That Hawk Again
IN recent articles in the Atlantic Monthly, it seems that the problems of the soaring hawk have been unduly involved by improbable, if ingenious, explanations.
The soaring hawk, to all intents and purposes, is the same as a gliding aeroplane (an aeroplane with the motor idle). A gliding aeroplane descends at an angle, say, for illustration, of one in seven — not an unusual figure. In other words, if the machine were at an altitude of a mile, it could glide, on a perfectly still day, seven miles, measured horizontally, before landing. Now the hawk, unburdened with a heavy engine, fuel, and passengers, and hence having a much greater wing-surface in proportion to its weight, can glide at a much lesser angle than one in seven; but, for the sake of argument, let us assume that the hawk glides at an angle of one in ten. Then a wind which is blowing upward at an angle of one in ten would allow the bird to glide without losing height, and a wind blowing upward at a greater angle would allow the bird to rise while gliding.
The upward, as well as the downward, currents in winds are caused by the hills, the wind following the contour of the ground for several hundred feet up. Whether the bird is flying straight or in a circle does not alter the principle. He can, of course, spiral about a vertical axis even though the wind may be blowing, by shortening his turn, or decreasing his speed, while flying down wind, — that is, in the same direction as the wind, — or by lengthening his turn, or increasing his speed, while flying up wind. This covers the case of soaring in a wind.
When there is no wind, there are frequently upward currents, caused by the sun heating the air near the ground, which becomes less dense and therefore rises; or the upward motion can be caused in a zone where opposite winds meet. This upward movement does not have to be very brisk, and therefore would be unnoticeable to anyone on the ground. To illustrate by simple figures, assume that the hawk glides at an angle of one in ten, at ten miles an hour. Then, if he were at a height of one mile, and were to glide in a straight line on a windless day, he would strike ground ten miles away in one hour’s time, and he would have also descended a mile in altitude. But if, in that time, the air had been ascending at the rate of one mile (the hawk’s starting height) in one hour (the duration of the flight), the bird would not have lost any height. Increase the upward speed of the air, and the hawk would have gained height. Under actual conditions, the hawk spirals round, often in a rising column of air. No doubt many observers have noticed how he will sometimes get out of the column, and lose height in a downward swoop, only to find the column and again continue his upward course.
Sea-gulls following a boat illustrate gliding without losing height. The boat, in passing through the air, makes a good many eddies, among them, upward currents. When the birds encounter these, they can glide horizontally. Away from the ship, they do not glide without descending, because they lack the upward current made by the vessel. The same condition exists when a following wind blows at the same speed as the ship, indicated by the smoke rising straight upward. There are no eddies; hence, the birds must use their wings. Of course, at sea there are upward currents such as exist on land, caused by heat, or by opposite winds meeting; but the sea-gull, unlike the hawk, does not use them in soaring.
That upward currents are of importance is very evident to aviators. When flying on a hot day, over a body of water where heat-radiation is intense, the machine will rise five hundred feet or more in a minute or two, without any change of controls on the part of the pilot. Again, when about to land, upward currents will occasionally hold the machine off the ground perceptibly. On the other hand, downward currents are just as often encountered, allowing the machine to drop; but these currents are naturally diverted into a horizontal direction near the ground, and consequently are not a source of danger. In the early days of aviation, before the aeroplane had yet flown, one of the Wright brothers, in a glider, remained almost stationary in the air for nearly a minute, supported by an upward current. It is, therefore, not surprising that birds with large wing-surfaces in proportion to weight, with a natural instinct for, and a great sensitiveness to, the air-currents, can make use of them to fly upward without muscular effort.