Jodel D9 Bebe

29 January 2016

A rectangular piece of foam is weak in two directions: Span-wise, i.e. the wings may fold, and torsion-wise, i.e. the wings get twisted. It might not happen to a small and light model, but accidents will happen and as the foam sheets got weakened, the ensuing flights can not be good. Curve the rectangular piece of foam and I will get better bending and torsion strength. It will be better than a flat rectangular piece but limited more by the compressibility of the foam. I get better strength because of the curve and a better airfoil but all these advantage goes awry if the airfoil straightens since there is nothing to hold it in that geometry. What is needed is some ribs providing chord-wise strength. A rib is a vertical member providing chord-wise and vertical strengths. How about having horizontal spacers instead of vertical ribs? The material are laid in the position where maximum use can be had to resist the forces. Another horizontal spacer span-wise can be added between the top curved sheet and the bottom horizontal flat piece. This spar will bind the two surfaces together. Span-wise strength can be improved at the leading and trailing edges, not necessary at the spar position only. If I do up a rectangular wing like that, I can add flat sheet tips to form the U-hedral. The ends of the rectangular wing will have the horizontal spacer and the whole can be chamferred to accept the tip plates.

Wing

Chamfer opposing edges of a rectangular piece of foam sheet, the chamfers are to meet the leading edge and the trailing edge of the curved foil. This chamfered sheet is cut into 3 portions. A pair of perhaps 15mm width will be for the tips and the last portion for the middle section. Place them to their positions on plan.
Glue a span-wise flat foam spar to these 3 pieces. The simplest shape would be a rectangle piece, but it is possible to have it wider at the centre and tapering to the ends
Cut, colour and add registration numbers to the rectangular wing, hand curve the panel. Or, leave the colouring and registration last.
Glue the spar and 3 spacers to the curved panel, start at the leading edge, rolling the curved panel on until the trailing edge.
Leave the centre wing assembly flat on the building board to dry/cure.
Transfer the airfoil shape of the rectangular wing to a piece of paper, that will be the template to cut out the saddle at the fuselage.
Slice the ends at an angle and glue the 2 tips.
Most of the lift will come from the centre section and the tips mainly comes into play when they are functioning as dihedral or when the angle of attack gets too high for the centre section.
When the wing is fully assembled, without the undercarriage, I think that some bottom tissue or plastic covering will be good. A curved foil is great, if the air behaves. But protrusions, as presented by the horizontal spar cannot be good for the foil, so a covering of some sort to "smooth" over and make the air behaves more predictably would be better than none.

Fuselage

The box fuselage is folded and glued with some formers in the shape of an inverted U. The pre-hinged tails are glued to the box fuselage.
The top of the box fuselage, where the cockpit is, has a reactangle cutout for the servos to protrude. The pushrods will run back to the tail surfaces.
The bottom can be covered but it is non-load bearing since fuselage formers are in placed and the fuselage doesn't have to be too strong.
The nose deck and the rear deck are non-loadbearing conical pieces of rolled paper with formers.
The nose deck has the wind screen glued on and is removable with the use of 3mm magnets so it functions as a battery hatch.
The rear deck has cutouts to clear the pushrods and is glued to the box fuselage.
The motor block is a slotted piece of foam block glued to the box fuselage.
The nose block is a rolled piece of paper on a nose ring glued to the box fuselage, enclosing the motor.

Undercarriage

The undercarriage wire is in one piece bonded to a spreader and the spreader glued to the bottom of the wing's centre section.

11 January 2016 (Folded Wings)

This idea of folding a wing with an airfoil shape and dihedral keeps appearing in my head, so to rid of it, I did a paper mock up.

I constructed the lines of one wing tip panel and then I folded the paper so the other wing would be similarly cut by the scissors. The top skin was then curled.

This was the result.










The slits should be very small. In this case, the centre folded up to almost right angle.
There is little twist to the panel. I guess it is because it is a shell.
Despite curl being applied to the top skin, the airfoil tend to be symmetrical.
Spar is required to hold the dihedral.
Ribs are required at dihedral joints, it would prevent the bottom sheet from developing a curl like the top sheet.

Foam construction

V-cut the folding lines on the lower skin so that the external edges can be kept sharp and continuous.
Draw a span wise section at the wings' thickest section. Not only can you figure out the spar shape, the depth of the dihedral cut-out to the top skincan can be determined. When the depth of the dihedral cut out is determined, a transposition of a 'flattened' shape of the airfoil would give the right shape of the cut outs at the top skin.
There should be a rib at each dihedral joint for the top skin to bond to, keeping shape and also to strenghten (shear force between top and bottom skin) the wing.

Jodel D9 Bebe

If the wings of the Jodel D9 Bebe is to be constructed, and the model to be powered and controlled by WLToys' equipment, I think 2mm foam sheet from Daiso is enough.
The tips would not require reinforcement because in flight, the loading will be very light. Crashed landings on either tip might break it but the spar can be strengthened for this mishap by inserting thin strips of stiff material, e.g. CF strip.
What colour and markings though?
I'd like to do a F-PAAT, but I don't know the base colours. I suspect it is white. Shame, I'd rather have it more colourful then white.

Here's a collection of photos of the first Bebe, the F-PEPF, from the internet. I like the tractor paint remark, because I have a piece of 2mm foam sheet which is something like that. Maybe it is possible for F-PAAT to be in tractor orange as well?