Dynamic Soaring

'To identify birds in the air their characteristics of flight can be most helpful, allowing one to distinguish between different families and even species. When watching a bird on the wing one needs to be asking all sorts of questions. For example, does it flap intermittently and glide, soar, wheel and bank like an albatross or petrel? Or does it flap almost continuously like a skua? Is flapping rapid and with stiff wings like that of a shearwater, or is it slow with flexible, graceful wing movements like that of a gull? Is flight light, buoyant and tern-like, or heavy and ponderous like that of a giant fulmar? One gets to know that a shearwater's flight is rapid, a skua's is deliberate, and a gull's is leisurely. If it's erratic it'll probably be a prion, and if it's direct, a cormorant. We must also note how high the bird flies: just above the waves like a storm petrel, above the horizon like a gadfly petrel, at deck height like a Cape pigeon, or high over the masthead as a gull sometimes does. And then there's swimming style. That can tell you a lot too. Does it porpoise in and out of the water like a penguin, or does it ride on the surface? Does it dive and plunge, or remain visible at all times? Does it ride low in the water with only the head, neck and back exposed, or does it bob buoyantly like….'

'I say, old chap. Sorry to interrupt the eloquent flow, but what's that one over there? Much bigger than these other specimens. Doesn't seem to flap its wings at all.'

'Ah, well done Professor.' He didn't need his binoculars. 'Our first albatross, and a wanderer at that. Diomedea exulans. You'll make a good bird spotter.'

'Can I use your glasses? Gee, but that's fantastic.' Henry was excited. 'Massive wings, hey? And it just never flaps. Incredible! How does it do it?'

'The largest of the flying seabirds. Those long narrow wings can span in excess of eleven feet. Beautiful to watch, isn't it? But they need strong winds for that effortless gliding flight. If you watch for a while you'll see it follows a regular pattern in the air, rising into the wind, coasting across it, and then losing altitude but gaining speed while it dips to leeward and banks to turn and rise into the wind once more. In this way it's capable of planing on those flexed pinions for days at a time, never flapping, but making slight adjustments at the wrist and elbows to change effective wing area.’

……

When Fred Kelly and Jimmy Potsherd also began to evince a more than passing interest in poetry Henry felt obliged to steer the conversation back to the original topic.

'Bob, you were saying its possible for an albatross to fly for days without flapping. Without rest, without sleep, without food. I find this truly amazing. Where does the energy come from? This borders on the supernatural. How does it do it?'

'It was Lord Rayleigh, back in 1883, who first offered a feasible explanation of the principles involved. As he pointed out, the energy necessary to maintain flight can be derived from the variability of the wind. The velocity of the bird relative to the air, not its velocity relative to the ground, determines the forces acting on it. The bird can at any time, by climbing upward, turn some of its kinetic energy into potential energy and vice versa. Put simply, this dynamic soaring, as it's called, is merely a process of correcting for the turbulence in the air in such a way that potential energy is gained. I hope I'm not boring you. Some people find this detailed analysis of so simple and basic a phenomenon just too remote and tiresomely technical.'

On behalf of them all Henry assured him of their keen interest and urged him to continue. He noticed Sammy Coolrich was lighting up her first joint of the day. It was that top grade Durban Poison he had acquired for her and which had elicited her delighted approval. He now mistook the faraway expression in her eyes as she exhaled to be pure narcotic pleasure, when in fact it was generated more by a contemplative process taking place than by the effects of cannabis sativa. This process had been triggered when the ornithologist alluded to relativity.

'Well, as I was saying, a bird can gain energy from the small-scale turbulence of the air merely by making minute adjustments to its flight. And the best example of dynamic soaring is provided by the albatross, which uses the gradient of wind velocities near the surface of the sea. We really are privileged to have this demonstrated to us right now as I speak.' Indeed, the huge white bird had moved in closer to the ship and seemed to be tracking their progress in order to perform its graceful artistry just for them. 'It is known that the velocity of the wind diminishes down to the surface of the water because of the influence of friction at the surface and of eddy motion in the air; by a variety of turning manoeuvres the albatross can take good advantage of such a gradient. At the end of a downward glide, with the wind behind it, it nears the surface of the water. In order to gain altitude it turns and faces upwind (into the wind); the initial speed gained during the preceding glide generates lift and the bird climbs. During the climb the bird's air speed (but not the speed relative to the water below) remains constant because of the progressively increasing velocity of the wind at higher levels. Upon reaching a certain altitude, at a level where the bird cannot climb higher without losing air speed, the albatross turns and faces downwind (goes in the direction of the wind) to begin its downward glide. At this point the bird begins to gain air speed for two reasons: one, the force of gravity (the potential energy of height) becomes the kinetic energy of motion and two, it is entering air which is moving more slowly over the water. Therefore, as the albatross enters the lower altitudes it continues to gain speed relative to the progressively diminishing tail winds. When the bird again nears sea level, it turns upwind and repeats the manoeuvre. The natural way to combine these effects is to describe circles in an inclined plane, always descending when moving to leeward and ascending when moving to windward.

In strong winds a very swift bird like the albatross (whose average speed is 72ft. per second) can extract sufficient energy from the air to enable it to glide for considerable distances in the troughs of waves.'

(This extract is taken from Section 85 of The Life of Henry Fuckit, which can be read here. The discussion about dynamic soaring is based on an article in the 1972 edition of Gough Island’s Encyclopaedia Britannica. With advances in Physics the explanation has been revised over the years to include factors such as the aerodynamic forces acting on the bird, the effect of acceleration, the effect of the wind gradient, and the transfer of momentum between the bird and the wind.)a

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