Special Forces Get Stealth Robocopter

By David Hambling
November 21, 2008 | 12:22:17 PM
Categories: Copters, Drones, Planes, Copters, Blimps
Courtesy Of Wired Blog Network

This month, U.S. Special Forces Command is quietly taking delivery of a radical new drone: the Boeing A160T Hummingbird, which rewrites the rules for helicopters. Thanks to a remarkable piece of design, the Hummingbird can go further, longer, higher – and quieter – than anything else around.

According to Jane's, 10 Hummingbirds are supposed to be delivered this month, under a joint SOCOM-DARPA program known as the Special Operations Long Endurance Demonstration (SLED).

"The Hummingbird is designed to fly 2,500 nautical miles with endurance in excess of 24 hours and a payload of more than 300 pounds. The autonomously-flown A160 is 35 feet long with a 36-foot rotor diameter," according to Hummingbird-maker Boeing's rather brief entry on the craft. "It will fly at an estimated top speed of 140 knots at ceilings up to 30,000 feet, which is about 10,000 feet higher than conventional helicopters can fly today.

This impressive performance is achieved by reworking traditional helicopter design. Normal helicopters work at a fixed rate of rotor revolutions per minute. Rotors are flexible, articulated and have a complex pattern of vibration; changing speed would cause potentially dangerous vibration. In addition, the rotor speed is generally as high as possible; this is an advantage when the helicopter is moving fast, so that the retreating blade still provides some lift. As a result, normal helicopters are noisy (sound is directly related to rotor speed) and wasteful, as most of the time the high rotor speed is not essential.

The key feature of the A160 is its redesigned rotor blade:

To avoid vibration problems, the rotor blades are light and stiff, and their stiffness in flap, lag and torsion is progressively reduced from root to tip, so that the tips are more flexible than the root. This is made possible by the use of tailored carbon fiber construction. The A160 rotor is hingeless and rigid, and has a larger diameter and lower disc loading than a conventional helicopter rotor with the same maximum lift.

This allows the A160 to operate at a wide range of different RPM -- from a high rate for maximum speed, to a very slow and quiet mode that is highly fuel-efficient. The early prototypes were fitted with a modified Subaru automotive engine (!). Later versions had the Pratt & Whitney PW207D turboshaft, but a more efficient engine may be in the works.

The 24-hour endurance mentioned in Boeing's website is only the beginning: some developers have talked about 48-hour unrefueled flights – and a service ceiling going all the way up to 55,000 feet. Earlier this year an A160 was reported to have hovered out of ground effect at 20,000 feet.

So what are the new Special Forces birds going to be used for? According the Jane's, the Hummingbirds are expected to be fitted with a new ground surveillance radar called FORESTER (Foliage Penetration Reconnaissance, Surveillance, Tracking and Engagement Radar). This is another DARPA project which the developers say will "provide robust, wide-area, all-weather, persistent stand-off coverage of moving vehicles and dismounted troops under foliage."

The Hummingbird also took part in exercises at Fort Dix earlier this year demonstrating its networking capabilities:

For the first time, Boeing's A160T Hummingbird UAV used an electro-optical/infrared sensor to increase ISR [Intelligence, Surveillance Reconnaissance ] and situational awareness for other airborne and ground assets while bridging and strengthening air/ground communications networks.

The idea seems to be for the Hummingbird to quietly locate and pinpoint opposing forces, beaming back information while also acting as an airborne communications relay.

Most of this can be carried out well enough by fixed-wing drones – though I suspect the Hummingbird is quieter than most. What is not mentioned is the possibility of an armed version; while a Hellfire might be too much to carry, there are now plenty of lightweight missiles tailored to small UAVs. Small glide bombs, such as the 35-pound Viper Strike, could give significant stand-off range, and would not compromise stealth.

The real advantage of a rotorcraft – apart from being able to sneak around the treeline, quiet and unseen – is vertical takeoff and landing, This will make it useful for the other role mentioned by Boeing, precision re-supply. Three hundred pounds is quite a useful load for a Special Forces team; it's also enough for the Hummingbird to be used for emergency casualty evacuation (though it is not rated for this – yet).

All in all, the A160T Hummingbird should be quite an asset; no doubt Danger Room readers will have their own ideas about the sort of missions it could be used for. It will be interesting to see how quickly the other services start buying their own Hummingbirds.

(Picture: Boeing)