Bird wings act as a suspension system that rejects gusts
Scientists from the RVC and the University of Bristol have discovered how birds are able to fly in gusty conditions – findings that could inform the development of bio-inspired small-scale aircraft.
We thought there might be something birds can teach us about coping with turbulence, so we invited Lily the barn owl, Sasha the tawny eagle, Ellie the goshawk and some of their friends to fly through gusts we made in our laboratory.
Inside our purpose-built flight lab, we created an artificial gust by building a wind tunnel on its side, so that the blowing end was pointing at the ceiling. Firstly, we had the fans running very slowly so the birds almost didn’t notice the gust. As they grew in confidence, we turned up the fans so the gust almost equalled the birds’ forward speed. The birds flew down the track, began to glide on the approach to landing and then experienced a strong gust of wind from below. For the birds, it was no problem at all. They were barely knocked off course.
This is due to two properties of birds’ wings. The first is the centre of pressure – the location where the gust pushes on each wing if you think of it as acting at a single point. The second is the centre of percussion - the ‘sweet spot’.
If you hit a ball close to the very far end of a bat, the handle jerks forward out of your hand. Conversely, if you hit the ball close to the handle, the handle jars backwards in your hand. This must mean that in between these impact locations there is a point where the handle neither jerks forwards nor jars backwards: this is the sweet spot. An impulse hitting here jolts the bat both in the direction of the impulse (translation) and around your hands (rotation). These two responses cancel each other at the handle, and you can hit a cricket ball, baseball, or golf ball a really long way.
The impact of the ball is now the extra force applied by the gust at the centre of pressure, and the handle is the bird’s shoulder. If the gust force coincides with the centre of percussion, then the wing rotates around the shoulder joint without jolting the body up or down.
It is a suspension system, and it works by decoupling the mass of the wings from the mass of the body. The gust is permitted to accelerate the wings upwards but only in a way that will not transfer the translational force and rotational torque to the body of the bird. Similarly, car suspension is set up so that the forces and torques from bumps on the road don’t cause your body to bump about too much, even while the chassis gets jolted. Car design can even reach the extent of minimizing jolts experienced by the driver’s head, leaving the rest of the body to wobble a bit.
We built a prototype glider and flew it through the same gusty conditions. It was very important to give it the proper physical characteristics; not feathers, but specially designed hinging wings:
- The centre of pressure must align with the centre of percussion. We achieved this by adding nuts and bolts to just the right point along the wing;
- Each wing must be fixed to the fuselage by a hinge representing the bird’s shoulder joint, rather than fused rigidly;
- Each wing must be held in place in flight by a spring that supports Weight during flight but allows movement around the hinge if the aircraft enters a gust. (For the budding physicists: it turns out that the best kind of spring is an unusual one, where the force doesn’t change with length. Muscles, such as the pectoralis flight muscles of birds, are particularly good for achieving this property, but here we used long elastic bands!)
We thank Masateru Maeda and Steve Amos for helpful discussions, Nathan Phillips for helpful feedback and assistance during set-up and data collection and Maja Lorenc for assistance during data collection. We also thank Lloyd and Rose Buck for their falconry expertise during the flight testing. We thank Tony Lapsansky and our anonymous reviewers for helpful feedback on the manuscript.
The team at the University of Bristol: Jonathan Stevenson, Nicholas Durston and Shane Windsor.
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