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Aviation History
1909
1909 - 0691.PDF
OCTOBER 30, 1909 HOW TO GLIDE. The 1902 Model. THE 1902 pattern was a double-deck machine having two surfaces each 32 ft. from tip to tip, and 5 ft. from front to rear. The total area of the main surfaces was about 305 sq. ft. The front rudder spread 15 sq. ft. additional, and the vertical tail about 12 sq. ft., which was subsequently reduced to 6 sq. ft. The weight was 116^ lbs. Including the operator, the total weight was from 250 to 260 lbs. It was built to withstand hard usage, and in nearly a thousand glides was injured but once. It repeatedly withstood without damage the immense strains arising from landing at full speed in a slight hollow where only the tips of the wings touched the earth, the entire weight of machine and operator being suspended between. The practice ground at the Kill Devil hills consists of a level plain of bare sand, from which arises a group of Turning to the right. A view from behind oE the later 1902model after it had its rear tail converted into a rudder. detached hills or mounds formed of sand heaped up by the winds. These hills are constantly changing in height and slope, according to the direction and force of the prevailing winds. The three which we use for gliding experiments are known as the Big Hill, the Little Hill and the West Hill, and have heights of 100 ft., 30 ft. and 60 ft. respectively. In accordance with our custom of beginning operations with the greatest possible caution, we selected the Little Hill as the field of our first experi- ments, and began by flying the machine as a kite. The object of this was to determine whether or not it would be capable of soaring in a wind having an upward trend of a. trifle over 70, which was the slope of the hill up which the current was flowing. How to Test a Glider, When I speak of soaring, I mean not only that the weight of the machine is fully sustained, but also that the direction of the pressure upon the wings is such that the propelling and the retarding forces are exactly in balance ; in other words, the resultant of all the pressures is exactly vertical, and therefore without any unbalanced horizontal component, A kite is soaring when the string stands exactly vertical, this showing that there is no backward pull. The phenomenon is exhibited only when the kite is flown in a rising current of air. In principle soaring is exactly equivalent to gliding, the practical difference being that in one case the wind moves with an upward By WILBUR WRIGHT. (Continuedfrom page 673.) trend against a motionless surface, while in the other the surface moves with a downward trend against motionless air. The reactions are identical. The soaring of birds consists in gliding downwards through a rising current of air which has a rate ot ascent equal to the bird's relative rate of descent. Testing a gliding machine as a kite on a suitable slope, with just enough wind to sustain the machine at its most favourable angle of incidence, is one of the most satisfactory methods of determining its efficiency. In soaring, the kite must fly steadily with the string vertical or a little to the front. Merely darting up to this position for an instant is not soaring. On trial we found that the machine would soar on the side of a hill having a slope of about 7°, whenever the wind was of proper force to keep the angle of incidence between 4° and 8°. If the wind became too strong or too weak the ropes would incline to leeward. In one test, forming the subject of an illustration, the surfaces are inclined 40 above the horizon. Since the wind had an upward trend of 70, the total angle of incidence was n°, which is outside the limits specified. On steeper slopes the ropes inclined to windward quite strong. In experi- menting on this plan, it is essential that a uniform slope be found which will give the air current a rising trend just sufficient to cause the kite string to stand vertical. Then both gravity and the pull on the string, which together provide the force counteracting the wind pressure on the surfaces, are applied in a single direction. It is, therefore, not material what proportion of the total counteracting force is due to each of the several components nor even what is their total amount, because the experiments are exclusively for the purpose of determin- direction of the pressure on the surfaces by When the kite ing the observing the direction of the reaction, string inclines to windward the slope is too steep, if to leeward not steep enough. But it is not advisable to attempt to determine how much the slope varies from the proper amount by observing the angle of the string from the vertical, for when the pull of the string differs in direction from that of gravity, it becomes necessary to know not only the angle, but also the exact amount of the pull and the proportion which it bears to the weight of the kite. It is, therefore, advisable to hunt a better slope rather than attempt to make so many observations. A Little Adventure. The kite experiments having shown that it ought to be possible to glide on the 70 slope, we next proceeded to try it. Although on this first day it was not considered advisable to venture upon any absolutely free flights, the machine soon demonstrated its ability to glide with this angle of descent. At a later period we made more than a hundred flights the full length of this slope and landed a short distance out on the level ground. On the second day the machine was taken to the Big Hill, and regular gliding was commenced. The wind was somewhat brisk. In one flight the wind struck the machine from the left and began lifting the left wing in a decidedly alarming manner. Owing to the fact that in the new machine changes had been made in the mechanisms operating the 693
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