The world's first successful flight of a self-powered, rudderless, flapping aircraft has been achieved by engineers from AeroVironment.
The NAV, or nano air vehicle, operates by using two flapping wings, which also function as the rudder, elevators, ailerons and engine. With its two wings, the NAV is able to hover, move forward and backwards, and change its elevation. In flight, the NAV almost appears to replicate the movements of a hummingbird.
Simplified, most airplanes and helicopters today fly using the same principle: Air rushing over a shaped and immovable wing (or helicopter blade) produces an area of high pressure below the wing and an area of low pressure. The difference pushes the wing up in the direction of low pressure, creating lift.
An airplane is steered using adjustable flaps on the ends of the wings and at the tail, which also help steady the aircraft.
"It's extremely complicated and technically challenging to come up with ways to control an aircraft with two flapping wings,"said Matt Keennon of AeroVironment, "but this is the closest anyone has come to a rudderless, flapping aircraft."
AeroVironment wouldn't explain how the two wings accomplish flight, citing its contract with the Defense Advanced Research Projects Agency (DARPA).
However, Guido de Croon and Rick Ruijsink at Delft University of Technology in the Netherlands have their own theory about how AeroVironment's NAV flies. The two men, along with their colleagues at Delft University, have created three flapping-wing aircraft of their own, known as Delfly I, II and Micro.
The Delflys are flapping-wing aircraft, but differ significantly from AeroVironment's NAV. The Delflys have four flapping wings and a rudder, which in theory makes it easier to control than the rudderless, two-winged NAV.
According to Ruijsink and de Croon, the shape of the wings isn't quite as important for flapping-wing aircraft as it is for fixed-wing aircraft. While a fixed-wing uses a smooth flow of air over the wings to create lift, a flapping-wing separates the air into two flows.
The two European researchers also suspect that AeroVironment changes the wing's angle to move NAV forwards or backwards. Tipping the lead edge up would move the airflows to the rear of the wing, moving the aircraft backwards. To move forward, the lead edge of the wing likely tips down, moving the airflows, and the craft, forward.
Achieving flight with a rudderless, flapping-wing aircraft is certainly an accomplishment, but like any DARPA-funded project, the defense agency asks for more. Possible applications for this small, maneuverable aircraft could include spying on enemies or finding victims in a natural disaster.
According to a new contract DARPA recently granted AeroVironment, the final NAV should weigh 10 grams or less, have a wingspan of about three inches, withstand winds of eight feet per second and fly inside buildings over a half-mile away.
"The challenge is definitely in making it smaller," said de Croon. "The transmitter, steering, receiver, motor and wings all have to become smaller, making it more difficult to achieve lift."
AeroVironment wouldn't comment on their future efforts other than that they want to meet DARPA's goals. The team from the Netherlands has shrunk their flapping-wing aircraft into the Delfly Micro, but it has still faces its own challenges.
Creating a tiny flapping-wing aircraft that meets DARPA's specifications will be difficult, says AeroVironment's Keennon. "It's a combination of mechanical, aerodynamics (and) software, and they all have to be solved together."
