Boeing is poised to attempt a brace of world record endurance flights with its A160T Hummingbird unmanned air vehicle after installing new safeguards to prevent a flight control system failure which led to the loss of a prototype last December.

The accident put a three-month hold on an already aggressive test and demonstration schedule earmarked for the A160T through the rest of 2008. Yet Boeing remains confident it can meet its schedules, as well as set records for rotary UAV payload and endurance that it claims others will find difficult to match.

The record attempt flights will include a hover out of ground effect at 15,000 ft. and an 18-20-hr. flight with a 300.-lb payload. Together they form the final milestones of the Phase 1 demonstration which began in August 2003. Supported by the Defense Advanced Research Projects Agency (Darpa), the tests are intended to prove that a purpose-built, clean-sheet large vertical takeoff and landing unmanned air system (VUAS) can truly go the distance compared to other rotary UAVs that are generally derived from existing manned helicopters.

"We think we've got something different here," says Boeing Advanced Systems' business development director, Grady Eakin. "The range, endurance and payload are unique for a rotary-wing UAV, and we think it can provide a variety of missions all at the same time. We've proved we can get there quickly, stay a long time and fly to places that commanders think are important," he adds.

Although the A160T is aimed squarely at standard UAV roles such as reconnaissance, surveillance, communications relay, resupply and target acquisition, Boeing says the broader capabilities of the turbine-powered helicopter make it capable of much more. One of the initial test vehicles has been mocked-up with stub wings to carry up to eight AGM-114 Hellfire air-to-ground missiles, while another has flown with an aerodynamically-shaped pod large enough to evacuate a wounded soldier or transport a small robotic vehicle.

With Northrop Grumman's MQ-8B Fire Scout already destined for major VUAS roles with the U.S. Navy and Army, Boeing is seeking new niche opportunities for the A160T beginning with the U.S. Special Operations Command (Socom). Initial operational evaluations of the MQ-8B, a modified Schweizer 333, are planned for 2008, while first flight of the Army's MQ-8B variant is slated for the end of 2010, with initial operating capability scheduled for 2014. Demonstrations are also planned to the Navy. Operational tests of the A160T could, by contrast, begin within the next year or so, says Boeing.

Part of the challenge, says Eakin, is making potential users aware of the A160T's capabilities. "There [are] a variety of military and government users that haven't thought yet about how far this has flown and what it can do for them. We've just recently talked to a couple of potential customers and they are surprised that we can carry a couple of payloads, and fly far away from their basing scheme," he adds.

The 35 ft.-long A160T is powered by a Pratt & Whitney Canada PW207D turboshaft driving a 36-ft.-dia., four-blade rotor. The blades, like the fuselage itself, are made from lightweight carbon fiber composites, while the streamlined fuselage shell is designed for both low drag and reduced radar cross-section.

"It's significantly larger than any other VTOL [UAV], but it is significantly lighter as well," says Eakin. "We have a fairly high fuel fraction of more than 50%, which is slightly higher than other UAVs and manned helicopters." Empty weight is 2,500 lb. and the helicopter carries 2,600 lb. of fuel in large tanks clustered around the center of gravity. The forward tank, mounted just ahead of the mostly internally housed rotor mast, occupies almost the whole depth of the fuselage, while a second large tank is sandwiched beneath the engine and transmission housing and the bay in the belly for the retractable gear. Maximum takeoff weight is 6,500 lb. while the largest payload carried to date is around 1,090 lb.

However, it is the patented Optimum-Speed Rotor (OSR) concept and its accompanying two-speed transmission that Boeing believes represent the game-changing aspects of the A160T design in terms of performance and low noise. In conventional helicopters, the RPM of the rotors is normally set for a maximum forward speed at a maximum weight at a certain critical altitude. At maximum forward speed, the tip of the advancing blade is traveling at speeds just below Mach 1, which avoids the drag, vibration and noise encountered at higher transonic speeds. But this means that at any other flight conditions, particularly low-speed forward flight, the rotor RPM is higher than needed, creating additional drag and consuming extra fuel.

Several ways of getting around this conundrum have been developed successfully and tried in a few vehicles. These include the Lockheed XH-51A compound helicopter, which used both a fixed-wing and propulsive engine to enable rotor RPM to be reduced, and the more recent Bell-Boeing V-22, which tilts the rotor itself.

Other attempts have been made to improve helicopter maximum forward speeds by using two-speed gearboxes that allow the rotor to rotate at two RPM values while maintaining a constant engine RPM. However, Boeing believes the OSR goes beyond these attempts because it allows the RPM to be reduced for lower-speed flight conditions, substantially cutting fuel consumption and therefore increasing range and/or payload.

The key breakthrough in the OSR concept is the design and construction of the A160T's rigid, but extremely light composite rotors. These overcome the structural dynamics problems normally associated with significant changes of rotor RPM by having a continuously decreasing flap, lag and torsion stiffness, as well as mass, from the root to the tip. Weighing around 52 lb., each blade is constructed of a carbon-epoxy spar/shank and a leading edge made from the same material. The trailing edge is a lightweight section made of thin carbon-epoxy top and bottom skins and a full-depth honeycomb core. The blades are mounted in a steel, hingeless rotor system and move only in pitch through a beefed-up bearing system designed to resist moments that are substantially greater than those for an articulated rotor system.

Due to the blade stiffness and lightness, the OSR is able to operate over a wide RPM range at full rotor lift load, close to the rotor excitation frequencies. This is possible because of the rigidity of the blades in relation to the feathering or pitch axis.

Early analysis, conducted for the A160T's original developer, Calfornia-based Frontier Systems, suggested the potential for dramatic performance gains. Much of this has been proven during flight tests by Boeing, which acquired Frontier in 2004. On one flight in October 2007, an A160T flew for 12.1 hr. carrying a 500-lb. payload at an altitude of 5,000 ft., simulating a multisensor operational mission. When the helicopter landed, less than 60% of the maximum fuel had been consumed.

The reduction in required power levels for specific speeds and payloads also decreases the speed of the rotor and thereby cuts noise. The reduction in tip Mach number, to about 40-50% in some cases, of the advancing blade cuts rotor noise level by several dB. Flyover measurement tests show the A160T is roughly four times quieter than a Bell 407, further adding to its military utility.

The design has evolved through several iterations, the latest of which has seen an engine change from a six-cylinder Subaru automotive engine to the turbine. This also necessitated a switch to a two-speed transmission to help the turbine deal with the speed changes. Developed by Boeing's Philadelphia rotorcraft technology center, the gear uses electrically actuated carbon-carbon clutches to control a 2:1 sun-and-planet gear system and is designed for inflight changes. To date, all testing has been conducted in low gear; high gear tests are set for later this year. There is currently no target date for trials of the inflight switching feature.

First priority for the A160T test team is to evaluate the revised flight control system devised as a short-term fix to enable testing to resume, and a longer-term fix to eliminate any chances of a repeat accident caused by single-point control system failure scenarios.

"We're making short-term changes to try and improve the robustness of the current configuration within the limits of the architecture we have," says A160T Program Manager Jim Martin, who explains this includes the addition of "automated tools and diagnostic software." Longer-term solutions, still being defined, will include a modified flight control system architecture with increased levels of system redundancy.

Boeing's accident investigation board determined the accident occurred when sensor data in the flight computer stopped being updated in mid-flight. As a result, sensor feedback to the control system failed and "it departed controlled flight and impacted the ground at a near-vertical angle," says Eakin. "The flight computer was not receiving updates for some reason and unfortunately, post-accident, the avionics box was destroyed, so that forensic evidence is not available."

Boeing investigators have "revealed a number of areas that potentially could cause this processing thread to either stop receiving information or to stop processing information. However, because of a lack of forensic information we've been attacking all of those areas," says Martin. "The good news is we were able to recreate the aircraft response we saw by freezing this processing thread."

Once the Phase 1 milestone flights are completed in civil airspace around Boeing's flight test site at Victorville, Calif., demonstrations involving various advanced sensors for Darpa are planned to be conducted in restricted airspace. These are expected to begin with the agency's Forester foliage-penetration radar and later move on to the Argus-IS (autonomous real-time ground ubiquitous surveillance imaging system) wide-area video sensor.

Another projected Darpa-A160T payload is the Adaptive Conformal ESA Radar (Aacer). The agency adds that, "although the Aacer objective system is designed for operation on board the A160, the current plan is to conduct demonstration flight tests using a U.S. Army Black Hawk helicopter. Those flight tests are planned for this summer."

Other tasks will include demonstrations of a 1,000-lb.-payload cargo pod for Socom, plus Hellfire missile firings. Despite the holdup caused by the accident, Eakin says "all of the demos we'd planned are still on track, and there are no demonstrations delayed out of this year."

Developed by Darpa's Information Exploitation Office, the Forester (Foliage Penetration Reconnaissance, Surveillance, Tracking and Engagement Radar) is due to enter tests sometime in July or August under the revised schedule. Already tested in prototype form beneath a UH-60 Black Hawk, the large 21.5-ft.-long UHF synthetic aperture radar sensor is designed to detect and track moving dismounted soldiers and vehicles at a range of 20 mi. The ground moving target indicator element of the radar is designed to track people at a range of at least 30 mi.

Suspended beneath a hovering A160T, the sensor is supposed to provide standoff and persistent wide-area surveillance, and both warn of actual ambushes and identify possible sites. Darpa says a 90-deg. total scan from a typical A160T hovering at 20,000-ft. will, for example, cover a 155-mi. area at an update rate of once per 20-80 sec. The agency adds that on the A160T specifically, it will "provide detection of dismounts under foliage ranges out to more than 12 mi."

Working with prime contractor Syracuse Research Corp., Boeing and the Army have done fit checks for ground clearance for takeoff and landing. Ground tests of the transmitter, antenna and software for the radar have already been successfully demonstrated, says Darpa.

"The Forester is a rather large payload, is relatively heavy and has to be carried on a rotary-wing platform," says Eakin, who adds the A160T's retractable gear is a key enabler. "We have to be able to carry it an angle and if we had skids [instead of retractable legs] it wouldn't be a good fit for us."

Argus-IS tests on the A160T are scheduled "for the first half of calendar year 2010," says Darpa. Argus-IS consists of steerable sensor "beams" to simultaneously enable real-time monitoring and tracking. Carried in a 500-lb. pod, the stabilized gigapixel sensor will image an area with a radius of 1.6 naut. mi. from 13,000 ft. with a beam resolution of around 0.5 ft. and frame rate of at least 10 Hz. The sensor will provide a field of view of at least 45 deg., possibly through the use of four commercial complementary metal-oxide-semiconductor (CMOS) high-density focal plane arrays sharing a single set of optics and forming a mosaic image.

Socom plans to evaluate the A160T for a range of missions, including carrying payloads and sensors for long-range electro-optics and 3D laser radar. Future roles could even include "psych-ops" missions such as radio and television broadcasting and leaflet drops. Boeing plans to signal its intent to participate in these future Socom tests by displaying an all-black-painted A160T in the static park at this year's Farnborough air show.