Single surfaced curved foil with foam sheet structure and curved balsa strips

18 November 2020

Haha! I now think balsa is good for the leading and trailing edges.

For cantilever monoplane of upto 20" span, 30-60grammes weight:

LE 1/16" x 10mm balsa

TE 1/16" x 5mm balsa

Why?

CG is nearer to LE, so LE should be wider than TE.

TE is still important structurally, it should be stiff enough to resist flying flexing and static pull of the tissue covering.

Primary ribs 1/16" strip or 1/16" sheet.

Secondary ribs 2mmx2mm foam strips, because I would want it to be able to curve and all it does is support the tissue covering.

Structural points of the wing are at the root and tip, the attachment points to struts or tension lines, and then finally, enough interspersed within the span so that the LE and TE will maintain straightness and not bowed. Primary ribs are for this purpose.

If it is still too weak, then more primary ribs to be used, I'm sure it can be spaced more than 3" apart because the 1/16"x5mm TE should be stiff enough.

Secondary ribs are so light, they can be spaced at 1" apart. Afterall, they only need a tiny bit of glue on each end and having say 40 of such pieces on a 20" wing will only add say 1 gram? The point is they are light, multiple parallel lines is pleasing to look at and they support the tissue covering well enough.

So, here's how it can be done. Let's say I am doing an 18" wingspan model, 4" chord, no lift struts or tension lines, just cantilever.

From 1/16" sheet balsa:

• LE: 2 pcs 9"x10mm
• TE: 2 pcs 9"x5mm
• Primary ribs: 6 pcs of sliced ribs. Using a template, slice the ribs from a rectangle piece of 4" length., about 1/16" thick. 
• Dihedral setting rib: 1 pc of sliced ribs. Slice it thicker for more dihedral, and thinner for less dihedral. It is relative to the wing joiner width. If it is 1/16" thick, the rise is about 1:6.

From 2mm foam sheet:

• Secondary intermediate ribs: 14 pcs (4"-10mm-5mm)x2mm straight strips. To ensure all of the same length, cut a rectangular piece of the length and slice off at 2mm interval.
• Secondary root ribs: 2 pcs of the same length but 1" wide.
• Secondary tip ribs: 2 pcs of the same length but 10mm wide.

Wing panels:

• Lay, hold down and glue together the LE, TE, (optional tip) and all secondary ribs (1" spacing for intermediate ribs) over wing plan. We get a flat wing panel that looks a bit like a ladder.
• Glue primary ribs to underside of wing panel. We get a curved wing panel.

Wing joiner:

• Wing joiner sheet may be from balsa or foam. about 3/4" wide and slightly longer than 4" because the ends can be sanded off. Taper off the long edges at the bottom side, about 3/8" from each side for a rise of 1:6.
• Dihedral setting rib is glued to the centre of the wing joiner sheet on the bottom side. This sets the matching airfoil of the wing joiner.

I will have a single piece wing structure when both panels are glued to the under-side of the wing joiner. Cover the structure after sanding/trimming.

16 November 2020

Purpose

To make a single surface curve plate wing that is light enough and does not distort too much for an indoor RC model.

Problem and solution

A general outline of the wing planform to be constructed from foam sheet. The leading and trailing edges will be 2mm sheet foam. The width has to be wide enough because there is no other lateral spars. Both edges may be of same width as they are at the edges and between them they have to cater for the bending moment created by the mass and acceleration (up-down direction) of the entire model. Theoretically, I suppose that since the CG is not at the 50% of the chord, the width can be distributed according to the CG location, i.e. wider on the leading edge than the trailing edge.

Let's say the model's total mass is 30gram and it will experience 2G in normal flight. It means each  wing half will have to be strong enough to support 30gram. To be safe it need a safety factor of 2 for practical consideration, such as pulling out of a banking dive and to allow variance in construction material and workmanship. So each wing, port or starboard, need to be able to support 60gram and deflection has to be acceptable too.

Each wing half is a cantilever structure unless there is further support along the span of the wing half. The load of 60gram is spread over the span, not uniformly, but let's just use 60gram at the middle of the half wing span. The bending stress on the sheet leading and trailing edges will be highest at the root and tapers to near nothing at the wing tip. It will be great to have shorter span, support at the middle of the half span, and taper out the width of the leading and trailing edges. I will not bother with reducing the width at the tip because I'm sure the tip or thereabout will hit obstruction and making individual spacer (rib) is extra work.

Cantilever a rectangular sheet of foam. The cantilever length shall be half the span of the half-wing. Place 60gram on the free tip and observe how much is the deflection and whether I can accept that amount of deflection under tip load and which should return back (mostly) when the load is removed. The width is what is needed for that thickness of foam. Now, if CG is at 25%, I know the width of the leading edge will need to be about 3 times the width of the trailing edge, meaning that I should take 75% of the width of that experimental width to be my leading and trailing edge.

Another way of going about it is to use 1" width for both leading and trailing edges for upto 20" wing span model. Totally arbitrary, just what I feel to be sufficient.

Wing Construction (all material is 2mm foam unless otherwise stated)

1. lay out leading and trailing edges
2. glue root and tip spacers which shall be as wide as the leading and trailing edges if there is no other consideration.
3. glue intermediate spacers, maybe of about 4mm width.

1. Wet curl a piece of 1/16" balsa sheet.
2. When dry, cut 1/16" strips off the sheet.
3. Since it was cut from the same piece, the curled strips will be of the same length and curvature, roughly.

1. Glue the curled strips to the foam wing structure, at the underside of the root, the tip and some intermediate spacer especially when load bearing, such as intermediate supports.
2. Sand off the balsa strips to the foam wing structure and we now have a curved foil wing suitable for single surface covering. The curled balsa strips maintain the curvature.

1. Cut a rectangular piece of 2mm foam. Slightly longer than the curled strips.
2. Chamfer the long edges of the 2mm foam sheet. This and the curl strips used later will form the dihedral brace for the curved foil wing. Varying the width of the foam piece and thickness of the curled strips will give the dihedral angle. Let's say the 1/16" thickness of the curled strip is just nice with 1.5" width of 2mm foam.
3. Glue 2 curled balsa strips side by side along the middle of the 2mm foam rectangular piece. This curls the foam sheet.
4. Glue the port and starboard wing halves to the underside of this dihedral assembly.
5. Since I assume it is 1.5" width of dihedral piece, half of that is 0.75" and therefore the curled strip at the root is placed just less than 0.75" away.
6. to a rectangular piece of foam. This makes a very short curved foil which will be the dihedral brace. Sand the 
7.