Fort Hood, Texas, to Iraq aboard Lockheed C-5A transports and to rapidly make them ready for combat. “The war on terrorism is going to be a long one,” Cody told the AHS community. “You can help us by reducing operating costs and with commonality.”
He added that unmanned aerial vehicles (UAVs) “will never be the entire answer; we’ll always need manned systems.” Cody said experience has shown that UAVs require human monitoring because, in cases of “friendly fire” casualties from armed UAVs, the drones “didn’t ‘own’ their mistakes. There’s synergy [between manned and unmanned systems], but we gotta be careful.”
A Military Emphasis
The American Helicopter Society (AHS) 59th annual forum, held May 6 to 8 in Phoenix, featured a wide variety of presentations, covering the spectrum of technical disciplines involved in the design, production, operation and support of rotary wing and other VTOL aircraft.
Under the thematic banner “Vertical Flight Transformation,” AHS’s Forum 59 carried a heavy military emphasis, beginning with the opening general session in which a review of helicopter successes in Operation Iraqi Freedom took center stage. Overall, much of the highly technically oriented seminars that comprised Forum 59 had to do with the role of vertical flight in transforming the services into lighter, more mobile, agile and lethal 21st century fighting forces. Specifically, a number of sessions outlined concepts and developments applicable to emerging U.S. Army requirements for transformation to a new generation of highly maneuverable, high-speed rotorcraft. Still, a significant amount of material dealt with subjects that, although sponsored by and directly applicable to the military, clearly possess potential to cross over into the civil rotorcraft realm.
These included presentations on the measured and calculated effects of two camber-varying devices–one on the forward rotor blade edge and the other on the aft–in delaying the onset of retreating blade stall while reducing the transonic drag rise on the advancing blade. Two other sessions described how breakthroughs in simulation technology are producing advances in rotorcraft design modeling techniques and in cost-effective flight training.
Other sessions explored technologies and strategies for reducing generated rotorcraft noise and ameliorating public perception of helicopters as noisy. These focused upon public acceptance issues and refinements in measuring and assessing auditory emissions.
In addition to the major helicopter airframers, engine and avionics manufacturers, the 62 exhibitors included providers of mission-support equipment and computer software. Teletronics Technology of Bristol, Pa., held a briefing at its booth on a modular datalink system for two-way transmission of a wide variety of digital and analog data. Plano, Texas-based EDS, the company formed by Ross Perot (with which he is no longer associated), showcased its product lifecycle management software, Teamcenter. EDS specifically featured the software’s aerospace and defense application being used to allow Joint Strike Fighter project teams to integrate their product development efforts.