Sunday, August 1, 2010
Controlling UAS Flight
Smart guidance, navigation and control give unmanned rotorcraft autonomy adaptable on the battlefield or sea
Though vertical takeoff unmanned aircraft systems (UAS) have yet to make the combat contribution of fixed-wing UASs, four very different autonomous rotorcraft show near-term warfighting potential.
The 16-pound Honeywell T-Hawk ducted fan UAS helps the U.S. Army scout convoy routes and joint-service Explosive Ordnance Disposal (EOD) teams spot Improvised Explosive Devices in Iraq and Afghanistan. The 3,150-pound Northrop Grumman Fire Scout helicopter completed its first fleet deployment on a Navy frigate in April and enabled the U.S. Coast Guard to make a Caribbean drug bust.
In February and March, the 6,500-pound Boeing Hummingbird and 12,000-pound Kaman Aerospace/Lockheed Martin K-MAX helicopters at Dugway Proving Ground, Utah, showed the Marines an Immediate Cargo UAS capability that may resupply forward operating bases in Afghanistan.
All of the unmanned rotorcraft use smart guidance, navigation, and control packages to fly with minimum operator intervention, change objectives in flight, and land safely should ground control station (GCS) links be lost.
Predators, Shadows, and other fixed-wing UASs offer longer endurance and lower operating costs than rotorcraft for Intelligence, Surveillance and Reconnaissance (ISR) applications. Vertical takeoff and landing and hover-and-stare performance are nevertheless valuable qualities in some missions.
Army Brigade Combat Team Modernization plans include a company-level UAS like the T-Hawk carried and controlled by foot soldiers. The Navy expects unmanned Fire Scouts to share space on new Littoral Combat Ships with manned Seahawk helicopters. In the Dugway demonstration, the Marine Corps Warfighting Lab modeled a company-sized unmanned air resupply system to haul 10,000 pounds of cargo daily over a 150 nautical mile round trip and deliver sling loads within three meters of a set point.
None of the keyboard-programmed rotorcraft has a pilot in the loop.
“We can tell the aircraft what altitude, airspeed, and heading. I can’t tell it what power to apply or what angle of bank to fly,” explained Capt. Tim Dunigan, a helicopter pilot and the Naval Air Systems Command (NAVAIR) Fire Scout program manager. Fire Scout autonomous flight controls down-link 291 parameters on air vehicle status and health, but deployment feedback suggested they shouldn’t tell the operator too much. Dunigan acknowledged, “The engineers loved it, all of the data, but the Air Vehicle Operator (AVO), who is a fleet guy, just needs enough to be sure he can operate the aircraft.”
The T-Hawk Micro Air Vehicle (MAV) gives soldiers a backpack-portable UAS able to downlink imagery up to 10 kilometers away. The ducted fan MAV takes off vertically and tilts to sustain cruising lift on the lip of its composite duct. The fixed-pitch fan in the duct slows down as the vehicle transitions from takeoff to cruise, and moving thrust vanes steer the vehicle. Actuators from the medical industry provide the necessary control speed and bandwidth.
The Defense Advanced Research Projects Agency (DARPA) chose Honeywell to develop a MAV in 2003 for an Advanced Concept Technology Demonstration, but the compact ducted fan that could hover amid trees or buildings was inherently unstable. Stable flight required sophisticated control functions and Mission Adaptive Control Hierarchy (MACH) software integrated in Honeywell’s micro-electromechanical systems (MEMS) micrometer-sized machinery.
T-Hawk program manager Vaughn Fulton summarized, “What MEMS brought was the ability to run sophisticated flight controls and fully autonomous behaviors on a set of hardware small enough to meet the demands of a less-than 16-pound aircraft with the control and pointing accuracies envisioned for T-Hawk by the U.S. military.”
T-Hawks are integrated at the Honeywell Defense and Space facility in Albuquerque, N.M. Honeywell makes the T-Hawk Flight Management Unit (FMU) or flight computer, Inertial Measurement Unit (IMU), and Air Data Sensor System (ADSS), all with MEMS components from the company’s facility in Ridgeway, Minn. Together, the FMU, IMU and ADSS perform the functions of an Attitude Heading Reference System (AHRS). An off-the-shelf Rockwell Collins MPE-S GPS receiver provides navigation inputs.
Early in the MAV development, AHRS was a reversionary control mode with basic return-to-base-and-land functionality. The system can now complete missions in a GPS-denied environment. Today’s MQ-16B also has the sensor gimbal and aircraft behavior coupled, so an operator can steer the camera without regard to aircraft or gimbal orientation. The vehicle re-orients itself continuously to optimize control response and keep the gimbal away from mechanical stops.
Two MQ-8B Fire Scout Vertical Takeoff UAS (VTUAS) logged about 60 flight hours from the USS McInerney on a six-month operational deployment in the U.S. Southern Command/4th Fleet area of responsibility. The VTUAS platform was generally flown and fixed by Seahawk pilots and maintainers from squadron HSL-42, backed by Northrop Grumman representatives.
“We wanted the maintainers and operators on the Navy side to use it as they saw fit,” said Dunigan. “This was the first time people not connected with the program got a really good look at it.”
Fire Scout emerged from a 1999 Navy requirement. The first prototype built by Northrop Grumman around a Schweizer Aircraft light turbine helicopter demonstrated autonomous control but crashed in 2000. The follow-on RQ-8A with redundant electronics and more robust software first made hands-off ship landings in 2006.
Northrop Grumman Fire Scout Chief Engineer Bob Navarro observed, “It is a bit tough to control autonomously. The aircraft has to be capable of maintaining a hover, very low speed flight, where airspeed and angle of attack and sideslip sensors are not necessarily valuable.”
Fire Scout engineers developed flight controls on a manned testbed. “We have adapted all of the flight control inputs that a manned pilot would use for generating those hover and low-speed maneuver commands manually,” Navarro said. “We translated all of those into Guidance, Navigation and Control flight commands. That was the big challenge.”
The Fire Scout references the ship navigation system to fly its programmed mission. “We’re always pretty much in relative navigation mode between the aircraft and the ship,” said Navarro.
A recovery course brings the helicopter one-half to three-fourths nautical mile behind the ship to acquire the Sierra Nevada Unmanned Common Automatic Recovery System (UCARS). A transponder helps shipboard radar determine aircraft position, and a Recovery Data Link carries precise distance and slant range data between Fire Scout and moving ship. Ten to 15 feet from landing, the GE Intelligent Platforms vehicle management computer (VMC) begins to mimic ship motion to set the helicopter gently on its deck trap.
With far less dynamic interface testing and shipboard history than the manned Seahawk, the Fire Scout has a smaller launch and recovery envelope. VTUAS takeoffs and landings are conservatively limited to 2 degrees pitch and 5 degrees roll, but the envelope should expand with more deployments, according to Dunigan.
The early RQ-8A flight control computer with Intel-based processor was designed by Northrop Grumman. Today’s more powerful MQ-8B uses Power PC-based dual redundant VMCs made by GE Intelligent Platforms in Albuquerque. Fire Scout systems are integrated at the Northrop Grumman Unmanned Systems Center in Moss Point, Miss. Air vehicles are still built at the Sikorsky Schweizer plant in Horsehead, N.Y.
The RQ-8A and MQ-8B use the same Kearfott Corp. dual redundant AHRS INS/GPS for pitch, roll, heading and navigation references. Fire Scout air data sensors come from Honeywell and connect to the VMC via a serial interface. A Control Area Network interfaces the computer with flight control actuators and with the electrical power controls.
The Fire Scout VMC also has a Mil-Std-1553 interface for three Rockwell Collins ARC 210 radios. Though ARC-210 links are used to launch and recover the vehicle, a Tactical Common Data Link (TCDL) from L-3 Communications West provides bandwidth to control both the air vehicle and payload and carry real-time video to the GCS. The deployed MQ-8B had a FLIR Systems Brite Star II electro-optical/infrared payload and Automatic Identification System (AIS) to interrogate cooperative ships.
Last December, the MQ-8B demonstrated lost communications capability at sea. A return-to-base command loiters the helicopter, so the ship can place itself in the best position to re-establish the command link.
“The aircraft did exactly what it was supposed to do,” noted Dunigan.
Northrop Grumman, meanwhile, intends to adapt Fire Scout controls to the 6,000-pound Bell 407 helicopter and offer the resulting Fire-X UAS for Navy missions.
The next MQ-8B deployment is scheduled for January 2011 aboard the USS Halyburton to U.S. Central Command and the combat theater. Future Fire Scout detachments will probably stock assembled VMCs rather than individual circuit cards. Dual-trained Seahawk pilots may give way to enlisted Air Vehicle Operators.
“The H-60 pilot is kind of overkill for that kind of job,” said Dunigan.
To take people out of convoys and cockpits in Afghanistan, the Marine Corps Warfighting Laboratory at Quantico, Va., sponsored a non-competitive Immediate Cargo UAS demonstration at Dugway Proving Grounds.
“We just wanted to get an idea of what was out there and how it could be put into theater,” said project officer Capt. Amanda Mowry.
The Boeing Hummingbird with Optimum Speed Rotor and Kaman/Lockheed Martin K-MAX with intermeshing rotors each demonstrated high-altitude performance for mountain landing zones. Each moved 2,500 pounds within six hours over a 150 nm round trip, sustained beyond-line-of-sight communications, and recovered automatically from a simulated loss of communications.
The Boeing A160T Hummingbird was designed for ISR, but has carried a 1,000-pound cargo pod 600 nm. Changing rotor speed with airspeed, altitude, weight and load factor increases lift-to-drag ratios and extends endurance.
“There’s a lot that goes into that decision,” said A160 Deputy Program Manager Mike Lavorando. “Right now, it is manually controlled. We are automating that control. You’ll be able to select best endurance, best range, and the aircraft changes rotor speed based on its weight, altitude, flight condition.”
Frontier Systems in Irvine, Calif., now part of Boeing Advanced Rotorcraft, flew a piston-engined Hummingbird under DARPA contract in 2002. The digital fly-by-wire flight controls in today’s turbine A160T grew out of analog work done by DARPA and the Army Aviation Applied Technology Directorate.
“We started out in the Frontier days proving out the flight controls on a Robinson R-22 we called the Maverick,” recalled Lavorando. “That legacy still remains. We’ve certainly made improvements.”
Hummingbirds built in Mesa, Ariz., have redundant flight control computers and INS/GPS navigators. Production vehicles will have redundant actuators, Lavorando said.
The production Hummingbird leverages its research instrumentation. “We’ve retained a flight-test level of sensors on our production aircraft,” Lavorando said. “It’s an unmanned air vehicle, and the more sensor information you have and the more redundant information you have, the more control you have.”
At Dugway, the A160T showed it could place loads within 3 meters of a designated point consistently. Boeing built a portable GCS based on the Enhanced Position/Location Reporting System (EPLRS) radio that enabled a ground operator to change the delivery point using the view from nose and belly cameras.
The Hummingbird currently uses a Boeing proprietary command uplink but will adopt a min-TCDL in production. Engineers re-routed the A160T in flight via satcom and simulated a lost-link return to base.
“The aircraft has to know how to get back and land,” said Lavorando. “Because you’re never 100 percent sure the load is going to release, the aircraft has to assume the load is still attached … It knows how long the long-line is. It does a backward stair-step pattern like it’s still attached to the load and lands like that.”
Designed for sling load work, Kaman’s single-seat K-MAX first flew autonomous cargo demonstrations with a safety pilot aboard in 1999 for the Marines’ Broad-area Unmanned Responsive Resupply Operations (BURRO) effort and follow-on Army demonstrations.
BURRO introduced a cockpit floor pallet with control actuators from Huey target drones working K-MAX cyclic, collective and foot pedal linkages. A Hamilton Sundstrand flight control computer from the SH-2G(A) naval helicopter ran UAV software.
Kaman teamed with Lockheed Martin Systems Integration in Owego, N.Y., in 2007 to market the cargo UAS. For the Dugway demonstration, Lockheed Martin provided software fixes and a concept of operations, and Kaman integrated redundant flight controls and more responsive actuators. Work-ups for the demonstration saw the first K-MAX flights without a safety pilot.
“Up until that point, everything was single-string,” said Kaman UAS General Manager Terry Fogarty.
At Dugway, the unmanned helicopter with cargo carrousel delivered four 750-pound loads to different locations. The first three loads were delivered automatically, the last under manual control from a TCDL station.
An anticipated NAVAIR competition for a combat resupply UAS makes details of K-MAX and Hummingbird controls proprietary. However, Kaman has announced a contract from the Army Research, Development and Engineering Center in Huntsville, Ala., to give the unmanned K-MAX redundant flight control actuators.