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Flying Eye in the Sky

March 2007
By Robert K. Ackerman
E-mail About the Author

Scientists at the U.S. Air Force Research Laboratory test a composite radar antenna that can serve as the skin of an aircraft. Breakthroughs in low- and high-band radar antenna technology are clearing the way for aircraft exteriors built largely of sensors.
Radar advances clear the way for long-duration sensor aircraft.

The next new aircraft to roll out of the U.S. Air Force hangar may be a powered sensor. Scientists at the U.S. Air Force Research Laboratory at Wright-Patterson Air Force Base, Ohio, have developed radar arrays that can serve as aircraft skin and as structural components. Their research is opening up entirely new radar capabilities as well as materials advances.

This work is emerging from the SensorCraft program, which aims to produce a high-altitude unmanned air vehicle (UAV) that could loiter over an area of interest for 40 to 50 hours (SIGNAL, February 2001). While this program is geared toward producing a vehicle in the next decade, some of the advances could be implemented in existing aircraft well before delivery of the specialized surveillance and reconnaissance vehicle.

The key to the SensorCraft lies in its skin. Instead of being an aircraft equipped with advanced sensors, the vehicle would be an aircraft built out of advanced sensors. John Perdzock, SensorCraft lead in the Air Vehicles Directorate of the U.S. Air Force Research Laboratory, explains that the program’s radar research largely has focused on breakthroughs in two areas: low-band and high-band antennas. An original aircraft design featured a joint wing configuration that resembled two letter “Vs” attached at the ends. This approach was favored for placing radar antennas on the outside of the aircraft so that the radar could scan completely around the aircraft’s exterior.

But scientists at the laboratory now are exploring a concept known as Endfire for a low-band radar element to perform 360-degree scanning. Perdzock explains that a traditional flat-panel radar antenna emits a signal perpendicular to its surface. Endfire technology would emit a radar signal parallel to the plane of the surface.

That radar energy can be emitted from all four sides of the flat panel, Perdzock continues. The signal is not continuous, but the technology can scan around the four sides. So, designers can build larger electronically scanned arrays out of multiple Endfire elements. This antenna would be embedded in the surface of the aircraft’s wing, and the energy would come out of the front or the back of the wing.

This new capability allows engineers to look at a totally different airframe design. Instead of the joint wing, the aircraft could have a more conventional swept wing configuration similar to that of the B-2.

Electrical testing has confirmed that the Endfire element will scan 360 degrees around the array. Engineers have built several different configurations of this technology, including a square array five elements x five elements.

Another array under construction would be up to 20 feet long x 10 feet wide. This rectangular array would be more consistent with the type of antenna that would form a wing, Perdzock observes. This large construct will undergo electrical testing to validate its antenna performance as well as structural testing to determine its capability as a wing component. These tests, beginning with the electrical assessment, should start later this year, Perdzock notes.

Similar work characterizes the high-band array, which operates in the X-band. Perdzock recounts that engineers have built arrays in a solid form that constitutes a load-bearing structure. The transmit/receiver part of the radar is bonded to that load-bearing antenna using conventional composite bonding technology. The result is a thin, compact X-band array that can go into the skin of an aircraft.

The large size of these antennas provides a higher gain, which in turn allows maximizing the power performance of all the aircraft’s radar antennas. Perdzock relates that scientists have developed a square-foot construct that addresses all of the array’s requirements. This one square foot of X-band radar features more than 200 elements, and a high percentage of these must work to ensure that the antenna functions properly. One of the program’s challenges has been to get a high yield out of that array, he observes.

Once that square foot has passed muster in the laboratory, scientists will build larger arrays. One array that is 3 feet x 1 foot already has been tested for primary structural load bearing. It withstood a primary load in excess of 50,000 pounds, Perdzock reports.

Another effort underway has produced a test article that is 20 feet long x 2 feet wide. This unit lacks the necessary electronics, but it features all of the antenna elements along with the bonded feed network that would be attached to the transmit/receive chips. The electronics were omitted to spare expense, Perdzock allows. Absent active radar components, this construct was tested last year successfully for withstanding primary aircraft structural loads.

The successful tests have demonstrated that this X-band antenna concept can be used in many aircraft areas, Perdzock says. These include tertiary loads such as a typical aircraft dome, secondary loads such as a weapons bay door or a fuselage door and primary loads such as a wing structure or a main fuselage panel.

“We’re quite excited about the capability structurally,” Perdzock declares. He adds that the next step is to verify the construct electrically.

The arrays are the key part of the aircraft, but the vehicle’s structure also is undergoing substantial engineering. Perdzock says that a “significant amount of activity” aims at maturing the basic airframe concepts and the related air vehicle technologies.

Beginning with the original concept of building an aircraft around a sensor package, engineers worked to determine just what would compose that package. Their work defining the low- and high-band radar constructs led to the two vehicle variants—the flying wing and the joined wing. The design of either aircraft would mandate a fairly large vehicle with a wingspan of from 150 feet to 200 feet, Perdzock points out.

A joined-wing technology demonstrator takes off at Wright-Patterson Air Force Base. This vehicle examined aircraft technologies in support of the SensorCraft program.
With that large conceptual design on paper, scientists now are identifying which technologies will be needed to realize the goals of the SensorCraft. The flying wing version, compared to the configuration of the B-2, would have a higher aspect ratio of wing to body than the bomber. Its wings would be very long, which in turn would cause the wings to flex considerably in flight. Too much flex would degrade the vehicle’s radar performance, so the aircraft’s structural mode interaction with its aerodynamic loads must be controlled.

One way of controlling these five structural modes is to stiffen the aircraft, but this runs the risk of adding weight that reduces the vehicle’s range and endurance. A research program aims to verify that designers can control those structural modes successfully. This will allow removing weight from the air vehicle.

Engineers have built a half-span model of the flying wing version. That half-span model is dynamically scaled to exhibit the same structural modes as the conceptual full-size aircraft. Tests in a transonic wind tunnel at the NASALangleyResearchCenter have stimulated the model’s structural modes. This allowed designers to determine that they could reduce the model’s structural response by a factor of about 2.5, Perdzock relates. This also will reduce the aircraft’s structural loads when it is built.

Subsequent tests will evaluate the half-span wing in a conventional pitch-rotation-plunge configuration. Whereas the wing was attached to the wind tunnel wall in the first series of tests, this testing will simulate free flight conditions.

The joined wing version, which also exhibits many of the same flexible structural mode characteristics, has been the subject of substantial analysis related to structural and aerodynamic modeling. Perdzock offers that future testing on the joined wing version will seek to achieve the same results as on the flying wing model. “We want to show that we can control all of those flexible modes in a wind tunnel,” he states. That effort is getting underway with wind tunnel tests beginning no earlier than autumn of this year.

The SensorCraft program includes a highly sophisticated sensor manager that will help avoid active sensor fratricide on the vehicle, Perdzock reports. The program is just beginning its development in concert with the laboratory’s Sensors Directorate. This device will control all of the vehicle’s sensor emissions.

Perdzock does not foresee sensor fratricide as a major problem, however. A long-duration, high-altitude UAV will not be bursting with available power. So it is not likely that the craft’s entire sensor suite would be operating at the same time. Even so, the sensor manager will be a proactive piece of the avionics architecture determining which sensor operates and when. The laboratory is beginning a research effort to mechanize the low-band/high-band operating relationship, he notes.

The program’s testing regimen has not been without trial and error. Perdzock relates how the first attempt at a five-element array was a failure. Engineers went back to the drawing board to undertake a much more rigorous buildup to design and testing. The steps have taken longer than envisioned, but progress has been steady. For example, instead of designing the entire system and then bonding it for testing, engineers have inserted many intermediate steps for testing various components before bonding.

The biggest hurdle has been understanding the nature of “this fundamentally new way of doing business” with radars, Perdzock allows. “We’ve never attempted something as ambitious as a structural X-band antenna,” he states. Once all of these components are bonded, there will be no active moving parts. He compares it to an avionics box in which circuit boards flex and bend and wires come loose. The radar skin’s composite material will have to endure even greater stresses without sacrificing functionality.

These tests, which are designed to validate the maturity of both the arrays and the structural aspects of the vehicle, are generating results that are not limited just to the SensorCraft, Perdzock notes. The flat-panel structural arrays could be adapted to existing platforms such as wide-body aircraft. Virtually any radome, bay door, fuselage door, or generic panel or wing could serve as a low- or high-band radar antenna. The laboratory already is looking at opportunities to deploy these technologies before the SensorCraft is delivered.

“The structural arrays are scalable, so you could put them in conventional aircraft—not as large as SensorCraft is envisioned—and still get some significant level of functionality,” he says.


Web Resource
U.S. Air Force Research Laboratory Air Vehicles Directorate: