Street fighting planes

Funded by the US Air Force and NASA, University of Florida aerospace engineers have built prototypes of six inch to two foot drones capable of tight flight manoeuvres in urban environments. Their secret is seagull-inspired wings that morph, or change shape, during flight, transforming the planes’ stability and agility at the touch of a button on the operator’s remote control.

“If you fly in the urban canyon, through alleys, around parking garages and between buildings, you need to do sharp turns, spins and dives,” said Rick Lind, a UF assistant professor of mechanical and aerospace engineering who heads the project. “That means you need to change the shape of the aircraft during flight.”

The Air Force’s Predator Unmanned Aerial Vehicle and other military drones have been key to military operations in Iraq and Afghanistan. But the drones, which shoot surveillance images and sometimes also fire missiles, are designed to soar high above the landscape, limiting their ability to snoop up close in the windows, alleys, corners and other urban crevices of the tight neighbourhoods that define many cities, Lind said.

The UF planes are intended to correct this deficiency and add new capabilities, such as landing in tight spots during a mission. That could be useful, for example, if the planes, equipped with sensors for biological or chemical weapons, were investigating single buildings where the weapons were suspected of being made, Lind said.

Lind came to UF in 2001 from NASA, where one of his last projects involved modifying the wings of an F-18 fighter to change shape during flight. He drew on this research for the drones, but he also had another source of inspiration: the Wright brothers’ first plane. As Lind noted, unlike later planes, the wings of that biplane had no flaps, or ailerons. Instead, the brothers controlled the plane’s roll by using cables to twist the shape of the wings up and down during flight. Birds also change wing shape.

“Birds morph all the time, and they’re very agile,” Lind said. “There’s no reason we can’t achieve the same control that birds achieve.”

The first prototype in the three-year-old UF effort copied the Wright Brothers’ approach by using tiny motors to twist threads and move flexible wings. A traditional rudder and elevators on the tail, meanwhile, control up-and-down and side-to-side motions.

The downfall of the thread approach was that the wings could only be pulled down, not pushed up, which limited their capabilities. The next version replaced the threads with metal rods, allowing both up and down motion and improving performance.

The latest version, built by mechanical and aerospace engineering doctoral student Mujahid Abdulrahim, goes a step further. Impressed by seagulls’ ability to hover, dive and climb rapidly, Abdulrahim photographed the gulls close-up during flight. The images showed the gulls’ wings flexing at both their shoulder and elbow joints as they altered flight patterns.

Abdulrahim added this ability in the new prototype, with promising results. With the wings mimicking the gulls’ elbow in the down position, the plane loses stability but becomes highly manoeuvrable. With the wings in the elbow straight position, it glides well. And with the wings in the elbow up position, it’s highly controllable and easy to land.

Motors can transform the wings from the down to the up position in flight in 12 seconds, “fast enough to use in a city landscape,” Abdelrahim said.

The bird-like prototypes are said to be strikingly manoeuvrable, capable, for example, of completing three, 360-degree rolls in one second. An F-16 fighter jet can manage at least one roll per second, but three rolls would produce excessive gravity force, killing the pilot.

Flying in videotaped demonstrations, they are so agile they appear out of control at times, and indeed the planes require considerable talent by the remote control pilot. The Air Force and NASA have so far provided about $3 million for the UF research, a substantial portion of which is aimed at addressing that issue by making the planes easier to fly.