What is a cantilever wing? A wing that can support the weight of the aircraft by means of its internal structure, that's what, i.e. it has no need of external bracing wires or struts. As previously mentioned, we join the wings to each other and the fuselage by means of steel joiners which slide into, or through the fuselage and out into the wing roots, but how do we calculate how far into the wings they should go? Frankly, I don't have a clue, calculation-wise, but I do have plenty of practical experience upon which to draw. If you run your brass tube some four or five ribs out from the wing root, things should work out fine provided you have a strong enough spar system. In percentage terms, rather than trusting to the vagaries of rib spacing, it seems to work out that the joiners should be inset into the wing by about ten percent of the span of the individual wing panel. The important thing is that the joiner box should be attached with extreme firmness to the spars so that the stresses can be transferred out along the wing. It's not just for aerodynamic or aesthetic reasons that glider wings taper out to the tips, in engineering terms this is the best and most elegant way to construct them. In flight, the weight of the sailplane is spread evenly over the entire surface of the wings, and you can usually see that they flex upwards slightly in the air. Now, if you pick up your model on the ground by one wingtip and lift until the fuselage is completely clear of the deck you will see something entirely different...you might hear a lot of creaking and cracking, and the upper covering will wrinkle like Nora Batty's stockings. This is because the brunt of the weight of the fuselage is now being borne by the wingtips which is not something included in the original design parameters. The wings taper because the upward force applied out at the tips must be less than at the roots, to prevent what we have just witnessed. Being smaller in area, the wing generates less lift at the tips, thus applying less bending moment to the spar and, as we move towards the root, the area and therefore the lift increase proportionally, hence the elegance of the glider wing is profoundly visual as well as mathematical. The same rules should ideally be applied to the spar structure, with the essence of it of it tapering smoothly in size and strength as it journeys out from root to tip. But what of the wing joiner box, surely it has to come to a sudden end somewhere? That's right, and there's not much you can do about it except ensure that the structure is sufficiently over-engineered to cope with all the expected (and hopefully some of the unexpected) loads that may be applied to it. When it comes to a two-joiner system you can reach a more satisfying compromise: first you must consider where a single joiner does all its flexing...at the wing/fuz junction, where it is unsupported by the brass box, right? So, when you add a second joiner, it's this point that you are reinforcing, therefore the second joiner can be shorter within the wing root to avoid the sudden change of strength that would otherwise ensue.

How then, do we attach the joiner/s to the spar system? You must first look at the spar options; if the spar material is square in section, then the choices are limited, but if it is rectangular you can choose to lay it flat along the surface of the wing, or inset it vertically into the structure. If you choose the latter, then you are presented with the maximum of surface to which to attach the joiner box. It is well known that when you bond two materials together, the greatest strength is achieved in the shear plane, and it is thus in this instance that the forces are to be applied. So, two nice flat spars and a brass box, how do we best marry them together? The simplest way is to fabricate a plywood box to the full depth of the wing within which is contained the brass joiner box, (set to the right dihedral angle), the whole thing simply being epoxied to the spars. If you have two joiners, then one box can go to the front of the spars and the other to the rear. You could, of course, set the brass box between the spars and web both sides with ply, but in some models, such as the Habicht, this somewhat restricts the amount of dihedral that you are able to apply. While this is going on, of course, you have to leave out the ribs for this part of the wing which have to be separated into riblets and glued in afterwards, but that's another story.

As I have explained many times in the past, the wing is made torsionally rigid by the structure of the 'D' section. The 'D' section is simply that part of the wing that is bounded by the upper and lower spars and the upper and lower sheeting in front of the spars. Torsional rigidity is of extreme importance because the aerodynamic forces in flight tend to twist the wing, and this twisting moment increases proportionally with the airspeed. Also, when the ailerons are moved, the reaction forces that are created do their bit to twist the wing as well. To achieve this rigidity, the spars have to be locked together to prevent them moving laterally in opposition to each other when the twisting forces are applied. You may have heard of the term 'spar webbing' well, this is the stuff wot does the job. Simply, the spars are locked together with pieces of ply glued to them right down the length of the wing. If you wish to be elegant, you can do both sides of the spar out to about 1/3rd span, but I have given that sort of thing up now as being fairly irrelevant, as one application of 1/32" ply is plenty strong enough on its own.

Of course, there are other ways of constructing a built-up wing. One method is to make the spar entirely separate and add the front and rear structure afterwards. This allows you to fabricate the spar with an unbroken run of wood and provides a better strength to weight ratio than the previous method, although complications may arise when it comes to jigging the wing to maintain the correct section. (On my last version of the Habicht, the front part of the inner wing panel was made up from a foam core to allow for ding-proof handling in the hangar, and proved to be very strong with its balsa sheeting.)

CENTRE SECTION BOXES

For many gliders it a simple matter to run a straight joiner (or two) through the fuselage and to allow the wings to take up the dihedral angles. On some, however, such as machines that employ a gull wing, there simply isn't enough room for this to work, and we have to reverse the concept and have the joiners run straight in the wings and make up a box to accommodate the dihedral in the fuselage. The principle is exactly the same, with the brass boxes being sandwiched in ply plates with appropriate spacers cut to give the correct dihedral. The unseen snag set to catch the unwary is that the wing is usually rigged at quite a positive angle to the fuselage, and this may not allow the wing joiner box to mate up with a vertical former in the fuselage. (Not a problem with a glass fuz). My solution to this particular problem is to angle the spars in the wing to match.

AILERON ISSUES

One of the unexpected complications of wing construction lies in the construction and hingeing of the ailerons. Of the many ways of hingeing an aileron, the most common for built-up wings are the three following: Knuckle Joint, Frise, and Chamfered Butt-Joint. (I made that last name up because I don't know the correct terminology!)

The Knuckle Joint

The knuckle joint seems to be the most favoured method of mounting the ailerons to the wings, and is undoubtedly a very elegant solution if you can keep all the edges nice and straight. Basically, the aileron LE is rounded off and the top and bottom surfaces of the wing are extended slightly in the form of a shroud to seal the aileron/wing gap and cut down on parasitic drag. The aileron then has to be hinged at such a point that the LE rotates close to the wing without any snagging or binding.

So, how do we construct a shroud that will maintain a straight edge over a long distance, and at the same time resist the pull of the covering material? The answer is to use 1/64" ply over the TE of the wing in strips with the grain running parallel to the ribs. 1/16" balsa strips are glued in place over the top in the usual way, with the extra 1/32" being sanded off in the final building phase. It is important to insure that the TE is sanded to the correct shape to allow the shrouds to follow the shape of the chord, as there is plenty of opportunity here to lose the plot!

Frise ailerons pose a different set of constructional challenges...what happens here is that the ailerons are hinged at their bottom edge, and when they are in the down position the lower leading edge protrudes into the airflow deflecting air up through the wing/aileron gap. The idea is to reduce drag and at the same time allow the drag that is produced to counter the normal effects of adverse yaw which is a problem with all aileron- controlled wings. I had plenty of practice with this type of set up on the various Habichts that I built over the years, and the same simplified method seemed to work quite well for all of them. The hinges themselves were made up from two strips of Dural (hardened aluminium) and drilled and joined at the centre with either an eased off pop rivet, or sealed nut and bolt. Slits were prepared on the lower edges of the ailerons, and ply boxes in the wings. The hinges were then glued to the aileron first, and then the aileron slid into the wing to check for fit and free movement before being finally glued after covering and painting.

Ailerons of the chamfered butt-joint variety can be seen on many of Fred Slingsby’s products, such as the Prefect, T21 etc. In this instance the TE of the wing is chamfered to a point with the flat face of the aileron hingeing directly in the centre. The chamfer is achieved by the simple expedient of running a stringer along the centre of the aileron mounting spar on the wing, and covering it with fabric. Usually the gap is sealed with fabric to prevent the air on the lower and upper parts of then wing from combining and interfering with then wing's efficiency

Using the Robart type hinge

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AILERON CONSTRUCTION

By its very nature, especially on the older sailplanes, the long thin structure of an aileron does not lend itself to being torsionally stiff. It's worth bearing this in mind when you come to design your wing. The traditional way of construction is to build the aileron complete with the wing, and to cut it out afterwards. This has the advantage of making it easy to keep the correct shape of the aileron in relation to the wing and its sectional shape, but there's a certain amount of hassle involved in cutting it out and facing it off. Depending on how you plan to hinge it, you need to pre-embed your aileron top and bottom LE spars to allow for your chosen hingeing method. Torsional stiffness can only be attained by using the same basic method as used for the wing itself'...the good ol' 'D' section, except that in this case it really should be called the 'V' section. Take your uncompleted aileron and twist it back and forth, looking closely all the time at the LE spars. You will see that they are moving in time with your twisting motion laterally in opposition to each other. In the same way that the spar webbing stiffened the wing, facing the LE of the aileron will go some way towards achieving a similar result. In this case, however, the top and bottom surfaces of the ailerons may not be sheeted, so two elements of the 'V' section are missing. If the full-size ailerons are painted you can usually get away with adding some diagonal stiffeners, which will do the job nicely, but once again you may be faced with making a decision between scale accuracy and everyday practicality.

The method I have favoured in recent years is to make the ailerons separately from the wing, indeed on some models you may not have a choice if the ribs on the ailerons don't carry on in a straight line from the wing ribs. The idea is to make the aileron slightly oversize with full depth 1/8" ribs and then sand the thing back to a perfect fit once the construction is completed. (This is a very dusty business, don't even think of doing it in the bedroom). You start off with the root and tip ribs cyanoed in place on the LE, and then you add the TE followed by the remaining ribs and diagonal bracing. The advantage of this method is that you don't need to plot or draw up the aileron ribs, and not only is the process a quick one, but it's easy to ensure that the structure is straight and warp free in the process. (Even so, with ailerons as long as those on the Petrel, warps do tend to creep in once the covering has been added.

THE ALL-IMPORTANT TRAILING EDGE.

As you all know, a wing starts off pretty chunky at the front, and becomes extremely thin at the back as we struggle to maintain the correct shape for the chosen section. This means that the TE is liable to be a bit on the flimsy side and susceptible to the sort of handling damage that is all too easy to inflict. There are many traditional ways to construct this part of the wing, one of the oldest being to run a strip of balsa along the top and bottom surfaces, with maybe a strip of thin ply let into the rear of the ribs to beef things up. The problem with this idea is that it's a bit difficult to attain that nice straight TE that is a credit to the builder and it's not very strong either. Another traditional idea is to use a length of pre-shaped triangular moulding which is certainly a lot stronger in the larger widths, but not much use in the dimensions needed for scale authenticity, which is especially important with a see-through covering. Many years ago I built the 1/5 scale Minimoa from the Bob Banks plan (excellent, although not entirely accurate design... should still be available from Nexus) in which a method of TE construction was used that I've stuck with ever since. Bob specified the use of a thin strip of 1/16" spruce which was laminated either side with balsa sheet which could be sanded down to match the cap strips in the ribs. The result is tough and light and achieves both of these desirable effects with a thin TE that matches that of the full-size. Also, if your wing has a gull shape, it's an easy matter to laminate the TE over a former to get the correct shape.

JIGGING THE WING STRAIGHT.

When it come to constructing a gull wing on a flat building board, I have often heard of people making up a special surface that is bent or hinged to allow the gull wing to be built over it. Although a perfectly acceptable solution to the problem, it's not really necessary because any wing, whatever the shape can be simply jigged up on a flat surface with the minimum amount of fuss or hassle.

If you take three points along the length of the wing and support each of them with a piece of wood that mirrors the lower surface of the section at those points, and ensure that these three jigging pieces are parallel to each other on the building board, then you have the basis for building a perfectly straight wing. Here's the order of battle...

1. Build the basic wing structure i.e. the ribs, spars LE and TE's
2. Add the upper 'D' section sheeting. Up to this point you don't need to worry about jigging as the wing is not yet torsionally rigid.
3. Now the important phase: pin the jigging pieces to the board and lay the wing in place. Weigh it down carefully so that it sits snugly in place, and then add the upper part of the 'D' box sheeting.

As you have no doubt worked out, for a gull wing, the centre jigging piece is simply placed under the gull-joint and cut out to support the joint at the correct height taken from the plan. There are one or two subtleties, of course, you can angle the tip support to allow for geometric washout for instance, and you may have to add packing at the TE to allow for the fact that the rear sheeting and cap strips have yet to be added.

YES, BUT ARE THEY STRAIGHT?

Having religiously followed the foregoing, how can you be absolutely sure that the wings are twist-free? The answer lies of course in the Mk 1 eyeball, the same trusty implement that you must use throughout the construction process. Hold the wing panel out level in front of you with the TE facing towards you, and lift the TE until it becomes exactly parallel to the furthest point you can see on the underside of the wing. If you have no washout built in, the two should run pretty much parallel to each other, but there may be some strange looking things happening if the mainspar is a combination of a parallel and tapering shape. In this case I try to ensure that the TE appears to follow the centre line of the spar throughout the wing. If you are using a scale section throughout, the TE may do some odd things, on the full-size T21, for instance, the aileron TE takes on a pronounced dihedral as it follows the shape of the spar. Also, if you have built a gull-wing, you will notice that the outer panels seem to be at a more negative angle than the inboard panels...don't tear you hair out this is normal and merely an optical illusion. It doesn't take a lot of practice to get to the point where you can eye up a wing in an expert manner and tell instantly if there's a problem. The important, in fact, the vital thing is that both wings should look the same, and this is no truer than when they have to be aligned to each other as they fit to the fuselage. In this case you lift the rear of the fuselage slowly, or jig it up in the garden and swivel your eyes madly from wingtip to wingtip. You've got to be careful about what you say in print, but I think I can safely say this...using the forgoing methods, I have yet to build a set of wings that weren't true...there, I've said it, the next pair will no doubt resemble a pair of aeronautical bananas...

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