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Aviation History
1922
1922 - 0578.PDF
OCTOBER 5, 1922 corresponding to the various gliding angles and gliding speeds, for the assumed wing loading of 2 lbs./sq. ft. Owing to its popularity for power-driven aircraft and its good efficiency the section known as RAF 15 might be expected to be suitable for a glider. It is observed, however, that the lift coefficients corresponding to the high values of L/D are small, and possibly the rate of descent will therefore be fairly high. The first section whose data are tabulated is the RAF 15, and it will be seen that the highest value of (L/D)2 X AL IS 75-7 and that this value occurs at an angle and lift coefficient which correspond to a speed of 36-4 m.p.h. when the wing loading is 2 lbs. /sq. ft. The rate of descent is 3-32 ft./sec. For this wing loading the RAF 15 section gives a landing speed of 27-6 m.p.h. It was mentioned at the beginning of these notes that the lowest rate of descent would not necessarily be obtained at the angle giving the flattest glide, and it will be seen that for RAF 15 this statement is borne out. The maximum L/D of this section occurs at 40, but the corresponding gliding speed is 41-6 m.p.h., which gives a rate of descent of 3-58 ft./sec. For the section Gottingen No. 441 the lowest rate of descent is 3-38 ft./sec, but this corresponds to a speed of 30-6 m.p.h., and the landing speed is lower, being 22 • I m.p.h. Particulars of Gottingen No. 441 section were published in our issue of September 21. It has, we believe, been extensively used on German gliders, and it will therefore be of interest to examine several British sections so as to discover whether or not an even better section can be found. The section known as airscrew No. 3, the data for which were published in Reports and Memoranda No. 322, shows extraordinarily high values of (L/D)2 x AL, the best being 118 as compared with the 75-7 of RAF 15 and 74-4 of the Gottingen 441. The lowest rate of descent is 2-68 ft./sec, and the rate does not change greatly over a range of from 20 to 8° incidence. This section is somewhat thin, and would require a braced wing structure. As a braced monoplane, however, airscrew No. 3 should be very suitable. Airscrew No. 4 gives a very slightly higher rate of descent, and its L/D (or gliding angle) is not quite so good. It is, however, a much thicker section (maximum thickness 0-127 of the chord), and could be used on a cantilever monoplane. It gives a landing speed of but 21 -5 m.p.h. Airscrew No. 5 gives a slightly higher rate of descent, and its best gliding angle is 1 in 14 • 7. It is, however, a very thick section, and might be used in the centre of a large- span wing, end sections of a thinner section being attached to it. The foregoing remarks should help to give those who have not previously given the subject much thought a general idea of the things to be aimed at in choosing a section for a glider. It will readily be understood that other considerations than aerodynamic ones have to be considered. Thus a thin wing will be heavier per unit of area than a thick wing for the same strength, and this has to be taken into account in deciding the respective merits. As to the actual piloting of a glider, a few general remarks may not be without interest. If our glider is fitted with airscrew No. 4 section, and we wish to remain in the air as long as possible, we would obviously fly the machine at the speed corresponding to the lowest rate of descent, i.e., at 32 m.p.h. Even in a gusty wind the pilot would try to keep the machine at this speed ; that is to say, in a gust he would elevate and in a lull he would push the stick forward. Gliding is largely a matter of flying at a constant speed, and in this connection it is of interest to note that Smith and Sons have already introduced an air-speed indicator which reads from 10 m.p.h. upwards, and is therefore specially suitable for gliders. If a pilot wished to cover the longest possible distance, he would fly at the angle giving the flattest glide, i.e., at maximum L/D. This applies to flying in still air. In a wind the pilot would attempt to gain height while gliding into the wind, and when he felt himself getting outside the region of ascending air currents he would turn down wind and glide for as long a distance as possible. Angle of Incidence (Legs.) 4 5 6 8 —2 0 2 4 2 3 4 6 8 2 3 4 6 2 4 6 8 L/D. &.. R.A.F. 15 (L/D)* (L/D)*. X ftL. (Absolute.) 17-1 16-7 16-1 i4'3 15-2 14-6 13-3 12-2 19-7 19-4 18-3 16-5 14-8 17-5 17-5 17-1 15-5 14-7 140 13-0 12-0 •226 •260 •296 • 369 292 66-o 279 72-5 259 75-7 202 74•5 Landing speed . Gottingen No. 441 •275 •34 •42 •48 231 63-5 213 72-5 177 74-4 149 71-6 Landing speed . Airscrew No. 3 •274 •313 •35i •425 •490 388 106-4 376 118-0 335 "8-o 272 116-0 219 107-4 Landing speed Airscrew No. 4 •308 •345 •384 •453 306 94-5 306 106-0 292 1120 240 109-0 Landing speed Airscrew No. 5 •39i •462 •536 •599 216 84-5 196 90-5 169 90-6 144 863 Landing speed V. (m.p.h.) 41 -6 38-9 36-4 32-6 . 27-6 37-7 34-o 30-6 28-6 .22-1 37-8 35-4 33-4 30-3 28-3 •• 25-3 35-7 33-5 32-0 29-4 • • 21-5 3i-7 29-1 27-1 25-6 . . 22-2 Vv. ft./s. 3-58 3-42 3-32 3-36 3-65 3-42 3-38 3'44 2-82 2-68 2-68 2 "JO 2-8l 3-o 2-83 2-75 2-79 3-17 3-06 3-°5 3-14 THE NEW DE HAVILLAND GLIDER Parasol Monoplane with Wire Bracing THE first of the two monoplane gliders which are being constructed at the Stag Lane works of the de Havilland Aircraft Co. will be finished this week, and it is hoped that some preliminary test flights may be made during the next week or so. On Monday we paid a visit to the D.H. works, and the glider was then erected and the bracing wires of the wings were being rigged and finishing touches given to various minor parts. At present the machine is fitted with a vee under-carriage, but probably later on, when the pilot has got used to the machine, a modified form, fitted inside the fuselage, will replace the present tubular vees so -as to save resistance. From the accompanying scale drawings it will be seen that the de Havilland glider is characterised by a parasol mono plane wing of very high aspect ratio (n to 1), and that in spite of the fact that the wing has external wire bracing the appearance is very clean. The wing section used is R.A.F. 15, but a slight departure from the standard section has been made by stepping down slightly the ordinates of the top surface. This has been done in order to reduce spar weight, as it was found that with the spar section employed (I-section) the standard section would give rather too heavy spars. Both front and rear spars are of spruce, and the ribs are built up of spruce flanges, approximately £ in. square, the form of which is preserved by webs or distance pieces tacked to one side of the spruce flanges, as shown in one of our sketches. ff The internal bracing is in the form of small-gauge piano wire, and the use of wire strainers has been avoided by using U-bolts passing horizontally through the spars. The system of bracing is, however, different from that employed in the old Bleriots, inasmuch as alternate bays have the drag wires anchored to ordinary wiring plates. Thus the trueing-up is not quite the work of art it used to be in the Bleriotsjof old. The monoplane wing is very simply mounted on the fuselage by two eye bolts engaging with corresponding forked end bolts at the apices of two formers or bulkheads rising up from the top longerons of the fuselage proper. The wing thus rests on the fuselage at two points only, and these on the centre line. The bracing is therefore relied upon to maintain the wing in its proper transverse position relatively to the fuselage. As far as the stresses in the wing are concerned, the spars may be regarded as continuous beams, as the two end sections are not pin-jointed to the large-span centre section.
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