Small robotic spy-planes have been developed that use shape-shifting wings to switch from being stable gliders to ultra-manoeuvrable fliers.
The articulated wings – with a span of 60 centimetres – were inspired by the way seagulls alter their wing-shape during flight, says Rick Lind, an aerospace engineer at the University of Florida, in Gainesville, US.
The robot plane, or drone, has a joint halfway along the leading edge of both its wings. Actuators at this “elbow” joint and at the “shoulder” joint of each wing, where it connects to the fuselage, allow the wing structure to shift from an “M” to a “W” configuration, as viewed from the front or rear.
The tiny surveillance drone mimics the changing wing shape used by seagulls to switch between agile and stable configurations (Image: Kristen Bartlett/University of Florida)
With the elbow lowered, the “W” configuration produces a highly manoeuvrable aerodynamic shape, says Lind. In contrast, the high elbow “M” shape is less manoeuvrable but highly stable, perfect for gliding and conserving battery power.
The change in flight performance comes partly from a shift in the aircraft’s centre of gravity, says Lind: “With the wing position higher than the centre of gravity it is stable like a pendulum.” Reversing this – the "W" position – produces low stability, like an inverted pendulum. See a movie of the craft showing off its different in-flight configurations, here (Mpeg format).
Rolling, rolling, rolling
It is unlikely that this design would scale up to larger planes because the strength required of the structures would make the wings too heavy, says Lind.
The airflow characteristics at the low speed small planes fly at are very different to those at high speed and not very well understood, he adds. That is one reason his team is investigating novel designs.
It is difficult to quantify how manoeuvrable the new drones are. But during flight tests they have been capable of performing three continuous 360° rolls in 1 second. F-16 fighter jets can carry out one roll per second but have safeguards to prevent more than this in case the pilot passes out through g-force effects. But even without these safeguards Lind, a former NASA engineer, doubts F-16 could match his drones’ performance.
The drones are being developed for use in an urban landscape. Lind sees his planes being used for surveillance and security purposes. They could carry sensors to the location of a suspected bioterrorism attack, for example.
The current prototype already has basic autopilot capabilities that allow it navigate through six different waypoints under its own control. But this is with the wings fixed in a single configuration. “The challenge now is to develop autopilot software that makes use of this enhanced agility,” says Lind.