by Chris Williams
IN THE BEGINNING
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
Before 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.
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.
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:
1: Safe handling at low speed;
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.
There 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
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.