From Razorplane

/From Razorplane
From Razorplane2018-07-19T12:37:22+00:00

by Chris Williams

IN THE BEGINNING

For many years now I have preached to any

Rick Briggs with his Petrel

Rick Briggs with his Petrel

who would listen that, for those who wish to attain the ultimate in satisfaction from our hobby, you have to be able to design your own models. People would look at me in a certain way, cough and change the subject but I still maintain that it’s true.

Over the years I pondered on the wisdom of writing about the subject, but always there were other things to do and a living to be made. Then one day, Rick Briggs emailed me from Southern California, asking about the availability of plans for the Slingsby Petrel. I replied that, even if plans could be located, they would probably be to the wrong scale and designed back in the Dark Ages, when men were men and good glide angle values could be counted on the fingers of one hand. No, I assured him, his best bet was to design the thing himself, and if necessary I would lend the necessary information and encouragement. Rick, who was then VP of the International Scale Soaring Association, is also obviously a man who relishes a challenge and soon the emails were flying as we sorted out the preliminaries. So, here at the beginning of a cross?Atlantic adventure, what better time to both chronicle the project and at the same time explain to all who may be interested, just exactly how to set yourself free…

THE SUBTLE ART OF COMPROMISE
One of the very first decisions to be taken concerns that of wing section choice. The Scale Purist would argue that to use anything but the original section is no less than sacrilege and I’m not going to argue with that, after all, even I’ve heard of the Spanish Inquisition. Standing next in line is the Fundamentalist, who argues that not only should the model be externally accurate, but, if I may use such an indelicate expression, its innards too, and one can only admire such true dedication. At the other end of the scale, we will find the Unbeliever, or Infidel (he probably flies F3JB) and I like to think of myself as somewhere between the two extremes. I believe that a scale model, as well as resembling as closely as possible the full size, should fly like it too, and to do this you will need to apply some Aeronautical Diplomacy ? you will need to be able to compromise. The sad fact is that in competition, or at least, competition that puts an emphasis on performance, a model that has been subjected to a careful and sympathetic programme of compromise, will more than pull back any points lost in static due to its superior performance. But that’s just competition, and I guess the whole question is more fundamental than that.

We all know that the construction process is a rewarding and satisfying one, but so is the art of flying … surely the ultimate goal is to design a machine that allows you to experience the full gamut of the Scale Experience? Well, that’s how I see it, and in this and all that follows, I hope to show you some of the ways of achieving this goal. Oh, and there’s something else I should point out; the methods and practices that I employ are not the only ones out there that you can use, they are just the ones that by trial and error work the best for me.

It’s up to every individual to define as he goes along, what works best for him too.

THE DRAWING BOARD
DesignBefore we can even think about cutting wood, we have to come up with a workable set of drawings to the scale that we have chosen. To the uninitiated, just the simple task of putting pen to paper can be a daunting one. Despite what you may think, you don’t need the skills of a professional draughtsman to do the job, far from it. (Although I sure wish I’d paid more attention in the technical drawing classes at school).All that you are seeking to do is to lay down on paper the information you require to get the job done. It doesn’t have to be pretty, it doesn’t have to be neat, just as long ?as it makes sense to you. You don’t need a vast array of tools either, just a few basic, common?or?garden items. Let’s list ’em..

1: A couple of pencils and a fine felt tip pen when you are sure you got it right and you want to make it permanent.
2: A set square, or preferably two, one large, one small.
3: A long steel ruler, at least one metre long, and a shorter rule with metric measurements (more of this later) 4: A set of French Curves, and please, no jokes about the Au paire. These are handy, but when you need to draw curves you can also bend a suitable piece of spruce strip around some pins, especially for the longer ones.
5: Paper: You can tape together pages of A3 Cartridge paper or similar for the fuselage, but it’s best to use tracing paper for the wings so that you can turn it round and build the other wing. .And that’s about all you need, we’re not yet into the realms of rocket science ? right?

THREE VIEWS AND A PHOTOGRAPH
Most of the fundamental decisions are going to be made from the information gleaned from the 3?view and whatever photographs you have managed to scrape together. Take a look at a 3?view of the Petrel ( Martin Simons’ excellent new book ‘Slingsby Sailplanes’ is a must for every serious modeller’s bookshelf). Here are the first issues to which we have to pay attention.

A Typical 3 view

Many pictures of full size gliders can be found on the web

The first thing that you might notice is that the Petrel has an extremely thick section at the root; it is, in fact one of the characteristics that defines this particular machine. Here then, is our first and most important conflict. If we use the full?size section, it will fly like a brick with struts on, yet if we use a modern one, it ain’t going to look right. My solution is this: using the Quabeck 3.5112 (3.5 camber, 12% thickness) section, that has served me so well over the last half?dozen models, I plan to thicken the root to scale thickness (17%) and to transition it back to 12% at the gull join and build the rest of the wing as per normal. This typical compromise should retain the model’s character and still provide it with a reasonable performance.  That, you might think, is easy for me to say, but what do I do if this is my first attempt at own?design? My advice would be to seek advice ? there’s plenty around. You should also look at machines that are similar in size and aspect ratio to yours; if they fly well, ask the owner what section is employed and then consider using it yourself. (That’s precisely how 1 came to discover the Quabeck). Remember that factors vary considerably from ‘plane to ‘plane, and the two most important considerations are:RSD1 1

  1. Safe handling at low speed
  2. Efficiency.

WHICH ONE TO MODEL?

There are two Petrels in existence, one with a conventional stab, and one with the all-moving variety. To my eyes at least, the all-moving stab is by far the most elegant, and there is the added advantage of easy rigging too.

Delving a little further into the subject, it becomes apparent that the chosen glider has had a modified canopy fitted at some time, and in common with many sailplanes, the compound curve has been sacrificed to simplified manufacture and the result, quite frankly, is not that pretty So, we can get photos of this machine for detail work as it still exists, but we will need to represent it much further back in time when it sported its original canopy.

HOW BIG?

here is no doubt that the larger the model, the better and more realistically it will fly, but once again, no doubt, compromises will have to be made. The size of your car may seem a prosaic factor, but you do have to get it to the flying site. A large model may seem intimidating at first, but it is surprising how soon you can become used to it, and regard it as perfectly normal. At first, i thought it was going to be impossible to hand?launch my monster Slingsby T21, but after a few sessions, it soon became perfectly natural to do so. To my mind, 1/5th scale is as small as you can make a model of a single?seat sailplane and still get a reasonable performance. There are smaller models about, of course, but they tend to fly at unrealistically fast speeds. The problem with such models is that weight and wing loading become much harder to control, and the main penalty that results is a degrading of those safe handling qualities that we find so desirable. So, if you are planning a small model, you are going to have to pay particular attention to ways of keeping the structure light. For quarter scale or larger, the sums work out much more to our benefit, which means, to quote an example, that the balsa to be selected for the wing sheeting can be hard, and therefore your fingers won’t go through it when you carry the model out of the winner’s circle!

PENCIL SHARPENING TIME

Pencil
Right, enough of this theory for the moment, because I can sense that you are impatient to get to work. How do we get the model’s outline down on paper? We’ll start by looking at a way of making this task much simpler.

The Petrel drawing in the book comes to us at A4 size. The first job is to enlarge it to make measuring easier and to cut down on translation errors.

The easy and obvious option is to get it enlarged up to A3 via that marvel of the modem age; the photocopier. This is where serendipity plays its part. Let’s say that, like me, you are planning to build your model to a scale of 1:3.5.The first calculation that we need to make is the work out what the scale of our enlarged three?view is in relation to our chosen model size. So, we measure the length of the fuselage for instance, and divide that into the proposed fuselage length on our model. In this instance, (and this applies to all the drawings in the book) it works out that the drawing is exactly 1/10th of the size of the model. This means that everything you measure on the drawing can be transferred to the plan with a simple calculation ? child’s play, in fact, if you are using a metric rule.

Take fuselage frame 5 for instance, at the rear of the canopy. It measures 27mm on the drawing and so will need to measure 270mm on the plan. Simple, eh? But what if you are working to 1/4 scale? This gives you a scaling factor from A3 of 8.8.The same measurement in this scale means that 27 has to be multiplied by 8.8 ? OK, no big deal with a calculator, but remember that there’s hundreds of these scaling?up calculations to be made, so why not make life easier? So, for 1/4 or any other scale, instead of enlarging the original drawing up to A3, why not juggle the figures so that you end up with a scaling factor of 10.This should not be a complicated task, thanks to the percentage adjustments of enlargement to be found on most of today’s photocopiers. Back, then, to that freshly sharpened pencil…

You start with the datum line from below the fuselage on the 3?view. Draw this to the specified length, mark off the former stations and draw the vertical lines with your set?square. Measure off the points where the lines intersect the fuselage lines, remembering that for straight lines you only need to plot each end of it. At the front of the fuselage, where the curves are more pronounced, it always a good idea to add more stations, thus giving you more measurement points and a more accurate curved line. For straight lines that run at an angle, such as the front of the fin, you can simply extend the line until it intersects the fuselage outline, measure the points of intersection and transfer them to your plan. You now have the basic outline.

The next job is to decide on your wing section and make up a template of the wing root, as it will be when it is fully sheeted. Depending on the section of choice, you are going to juggle three factors against each other ? the fuselage datum line, the section mean line, and the stabiliser mean line. As a good starting point, you should set the wing at a similar angle to the full size mean line, if you can ascertain it, and set the stab at some 2 degrees positive to the wing. Laying your section template on the plan over the proposed mean line and drawing around it in pencil will give you the basis from which you can proceed. If you simply lay your template parallel to the fuselage and set the stab relative to it, you will end up with an aeroplane that flies nose?high, because the wing has to fly at a positive angle of attack in normal flight, to achieve the necessary lift

The stab is set at a positive angle to the wing, because this set up gives us longitudinal stability in flight. This means that if we take our hands off the controls to, say, negotiate a farm gate, the model will not make a sudden dive for the ground or rear up and stall when our backs are turned, assuming, of course, that the CG is correct and the model is trimmed out.

GETTING INTO THE THIRD DIMENSION

OK, that’s the easy two?dimensional bit taken care of, now for what is undoubtedly the only really difficult part of the process. Like everything else on the glider, the fuselage occupies three dimensions, and with the compound curves to be typically found on a machine like the Petrel, we have to work out a way to draw them. If you are fortunate enough to be working from a 3-view as good as Martin’s, you will see that the cross sectional shapes are drawn at the various stations along the fuselage length.

This time around, I have had a new tool to play with, an extremely cheap and humble A4 flatbed scanner, costing the same as about a couple of mini?servos. In many ways, this machine is similar to the photocopier, although not as quick. On the other hand, have you ever stood at the front of the queue in the copy shop and told the harassed girl behind the counter that you want this drawing blown up sixteen times by increasing percentage amounts? Then you tell her there may be some juggling around to get the things to fit on to A3, but it’s all right, cos you’re not in any particular hurry? Well, actually, neither have I, as I’ve always conned my wife into doing it …

We know that it’s easy to enlarge straight lines, but what about curved ones? Taking the cross?sectional drawings and putting them into the scanner, the first thing you notice is that the machine allows you to select just that particular part of the document that you want to scan.

Then you realise that not only does it allow you to enlarge by up to 250% each time, you can also set it to any percentage that you want. Going through the process twice and printing the results, you end up with a fairly large outline of your proposed former which is still some way short of the right size and which has somehow grown these enormous thick lines.

The trick at this stage is to turn the paper over and draw around the outside of the shape, which, assuming you are not using paper that is too thick will be visible through the other side. Now divide the height of the former, as it should be on your plan, by the height of the former you have just drawn around. Let’s say that the size you want is 23cm and that your drawing has reached 17cm. 23 divided by 17 comes to 1.3529. Simply ignore, or round up the last two figures to give you 1.35 This means that you simply set the scanner to 135% and the former will come out at exactly the right size. The same process applies to the photocopier of course, but you’re really going to make an enemy of that girl in the shop!

It wasn’t quite that simple though, when I came to do it Many of the formers were too wide or not wide enough, due, no doubt, to the inevitable creeping in of some scaling?up errors. This is quite simply overcome by drawing a top view of the model’s fuselage and taking width measurements from this. When you measure up the expansion factor for the vertical axis, measure up for width too, and if there is a difference, well, the scanner allows you to enlarge in differing amounts in the horizontal and vertical axes. As with so many things in Life, the telling seems much more complicated than the doing, and you will find that after the initial head scratching, the process will reveal itself to be relatively simple.

The problem with this type of fuselage is that the designers of the full?size keep applying different rules to their designs. Some have a semi?elliptical shape that stays the same throughout the length of the fuselage, in which case you can make up one former and enlarge or reduce to you heart’s content, to produce all the others. This won’t work on the Petrel though, because if you look closely you will see that the fuselage formers change in shape towards the rear, becoming almost circular at the top. It is important to include the horizontal line in your original scan, and keep it on through to the end, as this will give you the point at which you will place your longeron, against which the ply sheeting will butt.


(Alternatively, you can lay the longeron along the formers once they are temporarily pinned to the plan, and mark out the points at which they will be slotted).At the end of the day, when you have drawn out the formers and then proceeded to cut them out, you will find, when you pin them to the appropriate parts of the plan, that a certain amount of irregularity; will have occurred when you cast your eye along the length of the fuselage. This is not surprising when you consider the errors that are liable to creep in when scaling up from such a small drawing. Now you will have to be patient, adding (with wood and cyano) and subtracting (with a sanding block) until a smooth transition is finally achieved. As 1 said before, this is undoubtedly the most difficult part of the entire project, but once it is done, you will be well on your way. If you should decide to choose a prototype with a slab-sided fuselage, things will of course become much simpler without the complications of those compound curves.

Jumping ahead a little now, but still dealing with the scanning process let’s look at the stab. Drawing an enlarged conventional stab, which is mostly straight lines, is a simple enough process, but what about the elliptical one which is to be found on one of the surviving Petrels? This too I was able to enlarge via the scanner in a similar process, only this time it had to be spread over two pages of A4. If you scan past the area which has to be joined, it is a simple matter to accurately tape the two pieces of paper together, by measuring the length of the straight lines, and adjusting the two pieces of paper until they are right Even the wing templates can be processed this way. Remember way back at the beginning of this piece, when 1 talked about thickening up the Quabeck section at the wing root? One thing the scanner does, that the photocopier doesn’t, is to reduce or enlarge in either the ‘x’ or ‘y’ axis, individually. Having blown up the section to the root size, it was then put through the scanner to be enlarged in the vertical direction only, turning it into what I guess you could call Quabeck 3.5/17. Well, already this short essay has turned into a chapter, so before we reach the status of a book, I’d better turn the word processor off and go cut some wood.

RSD2 6

I have explained how to thicken up a wing section drawing by manipulating it in a scanner to achieve the desired result. Since then it has been pointed out that this process also distorts the camber line and that the results may not be in step with those expected from the original section. Taking the last point first, you really wouldn’t expect the same results from any section thickened from 12-17%, the point really being that we are trying to grope our way towards a suitable compromise between appearance, which is of prime importance in scale, and efficiency which is vital if you want any sort of decent performance as a reward for all your efforts. As I explained last time out, my method is to Suck, (float a few ideas and translate them into drawings) and See…(put glue to wood and throw it off a hill.) In the case of the Petrel it all worked out very well indeed, in fact I rate this as my most successful model yet.

This distortion of the camber line has niggled at me for some time, so during the writing of this article I called up the section in Compufoil, my new computer airfoil-plotting programme, and printed off HQ 3.5/12 thickened to 17%. (This programme allows such a thickening whilst maintaining the camber, or at least, that’s my assumption.) Then I scanned the section at its original thickness and thickened it as previously described in the scanner. On a section of approximately 8″ in length I could see no visible difference when one section was laid over the other, although at a larger scale you may have been able to detect something. Given the inexact nature of wooden construction, I seriously doubt that there is any measurable difference, and, given that the plan was for the section to thin to 12% at the gull join anyway, I really don’t think there’s too much to worry about here.

The article in the last issue was written quite some time ago and languished in the vaults for many months, since then I have discovered Compufoil. Now, for those with a PC and the desire to design models, a programme like compufoil is an absolute must when you consider the benefits to be found therein. Look at the current and traditional alternative, the Sandwich Method for making wing ribs, whereby a pile of balsa blanks are squashed between two templates and carved and sanded into submission in a great cloud of dust. Imagine being able to design your wing on your monitor, to establish all the important parameters such as the size and positioning of the spars, the thickness of the sheeting material, the positioning of the aileron drag spar and so on. Needless to say you can instantly call up your section of choice from a large library of sections, you can modify it, change the thickness, the camber, or both, plus a pile of other thing too numerous to mention. Now for the good bit…you can then print out all the ribs for you wing onto A4 sticky label paper, cut them out and stick paper to balsa, why, you can cut this little lot out with so little dust, the wife will hardly notice that you’re not polishing the French windows.

For me there’s only one catch, the programme is wing-specific, and it seems to me that there enormous scope here for a fuselage plotting programme, after all, the fuz is far more complicated than the fairly geometrically simple wings. Then of course, maybe there already is such a programme, in which case why doesn’t somebody whisper in my shell like? (An e-mail on this subject to Compufoil-Meister Eric Sanders resulted in him sending me a file for a mathematically described circle. It seems that this can be manipulated within the programme to enable the lofting of ellipses as formers, so we’ll have to see.)

Don’t think that the process is entirely without hassle mind you…anyone who has used any computer programme will know what I mean. You will need to become familiar with this or any such application, and there will undoubtedly be frustrations along the way. Clubmate John Kerley bent my arm and persuaded me to plot the ribs for his Slingsby Prefect a little while back, and it was not all plane sailing. The problems involved a lack of knowledge of and practice with the programme, but it was interesting to say the least to see the possibilities that were thrown up. John decided to forgo the usual cap strips and have the LE sheeting cut into the front of the ribs. This we were able to achieve after a few false starts, and after having seen the built-up wings in the flesh I am impressed all over again with the programme’s possibilities.

CONSTRUCTION BASICS

So, you’ve drawn up the basic shape of your glider, you’ve decided on the size of the model, and, hopefully, you have some idea of the projected all up weight. Starting with the most interesting bit, the fuselage (and you only have to build one of ’em) how do you get from a two-dimensional drawing to a three-dimensional object.

SLAB- SIDED CONSTRUCTION

RSD2 1

The author with his Slingsby T-46

There’s a lot to be said for choosing a subject with a flat-sided fuselage, especially if you’re new to the own-design process. (We’re talking here about such models as the Slingsby T31, the Tutor, the Grunau baby and such like.) The mission-profile here is a relatively straightforward one…simply build the two side frames flat on the plan, and then fix them to a couple of formers around the centre-section area, draw in the nose and tail with suitable formers to get the top-view curves, and add turtle-decking and stringers/sheeting as necessary. Where, you might ask, is the catch? Well, the one thing that most usually strays from the straight and narrow is the top-view alignment of the two sides, not to put too fine a point on it, it’s tragically easy for your fuselage to resemble the shape of a banana if you’re not too careful. There are ways to avoid this scenario however, and you don’t need to be a brain surgeon to apply them. One way is to start by drawing a straight line on your building board. Draw centre lines vertically through all of your formers. Now, when in the first stages of glueing the two sides together you can simply lay the whole structure over the line and ensure that the centres of the formers correctly follow it. This construction process has been in use since Aeromodelling began, and although relatively simple, gives the builder the same sense of deep satisfaction that comes with the more complicated forms.

STEEL FRAME STRUCTURES

Construction

A full size Schleicher Ka-7 steel fuselage

In between the early wooden gliders and their modern glass counterparts, there is a whole range of sailplanes whose fuselages are made from welded metal tubing. Some of these are slab sided, but many are of a more complex shape that does not lend itself very well to the previous construction method. Some examples are: K18, K8, Bergefalke 2 etc. Although I have seen some beautifully crafted models actually made from metal tubing, most of these types of models are rendered using a wooden framework to represent the full-size. So, how does one assemble this disparate arrangement of stringers and spars? The answer is to use a jig, and although its construction may seem to add a daunting extra burden to the building process, there isn’t actually that much extra work.

The principle is a simple one: from a wooden base you glue in place supports that mirror the bottoms of the formers in your fuselage. The formers sit in these supports and are held vertically with an arrangement of cross-supports which themselves are fixed to two horizontal rails, which run fore and aft each side of the fuselage. The drawing up of the supports is easy; you simply extend your former centre line down to the baseline when you are drawing up the formers, and draw the supports at the same time. Once you have the formers set up in the jig, you start to add the longerons, and it’s quite fascinating to see a complex structure take shape before your very eyes. The fuz stays in the jig to the point at which it becomes safely rigid, then you can remove it to gain easier access.

Once again, keeping the fuselage straight is an issue; a line drawn through the centre of the jig base, lines drawn vertically through the formers, and, to be absolutely sure a thread of cotton or electrical wire run from the front to the rear of the jig on top of the fuselage will ensure that things do not become derailed and add to the stress that is such a feature of modern living.

One further point involves those longerons that show through the fabric surfaces. If the full-size used round-section tube it follows that the part of the tube on which the fabric rests will show its shape to the external gaze. So, it would be best where possible for the wooden longerons to have a consistently smooth round surface too, and you can achieve this by using half-round hardwood dowel from the mouldings section in your local DIY store, and glueing it in the appropriate places over your square longerons. At the same time you will also need to remember to scallop out your formers in the spaces between the longerons to prevent them showing through the fabric.

COMPOUND CURVATURES

There is no doubt that the best looking, and hardest to achieve in fuselage shapes, are those that have the smooth, rounded contours that are associated with designers’ efforts at streamlining. Most of the wooden gliders from the ’50’s are built like this, gliders like the Oly 2B, the later models in the Slingsby range and the K6, to name but a few. In reality, a lot of it was faked…ply was wrapped flat around a series of fuselage frames, the final shape of which cunningly suggested a smooth curve… well, why should we be surprised, it’s extremely difficult to get plywood to bend in more than one dimension. Attempts were made to get a true compound curvature by steaming the wood in a press.Bowlus
The pod-and-boom Super Albatross being a case in point, which was sold very successfully as a kit with its pod halves pre-manufactured in this fashion.

Traditionally, to build this type of fuselage for a model, it is built in two halves. Imagine, if you will, sawing a completed fuselage in half through a line drawn vertically through its length down the middle. If you were to lay a series of keels flat on the board, glue in the half formers and a few stringers until the half shell was reasonably rigid, then remove it from the board and repeat the process for the other side then you have in a nutshell the method by which I personally prefer to build such a fuselage. There are, however, many variations. Some people prefer to sheet the entire half shell whilst still on the board, then build the other side separately and join the two sides afterwards. There is also the issue of strength; if you sheet the fuselage in a series of flat panels as per the full-size, the only longitudinal strength available to the fuselage lies in the number of stringers that are designed in, and I have seen models of this nature consistently crack a joint after a heavy landing. My method is to sheet the rear of the fuselage in ply that runs as far as possible in one length from the wing LE position to the tail. The front is sheeted with planks of 1/16″ ply, and it takes a pretty heavy landing to stress that lot!

STRENGTH VERSUS WEIGHTant
This is the designer’s true test of skill, to find the best balance between a structure that can take all the slings and arrows of outrageous gravity, and yet still be light enough to lift off the ground without the need for a huge leather belt around the waist to hold the vital organs in place. Let’s start with that most crucial of design decisions the wing joiner. I’m not going to talk about round steel joiners here, as I have very little experience of them, (although they work very well in the foam-winged models I have flown) but instead of the almost universally accepted flat steel bar that has served us so well. Say you expect your <img class=”size-full wp-image-4983″ src=”https://scalesoaring.co.uk/wp-content/uploads/2018/07/RSD2-2.jpg” alt=”” width=”400″ height=”300″ /> The strutted T-46 after finishing
model to weigh in the region of twelve pounds or so, then you should be able to get away with using only one. I’ll immediately qualify this by pointing out that if you intend to whizz your model about at high speed in strong winds, then you might need to reconsider. The furthest I have stretched this was with my seventeen pound Bocian which had only one joiner and wasn’t too bad in this respect. On the other hand, if I fly my thirteen-pound Bergefalke in a strong wing, the wings do tend to flap a little bit. All of my 3.5 scale model which are twenty pounds or over are fitted with two joiners, the only exception being the Condor which has a three-piece wing, each outer panel having its own single joiner.

If your model is going to have a strutted wing, then it makes sense to have the struts working for a living rather than just bits of heavy decoration: this allows you to save weight at the centre section where the wings need only to be braced fore and aft. But hang on though, what about rigging? A true-to-scale system can be awfully fiddly to rig so how about a compromise? On my T21 I utilised a short steel wing joiner with bolt-on struts. This allows me to slide the wings in place where they will sit unsupported whilst it is then relatively east to bolt on the struts. The struts themselves can be made from a piano wire core with modified electrical connectors soldered at each end to cope with the loads under tension, and with spruce and balsa fairings to make up the streamlined shape. (In fact, on the T21 Gelutong sheet and carbon fibre rovings were glassed together to make the strut strong enough to cope with any compression loads that might result from an embarrassingly heavy landing.)

The strutted T-46 after finishing

model to weigh in the region of twelve pounds or so, then you should be able to get away with using only one. I’ll immediately qualify this by pointing out that if you intend to whizz your model about at high speed in strong winds, then you might need to reconsider. The furthest I have stretched this was with my seventeen pound Bocian which had only one joiner and wasn’t too bad in this respect. On the other hand, if I fly my thirteen-pound Bergefalke in a strong wing, the wings do tend to flap a little bit. All of my 3.5 scale model which are twenty pounds or over are fitted with two joiners, the only exception being the Condor which has a three-piece wing, each outer panel having its own single joiner.

If your model is going to have a strutted wing, then it makes sense to have the struts working for a living rather than just bits of heavy decoration: this allows you to save weight at the centre section where the wings need only to be braced fore and aft. But hang on though, what about rigging? A true-to-scale system can be awfully fiddly to rig so how about a compromise? On my T21 I utilised a short steel wing joiner with bolt-on struts. This allows me to slide the wings in place where they will sit unsupported whilst it is then relatively east to bolt on the struts. The struts themselves can be made from a piano wire core with modified electrical connectors soldered at each end to cope with the loads under tension, and with spruce and balsa fairings to make up the streamlined shape. (In fact, on the T21 Gelutong sheet and carbon fibre rovings were glassed together to make the strut strong enough to cope with any compression loads that might result from an embarrassingly heavy landing.)

The joiners have to apportion the flight loads to the spars, so what should they be made of? Most of my earlier 1/4 scale models had 1/4″ (6mm) spruce spars and seemed entirely adequate. These days I prefer to be a bit more elegant and use tapered spars for my larger machines, usually 1/2″ by 1/4″ at the root, tapering to 1/4 sq. out at the tip. Either way, when you choose your wood, eye it up carefully to ensure that it has no knots or splits that might later on prove to be a weak point. It is important whatever the size of the model to lock the spars together with strips of webbing to achieve torsional as well as lateral strength, and for me, 1/32 ply is the ideal material. How do you taper spars, I hear you ask? First of all you need a good straight edge as long as the spar itself. Then you need to affix the spar to your building board against the straight-edge to ensure that it is perfectly straight. Now mark out the reduction in thickness at one end and with yet another straight edge, draw a line from one end to the other. The difficult bit is now over…all that’s left is to clamp the wood into a Workmate or something similar and attack it with a razor plane followed by a long sanding block with some 40 grit attached. Ten to fifteen minutes per spar should see the job done.

Sometimes the question is asked about wood selection, balsa wood that is. This is a matter that goes back to the earliest days of Aeromodelling when free-flight models had to be light to fly and the quest for lightweight structures was uppermost in everyone’s mind. Conventional wisdom tells us, for instance, that ‘C’ (quarter) grain balsa is best for ribs, the reason being that because the grain runs in more than one direction it is better able to resist splitting along the grain. After the construction of a long run of large scale wooden sailplanes, my experience is that it is better to select wood for strength than for lightness, although it is a matter of common sense to keep the structures at the rear end as light as possible. A built up quarter scale wing covered in hard balsa sheet for the ‘D’ section is still pretty damn light for it’s size, and this sort of balsa selection stops your pudgy digits from going through the sheeting when you lift the model up hurriedly to avoid that angry bull. When it comes to filling in gaps in structures with 1/2″ balsa sheet, such as at the wingtips, or tailplane centre-section, you can afford to err on the side of lightness, and this of course applies to any part of the structure where strength is not the deciding factor.

When trying to set the balance between strength and weight, there is one universal rule that will help to guide you. The smaller the model, the more you have to move the balance towards weight saving, the larger the model the less it matters. Of, course, the larger model hits the ground with more of a thump, so you can see the conundrum..

RSD2 4

The author’s Minimoa in her elements

Click edit button to change this text.

WING RETENTION SYSTEMS

NailingThere are basically two ways you can choose to keep the wings fixed to the fuselage; rigid attachment, and flexible attachment.

Rigid attachment, as it implies, involves the use of bolts and screws, or maybe metal pins as the situation demands. The advantage of such a system is that the wings are fixed with undeniable reliability, the downside being that with the onset of the rather sudden forces associated with an unplanned impact with the ground, the bolts and screws and pins turn out to be rather stronger than the structure to which they are attached. One way around this is to use ‘weak bolts’ i.e. plastic bolts or partially sawn through metal ones, the obvious problem being the job of calculating the flight forces that they must overcome, and the catastrophic arrival forces to which they must surrender.

The most reliable form of flexible attachment involves the use that dear old modeller’s friend, the humble rubber band. Two or three hooks on the roots of the wing, suitably aligned holes in the fuselage and you have a system that some people believe Icarus used to retain his crude wood -and -feather wings when he flew too close to the sun. (If cyano had been available then, he made still be alive today…well, you know what I mean) The big advantage of any flexible system is that it will absorb a lot of the forces involved in a gravity -based mishap, thus saving damage to the main structure. The downside being that in the sudden acceleration of say for instance, a winch launch, the wings may come out part-way under the stress, making this system unsuitable for plug and socket connections for the wing servos.

Some people, noticeably those that operate glass ships, have come with some ingenious systems utilising strong steel springs within the fuselage to combine the best of both worlds. The important thing to remember is the springs should allow the wings to be tensioned against each other, rather than fixed to the fuselage sides, as this could lead under extreme circumstances to the fuselage being ripped asunder.

Strutted glider pose a different set of problems: although the struts are simple to fabricate strongly enough to take the normal flight loads, once again in a heavy landing they can bow or snap under the sudden compression loads. Also, in normal i.e. not inverted flight, the forces operate to push the wings into each other, whilst in that heavy landing the forces are doing their best to separate them. A there usually isn’t much room in the centre section to use rubber bands, a different method must apply, and on my T21 that other reliable standby, wing-tape came to the rescue. Tape provides quite a reasonable compromise between rigidity and flexibility, making it very suitable for plug/socket connection systems, which make for such quick and hassle-free rigging. Here the problem is that this tape is only available in black or white, eminently suitable for colour schemes where the wing roots are white (I’ve not seen a black sailplane yet!) but you’re stuffed if any other colour is involved. By the way, if you plan on taping your wing to your built-up wing root fairing, you’d better ensure that the two fairings are interconnected in some way, as experience has shown that tape is strong enough to rip off the entire fairing. It’s not a foolproof system, of course, what is? The other day I threw the Condor off a hill in a non-existent breeze. The resultant lack of lift caused to her hit the ground with a thump and then bounce out into the void where flying speed was quickly attained. Unfortunately, the thump had caused the tape holding the port outer wing panel to split, and the connectors became disconnected. This left the machine with one aileron inoperable and an airbrake semi-deployed, leading to a tense and untimely trip to the bottom.

RSD2 5

Ahh……… the satisfaction

So, no system is perfect and, as usual, the trick lies in successfully juggling the factors involved to reach a satisfactory conclusion. You may have noticed that there has been much stress laid on the necessity to design against the possibility of disaster…I’ll leave you to draw your own conclusions…

THE WINGS

RSD 3 2What 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.)

RSD 3 Joiner box 1RSD 3 Joiner box 2RSD 3 Joiner box 3

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!)
RSD 3 Knuckle jointedThe 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.

RSD 3 Robart Hinge

Using the Robart type hinge

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.

RSD 3 Aileron 1

RSD 3 Aileron 2

RSD 3 aileron 3

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…

RSD4 1

A DAY AT THE OFFICE

“I have a confession to make …I don’t particularly like the work involved in detailing a cockpit”.
” Well, Mr Williams” said the psychiatrist, “that’s not so bad, last week I had a guy on the same couch who thought he was a pair of curtains…” I looked at him steadily.
“I suppose you told him to pull himself together.” I don’t know, everyone these days seems to think he’s a comedian.

The fact still remains though, that cockpit detailing is something that I, and I suspect a lot of other folk, put off until the very last moment. Many people don’t bother at all, and are content to stuff a Cindy doll in the office and leave it at that. That’s OK by me though, because I know one of the Great Secrets of aeromodelling and it is simply that the builder of a model is his own customer. This means he can please himself what he does with his project and adjust the agenda to suit his (or her) own proclivities. So, why should you bother with the whole fiddly business anyway, assuming that you wish only to please yourself and to heck with what other people think? Well, it’s like this… scale modelling in some ways can be represented by construction of the humble onion, what I mean is, there are layers of satisfaction and achievement that peel away as you delve deeper into the subject. On the outside lies a simple scale model, scale in outline, at least according to the manufacturer, and of which the proud owner is happy enough to pole around the sky in a carefree fashion. Look, says the pilot, you can’t see any detail in the air anyway, so what the heck, and he’s quite right, of course. At the centre of the legume you will find represented a true scale model, complete to the last detail, or you would if there was such a thing as earthly perfection. Somewhere between all those eye-wrinkling layers lies you and me. Many people assume that a scale model is like an F3B or F3F machine, and that the sole reason for its existence lies in its performance in the air.

 

This is a perfectly valid point of view, except for one tiny factor: all model aircraft spend more time on the ground than in the air, and this is just as true for scale models. You may be having a crafty fag, or waiting for your frequency or looking for a convenient bush to vent the remnants of last night’s beer, meanwhile your model is sitting on the ground and is open to public inspection. You must have noticed yourself, if you have ever attended any sort of scale event, that next to sheer size, the one thing that attracts interest in a particular model is the amount of detailing that is on view. At its most basic level, the philosophical equation runs thus: the more you put in the way of involvement and effort, the more you will reap in the way of satisfaction and the sheer pleasure that is the reward of constructional creativity.

BEFORE THE HOW…THE HOW MUCH?
EuroThe whole key to successful cockpit detailing lies in deciding how much of the original you are going to replicate. If you wish to put in every rivet and bolt, go ahead, but it’s going to take months and months of patient effort, and all the time there will be ticking away at the back of your mind the scale modeller’s Damocles Sword of Doubt, framed in the question.. “Wot if I crash it, then all that effort will be wasted?”

At the most, I will spend a week or so of evenings sorting out the cockpit, and I believe in going mostly for effect rather than substance. This means that from a reasonable distance the appearance of scale has been achieved without the awful amount of work needed to achieve the actual result. Another point to consider is the snug nature in many cockpit apertures of the fit of the pilot figure. If the fit is, shall we say, tight, then whole chunks of the interior are hidden, leaving us with less work to carry out. (Why can’t you buy a fat pilot, then you’ll need hardly any detail at all?) Talking of the pilot figure, the most common mistake you will see concerns the little chap’s posture. Time and time again you will see pilots leaning right back and looking at the sky in the most unnatural way, this is usually because their poor little derrieres have nothing to sit on but a spikey fibreglass floor.

(Call in the RSPCA, I say!) Many folk believe that a true modeller makes his own pilot figures – I am not one of them. There are two manufacturers of scale pilots in the UK, A & H Models and Pete’s Pilots. They come in a variety of scales and it would be a good idea to check them out for size before buying, as both of these companies have differing ideas about what size a figure should be for a given scale. Recently arrived on the scene are Axel’s pilots from Germany which – although not exactly cheap – are superbly finished and detailed.

AND NOW…THE HOW
glider airspeedIf you had to choose the one item in the cockpit that is visually the most important – after the pilot that is – it surely must be the instrument panel. Here again, this is not necessarily a big task, as you can buy the instruments ready made, leaving you the simple job of making up the dashboard to which to apply them. There are many manufacturers from whom you can get most scales, and you should find a reasonable selection in any decent model shop.

There is a certain satisfaction in making your own, however, and this too, needn’t be overly complicated. Take those humble examples of UK coinage the penny and two-penny pieces. The latter are ideal for the larger sizes of model and can be processed thus… Drill holes in your ply or Aluminium dashboard so that the coins are a snug fit and can protrude just enough to give the impression of a proper instrument bezel. The simplest thing is to make up an identical backing piece to the dashboard which, when glued to it will allow you to simply retain the coins. Choosing the right thickness of dashboard material will then allow you to adjust the depth at which the coins will sit.
PanelClean the coins up with some thinners, or similar, and then spray them matt black. All that remains is to apply suitable instrument faces in the appropriate places. In the old days, this involved scratching out the instruments from bits of black card, but now life is much simpler. If you have access to a computer and a scanner, (if not, then surely someone in your club does) it’s a simple matter to scan instrument faces from a printed sheet and to correctly size and print them on to a sticky label..

You will notice that I have made no mention of glazing the instrument faces. This is because I believe that the darn things show up more clearly this way and look a whole lot neater. Effect, remember, rather than substance. It’s a very debatable point, accuracy versus neatness, and is worth a quick exploration. I have heard the criticism sometimes that our models are too ‘clean’ and would benefit from ‘dirtying up’, as they would be in real life. Apart from the fact that all aeroplanes leave the factory in mint condition, and we could easily claim to be modelling them in that period of their lives, there’s no reason why you can’t reproduce a distressed’ glider. Some of the photographs from which I am working at the moment on my current project, the Schweizer 2-32, show some sailplanes that are very distressed indeed, in fact they’re the aeronautical equivalent of a bunch of bag ladies .The problem is that they look …well, they don’t look very nice. There is something deep in the human soul that cries out for neatness, it’s probably a cry of rage at the universe’s apparent and innate chaos. So, if the full size machine that you’re replicating has a panel full of disparate and wonky- looking instruments, you can choose to faithfully reproduce them, or go for something a bit neater. One thing is for cast- iron certain; if you choose the former, nobody is going to come along and nod wisely and say ” mmmm, obviously this modeller has replicated the full-size panel here”. No sir, he’s going to turn to his mate and say, somewhat scathingly, “Phew, he made a right bog of that then…”.

dials

The next step to Scale Heaven is to consider mounting the canopy frame on hinges, assuming that this happens on the full size. Yes, it’s a little fiddly, because you have to achieve a good fit between the canopy frame and the cockpit aperture when the canopy is closed. The secret here is to hinge the thing up first and then do something about the fit.

The simplest solution if the frame stands a bit proud (and they usually do) is to use filler on the fuselage to even things out. Mask out the frame with a strip of masking tape to protect it, apply the filler with a flat piece of wood or plastic and wait a few minutes for it to go off. When it starts to go solid, run a knife blade through the fuz/canopy gap and remove the tape. Sand the filler smooth, and repeat the process if necessary. It should be perfectly feasible to feather’ the edges of the filler in to the fuselage with a sanding block so that there is no join – we do it on your motor cars all the time. Another good move is to fit locating pegs in the canopy frame which will mate up with holes in the edge of the cockpit aperture, thus ensuring that the canopy closes in exactly the same place every time. If you are keen enough to do this with a fibreglass fuselage, don’t forget to sand the areas to which you are going to apply the body filler first. The best filler to use is Polyester car body filler, which is easy to sand and designed precisely for such a purpose.

In an ideal world you will be building your glider from a set of photographs that you have taken yourself, I say ideal, because in this instance you will undoubtedly have taken care to shoot all of the interesting little details in the cockpit, right? If not, don’t worry too much, because there are some things that are common to most gliders in this area. Placards are favourite, little notices full of that sort of vital information that will prevent a pilot from drilling a hole in the ground due to lack of knowledge. Max. permissible weight of solo pilot to keep within the CG limits on a two-seater, max. permissible velocity before the wings fall off in a dive, max. permissible amount of Mars Bar wrappers that can be left under the seat before the elevator cables seize up, this is the very fabric of aviation.

But, I hear you cry, I don’t know what these figures are. So what? Effect, remember, effect. On one of my models I reduced my club insurance certificate, mounted it in a simple frame and placed it in the cockpit. If anyone has noticed it yet, they’ve kept pretty quiet about it! The fact is, you can look at an assortment of cockpit details from a variety of full size machines, copy those that are easiest to make, put them in your model’s cockpit for effect and count yourself very unlucky indeed to encounter anyone knowledgeable enough to offer any valid criticism. I am resolved, as soon as I have finished this article, to make up a placard to put into the Schweizer cockpit. It will bear the legend: “If you can read this, then you’re too damn close.” Let’s list the basic essential ingredients for a believable cockpit:

  • Dashboard/Instrument Panel (already covered).
    Pilot’s seat. Ignoring my own advice I spent an evening making up and painting a virtual armchair for the front pilot in my Schweizer. Well satisfied, I put it in place, sat the pilot figure upon it and watched it disappear, completely dwarfed by the pilot. A simple ply structure will often suffice, the seat’s main job being to position the pilot in a natural and believable position.
  • Cockpit Lining. It’s essential to line the inside of the fuselage in the cockpit area in order to hide either the bare fibreglass of a modern ship, or the usually non-scale structure of a built- up machine. There are several ways to do this, the simplest being the method employed in my Condor. In this instance, two pieces of card were cut to shape, sprayed a light green, and stuck with double-sided tape to the fuselage sides. That was a few years ago now, and it’s still holding up well.

My built-up fuselages usually have all the formers removed, with the interior being fibre-glassed for strength. It’s a pretty simple matter to make up some battens, curved on one side to follow the fuselage and flat on the other, glue them to the fuz and fit 1/32″ ply panels in place. I would normally reckon this to be one or two evening’s work at the most.

 

lining

Full size Minimoa showing sheated sides with Placard

If your model represents a steel tube framework structure such as can be found on gliders in the Schleicher range (K8, for instance), it may be worth considering lining the inside of the cockpit with ply and adding half-round hardwood dowel to represent the steel tubing. This worked reasonably well on my Bergefalke I – and had the added advantage of making stronger an area prone to damage in a prang. Finally, if you have a glass ship and simply can’t be fagged with any of this, there is a range of cockpit pans available from manufacturers such as EMS.

Typical Sutton type harness as used in pre war British gliders. This can easily be made up using a corded ribbon and servo fixing eyelets. The harness was passed over the pilots limbs and was secured by a pin which passed through the eyelets from the back and then locked in place at the front with an ‘R’ clip over the pin.

Safety Harness. These too, can be purchased over the counter, with very
realistic items such as belt buckles etc. Alternatively, if you can obtain some discarded garments of the type that support that part of a lady’s anatomy that is high up and at the front, then you will have saved yourself a trip to the
model shop, and a bob or two into the bargain. Soaking the straps (you really must discard the rest of the garment) in a weak solution of thinned black paint will, once they have dried, remove any trace of the word lingerie from these items.
Placards. Simply paste on to an A4 shaped piece of ply. Adding a strip of clear celluloid top and bottom completes the illusion.
Joystick. Again, these can be purchased ready made, or made up from brass tube or hardwood dowel. Try to get the pilot’s hands grasping the stick if possible.

There, it doesn’t sound like a lot does it, and it really isn’t, when you get down to it. Please don’t infer from what you have read so far that I’m against a more rigorous approach to cockpit detailing, my aim is to tempt those of you, that haven’t bothered with this process yet, to give it a go and discover that, like so much of scale modelling, this is yet another enjoyable and addictive process.

So, come with me for a walk around a new machine and see if you agree.

Starting at the rear we walk slowly in a clockwise direction. A partially obscured sun casts it’s light on the flying surfaces, and in the reflected light we can see those slight undulations and irregularities that hint at the structure underneath. This is not the bland, flat surface of a foam filled wing, but speaks volumes to the experienced eye of the work that has gone into achieving a convincing simulation of reality. From the side we can see the panel lines and rivets casting their slight shadows, understated, yet adding their small contribution to the overall effect.

Cockpit nimbus

Cockpit detail in Rosenthal’s Nimbus

The canopy is hinged open, resting against its restraining wire, and the pilot looks ahead and slightly to one side as if he can’t bear to look at the perfection of the instrument panel.  Cockpit detail in Rosenthal’s Nimbus.  From the front, the sailplane takes on a menacing look, as if it cannot wait to convert latent energy into effortless soaring flight. From the other side the light reflects in such a way as to cause all of these factors to seemingly combine in such a way that the illusion of reality suddenly snaps into place. Mere words cannot adequately describe the surge of satisfaction and well being that this sight promotes in the human heart, in fact…in fact….

“Mr Williams?” said the good doctor in tone of mild surprise, “where are you going in such a hurry?”
“Sorry Doc” I reply, “places to go, hills to climb, transmitters to charge, you know how it is…”
He closed his note book with a flourish of satisfaction.
“Well, he soon pulled himself together………”

Placard

Placard discus

Schweizer 232 3

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.

Libelle

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
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 Spar 2way 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.

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)Libelle Wing

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″.

WingJoint

Matching the section to the fuselage: Mask the root with 2″ tape; skim body whilst filler is still semi-soft; sand flush for a perfect finish.

WingJoint 2

Filler is applied over the root fairing & wing; ease joint open with a knife

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…