Build your first flying scale model - Chapter 6 - Flying surfaces

In this chapter we will construct the wings, tailplane and fin. The wings are built directly over the plan, which should be covered with cling film or something similar, as we did with the fuselage sides.

You will need to cut some strip from a light/medium sheet of 1/16″ balsa – enough 3/16″ wide for the leading edge, and tailplane cross pieces, and 1/4″ wide for the trailing edge. We can get away with using light wood because the sections are generous for an 18″ span model.

Begin by pinning the leading and trailing edges to the board. You can see I have dared to pin some of the bits through the balsa strip – with wood of this size and lightness splitting is never a problem. The wing tip pieces are next, and the edges of these need to be carefully trimmed and adjusted so they fit snugly. Make sure the outer edges at least cross the oulines drawn on the plan – we will sand back to the final shape after removal from the board.

I found it easier to fit the centre tip piece (W.2) last.

The wing ribs can now be added, gluing them to the leading and trailing edges. After you have cut the wing ribs out it is a good idea to smooth their upper surfaces with an emery board. In fact, if you want to be really fussy, you can pin the ribs together in a block, lining up the spar slots with a scrap bit of 1/18″ x 1/16″ strip, then sand the whole block together so all the ribs are exactly the same shape. I confess on this model I did not bother.

The photo above illustrates something strange about the ribs R.3 on the printed sheet. After cutting them out, you will notice that one of them is smaller than all the rest. I cannot believe it is a coincidence that the short rib looks just the right size for the outermost rib, where the trailing edge has started to curve. Strange that there is only one though. Maybe the original intent was to have two ribs called R.4 to use at this position. Anyway, I cut down one of the larger R.3 ribs to match the smaller one, and used them in the outermost positions.

If you are lucky, the ribs will be a nice snug fit between the leading and trailing edges, and they will stay in place without outside interference while the glue dries. Set them at roughly 90 degrees to the building board using Mk.1 eyeball. In this picture you can see the highly sophisticated “bottle of glue and scalpel” method of holding a loose rib in place.

The root rib is one where you do need to be accurate about setting the angle correctly. The guide from the plan was stuck to a scrap piece of balsa and cut out to use as shown here.

Next stage is to add the 1/16″ x 1/8″ spars. These were cut from medium hard 1/16″ sheet, as they have to take the main bending loads on the wings. You can leave excess at the root ribs, but stop them half way into the outermost rib as shown here. A separate section is added between here and the tip pieces.

Final stages are the addition of the two tip pieces and the gussets at the wing roots on the leading end trailing edges. I left the gussets where the spar meets the root rib until later in case I needed to make any surgical adjustments when setting the correct dihedral.

Speaking of gussets, make sure you get the grain in the correct direction – if you cut them from a strip like this you can’t go wrong.

After removal from the board, the next step is to sand the wing tips until the outline matches the plan. While you are at it, make sure both wings match each other. It is not unusual to find some discrepancies between left and right wings on a plan.

Here is a potential weak spot that needed filling with a scrap of 1/16″ sheet. If you read this before cutting the ribs out, you would be best leaving excess wood here on the outermost ribs to save a filling job later on.

Now it is time to shape the wing to a nice smooth aerofoil section. This view of the root rib shows the sort of thing you are aiming for.

At the trailing edge you are only removing wood from the top, and this photo gives you an idea of how far to keep sanding. A sharp trailing edge will look better, and of course the more wood you sand off, the lighter the wing will be.

There is less wood to remove on the leading edge, and unlike the trailing edge, this is rounded top and bottom. Make sure the profile of the ribs is smooth and blends back without any steps to the leading and trailing edges. Also try to get the upper spar flush with the tops of the ribs.

Before gluing the wing halves together, check that the two root ribs match as closely as possible in shape – it will look better afterwards.

I could not see a recommended tip dihedral on the plan, and test fitting indicated a total of about 2.75″ on one tip (so 1 3/8″ per tip). In my opinion this is excessive, and in fact I am sure you could get away with just 1/2″ per tip if you were going for optimum scale appearance (the real Swift had no dihedral at all). Taking a compromise view, I would suggest 1″ per tip, meaning 2″ total when we prop one wing up.

In order to reduce the natural dihedral of my over-tilted root ribs, I just sanded them a bit to adjust the angle of the faces. You can see that I pinned one wing panel to the building board, and propped up the tip of the other one with a 2″ wide balsa offcut while the glue dried.

Important update, April 2013

Andrew Darby was kind enough to send me a scan of the instruction sheet from an original Comper Swift kit, which you can download here (580 KB). This contains some information not on the plan itself. One important piece of missing information is the dihedral figure for the wing. I used 1″ per wingtip, but Phil Smith actually specified 3/4″, so the choice is yours!

Final gussets are now added to the centre section. You can see I have added four at the spar join instead of two. Two would have been enough to reinforce the dihedral join, but four gives a better chance of not getting wrinkles here after covering. The extra two gussets at the front are purely added as an anti-wrinkle aid – these corners are prime wrinkle territory.

Here is everything sanded smooth.

We can turn our attention to the tailplane now, which is almost entirely constructed using parts cut from the printed sheets. Hopefully you used nice light sheet to copy the parts onto, so we can have a nice light tailplane. A bit of weight saved at the rear will save a lot more at the front in terms of noseweight no longer needed.

You should be used to the drill now – cover the plan with clingfilm and pin the parts down over the plan. As with the wings, Aliphatic wood glue was used throughout.

You will find a bit of adjustment necessary to make the parts fit snugly, but I found generally the bits went together fine.

Something I always do with flat plate tailplanes now is rub them gently on a sheet of wet and dry emery paper glued to a flat board until there are no steps to be felt when you rub your fingertips over them. This needs to be done both sides. I find this method much less fraught than trying to do it one join at a time using an emery board.

Here is the finished item after sanding the outline to match the plan, and rounding off all the edges.

The final piece is the fin, which simply needs cutting out and its edges rounding off as shown here.

Time for another weight check. All the bits made so far placed on the postal scale come to a total of just 9 grams, which is most satisfactory.

In the next chapter we will be looking at the stage you have all been looking forwards to (or secretly dreading) – covering!

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Build your first flying scale model – Chapter 6 – Flying surfaces

Build your first flying scale model - Chapter 6 - Flying surfaces

Important update, April 2013

You are currently on Chapter 6 of “Build your first flying scale model”