Orville Wright
Readers of U.S. Air Service may possibly have overlooked the fact that Orville Wright as long ago as 1911 set up a world’s record for soaring by hovering over the same spot for nine minutes forty-five seconds. A writer in Discovery, the British “Journal of Knowledge,” points out that Orville Wright’s achievement “possibly remains unequalled as a demonstration of control by all the long gliding and circling flights, up to three hours in duration, made in Germany during August of this year.”—Editor’s Note
The soaring experiments of the past year in Germany, France and England have excited much interest in this branch of aviation. Several of these flights have been so sensational as to mislead some into thinking that other means than a motor have been discovered for supporting and propelling an airplane from place to place. Because sailplanes based on standard type airplanes have not always led in these contests others have been misled into believing the data secured from wind tunnel experiments to be erroneous and of no value, and that soaring and gliding experiments will supplant the wind tunnel and laboratory experiments in the investigation of aerodynamic phenomena. In my opinion this is not true. The chief factors in soaring flight are the hill, the wind, the plane and the skill of the operator, ranking in importance about in the order named. The plane best adapted to one hill and one wind condition may be ill-suited to another hill and another wind condition. There is not now and probably never will be a type of soaring plane most suitable for all conditions of wind and hill, any more than there is a most suitable type of airplane for all conditions of speed and loading.
Hentzen’s flight2 of three hours six minutes without a motor is remarkable for its duration only. No new principle of flight was utilized. The longer duration of the flight was due in part to improvements in the glider itself, natural results of the progress made in aerodynamics in the last ten years; in part to the skill acquired in handling aircraft during this period; but especially to the more advantageous topography of the ground over which the flight was made.
For several reasons a ridge is better than a conical hill for purposes of soaring. If the wind is too light for support when facing directly into it, additional velocity can be obtained by quartering across the wind along the top of the ridge. On a conical hill this is scarcely possible, since lateral motion across the face of the hill will bring the machine into currents of air having less and less rising trend as the machine gets farther from the apex of the hill. But another disadvantage of the conical hill is the fact that the current of air is split by the hill, a part of the air flowing around it to the right and a part to the left. This sudden change in direction makes control of the machine difficult—so difficult in fact that often the ordinary controlling surfaces on an airplane are entirely inadequate.
These were the difficulties encountered in the soaring experiments at Kitty Hawk, N.C., in 1911.3 The machine with which the experiments were commenced had elevators and rudders abundantly powerful for all purposes of power flight, yet under the conditions encountered on the conical face of the Big Kill Devil Hill these controls were so powerless as to allow, on one occasion, the machine to be turned completely over on its back. After the effectiveness of the controls had been greatly increased a number of flights were made of more than five minutes duration, the longest of which was nine minutes, forty-five seconds. This was the record for soaring flight from 1911 till 1921 when it was beaten in Germany by Herr Harth,4 who remained in the air twenty-one minutes without a motor, and this year by Herr Hentzen with a flight of three hours and six minutes.
The French held a soaring contest in August of this year in which the longest flight was five minutes, eighteen seconds—-inconsiderable in comparison with the German record, and not even so good as the American record of eleven years ago. But a mere record of time is not a certain criterion of the efficiency of the machine used nor of the skill of the operator, when records are made on different hills and under different wind conditions, since the wind and the hill are the two most important factors in soaring.5
No soaring flight has been made as yet by man, nor, as far as my own observation goes, by bird, in other than rising currents of air. The theory has been advanced that birds sometimes soar in horizontal winds, without any advantage from rising trends, by utilizing the irregularities in the velocity of the wind. Mouillard,6 the great French pioneer student of soaring flight, in “The Empire of the Air,” published in 1881, was the first, I believe, to put forth this theory. He explained the principle as follows:
The wind gust is the very essence of the uprise; it is the magic wand, which striking the child’s hoop, keeps it upright in rolling, drives it along, or raises it up to overleap elevations on its way. Suppose the toy to be placed on a steep inclined descent; gravity will cause it to roll to the bottom. If beyond this an ascending plane follows, the hoop, urged forward by momentum of acquired velocity, will rise to a height equal to that of fall, minus the losses by friction on the soil and by air resistance.
Let us suppose further, when the hoop is about to ascend, we can displace the ascending plane, in contrary direction to the toy’s course, so that the plane shall glide under the hoop, then we would still more assist the ascension.
Mouillard here states clearly the principle, which he imagined to be involved in soaring, but he gives no figures to show how great must be the irregularities in the wind to sustain the bird.
In 1893 Dr. Langley published his well-known paper, “The Internal Work of the Wind,” in which he supported Mouillard in this theory. He repeated at greater length, though scarcely more clearly, the proposition laid down by Mouillard. Langley furnished some measurements of irregularities found in winds of different velocities, but he made no attempt to calculate the amount of support a bird would be able to secure from them. The rate of acceleration in winds of ten to fifteen miles average velocity, as shown in his charts, was less than five per cent of what would be required to sustain a bird in soaring flight.
In calm air the buzzard is able to glide on a path descending about one foot in each eight feet forward. In other words, its resistance to forward travel is equal to one-eighth of its weight, and its velocity when gliding on a horizontal course will be retarded at a rate equal to
g (32 ft. per sec.)/8
or four feet per second. It follows that if the velocity of the wind increased constantly at the rate of four feet per second, the buzzard would be able to just sustain itself. But wind gusts are composed of retardations as well as accelerations in velocity. Since approximately only one-half of the time is occupied in acceleration the actual rate of acceleration would have to be much more than four feet per second in order that the average would be equal to that amount.
Soaring in rising trends of air deflected upward by hills, trees, waves, etc., always occurs on windy days. But it is well known that over level ground, where there are no hills or other obstacles to deflect the air upward, soaring is done on comparatively calm days and not on windy days when the fluctuations in the velocity of the wind are greatest.
I have seen thousands of buzzards in soaring flight over level ground on calm, sunny days, but I have yet to see one case of soaring over this same ground on a windy day with the sky overcast. If the bird depended on getting its support from the “internal work of the wind” it would do its soaring on days when the fluctuations of the wind were greatest, instead of on days when they were least. My brother and I calculated many years ago the support to be obtained from gusts of greater intensity than those actually encountered in nature. I believe that anyone who takes the trouble to make this calculation will be convinced that the explanation of soaring flight is not to be found in the internal work of the wind.
On the other hand it is well known that soaring can be done in rising currents of air produced not by objects on the ground, but by the difference in temperature of the air at the surface and that above. These currents of rising air are most frequent on calm days when the sun is shining. We see them in the whirl winds which lift leaves and dust into the air. When you see one of these “whirls” stirring up the dust on a country road, look into the sky, a little to the direction toward which the wind is blowing. You will often find a buzzard circling there. He circles to keep within the area of the “whirl” which increases in diameter with height.
An aviator frequently runs into these rising currents and feels a slight “bump,” but as the diameter of the “whirl” is usually not much in excess of a hundred feet his machine is out of it in a second. With the lightly loaded, slow machines of ten to fifteen years ago these “bumps” were more pronounced and the distance the machine was lifted by one of them was much greater than that experienced by the fast machines of today. In 1910, while training some of the early aviators, I had an unusual experience of this kind near Montgomery, Ala.7
I had ascended to a height of a little over one-half mile and was descending when at a height of about fifteen hundred feet I suddenly discovered that I was not able to descend further, although my motor was throttled to the limit and the machine was pointed downward as steeply as I felt it safe to point it. I remained at a height of about fifteen hundred feet for a period of five minutes without making any appreciable descent. Suddenly the machine again began to descend and was on the ground in less than a minute.
The flight was made in an almost perfect calm. The descent was in a spiral of not more than five or six hundred feet diameter. This probably accounted for the long time the machine remained in the uptrend of air. No doubt, if I had steered out of the spiral into a straight course, I would have been out of the rising trend in a few seconds; but I did not think of this at the time. In fact, I was so astonished that I did not think at the time of any reason for the phenomenon. But it is evident the machine was in a whirlwind of unusual diameter, in which the air was rising as fast as the machine could descend. These whirlwinds and other rising trends of air are not present everywhere and on some days do not exist at all. We, therefore, cannot hope to get much use of them as a means of travel.
The news reports of the contests abroad have created in the public mind an exaggerated impression of the importance of this kind of flying. My brother and I, as well as Lilienthal, Chanute and others before us, experimented in this way. We found, however, that while it was a most delightful sport, and furnished a safe and cheap means of acquiring skill in operating an airplane, it was too slow and expensive as a means of obtaining scientific data for the design of aircraft. I feel safe in predicting that it will never rival the powered airplane as a means of transportation nor the wind tunnel as a means of obtaining more exact scientific knowledge in aerodynamics.
1. U.S. Air Service, December 1922, 7–9.
2. Heinrich Hentzen set a glider duration world record of three hours, six minutes, on August 26, 1922.
3. The last of the Wrights’ flying at Kitty Hawk was done by Orville in 1911 with a new glider. The aircraft was built to test the brothers’ prototype automatic stability device. But several reporters were there when Orville and his party arrived, so to avoid premature publicity, he decided not to test the automatic pilot. The flights were thus confined to testing the glider and experimenting with soaring. After some test flights and modifications to the glider, Orville made a spectacular flight of nine minutes, forty-five seconds, on October 24.
4. The precise time of Frederic Harth’s flight was twenty-one minutes, thirty seconds.
5. [This footnote appeared with the original article:] Hentzen’s record has been broken lately in the Daily Mail contest at Lewes, England, by the Frenchman [Alexis] Maneyrol with a record of three hours twenty-one minutes.
6. See Wilbur Wright’s comments on the work of Louis-Pierre Mouillard in document 38: “What Mouillard Did.”
7. In the spring of 1910, Orville Wright trained pilots at a winter flying site in Montgomery, Alabama, for the brothers’ newly created Wright exhibition team and for the U.S. military, which had recently contracted with the Wrights to train army pilots. The flights were made on the site of the present Maxwell Air Force Base.