In some early experiments in 1887 with the small models without motor power, which have not been particularly described, two pairs of wings, in the same plane, were employed for reasons connected with stability. Afterward, in many of the rubber-driven motor models, which have been described in Chapter II ◊, two large front wings were employed and the following pair were diminished into what may properly be called a tail. This plan was a retrogression in design, and it was pursued by the writer with a pertinacity which was not justified by the results obtained, being used even on the early rubber-driven models.
In this construction, it will be observed that the flat tail was in fact not only a guiding but a sustaining surface, since it bore its own share of the weight. It was not until a much later date (November, 1895) that the writer returned to his earlier construction of two pairs of wings in the same plane bearing the whole weight of the aerodrome, to which was now added a flat tail, whose function was not to support, but wholly to guide. This was developed into the final construction by the addition of a vertical rudder or rudders.
The present chapter is not concerned with the history of the earlier attempts with small models, or of those numerous constructions of sustaining surfaces which were never put to actual trial; nor does it give any description of the experiments which were made in placing one set of surfaces over the other, according to a method suggested in “Experiments in Aerodynamics.”27
The experiments in “Aerodynamics,” and the theoretical considerations given in Chapter V ◊ on sustaining surfaces, would never alone have led to the construction which was finally reached, which was largely due to the hard lessons taught by incessant accident and failure in the field. The present chapter, therefore, should be read in connection not only with the pages of “Aerodynamics,” but with Chapters V and IX ◊ of this book.
It is to be remembered that, while the center of gravity of the aerodrome could be determined readily and exactly, the center of pressure could be determined only approximately in advance of trial in actual flight. The positions [p081] of the supporting surfaces given in this chapter are, then, approximations made from rules for “balancing,” i. e., for obtaining equilibrium in actual flight, rules which are in fact tentative, since they are founded on a priori considerations with partial correction from the empirical knowledge gained by previous field trials. For these rules see Chapter VI ◊.
With reference to the supporting and guiding surfaces of Aerodromes Nos. 4, 5, and 6, Aerodrome No. 4, in its earliest condition mentioned in the preceding chapter, was taken into the field, but never brought to trial in the air. It is sufficient to say that in the largest of the three sets of wings constructed, each wing was 17 × 51 inches, and therefore contained about six square feet, so that with the tail (which was at this time a supporting surface), whose area was one-half that of the two wings, the total supporting surface was 18 square feet, or since the flying weight was 9.1 pounds, the proportion of surface to weight was somewhat less than 2 square feet to the pound. The wings were at this time ribless, it being expected that the silk cover which was purposely left loose would take its curve from the air filling it, which subsequent experience has shown would have led to certain disaster if the aerodrome had been launched. It may be added that there was a vertical rudder of what is now seen to have been a wholly inadequate size. These remarks may be applied with little modification to the attempted flight with No. 4 on May 25, except that the vertical rudder had been made larger, but was still much too small.
From the account of the field trials to be given in Chapter IX ◊, it will be seen that in numerous attempts at flight prior to October 6, 1894, the cause of failure can in every instance be traced to imperfections more fundamental than those of the sustaining surfaces, either the launching device or some other part failing to work satisfactorily. I therefore commence a description of the sustaining surfaces with those of Nos. 4 and 5 as used on that day.
The construction of the wings of No. 4 and No. 5, which were nearly identical, is shown in Fig. A Plate 16. A rod of hickory, tapering from 12 inch in diameter at the larger end to 14 inch at the smaller, was steamed and bent, as shown in the drawing, to form the main front rib of the wing. This was firmly clamped to the midrod, and to the rib in turn were attached a number of cross-ribs of hickory, slightly curved, the inner one of which was fastened to the hull at its inner extremity, while the whole was covered with silk. The length of each wing was 162 cm. (63.75 inches), and the width 54 cm. (21.25 inches). The tail was plane and equal in area to one of the wings, so that the joint area of the wings and tail was 2.62 square metres (28.2 sq. ft.). [p082]
Each wing was attached to the midrod by a single clamp, different forms of which are shown at F, G, H, I (Fig. 16). The clamp consisted of two short split tubes, into which the main front ribs were securely clamped by means of screws. They were set at an angle and united to a grooved frame, by which the wings could be readily attached to a second piece clamped about the midrod. The tail clamp, like the wing clamp, was composed of two pieces, sliding one upon the other, but as the tail formed a single surface, one part was permanently attached to it. Clamps F, G were fitted to aerodrome No. 4, and H, I to No. 5. The wings were set at a diedral angle of about 150°, but as they were not guyed in any way, this angle in flight and under the upward pressure of the air probably became much less. The tail was plane but ribbed like the wings.
In preparing the machine for flight, the wings and tail of No. 4 were set at a very small root angle with the midrod, perhaps not exceeding 3°, but while this angle might be maintained at the firmly held root of the wing, it was later seen to be probable that the extremity of the wing was flexed by the upward pressure of the air after launching, though the full extent and evil effect of this flexure was not recognized at the time. In the approximative calculations for “balance,” made at this time, the tail was treated as bearing 13 of the weight of the aerodrome, as it was 13 of the supporting area, for though it was recognized that its position in the “lee” of the wings rendered it less efficient, the degree of this diminution of efficiency was not realized. A vertical rudder 20 cm. × 70 cm. (8 in. × 28 in.), with an area of 0.14 metres (1.5 sq. ft.) was used. [p083]
The particulars of the launch will be found in Chapter IX ◊. In the present connection, it is sufficient to say that though launched with the requisite velocity and without accident, it fell into the water at a distance of about 15 metres (49 feet) with the midrod nearly horizontal, the combined effect of engines and initial impulse having in fact kept it in the air for less than two seconds. The true cause of this failure not then being recognized, it was attributed to the angle of the wings with the midrod having been too small.
The launch of No. 5 followed almost immediately, but taking warning by the supposed cause of failure of No. 4, its wings were set at a root angle of 20°, and a hurried adjustment was made to secure greater rigidity, the tip being partly secured against twisting by a light cross-piece, and guyed so that the wing as a whole was not only at a greater angle, but stiffer than in the case of No. 4. These changes it was hoped would cause the aerodrome to advance at a considerable initial angle with the horizontal, and it did so, for instantly after the launch, as the aerodrome escaped from its bonds into free air, the inclination of the midrod increased until it stood at about 60°, when the machine, after struggling a moment to maintain itself, slid backward into the water (with its engines working at full speed) after advancing about 12 metres (39 feet), and remaining in the air about 3 seconds.
On the whole, the result of the first actual trial of an aerodrome in the field was disconcerting, for unless the result was due to the wings being placed in a position wholly unfavorable to support, there seemed to be no doubt that either the engine power or the supporting surface was insufficient. Now this engine power was by computation between three and four times what was necessary to support the aerodrome in horizontal flight at an angle of 20°, and after making every allowance for slip, there should have been still an excess of power for the first flight of No. 4, whereas actual trial indicated that it was insufficient. But on the other hand, the experiment with No. 5, which momentarily held its position in the air at an angle of 60°, seemed to indicate that the engine power was abundant, and that the failure must be traced to some other cause.
As a result of these experiments it was concluded, “that it is an all-important thing that the angle of the front wing shall be correct, and that this cannot be calculated unless it is known how much the tip will turn up under pressure of the weight.” I felt, then, that I had learned something from the failures as to the need of greater rigidity of the wings, though how to obtain this without adding to their weight was a trying problem. It was thus at an early stage suspected that the evil to be guarded against in wing construction was the distortion of the form of the wing under pressure, chiefly by torsion, which is specially hard to provide against without a construction which is [p084] necessarily heavy. This suspicion was a correct one, though the full extent of the evil was not yet surmised.
In the light of subsequent experiment it may now be confidently stated that the trouble was with the wings, which at the moment after launching were flexed wholly out of the shape which they were designed to have, and which they retained up to that critical moment.
After returning to Washington, one of the wings was inverted, and a quantity of sand, equal in weight to the pressure upon the wing in flight, was added, under which the yielding at the tip amounted to 65°, or from +20° to −45°, showing that the wings were entirely too weak to sustain the aerodrome.
In speaking of the efforts to strengthen the wings, it must be constantly remembered that this could hardly be done in any way which did not involve increased weight; that is, it could hardly be done at all, since increased weight was forbidden.
The first attempt at systematic guying was made on October 27. As shown in Fig. B, Plate 16, two guy-posts extending beneath the midrod were connected by guy-wires with the outer extremities of the wing, by means of which it was sought to hold the wing in place and prevent its extremity from twisting upward, while a third wire connecting with the bowsprit prevented its moving backward. In addition, two aluminum wires, stretched across above from wing to wing, kept the lower guys tight.
On October 27, Aerodrome No. 5, equipped with large new wings and tail, having a combined area of 3.7 square metres (40 sq. ft.), the wings being each 64 cm. × 192 cm. (25.25 in. × 75.75 in.), turned sharply and completely round, apparently through some internal current of the main wind against which it was advancing. Owing to this almost instantaneous turn, it lost headway and came down. This led to the subsequent construction and use of a much larger vertical rudder, intended to prevent in future any such sudden pivoting and consequent loss of momentum. The wings showed a tendency to “pocket”28 and bag, which indicated some serious fault in their construction.
As a result of these experiments, it was decided on October 29 to attempt to make the wings stiffer (though their weight was almost prohibitory), by inserting more cross-pieces, cross-pinning and guying them so as to make them more rigid as a whole, and less liable to pocket.
At this time an automatic device in the form of a sliding tail was designed, which it was thought would cause the center of pressure to move backward when the aerodrome reared, and forward when it plunged downward, but the device, though afterward constructed, was never brought to trial in the field.
Aerodrome No. 5, equipped with a new set of wings similar to those used [p085] on October 27, and guyed as in the previous experiment, was again launched on November 21, with the results recorded in Chapter IX ◊. The failure was attributed to the twisting of the wings under pressure to such an extent that not only was their effective area greatly reduced, but the outer portions were upturned so as to catch the air upon the upper surfaces, the result being in part a downward pressure.
On the following day a pair of the wings was inverted and a weight of sand equal to the air pressure to which they were subjected in flight, was distributed over their surfaces. Under the action of this, the twisting of the wing was seen to increase from the root, which was held with comparative rigidity, up to the tip, where in spite of the cross-ribs it amounted to 45°. The resistance to torsion lay chiefly in the front rib, which, in addition, could be bent easily, allowing the surface to become distorted with great loss of lifting power.
The experiments of 1894 had demonstrated the urgent necessity for greater rigidity in the sustaining surfaces, which might, as it seemed, be obtained either by increasing the strength of the framing (which meant additional weight) or by resorting to some new and untried construction, or by a proper system of guying. Guying seemingly offered the most feasible solution of the problem; but although the system of wire guying was thoroughly tried, the result was very unsatisfactory, as the wings continued to twist and bag in a way that was extremely discouraging.
I accordingly had recourse in 1895 to the system of wooden guy-sticks shown in Fig. D, Plate 16, which necessarily added greatly to the weight of the sustaining surfaces. Each wing was separately strengthened by means of a light rod of spruce, in cross-section about the size of the main front rib, extending across the upper surface of the wing, at a distance of about one-third the width of the wing behind the front rib. It was tied to each of the cross-ribs and to the outer bent portion of the front rib, and at its root was fastened to the frame of the aerodrome.
This effectually prevented the bending of the front rib and the consequent bagging of the cover, and to that extent marked a decided advance in wing construction. But it was faulty, in that, not being supplemented by wire guying, it offered little resistance to the twisting of the wing about the main front rib, the rear tip of the wing being free to turn up under pressure, as it had done on former occasions. A similar guy-stick was stretched across the tail. To guard against torsion, rods extending diagonally across the wings and tail were used, which, with the aid of the guy-sticks just described, prevented the surfaces from twisting greatly. In addition, a rod joining the front ribs and stretching across from wing to wing tended to maintain a fixed diedral angle. [p086]
The wings as thus guyed were rigid enough, and in the field-trials of No. 5 on May 8 and June 6, did not yield noticeably under pressure, and there seemed to be no serious default in their lifting power, but the guy-sticks were heavy and the system was not again employed. The wings used in these trials, shown in Fig. C, Plate 16, had a frame of hickory, consisting of a front rib and nine cross-ribs, over which the silk was tightly stretched. The curvature of the wings, which is shown in the cross-sectional drawing, had a rise of about one-twelfth the width, the highest point of curvature occurring about one-fourth the distance from front to rear. Each wing was 64 cm. × 192 cm. (25.25 in. × 75.75 in.), the two with the tail, in surface equal to a single wing, having an area of 3.7 square metres (40 sq. ft.). The combined weight of the wings was 1150 grammes (2.53 pounds), and of the tail, 583 grammes (1.28 pounds).
The evolution of a vertical rudder had meanwhile been going steadily forward. Those first used had been small, rectangular, stiff, and heavy, but in the experiments of May 8 a much lighter and larger construction, consisting of a frame 92 cm. × 76 cm. (36 in. × 30 in.) covered with paper, was used, and on June 7 this was replaced by a long, diamond-shaped rudder, having a spruce frame covered with silk, very light and seemingly more effective than any hitherto used.
I had in the meantime designed a “tail-rudder,” consisting of a horizontal tail and vertical rudder combined, each having an area of about 0.6 square metres (6.5 sq. ft.) which, however, was not used until 1896.
In August was begun the construction of a deeply curved and arched pair of wings for No. 4, which consisted of a light framing of spruce elaborately guyed and covered with gold-beater’s skin drawn tight as a drum-head with pyroxelene varnish. In their construction a new feature, foreshadowed in the method of guying the separate wings used in the field-trials of May and June, was introduced, which was adopted in all subsequent constructions—the guy-stick, previously described as stretching lengthwise across the wing being now made a part of the wing itself, which was thus provided with two longitudinal ribs instead of one. The additional rib occupied a central position, and like the front rib was attached to the midrod by means of a strong wing clamp. Its outer end was united to the front rib, which was here bent into a quadrant of a circle. This pair of wings had an expanse of 435 cm. (14.3 feet), an area of 2.5 square metres (26.8 sq. ft.), a weight of 660 grammes (1.45 pounds), and a depth of curvature equal to one-tenth their width.
This construction offered a two-fold advantage in its resistance to both torsion and bagging, for as the pressures upon the wing were nearly balanced about the middle rib, the tendency to twist was reduced to a minimum, while the bagging, which results from the bending of the framework, as distinct from [p087] its twisting, was greatly reduced by the manner in which the frame was put together, the whole construction permitting a return to the system of wire guying at first adopted, which had been found inapplicable to a wing having but a single longitudinal rib forming its front margin. When completed, the wings were strongly guyed with piano wire, both above and below, to guy-posts attached to the midrod, and each cross-rib was separately guyed with wire chords. Although these wings had cost much in time and labor, and contained many points of improvement, they were eventually found to be too weak to support the aerodrome, and were therefore abandoned without a trial in the field.
For the plane horizontal tail hitherto used a pair of curved wings was substituted, similar in all respects to those just described, but having only half their area, and these were later replaced by a pair equal in size and in every way the counterpart of the front wings. The tail as hitherto used accordingly disappeared, and gave place to another having a wholly different function to perform; for while the old tail, like the rear pair of wings which superseded it, was intended to bear a definite part of the weight of the aerodrome, the new tail which was now added behind the rear pair of wings was not supposed to bear any part whatever of the weight, but to act solely as a guide, and this new feature, first introduced in October, 1895, was continued to the end.
This arrangement of the surfaces is quite different from that adopted by Pénaud in 1872, in which the tail became automatic in its action through its small angle of elevation as compared with that of the wings, while still acting as a supporting surface, whereas in the present arrangement the function of the tail was solely one of guidance. This, I believe, was one of the important changes which perhaps as much as any other led to final success.
During the fall of 1895 a large number of experiments were made both in free flight with gliding models, and in constrained flight with the whirling-table, to determine the relative lifting power of the front and rear wings per unit of area, and from these the following new rules were deduced for finding the center of pressure:
If a following wing is the size of the leader, assume that its efficiency is 66 per cent per unit of surface.
If it is half the size of the leader, assume that its efficiency is 50 per cent per unit of surface.
If it is half as large again as the leader, assume that its efficiency is 80 per cent per unit of surface.
For intermediate sizes of surface, proportionate values per unit of surface may be assumed.
If we consider the area of the front wing to be unity, and that of the rear wing to be n, and if m be the efficiency of the rear wing per unit of surface, [p088] the above is expressed in the following formulæ, which it will be remembered take account only of wings following each other in the same or nearly the same plane, and are not applicable where one wing is either above or below the plane of the other. In the formulæ, CP is the resultant center of pressure upon both wings expressed in the notation described in Chapter II ◊, CPfw is the center of pressure of the front wings, and CPrw the center of pressure of the rear wings.
If the value of n lie between one-half and unity,
while if the value of n lie between unity and 112,
In either case
where the leading and following wings are equal
The steady flight of one of the gliding models referred to led to the construction of a new set of wings for No. 5, patterned after those used on the gliding model. These wings, shown in Plate 17, were rectangular in outline, 200 cm. × 80 cm. (6.56 ft. × 2.62 ft.), each wing having an area of 1.6 square metres (17.1 sq. ft.) They were constructed with spruce framing covered with China silk, and were strongly guyed with piano wire in much the same manner as the light, skin-covered wings already described, which had preceded them. The combined weight of the two pair was 1950 grammes (4.3 pounds).
The long central rib was now much the larger of the two which, as in the preceding wing, formed the foundation of the structure. It occupied a position two-fifths the distance from front to rear, and presumably coincided at all points with the center of pressure of fore and aft sections of the wings, so that the pressure in front of the rib was at all points balanced by the pressure in the rear, and there was consequently little tendency in the wing to twist under pressure of the wind. The two main ribs were rigidly connected by cross-ribs of spruce, 20 cm. (8 inches) apart, steamed and bent to the desired form. The curvature of these ribs was the same for all, and in depth was one-twelfth the width of the wing, while the highest point of curvature was one-sixth of the distance from front to rear, these ratios having been chosen as approximating those found in the wing of the soaring bird. These wings were subsequently used in the first successful flights of the following year.
During the year 1895 but two field-trials were made with the steam aerodromes, and neither of these was successful; but a great step forward had been taken in the construction, guying and arrangement of the sustaining surfaces. The wings had been made stronger with no increase in weight per unit of area. On the contrary, the ratio of weight of sustaining surfaces to area had been actually reduced from 43 to 28 grammes per square foot, so that the surfaces were both lighter and stronger.
Two longitudinal ribs had taken the place of the single one before used, a second wing clamp had been added to correspond to the midrib, the difficult problem of torsion had been effectually solved, the system of guying greatly improved, and it appeared that in the next trial the wings might be expected to bear the weight of the aerodrome without serious distortion.
In January, 1896, two new pairs of wings were designed for No. 6, and in order to give a greater efficiency to the rear wings, they were made larger than the front ones, the area of the latter being 22 square feet, and of the former 27 square feet, and whereas the width of each wing had formerly been one-third of its length, it was now increased to two-fifths to correspond to those of No. 5.
The progress made in construction and guying is shown by the fact that when on January 28 one pair of the wings of No. 5 was inverted and sanded, the yielding at the tip was less than 5° greater than at the root, whereas at one time it had been 65°. A similar test applied to a pair of wings of No. 6 on March 4 gave even better results, as the yield at the root was but 1° 45′, and at the tip 2° 30′.
The successive stages of the development of the wing clamps are shown in Fig. 16. In its final form the front wing clamp, or that which held the main front rib, shown at AB (1896), had adjustable sliding pieces, by means of which the wings could be set at any desired angle of elevation, the wing as a whole revolving about the rear wing clamp, shown at CD (1896).
The general system of guying the wings, as shown in Plate 17, had been greatly improved. In the present form a bowsprit and guy-posts firmly attached to the midrod furnished points of attachment for the piano wires with which the wings were guyed and held rigidly in place, other wires being stretched across from wing to wing so as to maintain them at a constant diedral angle of about 150°. The clamps by which the guy-posts were attached to the midrod, are shown at EF (Fig. 16).
In the successful flights of No. 5 on May 6, the completed wings already described weighed together 1950 grammes (4.29 pounds), and had a total sustaining area of 6.4 square metres (68.8 square feet), the flying weight of the [p090] aerodrome was 11,775 grammes (26 pounds), and the sustaining surfaces therefore amounted to 2.6 square feet to the pound, which, as the event proved, was amply sufficient.
The “tail-rudder,” shown in Plate 17, comprised a vertical and horizontal surface of silk intersecting in a central rod or axis, having a length of 115 cm. (3.8 feet). The framing was of spruce and consisted of two sets of four arms, each radiating from the central rod, the hexagonal outline of the surfaces being formed of piano wire, over which the silk was drawn and sewed. The area of each surface was about 0.6 square metres (6.45 square feet), and the total weight was 371 grammes (0.8 pounds).
A flat steel spring inserted in the forward end between the rudder and the midrod gave it a certain desirable degree of elasticity in a vertical direction. The rudder was held in place by a pin passing through the midrod, and was so set as to coincide with the line of direct flight, its purpose, as already explained, being to guide the aerodrome, but to take no part in its sustention.
In balancing Aerodrome No. 5 on May 6, the wings were so adjusted that in accordance with the notation given above, p. 15:
and as the wings were of equal size, from what has preceded in the present
The center of gravity was located at 1497, so that there should have been a very slight tendency on the part of the aerodrome to rise, as was actually the case. The formula was perhaps not quite so accurate as the prolonged flight of the aerodrome would seem to indicate, as it takes no account of the thrust of the propellers, which in action tended to elevate the aerodrome in front while their resistance would tend to depress it when they had ceased to revolve, which consideration accounts for the action of the aerodrome on May 6, as described in Chapter IX ◊. The formula may, however, be regarded as approximately correct.
In the final successful trial with No. 6 on November 28, 1896, the wings used were similar in general construction and manner of guying to those of No. 5 on May 6, but, as shown in the photograph (Plate 29A, Chapter X), the front rib at its outer extremity was bent to a quadrant to connect with the midrib, this construction being somewhat stronger than that adopted in the wings of No. 5. The curvature was but one-eighteenth of the width of the wing instead of one-twelfth as in No. 5. The front and rear pairs were similar and equal and had a combined area of 5 square metres (54 sq. ft.), and a weight of 2154 grammes [p091] (4.74 lbs.). The flying weight of the aerodrome was 12,120 grammes (26.7 lbs.), the sustaining surface thus amounting to slightly more than 2 square feet to the pound.
The position of the wings, in accordance with the notation adopted, was
Since the wings were equal in size,
The center of gravity was located at 1484, which was 3.5 cm. in the rear of the center of pressure. The flight was approximately horizontal, and the setting seems to have been as accurate as could be desired. The angle of elevation of the wings at the root was 10° 30′, and so well were they guyed that there was no visible yielding at any point during the flight. As the midrod during flight was approximately horizontal the angle of elevation of the wings may be taken as 10° 30′; the efficiency of the rear wings was two-thirds that of the front wings, and the effective area was therefore 27+27×23 = 45 square feet.
The wings being very nearly plane we have therefore the data for determining the soaring speed from the formula of “Aerodynamics” (Chapter VI, p. 60).
in which W = 26.7 pounds; A = 45 sq. ft.; k = 0.00166; α = 10° 30′; F (α) cos α = 0.353. By substituting these values in the formula we obtain V = 32 feet per second.
The speed actually attained, however, was about 30 miles an hour, or 44 feet per second, which seems to indicate that the angle of elevation under pressure was reduced to much less than 10° 30′. For a velocity of 44 feet per second, the theoretical value of α would be but 6°. In this calculation, however, the hull resistance and that of the system of guy-wires, which must have been comparatively large, has been omitted. It would appear, therefore, that the actual results obtainable in flight are much more favorable than calculations based on experimental data would presuppose.