wing geometry

a puzzle...

How do birds with small tails - for example seagulls - achieve stable flight? Presumably there must be some aerodynamic features that are giving the bird stability in the three main axes, or is it simply that the brain of the bird is constantly correcting instabilities as they arise? And then the tip panels of the birds' wings droop downwards (anhedral) which would seem to increase the risk of instability - in contrast many conventional aircraft have wings angled upwards (dihedral) to provide lateral stability.

a clue from hang gliders:

The hang glider wing forms two 'billows', one on each side of the midline. The underside of each wing forms a conical surface, the centreline of which is angled towards the nose of the hang glider. This arrangement, together with the low centre of gravity provided by the weight of the pilot, gives the hang glider stability around all three axes (pitch, roll, and yaw).

In the early 80s I began to wonder whether the wings of birds such as seagulls conformed to a conical geometry like that of hang gliders. A series of models with wings built over conical jigs confirmed that this arrangement imparts stability during gliding flight - no additional surfaces such as a tailplane are required. Furthermore, it became clear that if the joint axes of the skeleton supporting the bird's wing were set perpendicular to the conical form, the wing is able to extend and flex whilst still retaining the conical geometry required for stability.

wing development

If, as suggested above, normal flight depends on the precise orientation and relative movement of the skeletal elements forming the wing, I began to think about how this could be achieved during development of the wing before hatching. Can anything provide a template for the developing wing to ensure that all the joint axes are correctly orientated? Looking at illustrations of bird embryos, it appears possible that the thorax provides just such a surface against which the wings can develop appropriately. The sketches alongside show chick embryos at different stages of wing development.

"Ex ovo omnia..."

I was pleased when I had these ideas, feeling that I had gained a fresh insight into the arrangement of some birds' wings. I discovered later that similar ideas have been discussed before - see for example Weyl (1945a, b). As Jonathan Swift reminded us:

"Our observations are old men's memories..."

Weyl, A.R. (1945a) Stability of tailless aeroplanes. Aircraft engineering, 17, 73-81.
Weyl, A.R. (1945b) Wing tips for tailless aeroplanes. Aircraft Engineering, 17, 259-266.