As this barn owl glides through the darkness a sudden flash of light illuminates
over 20,000 helium filled soap bubbles, suspended in the air. Researchers can track the bubbles as they
swirl in the owl’s wake, and they’re using this to study the bird’s flight in unprecedented
levels of detail, revealing unexpected ways in which
birds generate lift and reduce drag while gliding. The first task is to persuade the birds
to fly through the cloud of bubbles. If they can see the bubbles,
they try and avoid them, so the researchers have to snap
the lights on right at the last minute. That’s the idea anyway. When the birds do glide through smoothly, they leave vortices swirling in the air behind them. The movement of each bubble is then tracked
and recorded by a computer to visualise the air flow. As expected, the researchers saw vortices
spinning down from the wing tips. This helps provide lift. But the swirling bubbles
revealed something surprising: a second pair of vortices
coming down from the tail. These also generate lift and because they come from the tail,
the lift is spread more evenly over the bird’s body, reducing overall drag. This is different from the way something
like this toy plane works. It has fixed, solid wings
and uses its tail for stabilisation. A tail that generated lift
would make it very unstable. But birds are able to constantly move and
adjust their bodies for stability, so they can use their tails to make extra
lift and decrease drag. Engineers might be able to copy this trick for use in aircraft that
actively stabilise themselves. It wouldn’t work in the same way in
something like a passenger plane, which is much larger and faster, but for small, slow-flying craft,
like a gliding drone, designers could make good use of this
aerodynamic trick borrowed from birds.