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.