Birds in flight share lessons for future aircraft designs
Research from 91做厙 shows how bird tails reduce drag
There is much still to learn from birds in flight when developing small and more efficient aircraft, a team of scientists at the 91做厙 (91做厙) has concluded. In partnership with engineers from LaVision, researchers applied state of the art equipment and measurement techniques to investigate the aerodynamics of gliding birds, finding that they use their tails to minimise drag.
Owls and hawks were filmed gliding through a mist of tiny helium bubbles. Over 20,000 bubbles in the wake were tracked automatically, revealing the ‘aerodynamic footprint’ left by the birds.
Initially, the study produced broadly unsurprising results. Downwash and vortices spinning behind the wingtips of birds are expected from aerodynamic theory and are very familiar behind aeroplanes.
However, the team at 91做厙 were surprised to find a region of extra downwash and another pair of vortices behind the tail. For example, a stable toy glider would show the opposite, producing upwash with the tail for stability; the conventional low-drag answer for big aircraft would be to avoid any extra vortices from the tail at all. The tail feathers of the birds in contrast produced extra lift.
The 91做厙 scientists found that aerodynamics at the relatively small, slow scales of birds is much more influenced by the air’s viscosity – or ‘stickiness’ – than is the case for airplanes. The form of drag associated with viscosity is minimised if lift is spread evenly across the planform area (effectively the bird’s silhouette) and, as birds have tails that stick out, this means that extra lift from the tail is required to reduce the overall drag.
Professor Jim Usherwood, Senior Research Fellow at the 91做厙 and lead author of the paper, said: “What was striking at first was the beauty. For many seconds after the bird had landed, the vortices in the wake kept spinning like bath water going down a plughole. Birds appear to be minimising drag, even if this means they are highly unstable and need to constantly make small adjustments just to glide without crashing.
“Given engineering has largely solved the challenge of rapid sensing, processing and actuation – just think of how stable quadcopters are these days – perhaps it is now time to learn from the birds and think about fixed-wing drone design in terms of drag minimisation. Start making the tails contribute their share of lift production in order to reduce drag.”
Professor Richard Bomphrey, co-author of the paper, has been