ANOTHER LOOK AT WING CONSTRUCTION

Take a wing, any wing...Ah, I see you've chosen an all-moulded one, nice, isn't it? Look at the perfection of the surface finish, look at the accuracy of the airfoil profile, and notice that damned great hole in your wallet. As this wing represents aerodynamic perfection, near enough that is, let's use it as a benchmark against which to judge others. Coming down the scale a bit, there's the foam/obechi wing with optional lashings of carbon fibre. Look at the smooth surfaces, look at the accuracy of the airfoil profile, and notice the look of reduced strain on the face of your bank manager. At the bottom of the pile, aerodynamically speaking, is your poor sad old built up balsa wing: Look at the wavy surfaces, what happened to the profile, still, at least it was relatively cheap to build, right? Normally, a built up wing is utilised to replicate a full-size wooden glider, so its imperfections generally speaking mirror the imperfections of the subject, so no worries there then. But what if you wanted to build a wing for a glass ship, could it be done with a built up balsa jobbie? Hoy, I hear the plump-walletted amongst you say, wot's he talkin' about then, you just writes out a cheque and swaps the thing for a set of wings, wot's the fuss? This is true of course but what if you wallet hasn't been fattened up for plucking at Christmas? In this Pentium Processed, Short-Term Contract, High-Cost -of-Living age hear at the dawn of the brave new mmm, mmmm, mmi- (I simply can't bear to see that word any more) age, modelling pennies can be hard to come by for some folks, and your average glass ship kit represents a pretty big investment of resources. Besides, there are plenty of modern glass-fibre fuselages out there in the market place, and if it were possible to build a cheap set of wings to go with it, you'd have a pretty cost-effective way of getting into the slippery ship stakes.

Below: the Libelle displays it's wings described in this article

How did Williams get started on this subject, then? It all began with my recently finished Schweizer 2-32, which has a set of fully built-up balsa-sheeted wings. Things really came to a head with the refurbishment of my old 3rd scale 205 Club Libelle which has been languishing in the roof these many years. How much for a set of ready made wings then, do you reckon? An informed guess would place the figure at about £250, although you could get some simpler ones made up locally for a fair bit less. But wait, that's what I did the first time, and although inexpensive, they were less than ideal. A built-up wing, when you think about it gives many more options: full access to the wing joiner assembly for starters, and in the case of the Libelle, plenty of constructional choice for the complicated flap/brake assembly that takes up damn near the whole wing. Its difficult to quantify the price of the wings that I finally built, but up to the sheeted ready-to-cover stage I don't suppose I was into the bank for much more than about £35, so already I sense a quickening of interest amongst the more impecunious of you.

It's not just about money, of course, there's the challenge and enjoyment of the building process too, something they've not yet built into a computer game!

After the loss of my aerobatic LO 100 due to an unexplained loss of control, it was decided that the Schweizer would have to have a beefed up wing so that I could throw her about a bit by way of recompense. How to do it, that was the question. The first thing I looked at was the spar, after all, this is the main structural component through which all the flight loads must be tamed. On this model, then, was tried the process of building the spar separately in one piece and then building the wing around it. (This is not a new idea by any means, but more anon.) With the construction of the Libelle wings, things really came together with the abandonment of the building board, for the construction of the basic wing, at least prior to the final sheeting anyway. Also abandoned were the last vestiges of traditionalism with the doing away of that faithful old standby 1/16" balsa sheet for something more substantial. It was truly amazing how much more solid the final wing sheeting became with such a slight increase in material thickness, giving a surface not too dissimilar to that of a foam wing.

What is a traditional wing structure anyway?

For me it has always consisted of two spruce spars let into the one-piece wing ribs, which are locked together with individual pieces of 1/32" ply webbing. The ribs are from 1/16" balsa as is the sheeting, top and bottom, and this process has stood me in reasonably good stead up to now. But times change, now I need more beef in the construction, so let's take a look at the process without further ado...

FIRST, THE SPAR

For the sake of simplicity and strength combined, the spar is made up from four pieces, although on a large wing, some of those pieces will have to be joined from smaller bits. As before, the top and bottom spars from spruce are typically made up from 1/2" x 1/4" lengths tapered down to 1/4 sq. at the tip. (The best way to buy this material is from the mouldings box in your local DIY store as they then come in 8' lengths.) These sub-spars are sandwiched between to lengths of 1/16" ply to the full length of the wing, this way you get added strength under bending loads from the ply that you wouldn't get from the traditional individual pieces of spar webbing, yet the spars are still securely locked together. So, there's the spar, which after sanding looks pretty neat, but how are we going to join it through the fuselage to the other wing? Even with the six degrees or so of standard dihedral, there's not enough space in some of the thinner sections to allow for the dihedral by having a box between the sub spars, so the joiners will have to go to the outside of the spar. Actually, it couldn't be simpler, you cut out a support piece from 1/8"ply with the dihedral angled in, epoxy it and the 14mm brass box on to the spar and close it off with a 1/16" ply plate. If you are building a larger model and need two joiner bars, be absolutely sure that the second is parallel with the first. If you are wondering how to tell whether you need an extra bar or not, as a general guide models up to about thirteen pounds can usually cope quite adequately with one, any heavier and you would be better off using two unless you really only intend to pussycat your glider around on a calm day. (This is based in the 14mm steel joiner and brass box system). If you are further wondering why I don't use a round joiner system which allows the wing to be stiff fore and aft as well as up and down, this is because I believe that such a system would place intolerable loads on the structure of a built-up wing in the sure and eventual instance of a sudden forward deceleration. (No, I'm not talking about slamming the brakes on in the car.) Undoubtedly someone out there will eventually prove me wrong before too long. (Refer to pictures in Part 3)

PLEASE, NO JOKES ABOUT SPARE RIBS.

One big factor that I haven't so far mentioned that has allowed such constructional experimentation to take place is that new weapon in the Williams' armoury, the computer airfoil-plotting programme, Compufoil. You could achieve the job I suppose with the old Sandwich Method, but obviously with a damn great spar inserted through the wing, the ribs will have to be accurately cut into two, and that might prove a tad difficult. Compufoil allows you to print out the ribs on to A4 sticky labels and plonk them directly onto the balsa. Additionally, you can plot the full depth spar, which gives you somewhere accurate to cut to, and as if that weren't enough, vertical station lines are also printed along the rib which gives you more control over cutting out the front and rear of the ribs for the LE and TE's. With the Libelle, I also used the facility that allows you to print out the wing plan, involving the sticking together of a number of sheets of A4. Although I thought I had been careful with this process, the spar drawing didn't look any too straight, although this was easily rectified by overdrawing the spar with a straight edge. You see, this is where tradition gets the heave-ho in no uncertain manner, as the wing is not going to be built over the plan, but 'in the hand' as it were, at least until the final top sheeting is glued into place. All you need the plan for is for transferring the rib positions to the spar.

So, before construction commences, here's what we need to have done...

Kit up the whole wing, including the riblets and the LE and TE's. Mark out the positions of the ribs on both sides of the spar from the plan. Needless to say, for this and all the foregoing stuff you will need the services of a nice long metal straight edge for drawing and marking out and a small set square or similar for setting the ribs at right angles to the spar. Stock up on Cyano!

THE CONSTRUCTION PROCESS

1. Lay the spar flat on the board and add the following rear riblets: root and inner aileron aperture and tip riblet. Eye them up to see that they are equally aligned. Add the TE and the aileron mounting spar. If these two spars have been cut absolutely straight, they will provide a guide for the remaining ribs. Add a couple more strategically placed riblets to fix the TE's into a straight line and then add the remainder. I find that a medium viscosity cyano is best for this process as it allows you a little time to move things about for the best fit. (Hopefully, you will have remembered to allow for the extra size of the wing joiner boxes)

2. Remove the structure from the board and repeat the process for the front half of the wing, only this time offer up templates for the first three ribs to ensure that sectional fidelity is maintained. (Easily done with Compufoil)

3. Make up the lower wing skin from 3/32" balsa sheet. One of the problems that can plague the sheeting process on or off the plan, is when the spar takes up a bit of a curve in the plan view, something that happens more often than not. This time I drew out the spar position on the lower sheeting, cyanoed half of it straight and then cyanoed the rest. It is a fairly simple matter to white glue the ribs and LE and TE's to the sheet afterwards. Remember, you don't need to jig the wing yet.

4. Add all the necessary services (drill holes in the front riblets first, remember to take the wiring loom.) Cut out the holes for the servos access areas etc.

5. Lay the wing into three or four templates made up from 1/8" balsa and cut from the root, tip and middle ribs to ensure that it will be twist-free.

6. White glue in place the top sheeting, taping and pegging in place with some weights on top to hold it all in place.

Voila! This is the process by which the Libelle wings were made and I must say I'm rather chuffed with them. They seem to be pretty stiff, yet still light in weight, the materials were cheap and they were built over the Xmas holiday. Nor bad, eh?

There is one final twist to this tale...The biggest give-away for the built-up wing lies in the fact that it is almost impossible to hide the longitudinal joins in the wing sheeting. No matter what I have tried in the past in terms of different glues, taping the joint, not taping the joint, filling, sanding, whatever...the joint always shows slightly through the surface finish, bringing about a severe diminution of that well-deserved satisfaction that is the builder's due. This time I think I may have cracked it, and here's how...

1. When joining the lengths of 3/32" sheet together it seems best not to use tape with the cyano, as this generally causes a bit of a mess. Instead, using thick cyano to give you more time, shove the tongues of the two sheets together after coating one of them with glue, lay flat and scrape off the excess with a flat piece of wood, turn over and repeat for the other side. If the edges aren't together, use a drop of kicker to speed things up. So much for these joints, they'll still need filling and sanding in the usual way, but the spread out cyano will harden the area and help to prevent uneven sanding

2. The real problem lies with the lengthways joint and the best approach seems to be this: slightly bevel the edges of the sheets that will form the outer surface of the wing with a sanding block. Tape the sheets together on the other side with masking tape. Fold the two sheets into the open position, add thick cyano, lay flat on the board and wipe off the excess glue. Hold flat until the glue sets. The bevels will form a slight trench into which the body of the glue joint will exist, the wiped off glue will harden the surface area around the joint, and a smear of filler and some elbow grease should leave you with a flat, entirely joint -free surface. This works extremely well with the 3/32" sheet, although I'm not sure if it would be as effective with 1/16".

There may be one or two of you out there who, like myself, have enjoyed building vintage style models over the years, but who quite fancy the thought of a glass ship adding style and grace to the shelves in the garage. If the foregoing has tickled your fancy, take a dekko at the fuselages that are available and maybe give it a go.

If not, you'll just have to dig out that fat wallet again...

Continue to Part.......... 1 ........ 2 ......... 3 ..........4 ......... 5