Researchers have designed a new type of wing that could make small, fixed-wing drones far more stable and efficient. The new wing replaces the smooth contour found on the leading edges of most airplane wings with a thick, flat plate and a sharp leading edge. The design has distinct aerodynamic advantages at the scale of small drones. The new wing is far more stable than standard wings in the face of sudden wind gusts and other types of turbulence, which often wreak havoc on small aircraft. The wing also provides an aerodynamically efficient flight that translates into better battery life and longer flight times.
The idea for a wing that dispenses with the smooth contours of a normal wing's leading edge was inspired by natural flyers like birds and insects. A smooth leading edge helps to keep airflow firmly attached to the wing. But bird and insect wings have usually quite rough and sharp leading edges to promote separation of the airflow. Flow separation causes efficiency problems for large aircraft but it seems to work just fine for birds and insects.
The new wing — called the Separated Flow Airfoil — intentionally separates the flow at the leading edge, which somewhat counterintuitively causes the flow to reattach more consistently before reaching the trailing edge. That reattachment is aided by a small, rounded flap placed near the wing's trailing edge. The design enables more efficient, more stable flight at the scale of aircraft with wingspans of about a foot or less.
The reason the design works has to do with the characteristics at small scales of the boundary layer — the thin layer of air that's directly in contact with the wing. At the scale of passenger planes, the boundary layer is always turbulent, full of tiny swirls and vortices. That turbulence holds the boundary layer against the wing, keeping it firmly attached. At small scales, however, the boundary layer tends to be laminar. A laminar boundary layer separates easily from the wing and often never reattaches, which leads to increased drag and reduced lift.
Further complicating matters is the freestream turbulence — gusts of wind, vortices, and other disturbances in the surrounding air — that can suddenly induce turbulence in a boundary layer, which attaches the flow and induces a sudden jolt of increased lift. Rapid lift fluctuations can be more than a drone's control system can handle, leading to unstable flight. By separating the flow at the leading edge, it immediately becomes turbulent, which forces it to reattach at a consistent point regardless of atmospheric turbulence, providing more consistent lift and overall better performance.
Testing of the Separated Flow Airfoil in a wind tunnel showed that the design successfully smoothed out lift fluctuations associated with freestream turbulence. The team also performed wind tunnel tests of a small, propeller-driven drone equipped with the Separated Flow wing. Those tests showed that the increased aerodynamic efficiency resulted in a decreased minimum cruise power compared to standard miniature drones. That translates into extended battery life.
The Separated Flow wing can be far thicker than wings normally used in small drones. That makes the wings structurally stronger so subsystems like batteries, antennas, or solar panels can be integrated into the wing. That could reduce the size of an aerodynamically cumbersome fuselage — or eliminate the need for one altogether.