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. 
   

Dihedral or double dihedral, curved foil or bent foil

6 November 2020

New idea! Think of the wings as deformed foam sheets.

I'd like the root foil to be curved and the tip foil to be flat. The developed chord will mean root is wider than the tip. It won't be much but I think it is more pleasing that the wing does not appear to flare wider at the tip as might be the case where a rectangular blank was cut.

The wing will be at a positive AoA, the planform will need the taper at both edges, but let's just keep it either at the LE or the TE. The curved portion of the foil is more towards the LE, so let's just taper it at the LE for simplicity. Now we have a planform that is slightly wider at the root, and tapered on the LE only.

All we need now is a device to hold the roots together at the airfoil shape, at the dihedral angle, strengthened the area slightly and spread the bending moment outwards to the tip slightly. Let's simply call this a dihedral brace:

1. Cut 2 pieces of rectangular foam sheet with the same width as the root curved foil. 1 piece is maybe 1" long and the 2nd piece maybe 1/4".
2. Taper the long edges of the larger piece.
3. Curl both pieces chord-wise and glue the smaller piece to the centre of the longer piece.

To assemble the wing, glue one wing panel at a time to the bottom of the dihedral brace. There will be a gap between the wing panels as can be viewed on the underside, but the 1/4" piece will spread the load so it won't matter structually.

4 November 2020

Single dihedral or double dihedral or even triple dihedral?

Effectiveness of roll stability is a measure of area, angle and moment length. Hence, more dihedral joints mean less angle angle needed and more of the area will be for lifting, and furthermore, the rolling stability is spread gradual over the span of the wing, purportedly more efficient. However, in terms of difficulty: simple, double the trouble, and triple the trouble in that order, and in terms of weight, the more dihedral joints the greater the weight otherwise you compromise the strength and accuracy. Why kid yourself about your ability to achieve the maximal effect when we don't need it anyway nd can't really do it consistently?

So here it is, after some thought, on how to make a dihedral wing from sheet foam and with some form of bent airfoil. (Bent airfoil is just a substitute of a curved plate airfoil)

Why a bent airfoil instead of a flat plate? In the interest of slow flight, displaced air going down means a reactionary force going up but instead of achieving this through just a positive angle of attack, the rear portion just give it greater deflection than what is by the angle of attack alone. The step increase of deflection probably helps to reduce drag too.

A curved airfoil would probably be more efficient because the increased deflection is more gradual but perhaps it's just that bit harder. Curved foil is nice to look at. Bent foil may be slightly less efficient by logic, but at the size I am thinking of, does it matter? Yes, I can bend foam sheets, however there will be a tendency to have the high point at 50% chord and the curve may not match at the dihedral joint although you can also glue it in, but how do I get the curve at various chords along the span? To have the thickness of the airfoil gradually reduce from the root to the tip is also nice to have, how can you bend that? A creased line is ok in this aspect as the airfoil will flatten out towards the unsupported tip.  

A creased line is so much more straightforward and there is a better chance of both panels matching up.

So for simplicity, a bent airfoil.

Here's how I think I will make mine.

A bent sheet wing with dihedral

1. 2 wing blanks
2. Creased wing blanks at the point of bend
3. If spar is desired, glue triangular balsa spars
4. If reinforcement to LE and TE is desired, glue balsa reinforcement
5. Sand the wing root to half the dihedral angle after the LE and TE are taped on worktop and a packing at the creased point.
6. Glue the wing halves together then remove the packing pieces.
7. If further reinforcement for balsa spar is desired, cut a slit and CA a CF rod.
8. If a flat wing seating is desired, leave the packing pieces under the crease and glue a strip joining the LE and TE.

Sanding adverse? Well, if it is a bent foil, that means the front and rear part are straight and it is possible to make thin cuts with a straight edge. Just tiny sliver from the root. When both panels are identically cut, bringing the roots together would result in a dihedral joint. Another possible plus for bent foil.

 

A built up curved foil wing with dihedral

To be lighter, a ladder-style wing structure covered one side is probably the way to go.

Searching the internet always start with the LE and TE flat on the building board, then ribs, either whole or just strips glued to the 2 spars forming something like a ladder. To improve the structure, some gussets or even a 3rd spar is added. Since the LE and TE were flat on the board, the top surface doesn't quite flow neatly. Now, suppose that I only wish to have a top surface covered wing and I wish to improve the top surface, reducing the kinks. This is probably how I go about:

1. Use wider LE and TE strips.
2. Pack up the rear edge of the LE and the front edge of the TE at the rib locations.
3. Glue wingtips and root ribs, dihedral set.
4. Drop in the sliced ribs and glue. The packing pieces will act as a stop.
5. Add gussets etc.
6. Remove wing structure from board, trim and sand away bumps.
7. Cover each wing halves.
8. Join each wing halves.
9. Reinforce the wide LE and TE at the dihedral joint.

Polystyrene

 3 November 2020

Making a lightweight slab-sided fuselage

• Draw the fuselage profile on the polystyrene sheet and cut it out roughly.
• Draw the cut-out on the foam blank but remember to leave a perimeter of foam around the cutout.
• Carve and sand, round the edges of the foam blank before making the cut-outs.
• Do not cut across the perimeter foam around the cutout because glue is heavier and does not add strength.
• Insert and glue foam formers in the cutouts if the cutout is too long or too large. The idea is to ensure sufficient foam for the odd knocks without breaking. The foam will retain its shape.
• Cut 2 pieces of paper to cover the cutout hole.
• The 1st piece of paper is just about 3-5mm bigger all round then the cutout hole. 3-5mm is enough, anymore is just extra weight without adding strength.
• The 2nd piece is the same shape and size as the 1st piece except that if there shall be a hatch, for example, to allow battery or receiver removal, then cut a hole to the shape of the hatch on the 2nd paper piece. The hatch cover shall be 3-5mm bigger then the hatch.
• After gluing the paper pieces to both sides of the foam fuselage, it is time to cover the fuselage, perhaps with tissue. Remember to cover the hatch cover too.
• Devise how the hatch cover is hinged and held in place. Tape, loops, magnets can call be used.

To have a 'proper' cross section of the fuselage, just add blocks, formers and stringers before final shaping/sanding and covering. This adds weight.