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Aviation History
1931
1931 - 0096.PDF
SUPPLEMENT TOFLIGHT JANUARY 30, 1931 THE AIRCRAFT ENGINEER In the foregoing, all reinforcements are placed inside the body. Another classification of structures in which external reinforcements are used might be tabulated, but the above clearly shows what variations are possible, and in fact, examples of all these classes were on view in the Salon de VAviation. In some cases more than one, class of construc- tion was used in the same aircraft. Class 1 seems to be the most rational construction, as a complete structure is obtained and anything additional would appear to be superfluous. This would be the case if the inner structure were stressed to carry the loads independently of the rigid skin (as in a fuselage cited below). The skin, how- ever, may be an essential part of the structure as a stabiliser and stifiener to the individual members of the frame. Dia- gonal members in a rectangular panel mean smaller areas of unstifl'ened sheet, and are effective in improving external appearance and also give a more robust structure, but as a matter of fact, Class 1 was the construction most in evidence in the aircraft exhibited, notable among such constructions being Dt-woitine D.27, Dcwoitine D.35, Liore, Amiot, C. A.M.S. 80 and Nieuport-Delage. In other cases, whilst a large proportion of the complete, body was of the above construction, som e variations were made ; for example, in the Latecoere hydroplane, the last quarter of the body length was fabric- covered. The Dornier hull construction was of Class 1 over the rear half and Class 2 in front. Similarly, with the Caudron, except that as a third variant external stiffeners were used on both upper and lower surfaces. Class 2 type of construction was exhibited by S.P.C.A. and Wibault. The Junkers .Junior had Class 3 to itself, whilst as far as one remembers, the same remark applies to Nieuport-Delage Type 541 as regards Class 4. As would be expected, the exponents of Class 5 were few. Potez had sheets having shallow corrugations f-in. deep or so, probably at about 6-in. pitch ; the structure carrying these was fully braced. The Hanriot H.431 had corrugated detachable panels, held by turnbuttons to a complete strut and wire structure, square tubes suitably joined at the panel joints being used. Also on the Hanriot 215 corrugated panels were used on a braced structure over part of the fuselage, the after third being fabric covered. The most notable construction having the skin sandwiched between internal and external stiffeners was in the A.N.F. machine. Examination of the external appearance of machines in Class 1 proved particularly interesting. In some cases the appearance was most pleasing from the point of view of freedom from local buckles, in other cases such buckles were very pronounced. The futility of attempting to give reasons why metal- covered bodies should be produced in such a variety of ways is fairly obvious. Why have English designers who are developing steel aircraft produced wing spars of such diverse forms ? Simply because it is not usual for two people to think alike ; yet it would not be possible to say that any English interpretation of what a steel wing spar should be like is definitely superior to any other spar. Similarly, with the French monocoque bodies : criticism is possible, but the designer in each case could probably give a decided answer to our questions. The body of the smaller Nieuport-Delage was entirely free from buckles and had a beautiful finish : this, however, was obtained at the expense of much riveting, 22 longitu- dinally double rows of rivets being counted. The internal reinforcement consisted of large numbers of end-flanged channel sections, the transverse loops being semi-circular flanged channels laid over the longitudinals. The skin, lateral and longitudinal members were secured with cheese- headed bolts and nuts, as shown in Fig. 1, the shanks of the bolts passing through the longitudinal channel. It was not possible to see whether distance pieces were used or not. While the necessity for cutting either of the stiffeners was avoided, this arrangement did not inspire too much confidence, unless the hoops are called upon to do very little work, or if a little relative movement between these parts is of little consequence. Other methods adopted for reinforcing the metal skin are given later. A construction which appeared to be superior to the Nieu- port was to be found on the Dewoitine D.27 : here onh twelve double rows of rivets were used. The panels were approximately 18 in. by 9 in. and the whole was very free from buckles. The body curvature was about the same as on the Nieuport, hence closely-spaced stiffeners were not necessary for the purpose of bolstering up the plating against local deformation. The question of external appearance might be considered to be so closely bound up with external shape that, for example, local buckles in the thin flat plating would be absent in bodies of fairly sharp curvature, and that in " slab-sided " bodies the avoidance of such buckles would be difficult. As a matter of fact, some of the flat-sided exhibits were excellent in this respect, notably the Dornier, while the Liore boat was another example of excellence in the matter of freedom from local buckles, unsupported panels being up to Q in, by 1 ft. On the other hand, the Latecoere hydroplane plates, which were approximately 8 in., in width, were very badly buckled in places. Of course, thickness of plating has much to do with freedom from buckles, but in the three cases cited the plate thickness appeared to be about the same. No useful purpose will be served in describing in detail every difference between the numerous monocoque bodies exhibited. Possibly at future exhibitions we may get some indication of which constructions have proved the most successful. As stated above, the external appearance of the exhibit? varied considerably. The importance of this from a selling point of view cannot be ignored, and the desire to keep plating free from buckles may have led to what we have termed the Class 2 and other classes of construction, instead of the extremely simple construction we have called Class 1. The details of the reinforcements of the other classes are worth mentioning: the usual stiffener was round or rectangular sectioned channel having outwardlv extending flanges : in other cases, simple angles, or angles in which the edges were stiffened by bending. Other sections were used for this purpose, but the angles and channels were the most widely used. In constructions where the whole of the reinforcement lies inside the skin, the principal question arising is what shall be the scheme where the lateral and transverse members cross each other. In some cases it might be desirable to make the transverse members discontinuous at the hoops, or vice versa, having some simple fitting or joint at the point of juncture. An excellent example of this was seen in the Amiot three-engined machine. The construction can easily be seen from Fig- '-• This method of using a separate pressing was also found in some cases where a long ridge was raised in a sheet for purposes of stiffening (see Fig. 3). Probably due to breakage? at the ends of the ridges, these pressings offered themselves as a convenient solution. In other cases, similar ridges wen1 made without adopting this artifice at the ends. In some cases, a small sectional channel longitudinal passed through a larger dimensioned hoop, also of circular channel section, angle pressings being riveted to the main members at the point of crossing (Fig. 4). In other case*. small channels, acting as longitudinals, were attached to larger channels by means of fish plates and small angin- as in Fig. 5. Again, in the Caudron C.180, the skin reinforce ment was carried out by means of a number of channel secured together, and to the skin, in the manner shown ' Fig. 6, while some of the Hanriot coverings were stiffen^ by means of corrugations in the sheets, reinforced *' flanged circular channels riveted to the sheets at right' angles to the corrugations, as shown in Fig. 7. There i- obviously scope for considerable ingenuity in this direction- In one case, I believe on one of the Latecoere exhibits- ^ transverse formers appeared to be left out at the tail the machine. The bunching together of the longitude* * may have been sufficient for vertical stiffness in this case. 966
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