Chapter 14

 

Chapter 14

Wings: Sections, spars, tips, centre sections, fixings.

FIG. 33 shows some of the commonest wing sections in use for full-size aeroplanes, and others are usually only slight variations of these. R.A.F. 15 is typical of light biplane sections, and while being very good aerodynamically it is not very convenient for models. The trouble is that to make it strong enough for a model would result in too much weight. Clark Y is typical of the heavier biplanes like the Hart and Fury, and light strut-braced monoplanes like the Wicko and the Comper Swift. R.A.F. 34 is the type used on cantilever monoplanes like the Wellesley, Mosscraft and the Moth Minor. It is also used on some strut-braced monoplanes like the Lysander and Taylorcraft. Gottingen 436 is another section similar to Clark Y, though one writer considers it better, and is that most likely used on machines of German origin, such as the British Aircraft "Swallow."

Fig.33Fig.33

We need a number of wing ribs cut out to this section, enough to be spaced about 1 in. or 1 1/2in. apart along the wing. If the wing is parallel it is best to cut one rib to size out of 1/16 ply and use this as a template or pattern for cutting out the proper ribs. We can make these from 1/16 or 1/32 balsa. If the wing is tapered we shall have to cut out a number of different sized ribs, the length of each being obtained from the drawing. We shall require two of each size, one for each half of the wing. Getting these different sizes the right shape involves a little work. Anyone who has an enlarger for photographs can turn it to good account for this job.

One way is to make a tracing of a small rib that we can put in the enlarger in place of the negative, or a better way is to photograph a large drawing of the section and use the negative. We can now enlarge the rib to any size we like. There is no need to be critical about the sharpness of the image, and we can in fact use the focusing adjustment to get the image the exact size we require. We can use any odd bromide paper and need not be particular about the exposure as long as we can see the outline when developed. When the prints have been finished and dried we cut them out and use them as patterns for cutting the ribs.

We can also obtain blue prints or drawings of wing ribs of various sizes from some dealers that prove very useful. We can usually find a set of ribs to suit by adjusting the spacing of them. Suppose the wing tapers from 4in. to 2 1/2in., and the span of the tapered part of one wing, that is, on one side of the fuselage, is 15in. From the blue print we pick out a range of sections from 2 1/2 in. to 4 in. that vary 1/8in. at a time in length. The difference Between 2 1/2in. and 4 in. is 1 1/2in., and the variation is 1/8in. per rib, or 1/8in. between one rib and the next. There are twelve 1/8in. in 1 1/2in., so this means we shall have twelve spaces in 15in., so each space will be 1 1/4in. If the ribs on the blue print vary 1/4in. each instead of 1/8in., we must cut ribs to fit 2 1/2 in. apart, and cut by guess for those in between. It is fairly easy to guess the correct shapes if we cut out the rib roughly a bit on the large side and then hold it between the next rib smaller and next larger.

Table data. Profiles of AerofoilsTable data. Profiles of Aerofoils

Sometimes we may want to use a section for which we have no drawing, and in any case some of us prefer to draw them out ourselves to just the size we want. After all, it is quite easy enough when you have the profile dimensions, as they are called. A table of these dimensions is included, because they are a great advantage when dealing with certain tapered wings. The sections given are the most suitable of those we are likely to come across. The method of drawing these wing or aerofoil sections is illustrated in Fig. 33. We start off with the chord line, and mark the chord the length we require. Then we divide it up into percentages and erect perpendicular lines to correspond with the column of values headed per cent chord. To get the height of these lines we multiply the values in the other columns by the length of the chord. Sometimes these values will come above the chord lines for the bottom curve and sometimes below or on the line. The point marked 30 per cent chord is about the best place to arrange the main spar. Section R.A.F. 30 is symmetrical and is used for tail-planes and rudders.

If we are making a tapered wing we do not need to work out the dimensions for all the ribs; we can do it by drawing as shown in Fig. 34. We draw but the largest and smallest ribs on the lines XX and YY and equally divide up the space between the two to suit the number of ribs we are using. We now divide each rib into the same number of parts, say at every 15 per cent, as at A, B, C, D, E, and A2, B2, C2, D2, E2, and join up with straight lines.

Figs.34, 35, 36Figs.34, 35, 36

Next we draw a vertical line ZZ, and mark off the distance AA1 from Z to A1 and A2A3 from Z1 to A3. Now, by joining up the line A1A3 we find the height of each rib on the line AA1A2A3 by measuring the distance between the lines ZZ1 and A1A3 alongside that rib. The same, procedure is carried out for each of the points A, B, C, D, E, etc. The position of the main spar is shown for a wing tapering from 6 in. to 3 in.

Sometimes on a tapered wing the section is not constant, but varies in thickness/chord ratio like the B.A. "Swallow." It may be that the depth of the section at the tips is 10 per cent of the chord, and yet at the centre section it may be 20 per cent. In this case the same ordinates are used for plotting the wing section, but multiplied by 2, or if it is 15 per cent multiply by 1 1/2. By using the drawing method it is easy enough to get the depth of all the intermediate ribs. Making the rib or section thicker than is correct is called "blowing it up." When each rib has been drawn it is cut out and used as a pattern. If the wing is not tapered very sharply we can cut out the ribs satisfactorily by making patterns of the largest and smallest ribs from 1/8in. ply and then sandwich all the ribs, cut out roughly, between these two patterns. If we now cut across from one pattern to the other, as shown . in Fig. 35, all the ribs will be the correct shape. We can cut slots for spars at the same time.

If we use 1/32in. balsa for the ribs, when they are about 3 in. or more in length they will need stiffening. The amount of stiffening will depend on the strength of the trailing edge spar. With a strong spar we need not stiffen the ribs so much, but it is best to keep the spar on the light side and have strong ribs. One way we can deal with the ribs is to make small gussets of 1/32in. balsa about 3/4in. by 1/8in. and glue them in the corners between the ribs and the trailing edge spar. A better way, however, is to stick a strip of 1/8in. by 1/32in. balsa along the top or bottom, or both, edges of the rib to lap over the T.E. spar. See Fig. 36.

The trailing edge spar can be 1/8in. by 1/16in. balsa for a light one, or 1/4in. by 1/8in. for a heavier one. We can make a better spar, though, by forming a V section of two pieces of 1/32in. balsa 1/4in. wide, glued together. These spars will do for most models, providing the ribs are not more than 1 1/2in. apart, but the built-up V could be made half the size for a small model. If we use 1/8in. thick balsa for the trailing edge it will need tapering to a V section, and for this we can make up a gadget to assist us, and it is shown in Fig. 37. First we want two pieces of hard wood, such as oak, though deal would do. One piece should be 6 in. by 1 1/2in. by 3/8in. and the other 6 in. by 3/4in. by 1/8in. In the smaller piece we make two slots about 3/8in. long, and wide enough to allow a small bolt to slide along. In the larger piece we make two holes for the bolts, and enlarge them underneath so that we can sink the nuts in. The sketch makes it clear. On the edges we nail or screw strips of brass or steel about 1/32in. thick, to prevent damage to the wood. We want a large washer under the head of the bolt, so that it will not jam in the slot. By putting the balsa on the step we can shape it down with a knife or razor blade and finish with a piece of glasspaper. This is best in a strip about 1/2in. wide, glued on to a stick. Don't try to cut too much with the knife; it is just as easy with the glasspaper. If we want a tapered spar, we can make a parallel one and cut the surplus off the thick side.

Fig.37Fig.37

For the leading edge spar we can use 1/8in. by 1/16in. balsa or 1/8in. square, for almost any size model, providing we have ribs spaced less than 1 1/2in. apart. Ribs farther apart than this need a stronger leading edge and do not look so good in the finished model.

The main spar is the one we can most usefully vary for different-sized models, and by always making this as deep as possible the variation we require follows as a matter of course. One of the simplest ways we can make such a spar is to use 1/16in. sheet balsa about 1/16in. less in width than the depth of the ribs. In this way we get a tapered spar with a tapered wing, which is all to the good. We can fit spars and ribs together by cutting slots half-way down from the top of the spar, and corresponding slots in the bottom of the ribs.

We can make a better spar, though it is a little more trouble, by using 1/16in. or 3/32in. square balsa top and bottom, with a web of 1/32in. sheet glued across. Either this web will have to be in pieces between the ribs, or the ribs must be cut in two and stuck on fore and aft. Cutting the ribs is really the best. A spar made like this using 1/16in. square balsa is strong enough for a mddel 4 ft: span weighing 8 oz. The spars are shown in Fig. 36.

Fig.37a, Fig.37bFig.37a, Fig.37b

A new method of construction as far as spars and ribs are concerned is that used on the Airspeed Envoy, of which there is a photograph on page 12. The spar is cut from sheet to the full depth of the wing, and is slotted top and bottom for the ribs. The spar is lightened with oblong holes between the ribs, and the ribs also have oblong holes so that they can be threaded on to the spar and turned at right-angles to fit the slots. This will be clear from the photograph.

The wing is usually built in two parts, one for each side of the fuselage. In this way we can build each half on a flat board, and either join them together afterwards or attach them separately to each side of the fuselage. We can with advantage build the wing over a drawing (which must be protected with greaseproof paper), so that we get the ribs and spars in the right place. Spars and ribs can be held in place with pins stuck upright each side, and we must make sure everything is square and true while the glue dries. It is best, if we can manage it, to put spots of glue in the slots before the spars are put in, but unless we can work very quickly we shall be unable to do this with quick-drying glue. If the ribs have a flat bottom surface they will stand on the board nicely, and the T.E. spar can be pinned to the board, but if the ribs are a section like R.A-F. 34 we shall need some packing under this spar. For this we can use a strip or two of balsa to lift up the T.E. spar till it is level with the L.E. spar with the main spar resting on the board. Remember to make a wing for each side, but in the case of a parallel wing we may have no more difference than the end on which we fix the tips.

Well, now let us see how to make wing tips. One simple method that is very popular is to make them from sheet about 1/16in. thick. We cut the sheet into wide strips so that three or four can be joined together, as shown in Fig. 37a. The strips should be about 3/8in. or 1/2in. wide, and we can see how many to use by laying them on the drawing. The easiest way is to lay on the pieces that join up with the leading and trailing edges first, and pin them in place. Then we can join up in between with one or two more pieces, glued in place. Before cutting the outline to shape it is best to lift the parts gently from the board and put another spot of glue on each side of the joints. To get the shape we can lay a piece of tracing paper over the plan and trace the inside and outside lines, and cut out the paper to the pattern of the tip. We can glue this pattern to the balsa and cut it away each side. It is then ready to put on the wing.

Another method of making tips is also shown in Fig. 37b. To make this we stick pins all round the outline of the drawing, and put three or four pieces of 1/16 in. by 1/32in. balsa edge up on the plan, well glued together. The best way is to use one of the slower drying glues and stick the pieces together before putting them on the plan. The idea of using "slow" glue is that we can put the glue on, squeeze the pieces together, wipe off the surplus glue and bend them to shape before the glue begins to set. The pins on the inside of the bend hold the pieces together and to the shape of the bend. Put the middle ones in first. If the bend is very sharp we shall have to steam the balsa to be able to make it take the bend without cracking.

We can also make wing tips from light reed cane or bamboo bent to shape and steamed if necessary, bound and glued to the leading and trailing edges.

Fig.37cFig.37c

With some machines we shall be able to have the wing in one piece all the way across, held on top or underneath the fuselage with rubber bands. When we build it this way it is best to leave the main spar a bit long at the centre of the wing, so that when the two halves are fixed together the two main spars nearly touch. To fix the two halves together we put them on a flat board with the centre ribs the correct distance apart, which will most likely be the width of the fuselage, and raise the tips on blocks of wood, or match boxes or something similar. The main spar should be shaved off at the bottom, so that the ribs lie flat on the board. Fig. 37c shows the wing set up. We must put pins in to hold things in place while we put in the joining pieces. For these we use pieces of the same section as the leading and trailing edges, cut to length so that they are a good fit; that is, not tight nor loose enough to shake. To join the two parts of the main spar we can use a piece of 1/16in. sheet cut to a good fit and glued on the sides. If we have a large, heavy model we can use two pieces of sheet, one on each side. If we want an extra strong job we can cover the centre section with 1/64in. or 1/32in. sheet balsa. This will be useful if we are fitting the wing under the body. When fitting the wing this way, one of the best methods is to cut a piece out of the bottom of the fuselage to take it. Where we, cut away the bottom longeron we must insert a plate of sheet balsa about 1/16in. or 1/8in. thick. This is cut to the curve of the top of the wing, and should continue under the front of the wing to the point where the curve of the wing meets the line of the fuselage. All inside this curve should be covered with 1/32in. sheet balsa. The wing can then be held in place with a rubber band. The side plates should, of course, be put in when the fuselage is built in place of the longerons at that point (Fig. 15a).

There are many ways for us to choose from for fixing the wing to the fuselage, some of which will do for either high or low-wing machines. One such method is shown in Fig. 38. We can make the tongue 1/8in. to 1/4in. thick and 1 in. to 2 in. wide, according to the size of the model, and the box can be made from 1/16in. sheet let into the first two ribs. We can, if we wish, cut away some of the balsa in the tongue to make it lighter, but if so we must leave a strip about 1/4in. to 3/8in. wide along the centre that has no hole breaking into it. To get the shape of the curved ends of the tongue, we draw the rear curve with the point of a pair of compasses on the leading edge of the rib that fits against the fuselage. The front curve is drawn with the compasses on the trailing edge of the same rib. When the box is put into the wing we must not forget the dihedral angle.

Fig.38Fig.38

This tongue and box idea can also be used to make outboard wing panels detachable. These panels could be those outboard of the engine nacelles on a twin-engined machine, or outboard of wing struts.

Another idea is to use press studs or dress snaps as they are sometimes called. These can be sewn to the balsa, and when glued also are surprisingly strong. A better way, however, is to solder each part to the head of a small bolt and bolt it to the wing rib. This rib should be strengthened by putting a large thin ply washer under the nut.

Fig.39, Fig.40Fig.39, Fig.40

Where we have the whole of the wing detachable on strut-braced machines we really want some means of keeping the wing in place on the fuselage, and let the struts take the weight of the model. Fig. 39 shows how we can have small bamboo plugs on the wing panel to fit into holes in the centre section rib. It also shows the hole through the centre section through which a rubber band is passed to fit on the hooks on the wing. In use we put the band on one hook and pull it through the centre section and fit it on the other. The band should be short, so that it is tight with the wing in position. About the only way of getting the band through is to use a long wire hook that will pass right the way through.

The wing struts are the biggest problem, but the following method has been used most satisfactorily on a model "Lysander." In fact no fault has been found with it. For this a piece of wire was soldered on to the undercarriage, and projected about 3/4in. in the direction of the strut attachment on the wing. At the top of this was soldered part of a small press stud. Another piece of wire was fixed to a wing rib, and projected towards the undercarriage, and had a press stud soldered at the bottom. The other halves of the press studs were then pushed on, with a spot of oil on the knob part. The next part of the job was rather tedious, but well worth the care taken. The fuselage was packed up level by putting books and slips of balsa under the tail, and slips of balsa under the wheels. The wing was supported on a pile of books at the tip, and attached to the fuselage at the centre. The dihedral and incidence were adjusted by means of little wedges of balsa. A length of wire was then soldered to the outside parts of the press studs. The wires, which were about 22 s.w.g., were then faired (or streamlined) with balsa, leaving stepped ends, see Fig. 40. Struts for biplanes can be fixed up in the same way.

Fig.41, Fig.42Fig.41, Fig.42

Another way of making struts for biplanes is shown in Fig. 41. For this we bind wire hooks of about 22 or 24 s.w.g. to the main spars and put on a rubber band enclosed in a paper tube. In the sketch the tubes have been cut away at the ends- so that we can see the hooks. To make these paper tubes we need a former, that can be a piece of dowel, a pencil, a knitting needle, or if we want a streamlined strut we can make a former by sandpapering a piece of ' bamboo to shape. Round this former we put a piece of writing paper and then bind on strips of gummed paper tape as used for parcels. Writing paper or drawing paper can be used for the whole of the tube if we glue it well all over as it is wrapped round. Seccotine is very good for this job, as it can be spread all over with a wet finger. Damping the paper first also helps it to spread. If you use gummed tape, have a piece of sponge in a saucer of water for wetting it, or your tongue will get rather dry. As soon as the paper has been bound on to the former we must slip it off, because as it dries it shrinks, and if we allowed it to do that we should not be able to get it off without destroying the tube.

The other method shown makes use of a bamboo strut with a wire hook at each end that fits into a piece of tubing bound and glued to a rib or spar.

Fig. 42 shows another biplane strut fixing. The bamboo peg "A" could pass right through the balsa strut, the balsa being put on in halves. The slot for the rubber band could be covered with paper for most of its length, for neatness.

Fig.43Fig.43

The wing fixing on Mr. Hastings's " Fury " is rather smart, and is shown in Figs. 43 and 44.

The rubber bands holding the thread "lifting wires" tight pass through the fuselage in square balsa tubes. Similar tubes are used in the centre-section at the top. For the bottom wing fixing a strong rubber band is used with a hardwood sort of button on each end. This passes through the fuselage and pulls tight against the inside end ribs of the wing. These ribs are slotted to take the rubber.

Fig.44Fig.44

The Hawker Hurricane is very popular with builders of flying scale models. Here are three examples, all worthy of admiration for the skill exhibited in incorporating so much detailed work in small models.The Hawker Hurricane is very popular with builders of flying scale models. Here are three examples, all worthy of admiration for the skill exhibited in incorporating so much detailed work in small models.

 
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