Hi Simon,
first let's get a couple of points clear. Dynamic soaring is not so much free energy as getting energy at a profit. In particular, when the demand is not for drastic acceleration, but for leisurely cruising, birds can get where they want to go for a very low energy input, especially when they have anatomical adaptations particularly suited to the function. I suspect that the albatrosses are the most impressive extant examples of such adaptations, but I cannot express how I would love to have seen the living operation of some of the largest soaring animals of the past 250 million years or so. Nor of course, knowing little of either aerodynamics or ornithology, as I do, can I be sure whether any other living birds could rival the largest albatrosses in this respect. Frigate birds? Giant petrels?
In particular, apart from aerodynamic adaptations, I am thinking of the tendons that permit soaring birds such as albatrosses to hold their wings in the soaring attitude without significant energy input. We humans have analogous adaptations enabling us to stand on straightened legs with very little exertion. These adaptations involve mainly the joints, ligaments and tendons of our ankles, knees, and hips. Standing erect requires little muscular effort in most healthy people. In elephants and horses the adaptations are even more advanced, but the principle remains the same: once the attitude is adopted we can leave most of the maintenance to the connective tissue, relieving the muscles and nerves of all but minor corrections and adjustments.
Now, in the rest of this discussion I shall take such minor corrections, vitally important though they are, for granted, except where otherwise stated. However, as you shall see, there are situations that require definite exertion.
Let's first deal with and dismiss the question of flight at a large angle to the axis of the direction in which the wind is blowing. Flying cross-wind, if you like. Actually, in practice this might very well represent the birds' dominant flight mode, but as I see it, it presents us with the smallest conceptual challenge. Essentially what that comes down to is that the birds achieve their greatest stored energy in the form of momentum or altitude by dynamic soaring. Having achieved the necessary momentum in any convenient direction, the bird can efficiently adopt the desired direction by banking and gliding with very little loss of momentum. (There is of course the question of riding the eddies over the swells, but if I understand you correctly, you have no difficulty with that problem, nor with dynamic soaring in and out of the lee of swells in a stiff wind.)
That leaves us with two classes of cases to consider: upwind and downwind dynamic soaring.
In both of these cases it seems to me that in your description of your problem you are placing too little explicit emphasis on the role of inertia. Essentially, as I see it, what the bird is relying on in all circumstances under discussion, is the difference between the velocity of the bird and the velocity of the wind. I am sure that you are clearly aware of this, but I am not sure that you haven’t mentally dismissed the concept too casually, having once established it. As soon as the bird is, so to speak, in the same inertial frame as the surrounding air, it begins to fall out of the sky if it does not exert vigorous effort (after the fashion of a Hummingbird, or at least a windhover or a helicopter). None of these would be a useful model for the albatross! Of course, if the albatross achieves that notionally static inertial frame at a great height, it has considerable stored potential energy, and there is not much to worry about; it can simply draw on its account by stalling into a dive, and gliding off when it has attained the necessary airspeed. But never mind that. The important thing is that if it wishes to accumulate locomotive energy, it must seek and maintain an adequate velocity relative to the surrounding air.
As you imply, this generally requires velocity gradients in the air; wind shear if you like. The gradients need not in principle involve lower speed at the surface than at altitude, but by and large higher wind speed at greater altitude is what we expect in nature.
Now, let us suppose that at any point where there is wind shear, the bird is in one layer and at nearly the same velocity as the air. It now has two options: move into a strong headwind, or a strong tailwind.
In a strong tailwind the obvious option is to pick up speed while gliding at the best possible glide ratio, at least until the velocity relative to the wind becomes uncomfortably low. Sometimes the wind shear is very sharp, but for this purpose we would prefer a gradual change of velocity through a thick layer of air, which would permit a long glide before it is necessary to seek height.
The only expenditure of energy necessary to gain height once the bird has the speed, is to steer upwards to invest part of the momentum for climbing. Remember, we started high, then gained momentum for nearly nothing while gliding downwind. We can climb back to the same starting altitude while still having a comfortable profit in velocity and having covered a fair distance, possibly a few hundred metres.
Gliding downwind permits gaining momentum and distance.
Alternatively, we could turn upwind, using the wind to climb, which is a form of energy capture in the form of potential energy.
Right. So, once again: it is downwind for momentum and distance, upwind for storage of potential energy. And either or both could be copntinued as often as desired by repeatedly crossing a suitable wind shear boundary.
Sorry to be so long-winded about it, but I did not have time for anything shorter. I had to work this lot out on the fly, and I hope I didn’t mess it up too badly. Call back and shout at me if I got it wrong.
Cheers,
Jon
