A new wing design could make small fixed-wing drones more stable and efficient.
Credit: Breuer Lab/Brown University

Wing designs inspired by bird and insect wings could keep small drones stable in wind gusts and other types of turbulence. Developed by researchers from Brown University in Providence, Rhode Island, the wing replaces the smooth contour found on the leading edges of most airplane wings with a thick flat plate and a sharp leading edge. It may seem counterintuitive, but the design has aerodynamic advantages at the scale of small drones. In a paper published in Science Robotics, the researchers show that the new wing is far more stable than standard wings in turbulence that often wreak havoc on small aircraft. The wing also provides an aerodynamically efficient flight that translates into better battery life and longer flight times.

Smooth leading edges keep airflow firmly attached to the wing. Although bird and insect wings usually have rough and sharp leading edges to promote airflow separation, low separation causes efficiency problems for large aircraft.

“Animals at small scale don’t try to keep the flow attached,” says Kenny Breuer, a professor in Brown’s School of Engineering and the study’s senior author. “Once you stop trying to keep the flow constantly attached, it ironically makes some things easier.”

The new wing – dubbed the Separated Flow Airfoil – was designed by Matteo Di Luca, a graduate student at Brown and the study’s lead author. The idea is to intentionally separate the flow at the leading edge, which 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 12" or less.

Freestream turbulence – gusts of wind, vortices, and other disturbances in the surrounding air – 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 too much for a drone’s control system, leading to unstable flight.

The wing design dispenses with the contoured leading edge of most wings in favor of a sharper leading edge.

Separated Flow Airfoil wind tunnel testing showed the design smoothed out lift fluctuations associated with freestream turbulence.

Also, the Separated Flow wing can be much thicker than wings normally used in small drones. That makes the wings structurally stronger so subsystems such as 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.

Brown University