HOW BIRDS SOAR.

Shannon News, 12 October 1928, Page 4

HOW BIRDS SOAR.

PRINCIPLE APPLE TO MAN?

CARRYING PLANE.

(By W. Laurence Le Page)

"There be three things which arc top wcpderful for me, yea, four which I know not: The way of an eagle in the.air . . . . " (Proverbs 30: 18, 19). These words of Biblical origin, might wqII have, been uttered by the, "old seaman" who, in these columns, de* scribed the flight of the seabird as "one of the most wonderful things in the. world." In this sentiment we can heartily concur, though we find it difficult to agree that "there is a secret to their (scabirds') flight that man has not solved," nor can we contribute to the belief that "when we understand the secret of it we shall be able to fly without engines or propellers. '

True it is that the soaring or nonflapping . flight of birds is a remarkable phenomenon. Veterans of the seas, all.alike, tell the same story of the sea gulls and albatrosses which for days upon days follow in the wake of a ship or skim the waves thousands of miles from land, yet never ior an instant flapping their wings co sustain themselves. . . But let the reader be quite confident that there is nothing uncanny in this performance, nor does it go "contrary to all laws of physics." The phenomenon is explainable and its explanation has already had its effect upon the design of mancarrying aircraft. In addition to the non-flapping flight of sea birds, many tropical birds exhibit a remarkable display of soaring. The common Pariah Kite or Cheel of India is seldom seen in. flapping flight, but is constantly in the air dver the open plains and hills of Egypt and India, circling,around for hours, yet seldom, if ever, flapping its wings. Tne white scavenger vulture, the black vulture; and Others exhibit similar characteristics, and their flight has attracted the attentions of natural scientists and meteorologists for years. Light Loads for Wings, In searching for an explanation of the soaring of birds there are certain characteristics of all birds that indulge,, in this form of flight which immjjdiately attract • attention. Outstanding is the fact that all soaring birds, whether sea birds of birds of itropical species, have very lightly loaded wings. That is to say, their wings are large compared with the weight of the bird and thus in flight the load per square foot of wing surface, a criterion in estimating the Ayr ing characteristics of either birds or airplanes, is small, ranging from .55 to 1.23 pounds per square foot. This coupled with a high degree of what may be termed aerodynamic efficiency, enables such birds to glide, i.e., plane to earth, in still air, at a 1 very low rate of descent. This being the case, and since flying, is a,-phenomenon which is always relative to the air and not the earth, it will only be necessary to. establish the existence of a sufficient vertical trend in the wind, to explain non-flap-pings flight.' No doubt many of us have, noticed sea gulls aimlessly flying without flapping over the edge of a cliff when there has been a sea breeze •blowing. The presence of the cliff forces the wind up and the gijll has sufficient instinct to remain in the ro,gion of rising air currents and glide down the wind,, as It were, with the Tesult that no height is lost while it may, in fact, be gained, eyen though no effort is exerted on the part of the bird. This is soarinng flight in its simr plest form, but many observers have been led to search for .a more subtle explanation, for in many cases of soaring it has seemed difficult to prove the presence of rising currents. For example, in the tropical plains of India, Cheels are seen daily mounting the air in wide circles to great heights without flapping or exertion of any kind. Under such conditions the flat contour of the land is such as to dispel immediately any thought of a rising trend -in the wind. The fact, however, that the soaring of Cheels starts soon after sunrise and continues until sundown would indicate a close connection between this phenomenon and the heat of the sun.

Altitudinal Decrease in Temperature. It. is a well established physical fact that the normal temperature of the air decreases with height and that for equilibrium in the atmosphere there is a constant rate of decrease in temperature with altitude; known as 'adiabatic' rate. Under tropical conditions, however, measurements have shown that owing to the great heat of the sun there is sometimes as much as twenty degrees difference in tempraturo between the air at the surface of the ground and air only three feet up. The adiabatic decrease in temr perature is one degree for each 3000 feet, so that the abnormality of tropical conditions can well be appreciated.:

This marked temperature gradient causes the air to rise rapidly from the surface and it has been found that the rising currents thus created extend to heights of 500 to 10,000 feet. An efficient soaring bird like the Cheel and the scavenger vulture would require a vertical air current of no more than 1.5 to 2 feet per second to enable it to maintain altitude without wing flapping and such vertical velocities are.not at all infrequent in tlio tropics. This, type of non-flapping flrght hai been termed "sun soaring" because of the necessary influence of the sun upon the air <o ereat*. soarabu, con'Mtiorm-. Title soariig flight of certuii tropi-Ml dragon flies, observed by r fce eminent Dr Hankin, at Agra, India, Is nnother example of "sun-soarability" in whieh

the conditions are. so delicate that the merest-wind gust upsets soaring.. Those who hold to the belief that in the soaring flight of the larger birds lies, a secret which, when disclosed, will have revolutionary effects upon the design of our aircraft would do well to recall that, unlike an airplane, the bird has sensitivity in its wings, and thereby can veritably feel the air and its vagaries and take advantage thereof. Furthermore, the total weight of the heaviest bird is considerably less than the weight of the lighest airplane with its pilot, and disturbances in the air which are of sufficient magnitude to influence materially the flight of the one will have little effect on the other.

Bearing in mind these facts, it is not impossible to find some sound explanation of the non-flapping flight of such birds as the albatross and sea gull. " Sun-soarability,'' obviously has no part in any explanation of the flight of these birds, for there is nothing upon which to base the establishment of a vertical trend in the wind. Obviously, then, the bird indulging in nonflapping flight under such circumstances must be extracting energy from the very structure of the wind itself. It may be taken as a fact that it is just as impossible to derive energy continuously from a wind that is continuous in speed as it is .from a perfect »calm. However, most winds are very far from constant, and a wind of for" example, 20 miles an hour, will usually be continually changing in velocity, the extreme limits of the fluctuations being sometimes as much as 15 miles per hour apart. There is, in fact, no such ihing as a constant wind.

Constant Speed. A bird having weight, must of necessity have inertia, and its inertia is considerably greater than that of tne air. Thus, when the bird is moving through space its momentum will' hold its speed either constanr or perhaps gradually accelerating or decelerating but certainly not changing rapidly. Here, therefore, we have the necessary conditions for the bird to extract energy from the wind. (Wind is air in motion, and anything which 13 in motion possesses energy.) Suppose the sea gull to be gliding with the wind and at the same speed as the wind. No energy will be exchanged between the bird and the surrounding air. But as soon as the speed of the wind changes (gustiness sets in) the wind will work upon the bird; energy will be expended and do we need any persuasion to grant that the sea gull is able to harness this energy and put it to useful work in providing the necessary propulsion and lift to sustain flight? If we do, then a few minutes of calculation will convince us. The writer estimates that for the average sea gull a gust amplitude of only six miles per hour, which is present in almost any wind of 10-13 miles per hour velocity, is sufficient to permit soaring and gaining of altitude without the expenditure of energy on the part of the bird.

It should be borne in mind that wherever there is a change in velocity at coincidental points energy will be expended. Let us, therefore, analyse carefully the situation of the flight ol' the seabird which for days follows the ocean lin'er without ever flapping its wings except for purposes of maintaining stability. Suppose the ship to be travelling at 14 knots or that the relative speed of ship and wig..<l is 14 knots and that the breeze is blowinng from one quarter. The point of greatest shelter will, of course, be in the opposite quarter at the stern of the ship. Fourteen knots is a speed of 24 feet per second. At the water's edge in the sheltered spot there will be virtually no wind relative to the ship, so that all wind speeds are relative to the ship. For the sake of example, assume the deck of the ship to be 24 feet above the water line. Then there will be an increase of relative wind speed in the sheltered region from the water line to the deck of 24 feet per second. In other words, a nominal velocity gradient of one foot per second for each foot above the water line, which is easily sufficient to provide enough energy to sustain and propel an efficient bird like a sea gull or albatross.

In considering the possible influence which our knowledge of the mechanism of soaring flight in birds might have upon the design of man-carrying aircraft it must be remembered that" the air disturbances brought into play and harnessed by the bird are small compared with the size, weight, and wing loading of an airplane* Our aircraft will always need engines of one kind or another to enable eontrolled flight tq, all corners of the earth, but in answer to pessimists one can only point to the frequent flights of many hours' duration made by the German glider pilots flying motorless airplanes over the hills of the Ehone district, Germany, where rising currents caused by the rolling terrain are plentiful.