By L. HARGRAVE.
[Received Aug. 24. Read before the Royal Society of N. S. Wales, Sept. 6, 1899.]
There are many birds frequenting the southern oceans beyond the limits of the S.E. trade winds that are not adapted for soaring, and yet they circle, glide and swoop around without flapping their wings. These have been well called sailing birds; and it is one of their oft repeated evolutions that shows me, and I hope you, that sailing flight is not at all incomprehensible.
I will first point out that the tropics are the home of heavy short-winged birds, such as gannets, boobies, divers and small gulls. These seldom make any attempt to glide, much less sail or soar. The only exceptions I know are the frigate bird, and the boatswain bird; these two soar at high altitudes for long distances on motionless wings.
It is worthy of remark that large flocks of sailing birds accompany vessels running down their eastings and this gives an opportunity for observing whether sailing birds really can work to windward, this point can only be determined from on board a steamer going westward, and south of what I believe is the usual track from Australia to the Cape.
My own opinion is that sailing birds cannot make anything to windward except during the limited time that the sea is running in an opposite direction to the wind: and as an argument I call attention to the absence of sailing birds in the S.E. trades and attribute the scarcity to their inability to get out of the trades by standing to the S.W, if they get too far to leeward.
The most ordinary conditions for observing sailing birds are then the wind and sea are both aft. The waves are probably overtaking the ship and passing at about six knots. Large numbers of birds follow the vessel and make wide circuits on either aide of the wake; their interest is centered an garbage, and their efforts are directed to keeping astern, their weight and area are such that they must keep moving through the air at a nearly uniform speed in order that they may be supported; this velocity I estimate at forty miles per hour.
If you direct your attention to the position of a bird with regard to the wave surface, it will speedily be noticed to be nearly always on the rising side or face of the wave and moving apparently at right angles to the wave's course, but really diagonal to it.
The bird is going to leeward as fast as the wave is; and, if that speed is too great for its requirements it turns towards the crest, points one wing to the sky and uses its velocity to shoot upwards high above the back of the wave, and then descends to the trough of the following wave along the face of which it glides: the back of the wave is its peculiar aversion. Now there has been no flapping and the performance takes place with or without wind, all the bird requires is the wave.
As to the effect of the wave on the air, we will suppose the water to be quite flat and the air motionless, a heavy undulation comes on the scene, it has to pass, so it pushes the air up with its face, letting it fall again as its back glides onwards. The air on the face is slightly compressed, that on the back lowered in pressure, both operations taking power out of the wave and eventually largely contributing to its extinction.
The closer the bird is to the surface of the water, the firmer and more inelastic is the uplift of the rising air. The bird appears to almost feel the surface with the tip of its weather wing.
The case I wish you to consider is that of a sea-wave, for example one hundred and eighty feet long and ten feet high, travelling at eighteen knots, or say, thirty feet per second under calm air. This wave will raise all the air as it passes ten feet, at the mean rate of three and one-third feet per second. The rate will vary from zero in the trough, attaining its maximum velocity at half the wave height, or where the wave is steepest, and falling to zero at the crest. Let us suppose the maximum velocity of uplift of the air to be about four feet per second and the steepest part to be 10 degree slope.
Now according to Prof. S. P. Langley, a plane surface 30" x 4·8", weighing l.1 lbs. will glide on air, without losing its elevation, at a speed of 49·8 feet per second, if sloped 5 degrees. That is, the plane pushes 4·33 feet of air vertically downwards whilst it is translated 19·8 feet in 1 sec.
The same effect with regard to the position of the plane at the end of its journey of one second's duration is produced if the plane be sloped 5 degrees downwards, and the air through which it passes be pushed bodily upwards 4·53 feet in one second.
Now the air over our wave is being lifted about four feet per second; so if the 1.1 lb. plane were launched with 5 degree downward slope in the same direction the wave is travelling, from one foot above the steepest part of the wave, it would overrun the wave which has only a velocity of thirty feet per second. It would thus get out of the Air that is being lifted and shoot into the water in the trough. But if the aspect of the plane be changed so that it face 353 degrees either to the right or left of the track of the wave, its position above the mean sea level, and situation on the wave slope will be unaltered: and, if the wave was of unlimited width the plane would continue on its course till dashed ashore.
The plane is simply abstracting the power stored in the wave by a distant gale, and using it to counteract gravity. And if the work be continued long enough, or a multitude of planes be continually drawing on the reservoir of power, the wave must inevitably be flattened.
The velocity of 49.8 feet per second is sufficient to raise the plane to an elevation of thirty-eight feet in one and a half seconds, if its coarse be changed from horizontal to vertical, it there comes to rest And from a poise at this station the plane may swoop down, at great disadvantage if close to the back of the wave, at various slopes and directions till it cuts into the air that is being raised by the face of the following wave, which again enables it to resume its velocity.
Observe that the wave I instance in this example, is one of the low round topped sort that prevail in calm weather. If we were to base our calculations on a wave with a sharp crest approaching to the breaking dimensions, our plane would be travelling on its course through air having a velocity of uplift of 30 instead of 4·3 feet per second, if the wave slope were 45 degrees; and would need loading approximately to 76 lbs. per square foot to keep it down to its original mean height, and could be made of seven gauge wrought iron.
If we figure out the result with 2 degree angle of incidence, and a horizontal velocity of 65.6 feet per second, we find that the 1.1 lb. plane will be supported where the wave face is only 4.5 degree slope, giving a velocity of uplift of 2·289 feet per second, and will make a course 69 degrees 40 minutes to the right or left of that of the wave. Couple this with the fact that the head resistance of a sailing bird's form and the delicate arch of its wings are the survivals of untold numbers of cruder types, and no surprise should be felt at any intricate tactics pursued when further sided by the power derived from the wind and roughened sea.
This is the solution of the problem of a sailing bird's progression totally denuded of complications. It becomes a giant's task to compute the result when the effect of cross seas, wind at all angles and ever varying force, arched surfaces, head resistance, ratio of weight to area, and the intelligence of the guiding power crop up. These questions all combined, have been considered in the evolution of a sailing bird and must be reckoned with by the designer of a wave driven flying machine. I am not aware that anyone has attempted to show that sailing flight by wave-power alone is a practicable art, but even if some one else has done so, an observation from an independent source confirming old work cannot fail to be of interest.