Hello Again Simon.

Having inadvertently done what I should have done in the first place, I have slept on your questions and I hope that this will be a more compact, possibly more useful reply. For a start I can avoid spending so much time on discussing points you already are happy with.

As I see it your difficulty is:

>Bird is high, in say 20kts wind, dives down and increases it's own airspeed as it goes, glides a long way at the ocean level in calm air using it's own momentum. Then as it loses momentum (airspeed) it climbs again to gain height. THAT is where I am unstuck, HOW is it going from a low energy state (low airspeed) AND CLIMBING AT THE SAME TIME using just the wind shear to 'add energy'? I can't understand how a bird can dynamically soar (endlessly) given a flat surface and JUST a wind gradient. <

We can kill that easily (as I see it anyway, correct me if necessary) if we bear a few already familiar principles in mind at crucial points.

1: In the absence of waves and similar luxuries, the bird’s energy assets are height, and momentum relative to the air. “Groundspeed” is irrelevant, and in this case so is vertical shear (notionally). It does however expend its energy profit on lateral travel, which from our point of view is the main incentive.

2: As the bird lost its airspeed and height in gliding, it did not lose its available wind shear as long as it was in a generous wind gradient; as long as it kept enough airspeed or height, or is willing to do an occasional flap, it can change its altitude, and as soon as it does that it picks up airspeed again. The velocity might not have a convenient direction, but the bird can use its energy profit to correct for that.

3: So, as the bird reaches the end of its glide, it can use its initially low(-ish, not zero!) airspeed to climb, all at the price of a few small adjustments. As soon as it climbs it picks up some airspeed; not necessarily in a desired direction, but that doesn’t matter; it can use it to gain height, which it then can cash in on energy for progress in any direction.

4: The apparent violation of thermodynamics, “something for nothing”, is an illusion: the energy for most of its climb comes from moving through a shear gradient that contributes airspeed. It is now harvesting more energy than it is expending. In still air that would not apply. The bird can go on rising and falling as long as it likes, gaining energy on each pass across the gradient, and topping up on the occasional scrap of fish or carrion (whale dreck, drowned, pipe-smoking sailors and the like). You could consider surfing as an analogy – how can the surfer slide down a wave and run up a wave, and not run out of energy? Because by the nature of the mechanism he is harvesting energy from the wave all the time.

5: One could imagine a problem if the bird is in a high-speed consistent prevailing wind in the direction opposite to which it wanted to go. If it could not gain a higher airspeed than the wind speed near the surface, then it could not travel upwind at all. It would have to use its gains to move sideways as far as possible until it got out of that wind zone and could resume business as usual. In practice however, one would expect a strong swell in such a wind, and the bird could move upwind in the low windspeed in the lee of the swells, hopping over each swell as it came. Sound like hard work though!

I hope that is more useful.

Cheers,

Jon

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

HI John,

I wonder if you may be able to help me?  I've searched all over the inter tubes and can't find a satisfactory explanation of EXACTLY how albatrosses can dynamically soar on an (almost) flat ocean.  If there are big waves - no problem, I can understand how Dynamic Soaring works in that case, but how can they soar for so long, with no input of energy (via wing flapping)  using wind shear alone??  I understand in concept what is happening, they are utilising wind gradient or shear to swap air speed for height (and vice versa), but I have NOT been able to find a good detailed explanation of HOW this happens.

My understanding is:  Bird is high, in say 20kts wind, dives down and increases it's own airspeed as it goes, glides a long way at the ocean level in calm air using it's own momentum.  Then as it loses momentum (airspeed) it climbs again to gain height.  THAT is where I am unstuck, HOW is it going from a low energy state (low airspeed) AND CLIMBING AT THE SAME TIME using just the wind shear to 'add energy'?

I can't understand how a bird can dynamically soar (endlessly) given a flat surface and JUST a wind gradient.  Are you able to help me out here or perhaps point me to someone who could?  I've searched and searched and cannot find a sufficiently detailed explanation other than "albatrosses utilize the wind shear to dynamically soar" type answer..

ps - sorry I know this is really a new question rather than a reply but it seems like you may be able to help..  thanks

Thank  you!

Simon

Excellent answer thank you most kindly. I knew the answer but not the detail and the why.

Are you sure that your question reflects what it is that you wish to know? For a start, it depends on which bird you have in mind.  I leave it as an exercise for the advanced student to excogitate, explicate, and rationalise the case for the ratites and Spheniscidae, but what makes you think that the Trochilidae cannot do VTOL? They even are able to fly backwards, and do so more routinely and insouciantly than any VIFF-capable aircraft, such as a Harrier jet. And although the humming of their wings gives those birds their popular name, it is a lot quieter than the jet in any mode.

As for why other birds don't do it (though many, such as partridge, routinely exhibit an unnerving capacity for abrupt and noisy STOL), you will notice instantly of course, that the mode of take-off is associated with their mode of flight, which in turn is associated with their need for and application of flight. Not at all surprisingly the same applies to the insects, by the way, many of which, such as the dragonflies, bees,  and hover flies, can rival the hummingbirds at take-off and in flight in any direction. Insects with no such need tend to fly forwards, much as most birds do, and for much the same reason. 

Birds however, have larger Reynolds numbers than most insects, and nearly all flying birds fly by far the most efficiently and effectively when flying forward. Hovering is expensive and exhausting, and so is take-off.  VTOL is worst case because it not only is not efficient for most birds (though many of them can manage it in a pinch) but leaves them in mid air with no forward velocity and therefore with half the job of getting stably airborne still to do. For them it simply is an expensive, dangerous and unrewarding action, much as it is for a Harrier jet as well, by the way! Some of the smaller ducks, the dabblers such as wigeon and teal for example, can explode off the water as vertically as they like, by adding a tail flap to their first wingstroke, so as to use the water beneath as reaction mass. But they do not normally do so, for the same lack of incentive. They use the tail flap to help them off stilll water without a long take-off run.

For most birds and most insects, flying is a very utilitarian thing. They do what they need, and they adapt as they need. Or they die out. Sometimes they scrap abilities that they can do without, when they can substitute something else instead. For example, certain bees in Southern Africa at least, have lost the ability to hover, whereas most bees are great exponents of hovering for all sorts of purposes. These non-hovering bees are excellent at forward flight istead. So what do these bees do when males wish to stay in one spot when waiting for female company? The fly upwards and relax, fall back for several centimetres, then repeat the process, possibly for hours. And these bees bouncing up and down over the veld advertise their presence very selectivley to passing females. VTOL means nothing in their lives.

Birds are not great exponents of hovering either, unless it is a big thing in their lives for special purposes. For example, scavenging species of the smaller gulls oftn hover when inspecting their options above floating carrion. Terns and kingfishers hover when preparing to dive on fish. Kestrels and kites hover in a very similar manner when hunting mice or lizards. In fact, kestrels commonly are known as "windhovers", or equivalently as "windvalkies" in Afrikaans. But such behaviour, so closely related to VTOL, is atypical. Often when you see a bird hanging more or less motionless in the air, it really is doing dynamic soaring in a ripple of wind over a ridge.

Some birds go to the other extreme and certainly cannot do even STOL. Conspicuous examples include heavy species that have to stay aloft for long periods, sometimes for high journeys, sometimes sleeping on the wing,  such as geese, swans and albatrosses that can hardly take off at all without a long horizontal run or help from passing waves. Other examples include those magical, breathtaking fliers, the swifts.  Many cannot take off at all, unless they have a start getting into the air. They also cannot even hover, but fly. . . Oh they can fly!!!

And as for landing, most birds that have a lot of work to do in taking off, also have to take care in landing, or the energy that they had put into getting off the ground and up to speed may do them a mischief when they shed it again so as to come to rest at ground level.  The way in which say, a large pelican  water-skis to a halt is a beautiful display: Joe Cool showing off to the admiring multitude.

The way in which some albatrosses routinely crash-land is positively unnerving.

Now, what was it you really wanted to ask?

Jon