The Framework of Aeroplanes.

Progress, Volume V, Issue 6, 1 April 1910, Page 192

The Framework of Aeroplanes.

(' ' Engineering. ' ') Aeroplanes fall, generally, into two classes — the biplane and monoplane, the former having two main planes superposed, and the latter a single main plane. It will be convenient to deal with the former first, as the construction is the moie uniform. Figs 1, 2 and 3 show an elevation plan and end view respectively of tho frame of a biplane of ordinary constiuction divested of its chassis, engine, driving seats, i udders, etc It will be seen that the framing of each oi the main planes consists of two main cross members A A, connected by short longitudinals B; the plane itself, whose position is shown by the dotted lines, consists of numerous longitudinal battens (not shown) on which the fabric is stretched. These main planes are connected by vertical wood struts, C, and steel wire diagonal ties, W, in such a manner as to form a trussed girder. The struts are generally placed nearer to each other at the centre of the plane, where the weight of the engine, etc., has to be carried, than they are towards the sides; but their exact arrangement depends on the disposition of various other parts and on the ideas of the designer. The backbone forming the tail is constructed in a very similar manner to the framing of the main p>lanes, and consists of two top and two bottom members E, E, tied together with struts G and wires W in a similar manner to the main planes. In the majority of cases biplanes have the propeller placed on the centre line, and behind the main planes, and therefore the two parts of the backbone must be kept far enough apart to clear it, as shown. The wood usually used is either ash or pine, or in some eases both. Ash is considered the more reliable, as it bends considerably before breaking, but pine is the lighter. In some cases ash is used for the principal members, such as A, A, E, E, and for the struts C, near the engine, where the greatest strain comes, while pine is used for those struts which have less strain. It will be observed that in this construction there are some 25 to 30 vertical struts, and these have to be forced through the air at 45 miles an hour. They are therefore made of a section somewhat as shown in Pig. 4, to diminish, the air resistance, and in order to make them as light as possible they are made largest in the middle and tapered away at the top and bottom. The actual main planes, as also those of the tail, are constructed of battens fixed to the main members A, A, about lft. to Ift. 6in. apart, carrying the fabric. These are fixed in various ways, of which two common ones are shown in Figs. 5 and 6. In the former's case the battens are simply pieces of wood curved to the shape the planes are required to take, ana fixed to the under side of the main, members. In the latter the battens themselves are built up of thin pieces of pine with distance-pieces at intervals to keep them in shape, and to form the two into a girder, a transverse section near a distancepiece being shown in Fig. 7. In Fig. 5 the main members A. A are not enclosed in the fabric, and, therefore, the air-resistance should theoretically be greater than in Fig. 6, especially if the battens of tbe latter type are prolonged in front of the cross-piece and brought to a sharp edge, as

is usual in monoplanes. In practice, however, excellent results are obtained with machines having wings as Fig. 5; these are certainly cheaper to build, and possibly in practice lighter, as there is only one thickness of fabric. In many cases the longitudinal struts are curved to form part of the wing surface, while in others no special struts are provided, the battens themselves serving to keep the main pieces A A apart. The exact sizes of the various parts of a frame will obviously vary considerably with the size of the machine, weights carried, and other details. It would theoretically be possible to calculate the stresses on the various members and proportion them accordingly, but owing to the very great depth of the girder in proportion to the weight carried, this would probably, in most cases, give sizes impracticably small, as it is necessary that there should be a certain amount of local strength. An idea of the lightness of the whole structure can, however, be obtained from the fact that in an aeroplane of 30ft. spread across the wings the main members A A will be 1% in. square or slightly less, while the main longitudinals E E will probably be slightly smaller. The vertical struts C will be 2m. by lin. to 13/>l 3 /> in., bemg thickest at the middle of their length If the wings are built as in Fig. 5, the battens are about %in. by -K>in., with stiffer longitudinals at the uprights; while if built as in Fig 6, the materials may be under in. The wire tie-rods are usually about l-10m. in diameter, though some of the less important ones are somewhat thinner, and some constructors prefer to use a very large number of very thin wires Tightening screws are usual, but not by any means invariable, this being apparently a matter on which there is a difference of opinion among designers. The wires appear to be one of the parts of the machine which requiies a good deal of attention, and a coil of wire and a pair of pliers were very much in evidence in the aeroplane sheds at Blackpool. The breakages seem to be due to vibration, as the calculated stiesses must be, m most cases, very small. It is no doubt for this reason that the best results appear to be got with a soft grade of steel, the harder grades being more liable to break in spite of their higher tensile strength. "Piano wire," while often spoken of m connection with aeroplanes, is therefore not so much used, being very hard. In view of their liability to break, it would at fust sight appear dangeious to trust to wires, but m practice it is probable that with such an extensive system or trussing the breakage of one or two wires m no way endangers the whole structure. The wires are kept carefully oiled to prevent rust. The fianung for carrying the elevating plane in front is variously arranged in different makes, that shown in Fig. 1 being the arrangement adopted by Farman. In the Voisin machine, however, the longitudinal beaiers which carry the engine and driving seat are built up as a trussed girdei and are prolonged to cairy the elevating plane, as in Fig. 8; and this girder is cased in so as to form a protection to the steei sman. In the Wright machine the skids on which it rests on the ground are prolonged for the same purpose, and a system of wood tiusses with no wires is used (Fig. 9). If the wings are arranged so that they are warped to maintain the balance, the diagonal wires towards the outer ends of the after pair of main members A are omitted, and replaced by wires leading to the control-lever. In many biplanes, however, the main wings are not warped, balance being maintained by supplementary wings. The principal variations in the construction of the tail are those due to the nature of the surfaces to be supported and the position of the propeller. Where the latter is placed in the centre line and behind the mam planes, the backbone must of necessity be so arranged as to leave room for it. In some cases, however, there are two propellers driven by chains, one on each side of the centre line — e.g., the Wright and Cody machines — and in this case the backbone can be much more compact. In the case of the Wright machine, also, there is no supporting surface in the tail, and the rudders are comparatively near to the main planes; hence the backbone is shorter and of more simple construction. In the case of the latest 'Voisin aeroplane the engine is placed in front of the steersman, and the propeller is direct-coupled, working in front of the main planes, as in a monoplane, while the front elevating plane is dispensed with, and a hand-control to the tail plane substituted. The backbone is then made a compact trussed girder,

very much like that ojl a licic^l'hOj end ia carried right through from ' the tail to the front of the main planes, the engine, steersman's seat, etc., all being placed on it. In fact, the construction is much like that of a monoplane, with the exception that there are two main planes. The framing of the monoplane is in many ways much simpler than that of the biplane, as the wings need not be built into the machine in the same way as in the biplane, but are usually constructed as more or less separate units, which, are attached to the backbone. It is possibly in consequence of this that the frames of monoplanes present a very much greater divergence in their construction than those of biplanes. The construction of the monoplane is simplified by the fact that it is invariably controlled entirely by a tail, and has no elevating rudder in front, while the engine is mounted, as a rule, on the backbone itself, and has a directcoupled propeller working in front of the main plane. Hence the backbone can be of compact dimensions, as there is no need of special piovi&ion for the propeller to clear it. The construction which is most similar to the biplane is that in which the backbone is a similarly-trussed girder, an example of which is shown in Figs 10, 11, and 12, this being the general arrangement used by Bleriot and others. In this ease foiu main longitudinals E E <ire joined together by suitable wooden shuts C and wire-ties W,

the outside dimensions being usually laige enough for the steersman to sit inside, or from 18in. to 2ft. square at the front, tapering towards the tail. The main longitudinals are prolonged beyond the wings to form an attachment for the engine, and at the after-end there are suitable attachments for the tail. The wings can in this case be built as entirely separate portions fitting into two sockets in the main members. In order to sustain the upward pressure when in flight, a strut G is provided below the backbone, and from this ties H are stretched to the wings. A similar strut and ties are provided above the backbone to take the weight when the machine is at rest. In addition to these, there are ties J leading from the strut G to the after part of the wings for the purpose of altering their angle, these being under the control of the steersman. There are usually two main ties to each wing, as shown, and two to the after edge. These ties are often made of flat steel ribbon to diminish the air resistance. The lower strut sometimes forms part of the framing which carries the wheels. Although in the monoplane the depth of the girder forming the backbone is very much less than in the biplane, the main longitudinals are not, in practice, made heavier as a rule, but rather the reverse. The vertical struts are, on the whole, distinctly lighter, and, as they are shorter, this is doubtless justifiable. This being so, it is evident that the deep girder of the

biplane will not be any lighter than the shallow one of the monoplane, the reason being that full advantage cannot be taken of the depth in reducing the scantlings of the parts on account of the necessity for local strength. There are several modifications of the type of backbone shown in Figs. 10 and 11. In some cases the section is made triangular, the two bottom longitudinals being replaced by a single one. In the ' ' Antoinette not only is the section triangular, but the vertical struts and diagonal wire ties are replaced by diagonal wood struts, the general arrangement of these being as Tigs. 13 and 14. In some cases, instead of the backbone being built as an open girder, it is planked over with very thin wood, generally Vyin. mahogany, and this can, if desired, be formed into a sharp nose in front, as in a boat, to cut the wind In the "Antoinette" part of the length is planked with wood, and part covered with canvas, the planking being used for the part just aft of the main wings, where the strains are probably greatest. Several monoplanes have been built with framings varying consideiably from the wood and steel girder. The R.E.P., for instance, has a frame composed entirely of steel tube. The "Demoiselle," of Santos Dumont, originally had a single bamboo to foim the backbone and support the tail, the seat, etc. being really supported direct from the wings. This has, however, now given place to an ariangement of three bamboos, about 2in. in daimetei, connected with struts of steel tube and wire ties, the general arrangement being as Fig&. 15 and 16. The engine is fixed to the top longitudinal, while the beat consists of straps across the two bottom ones. The members L, L form part of the wings, and are built into the framing, and help to support the engine. The carrying-wheels are on h cioss-piece fixed to the lower bamboos. The construction of the main planes or wings of the monoplane varies somewhat from that of the biplane In the latter the wings simply consist ot nbs laid on the main framing, which is itself an elaborately trussed girder. In the former, however, the wings are generally made as separate units and must therefore have considerable stiength, as they are only attached to the mam structure at a few points. "While there are many variations in detail, the general structure consists of two main spars M and N (Fig. 11), which fit into sockets at O in the backbone. The forwaid of these sockets may be a tube going light across the backbone as shown, but the after one is usually of some more flexible type to allow of the warping of the wings, the greater pait of the weight being apparently intended to be borne by the front spar. Each of these spais is supported by the ties H and J, but as these aie not numerous, the spars must have considerable depth. To expose them would therefoie entail consideiable wind resistance, and the wings are theiefore so arranged that there aie suitably curved suifaces to enclose them, as shown in Fig. 17. The spar Mis m this case not placed at the extreme front edge of the plane, as is often the case in biplanes, but is sufficiently far from it to allow of the two surfaces being brought to a moie or less sharp edge, the exact sharpness varying very much with different makes. The mam spars are usually of wood, and various makers adopt very different sections to secure the greatest lightness for strength. Solid sections are sometimes used, as shown at N, and in other cases H sections, as shown at M. In some cases the spars are built-up lattice girders. Steel tube has also been employed, but it is not adapted for easy attachment. In some cases, where the wings are large, the system of having direct ties only to the wings is not considered enough, and they are in themselves trussed, as shown in Fig. 18. In this case struts P are provided about the mid length of the wing, and wires E lead from the root of the wings over the struts to a point near the tip. The main ties are then brought to the root of the strut P. The general construction is similar to the crosstrees of a sailing-boat, but it seems doubtful whether it would not be more satisfactory to arrange for taking the strains more directly, as the struts must entail some wind resistance. The framing of the various subsidiary planes, rudders, etc., is in general on the same lines as the main framing. Figs. 19 and 20, subjoined show the framing of a rudder, from which it will be seen that .it consists essentially of a frame S, having a cross-piece T for the attachment of the controlling wires, and that this is stayed to the frame of the rudder by various wires W.

Both surfaces of such a rudder are usually covered with fabric, and the number of battens to carry it varies with the size, as also does the number of wires. Comparing the various constructions of monoplane backbones, the triangular section should probably be slightly lighter than the square, as it eliminates one longitudinal. The result will, however, probably depend on whether the arrangements of the machine are such that it is as easy to attach the parts to the triangle as to the square. The system of diagonal wood struts also appears to be lighter in practice than the vertical struts and wire ties with their attachments. There seem to be distinct advantages in

planking over the backbone with this "wood. This planking weighs under %lb. per square foot, and it is probable that its weight can quite easily be saved out of the strutting, equal strength being assumed. It also forms a verysmooth exterior, with little air resistance. The bamboo framing is very interesting, as it is employed on the lightest flier yet made; but it may be very much doubted whether the lightness of the machine is due to its system of framing. In the Demoiselle the usual seat is replaced by a couple of straps, while many of the usual attachments, such as a spring-mounted chassis to

carry the steersman some way from the ground, etc., are dispensed With,; and it is probably mainly to this economy that its lightness is due. The wings are also very small and of very light construction, with the supporting wires very close together. Altogether the construction of aeroplane frames shows great ingenuity in the problem of combining lightness with strength, but it appears likely that in the future there will be still further improvement in this direction. Wood is probably the most promising material if properly used, and it is now possible to procure it ready made into convenient sections for light structures. For instance, in addition to hollow round

sections, which have long been famWiar in, small boat racing, it is now possible to get oval sections. The hollow sections are made by splitting the spars down the middle, hollowing them' out, and fixing them together again, partitions being left at intervals to stiffen the sides. The sections are milled, and often have bits left solid to stiffen, the flanges as shown. It would probably repay anyone designing aeroplane frames to study carefully the methods of construction used in small racing-boats, both in sailing boats and motors. In these the difference between winning and losing races is

almost entirely a question of weight; and there has been a very great deal of experimental work done, as there is plenty of competition, and no lack of money. The problems in the two cases are also very similar in the fact that the stresses on the structures are almost entirely produced by fluid pressures, except those caused by the vibrations of the motor. There must, of course, be many differences in construction, but there may be a good deal of information to be got as to the best materials for particular jobs. For instance, bamboo, which appears at first sight a most suitable material for light spars, has been largely superseded in boats by artificial, hollow, wooden lods. Single wires for shrouds,

etc., have also been found to be far less reliable than «trandecl wire, as the latter shows signs of weakness before breaking, while the former do not. The wager boat might also afford some lessons.' The hull of a sculling-boat -30ft. long only weighs about 201b., and carries a load of some 1601b., concentrated entirely in the middle of it, and is therefore probably lighter in proportion to the load carried than the backbone of any aeroplane. Its design is the result of exhaustive trial and error, and therefore well worthy of study.