How to weigh a flying bird

Do birds in flight weigh more, less or the same than they do at rest?
19 January 2015


If a sealed box was full of birds, and all of the birds took flight together, would the box weigh less?

The Discovery Channel's Mythbusters did a simple experiment and concluded it wouldn't, but now scientists from Stanford University have disagreed; moreover, writing in the Royal Society journal Interface they have developed a gadget that can answer this question directly. But the aim wasn't just to take on the Mythbusters.

The device has a serious application, because it can, for the first time, literally "weigh" a flying bird - or other objects. This means it can help us to improve our understanding of the mechanics of how things fly.

Previous measures of flight dynamics and forces relied on crude experiments using tethered birds or drones. "Remarkably," say the scientists behind the breakthrough, "there exists no method to measure these forces directly during free flight."

Assisted by two tame parrots called Ray and Gaga, the Stanford team led by David Lentink have successfully made a measuring device that can feel the individual wing beats as the birds flap their way from a perch on one side of a box to the other. The enclosure is equipped with sensors that can detect the minute pummelling of the surface by air molecules accelerated by the birds' wing movements as they fly.

The results show that the birds generate a force equivalent to about twice their weight on the downstroke, and then very little force on the upstroke as they reset their wings.

Apart from parrots, Lentink's team suggest that their system can be used to optimise other flying objects, like aircraft and drones. "Here, we demonstrate a new aerodynamic force platform (AFP) for non-intrusive aerodynamic force measurement in freely flying animals and robots."

So where does this leave the conundrum of the bird's flapping their wings to make a truck lighter? According to Lentink, it won't work. Because the birds apply double their weight during the down-beats of their wings, such a bird-laden trailer could become momentarily heavier, not lighter, if the cargo all took off at the same time.


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