Drones of all sorts are getting smaller and cheaper, and that’s great—it makes them more accessible to everyone, and opens up new use cases for which big expensive drones would be, you know, too big and expensive. The problem with very small drones, particularly those with fixed-wing designs, is that they tend to be inefficient fliers, and are very susceptible to wind gusts as well as air turbulence caused by objects that they might be flying close to. Unfortunately, designing for resilience and designing for efficiency are two different things: Efficient wings are long and thin, and resilient wings are short and fat. You can’t really do both at the same time, but that’s okay, because if you tried to make long and thin wings for micro aerial vehicles (MAVs) they’d likely just snap off. So stubby wings it is!
In a paper published this week in Science Robotics, researchers from Brown University and EPFL are presenting a new wing design that’s able to deliver both highly efficient flight and robustness to turbulence at the same time. A prototype 100-gram MAV using this wing design can fly for nearly 3 hours, which is four times longer than similar drones with conventional wings. How did they come up with a wing design that offered such a massive improvement? Well, they didn’t— they stole it, from birds.
Conventional airfoils work best when you have airflow that “sticks” to the wing over as much of the wing surface as possible. When flow over an airfoil separates from the surface of the wing, it leads to a bunch of turbulence over the wing and a loss of lift. Aircraft wings employ all kinds of tricks to minimize flow separation, like leading edge extensions and vortex generators. Flow separation can lead to abrupt changes in lift, to loss of control, and to stalls. Flow separation is bad.
For many large insects and small birds, though, flow separation is just how they roll. In fact, many small birds have wing features that have evolved specifically to cause flow separation right at the leading edge of the wing. Why would you want that if flow separation is bad? It turns out that flow separation is mostly bad for traditional airfoil designs, where it can be unpredictable and difficult to manage. But if you design a wing around flow separation, controlling where it happens and how the resulting turbulent flow over the wing is managed, things aren’t so bad. Actually, things can be pretty good. Since most of your wing is in turbulent airflow all the time, it’s highly resistant to any other turbulent air that your MAV might be flying through, which is a big problem for tiny outdoor fliers.
Photo of the MAV with the top surface of the wing removed to show how batteries and electronics are integrated inside. A diagram (bottom) shows the section of the bio-inspired airfoil, indicating how the flow separates at the sharp leading edge, transitions to turbulence, and reattaches over the flap.Image: Brown/EPFL/Science Robotics
In the MAV demonstrator created by the researchers, the wing (or SFA, for separated flow airfoil) is completely flat, like a piece of plywood, and the square front causes flow separation right at the leading edge of the wing. There’s an area of separated, turbulent flow over the front half of the wing, and then a rounded flap that hangs off the trailing edge of the wing pulls the flow back down again as air moving over the plate speeds up to pass over the flap.
You may have noticed that there’s an area over the front 40 percent of the wing where the flow has separated (called a “separation bubble”), lowering lift efficiency over that section of the wing. This does mean that the maximum aerodynamic efficiency of the SFA is somewhat lower than you can get with a more conventional airfoil, where separation bubbles are avoided and more of the wing generates lift. However, the SFA design more than makes up for this with its wing aspect ratio—the ratio of wing length to wing width. Low aspect ratio wings are short and fat, while high aspect ratio wings are long and thin, and the higher the aspect ratio, the more efficient the wing is.
The SFA MAV has wings with an aspect ratio of 6, while similarly sized MAVs have wings with aspect ratios of between 1 and 2.5. Since lift-to-drag ratio increases with aspect ratio, that makes a huge difference to efficiency. In general, you tend to see those stubby low aspect ratio wings on MAVs because it’s difficult to structurally support long, thin, high aspect ratio wings on small platforms. But since the SFA MAV has no use for the conventional aerodynamics of traditional contoured wings, it just uses high aspect ratio wings that are thick enough to support themselves, and this comes with some other benefits. Thick wings can be stuffed full of batteries, and with batteries (and other payload) in the wings, you don’t need a fuselage anymore. With a MAV that’s basically all wing, the propeller in front sends high speed airflow directly over the center section of the wing itself, boosting lift by 20 to 30 percent, which is huge.
The challenge moving forward, say the researchers, is that current modeling tools can’t really handle the complex aerodynamics of the separated flow wing. They’ve been doing experiments in a wind tunnel, but it’s difficult to optimize the design that way. Still, it seems like the potential for consistent, predictable performance even under turbulence, increased efficiency, and being able to stuff a bunch of payload directly into a chunky wing could be very, very useful for the next generation of micro (and nano) air vehicles.
“A bioinspired Separated Flow wing provides turbulence resilience and aerodynamic efficiency for miniature drones,” by Matteo Di Luca, Stefano Mintchev, Yunxing Su, Eric Shaw, and Kenneth Breuer from Brown University and EPFL, appears in Science Robotics.
[ Science Robotics ]