Learning your lines

This month we’re going to put some flesh on the bones by designing the fuselage, wings and tailplane.

THE FUSELAGE BOX

The first thing to do is draw the outline of the fuselage box. The front of the box will be F1, and the top will be on the horizontal datum line. The rear of the box should be set a little behind the elevator hinge line, though this point can vary from a 1/4” behind the hinge line (leaving just enough room for the elevator joiner in front of the rudder hinge) to somewhere beyond the end of the elevators. Let’s compromise and end the box 1” behind the hinge line.

When it comes to determining the line of the fuselage underside, one of the considerations that will affect the design is the material that you’re going to use to build the box. In this case, I’d suggest using 4” wide timber for the sides, so we’ll allow that dimension to define their maximum height. Then there’s the question of styling. You could set the bottom line parallel to the top, but we’ll go for something more interesting: the top and bottom of the fuselage will be parallel from F2 to the trailing edge, then the underside will taper up to within about 21/2” of the top side at the rear.

With this done, we can start setting out the formers. We’ll want one between F2 and the trailing edge of the wing, one at the trailing edge itself, and another at the leading edge of the tailplane. This leaves a space between the wing and tailplane that can be divided into three with two more formers.

PLAN VIEW

The next stage is to draw the plan view of the fuselage so, leaving enough room to add the undercarriage beneath the side elevation, draw a line parallel with the horizontal datum. Then, using your set square, drop lines from the formers on the side elevation and mark their position on this new datum line (Fig. 1). 

The thing to decide now, of course, is how wide the fuselage should be. A convenient width is 3”, because it allows room for three servos abreast, a normal size fuel tank, and plenty of room for an engine mount. The simplest thing to do, then, is to mark out the fuselage’s 3” width with two parallel lines drawn on either side of the datum line, extending the lines back as far as the trailing edge of the wing. That takes care of the fuselage front end, then.

To draw the taper from the trailing edge to the rear of the fuselage we can use a spline. The flexibility of this 1/8” sq. strip of spruce means that, having taped it down along the straight sections of the fuselage side, you can bend it so that it just touches the datum line at the rear of the fuselage. Once the spline is taped into position, it should describe a nice gentle curve from the trailing edge to the rear of the fuselage. Alternatively, you can draw a curve behind the trailing edge of the wing and then draw a straight line from the rear to touch this curve at a tangent.

Having now drawn a side and plan view, it’s a simple matter to draw in the lower part of the formers and add some structural detail to the plan view, such as the doublers and sides. I use 1/32” ply for my doublers, and 3/32” sheet for the sides. The doublers only extend about 3 inches behind the trailing edge of the wing, but they should always end in a taper; straight lines and sudden changes of section provide a focus for stresses, and make the model more likely to break in the event of a crash.

ADDING SOME STYLE

So far, our drawing could be of almost any model, but this is where the design process becomes exciting: you can now give the model a shape that will make it uniquely yours. You could, say, borrow a style from a jet, a crop duster (Fig. 2) or a racer. Or how about a warbird (Fig. 3)? You could lift a few lines from the P-51 - a suitably shaped fin, perhaps, a bubble canopy, a belly radiator, and a Mustang-ish cowl. Add an appropriate colour scheme and you have your own take on the famous fighter. Or maybe you could fit a radial cowl and a suitable canopy, add a wing-mounted undercarriage and change the colour scheme to make yourself a Harvard. More exciting still is the potential to create a new shape and style. All you have to do is imagine it and draw it.

TOP DECK

I say ‘all you have to do’, but drawing a shape is one thing, working out how to build it is quite another. Let’s start with the top of the fuselage and the formers that give it its shape. The simplest way to build the decking is by using a sheet of 1/4” or 1/2”  balsa, then fitting a canopy with a fairing running back to the fin. A variation on this approach is what I call the faceted top deck, in which the sides of the formers slope in and have a flat top. This is easy to both draw and build.

Things become a little more involved when you want to use formers with round tops. The first trick is to draw them accurately, which isn’t easy to do by hand. Moreover, proprietary ellipse templates are rarely big enough. A pair of compasses may help by allowing you to draw arcs, and if you have a combined scanner / printer you might be able to use that to enlarge or reduce a curve to suit. Remember, also, that when introducing round-topped decking to your design you can get away with a curved cross section but if you also want a curved profile you’ll probably have to plank in the top deck.

THRUST LINE

With the side view drawn, you can now return to the positioning of the thrust line, which we touched on last month. Leaving it until now, however, means that you can not only plan for the top line of the fuselage to flow into the spinner, but also decide how the engine needs to be mounted in order to both function and fit in with the model’s proposed shape.

Depending on the engine, a vertical installation can be problematic, causing difficulties in getting the fuel tank high enough to correctly feed the carb’. Equally, with the cylinder head in the horizontal plane the silencer will invariably foul the lower firewall or fuselage underside. The answer, I’ve found, is to angle the engine so that the silencer clears the fuselage side. Inverting it is, of course, another option, often allowing the silencer to clear the left side of the fuselage and providing you with the possibility of designing a very sleek cowl. Don’t forget, by the way, that when planning to fully enclose the engine, you need to ensure a good flow of air through your cowl.

WING DESIGN

Last month, we decided that our model would have a 50” wingspan and a constant chord of 10” - the two dimensions we need to start designing our wings.

The majority of built-up wings have the same construction: a leading edge, a main spar, a trailing edge spar and an aileron; the leading edge and the centre-section are covered with sheet and the ribs have cap strips. The reason that this approach is so universal is that it’s easy and works well. A very good reason for following suit with our wing. Now, let’s look at the construction of those components one piece at a time.

You can make the leading edge in several ways: a simple square strip set into the leading edge of the ribs, diamond fashion; a piece of rectangular strip glued to the flat front of the ribs or, my own favourite, a 1/8” sheet leading edge and a 1/8” sheet cap strip. This approach provides a really strong joint for the sheet and avoids having to trim things exactly to size (Fig. 4). 

The spar is normally set back from the leading edge by about 30 - 33% of the chord, which is also usually the point of maximum thickness of the wing section. For this size model spars will usually be 1/4” square, top and bottom, with 1/16” sheet webs. When combined with the sheeted leading edge, this makes for a D-box construction that is immensely rigid and strong.

When it comes to the ailerons, we can choose between two main types: the familiar strip design, or the inset aileron located out towards the wing tips. While inset ailerons look more realistic in many types of model, and can also be more effective, strip ailerons are much simpler to make - which is why we’ll opt for them in this build. The size of strip aileron can vary, but on a wing with a 10” chord, I’d make the chord of the control surfaces themselves somewhere between 11/4 and 11/2”. Depending on the wing section you may be able to find trailing edge stock that’s just the right size to make the ailerons. Otherwise, you’ll have to get busy with a razor plane, cutting some larger stock down to size!

You also have to decide how many ribs the wing needs. There will be one at the root, of course, one where the fuselage side will be, and then a number of evenly spaced ribs out to the tip. On a wing of the size we’re discussing here, I’d suggest that the rib spacing should be roughly 21/2”. You can vary this a little to suit your particular needs, but I would never set them more than 3” apart, nor closer than 2” unless it was to suit the requirements of a scale model.

Having worked our way out to the end of the wing, we now need something to stop the whole assembly from fraying - a wing tip! This is the bit that gets damaged in rough landings, remember, so you should try to design something that’s both sacrificial - so that it absorbs the stresses and prevents them moving inboard - and easy to repair. A simple tip made from soft sheet or block is the easiest solution; you can make up an outline from sheet parts joined together, or even laminate a tip from thin strips of 1/16” sheet.

The final factor to consider is dihedral. On a scale model this will be decided by the prototype, of course. With our sports model, however, we’re free to design in anything from 0 to produce a flat wing, through to several degrees of dihedral. I always think that a degree or two looks better than none.

DRAWING THE WINGS

Once you’ve decided on the construction of the wing, you can draw the wing plan - or rather plans. You should always draw the two wing panels separately so that you can build them at the same time. Drawing up the plan to show ribs, spars, sheeting, etc. will allow you to work out some of the finer details of construction, such as which ribs need to be cut away for the aileron servo, say, or have holes cut in them to create wiring runs (I’ll talk about aileron control systems next month). You’ll also need to design the wing joiner to give you the required dihedral, the angle of which can be incorporated in the drawing’s front elevation so that you can see how much the wing tip needs to be propped up when joining them.

TAIL FEATHERS

Compared to the wing, the tail surfaces are relatively easy to design. The figure often quoted for the fin and rudder area is 8% of the wing area, and a big rudder is essential for good flick manoeuvres and knife edge flight. I confess that, at the end of the day, I use the TLAR or ‘that looks about right’ system. I just draw what looks right, and it invariably seems to work.

The tailplane and elevators, meanwhile, should have a total area of about 15% of the wing area. Many designs have larger tailplanes, and I’ve also flown designs with tiny tailplanes without experiencing trouble. The thing to remember is that with a small tailplane the balance point should be further forward. However, in order that you can work all this out for yourself in future, let’s look at some figures.

The wing area of our model is 500 sq. in., 15% of which is 75 sq. in. Last month, I suggested that the tailplane should have a root chord of about 6”, with the flying surface tapering down to a chord of 4” at the tip; the chord of the elevators remains a constant 2”. This gives a mean chord - that is, an average chord - of 5”. If we make our tailplane span 15”, then we’ll hit our 15% target (5 x 15 = 75). Of course, you can play with permutations of these figures to vary the proportions of your tailplane design.

As far as constructing tailplanes goes, I favour 1/4” sheet, with medium hard material used for the fin and tailplane, and soft wood for the rudder and elevators.


PLAN FOR SUCCESS

Many people draw the essential lines with just a single line for ribs, spars and formers. This approach may work when you really know what you’re doing, but in the early stages you’ll find that it’s much better to draw the whole construction properly. By doing this, you can check that everything is actually going to fit and spot the things that might otherwise cause problems during construction.


BACK-UP BEFORE BUILDING

It’s a good idea to make a copy of your plan before you start building on it. I find that my plans aren’t even good for wrapping rubbish once they’ve been used!


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