Constructing a Dave Rees style wing
Building the Dave Rees designed D.H. Fox Moth introduced me to his construction methods, which combine light weight with impressive structural stiffness.
Especially for flying surfaces I don’t think there is a better way to reproduce a fabric covered structure in a flying scale model. Thus when it came to
designing a new open rubber model I incorporated all I had learned from the Fox Moth. I thought it might be helpful to document the construction of a
Rees style wing so you can see what’s involved. I don’t think there is any more work involved than in a conventional wing with scale rib spacing, and
you do save some time and balsa not having to cut out a large pile of conventional ribs.
Let’s start with the laminated wingtips, which of course can be used on any type of wing. Dave used 1/32” hot bent bamboo for his tips but I went with laminations instead, chiefly because it was a process I felt more comfortable with.
For a model this size (just under 30” span) I use 4 laminations of 1/64” x 1/16”. The strips were cut from a sheet of 1/64” sheet using a steel ruler
because it’s hard to use a balsa stripper with wood this thin.
The former for the wingtip was cut from 3/32" sheet, with a couple of shapes also cut to hold the outside of the laminated bundle in position (these are offset 1/16" from
the inner form shape). All edges are rubbed with an old candle to prevent sticking. You can see mine is well-used and now possesses a very impressive groove. Four strips
of 1/64" were cut to length (best to leave them a bit over-long)
I've soaked the strips in various ways over the years, but now immerse them in a shallow bowl or plate with a decent rim to contain the water, then
give them a couple of minutes in the microwave at full power. The boiling water softens the strips sufficiently for our purposes.
Now take one of the strips and apply a thin bead of wood glue along one side. The glue dispenser used here is the one supplied by the Vintage Model Company
in their kits, and very handy they are too. Mine has been refilled with Titebond Aliphatic.
A second strip is glued to the first. Next a bead of glue is added to the new strip and the process repeated to join the third and fourth strips.
As you can see, I now have a sandwich of four strips
Hold one end of the strips against the form with a pin, and start to work the laminations around the form. The outer balsa shapes are
helpful in doing this. It may be stating the obvious,
but note the four laminations shoud be vertical, not horizontal
The laminations are pulled round to the far side of the form and held in place with another outer piece.
The centre piece holds the laminations snugly against the form and each other. Not shown in the photo, but the laminations at the right hand end were later
held against the form with a pin before the glue dried.
Leave to dry overnight and then remove pins and peel away the outer formers.
If your waxing was thorough, the laminations should slide smoothly off the inner form. So, one down, three to go!
While we are laminating, let's turn our attention to the upper rib caps. Dave used 1/20" square for this, but the way I handle my models, this wouldn't last 5 minutes. So
instead I decided to slice mine from a curved laminated sheet of 2 x 1/32" light balsa. So the first thing you need is a form to curve the sheet over. The lower block has
1/32" ply sheet forms on both sides, with the profile copied from the upper wing rib shape (Dave used an 8% section on the Fox Moth, which I copied for this one). The ply
makes it easier to sand the block to shape and keep a constant profile across the block. The burn marks are due to over-enthusiastic use of a microwave
oven when building the Fox Moth. I have now learned my lesson!
So, here is the lower block with two pieces of light 1/32" balsa ready to be laminated - grain running left to right (so fore and aft on wing)
As with the laminations, I gave the sheet a couple of minutes boiling in water in the microwave. Make sure the plate doesn't boil dry.
A layer of Titebond original was applied to one of the sheets and the other placed on top. You can just see the cling film already placed on the forming block.
So, here is the sandwich with the laminations formed between the upper and lower blocks. Wide masking tape was wrapped round the blocks to hold them together.
Unfortunately, just leaving everything to set at room temperature isn't really an option, because the moisture can only escape from the edges of the block.
When I first tried this
method I waited two days, unwrapped the blocks, and the sheet was still really soggy. The solution to this problem is to use the microwave, but only
in short bursts, leaving time between them
for the sheet to cool down. I found 15 second bursts in a 900W microwave worked quite well. The first time you do it, very hot adhesive/water mix comes out of the edges
but it soon settles down. I found I needed something like 8 bursts to get everything reasonably set. Obviously, don't use anything metal to hold the blocks together.
Also, rubber bands don't work so well (ask me how I know!)
So, the moment of truth - time to separate the forms. You can just about see the cling film still on the upper block.
Here is the finished lamination ready for slicing
Before you strip any rib caps, use an emery board or sanding block to get one side of the sheet perfectly flat.
My trusty Master Airscrew balsa stripper does the stripping just fine - you just have to make sure you apply constant pressure towards the side of the stripper.
You may decide to cut some rib caps a little wider than 1/16, for instance to use as root ribs. Just wind the blade in or out as appropriate.
The result - a nice pile of identical upper rib cappings.
The basis of the wing is a flat frame built on the board as shown in this photo. Trailing edge is 1/16” x 5 mm, leading edge 3/32" x 1/8" deep and lower rib strips
1/16" wide x 1/20" deep. The tip laminations are propped up 3/32” at the outer edge and the front is fitted to the leading edge about half way up.
All the wood used here is light.
Next the two spars are added using relatively hard wood. The wing spar locations are determined by the strut positions (and hence spar positions) on the
full size plane, so one at the front, one at back. The rear spar will line up with the front of the aileron, assuming there is one. This particular
aircraft had ailerons on the lower wings only, but I'm building the upper wings first. For this wing the front spar is 1/16" x 4.5mm and
the rear spar 1/16" x 4 mm.
The diagonal braces are then added using medium 1/16” x 4 mm strips. These are also glued to the lower rib strips.
The upper rib caps have to be carefully cut to length and sliced at the rear so they sit neatly on the lower strips and lie flush with the front of the trailing edge. A cutting board
with lines printed on can help to determine the correct angle.
You will probably need a pair of pins at the front to hold the cap against the leading edge, and as you can see here I used a sanding block to hold the rear in position.
Note that the caps are not glued to the diagonal strips, they sit above them.
Use a dihedral jig to set the angle of the root rib - a very similar process to using a conventional rib.
Full set of rib caps now added
Some careful trimming of the spars was needed to fit this outermost upper rib (I didn't bother with a lower strip at this location)
The final addition is a vertical spacer in each rib joining the upper and lower strips at the high point – the unsupported centre of the rib caps is
a potential weak spot. There is a sneaky way of doing this which Tom Hallman developed - you glue in the end of a 1/16" strip of balsa with the grain
vertical, sized to fill the gap. Four strips have been installed here.
When the glue has gone tacky, just slice the strip off next to the rib along the grain and move onto the next one.
In this photo four of the vertical spacers have been added – I find this easier to do once the wing has been removed from the building board.
Finally, here is the completed wing, fully sanded, with a weight of 2.5 grams.
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