This page is particularly collaborative [especially with Bomphrey, Cheney, Song, Windsor, Stevenson]

Bird tails reduce drag

THIS PAPER describes a study measuring the motions of a mist of helium bubbles in the wake of two owls and a goswhawk trained by Lloyd and Rose Buck.

In collaboration with LaVision (Dierksheide and Nila) up to 20,000 0.3mm bubbles were tracked

From these observations, it is clear that there is additional downwash produced by the tail. This is not what a conventional or toy aircraft would do: these rely on ‘negative lift’ (a downward force) from the tail in order to provide passive stability in pitch, and the tail would produce an upwash. Using classical aerodynamic theory from the 1920s, then also state-of-the-art computational fluid dynamics (in THIS PAPER), we show that lift production by the tail considerably reduces the drag on the bird. Presumably birds do not rely on passive aerodynamic stability, and have either sufficient aeroelastic or active mechanisms to achieve steady gliding despite the lifting, drag-reducing action of the tail.

Wings and preflexes as inertial suspension

Unlike fixed-wing aircraft, the wings of even gliding birds are not rigidly attached to the body. THIS PAPER describes what happens when a bird glides through a very strong upgust.

A remarkable degree of suspension is clear: the wings accelerate upwards but the eyes, head and body continue, at least to begin with, in a straight line. The physics that allow this is simple, and might be applicable to aircraft: if the hinge at the shoulder supports the body weight during gliding, but does not change this force as it is deflected; and if the net aerodynamic ‘bump’ acts at the ‘center of percussion’ of the wing, then the upward and rotational motions of the wing cancel at the hinge, meaning that the body is not perturbed.

The demonstrator ‘wings’ are supported by strings at the centre of percussion, and connected to the body by hinges and elastic bungees. Impulses applied outboard of the centre of percussion result in a downward acceleration of the body; when applied inboard of the centre of percussion, the body accelerates up. But when applied at the centre of percussion, there is no reaction acceleration of the body. The action of the bungees mimics that of muscle before any reflex activation is possible; this is sometimes termed a ‘preflex’. This concept could be applied to fixed-wing aircraft, and is patented. Do contact jusherwood@rvc.ac.uk or rbomphrey@rvc.ac.uk if interested.

Link to 10 minute talk for the Royal Society Summer Science: Eagle Inspired Engineering.