Materials and Tools

30 October 2024

Added a bit about adding white glue to the finishing coat in 16 October 2024 posting.

24 October 2024

What kind of brush to apply the paint? Airbrush is not necessary for primer coating, base colour and washes, and I don't like cleanups, do you? 

16 October 2024

White glue and water based paint

White Glue

This material is hydrophilic and shrinks when dried, resulting in volume reduction so precise joints are needed for strength. It's flexible so it is better to absorb force and as a surface coating, it does not crack or craze. It can be re-softened, making it easy to clean; hands and tools can be washed in water and rubbing dried glue joints with a slightly-damp sponge/cloth, will remove excessive glue, make sanding less tedious. It is a good base for water-based paint, it can also become part of a barrier to protect polystyrene foam from foam-melting glue/paint. To prevent/reduce taut tissue from sagging too much in humid condition, a final fixative or lacquer can add stability and durability. Be aware that lacquer can craze.

Since white glue will dissolve in water, here's my idea for a rectangular wing.

Cut the rib template from plastic transparency sheet with a pair of scissors. The rib pattern will extend beyond the actual intended ribs, it extends to the entire chord of the wing. Yes, plastic transparency might not be suitable to be used as cutting template, but it can be used as a tracing template.

For flat bottomed airfoil, use a ruler and cut from 1/16" balsa sheet a strip of balsa, wider than the chord depth so that you get 2 straight edges and you can trace the top curvature of the rib template using the bottom of the template to align with each balsa edge, alternating the template and we minimise balsa waste.

Cut on the drawn curvature line, and stack the cut ribs and glue them together with white glue. When dried, this forms a block of balsa, roughly shaped, strong enough to handle as one but can be separated later with the magic of water.

Now sand the curved top, square the leading and trailing edges and slot for the wing spar.

Drop the entire perfectly shaped block into a bowl of water. Over time, the white glue will dissolve and the ribs will separate.

Water based paint

Water colour and poster paint are cheap, readily available and reversible. Acrylic is more expensive and while easily available, it is unreversible. All water based paint is easiest to clean up and are much safer to use. They work best on hydrophilic surfaces, only hydrophobic surfaces will require primer, so for tissue covered models, coated with white glue, no primer is required. 

Water colour is transparent and is best for washes, poster paint is opaque and is for accented lines and details. 

In planning the painting of tissued covered model, start with the basic tone, if it is coloured tissued, congratulations, as you already have a homogeneous coloured substrate, all it needs (if it is needed) is some washes to refine the basic tone. If it is plain tissue, it's not an issue, just apply washes with water colour to get the preferred tint. Tissue is translucent anyway, so a transparent colour will produce a striking dreamy effect. With water colour, progress from light to dark. The real plane is painted white? No problem, the tissue is white, apply slight tinted washes to homogenise and blur the tissue overlaps.

Poster paint is opaque and is great to cover up water colour mistakes or to make contrasting details. To ensure minimal paint, progress from light to dark because all else being equal, dark covers better than light.

If you spray coat (mist, not wet) the painted model with lacquer, the paint will be protected and will be durable. If you don't wish to apply lacquer, you can consider adding a tiny bit of white glue to the final paint coat/wash.

11 October 2024

Try as one might, if the former has to be precise, you can try to cut it carefully in one go but do not expect it to be perfect. Sanding down to an outline is an additional step and it will take quite a lot of time because one has to keep checking and make adjustment while sanding. 

How do you sand? Do you grip the former in one hand and use a sanding block or file in the other hand? Most would and I would, however it will still not be as precise then if you are able to guide the piece squarely across the sandpaper (not the tool across the work piece).

Grip the work piece against a square block, the forefinger is at the side of the work piece and the middle finger on the other side of the block. Rest the forefinger on the work piece to provide some stability and downward pressure. Slide the workpiece and block thus held, over sand paper and you can be assured of straight ninety degrees edges. Most importantly, you should grip it so that you can see the drawn outline on the work piece while sanding. 

8 November 2023

• 1mm: 1mm EPS: 20gsm
• 1mm: 1mm XPS: 30gsm
• 1mm: 1mm Depron: 40gsm
• 0.09mm: 1 pc Paper: 70gsm (A4 is 5g per piece)
• 1mm: 1mm Balsa: 160gsm (10lb/ft3, medium density)

Paper is 11 times thinner than any other 1mm material. Tissue is thinner. Weight of 1mm balsa is equivalent to 8mm EPS, 2.3mm XPS, 1.75mm Depron. 8mm EPS ought to be stronger than 1mm balsa. XPS (Extruded Polystryrene or Styrofoam) has its Compressive strength of 250 kPa, Tensile strength of 450 kPa and Shear strength of 250 kPa. Since I am taking 20gsm for 1mm EPS, while general EPS has compressive strength of between 40 and 72 kPa, the particular 1mm EPS ought to have compressive strength of 20/30*250kPa = 166 kPa.

Gms/m2:
Monokote 75
Doculam 42
Litespan 28
Cellophane 24
Reynolds 14

XPS (Extruded Polystyrene or Styrofoam) has its Compressive strength of 250 kPa, Tensile strength of 450 kPa and Shear strength of 250 kPa.

 Candidates for modelling use.

Covering:

• A4 paper, printer
• Store bought coloured paper and tissue

Structural:

• Expanded or extruded foam (EPS, XPS comes in various density)
• A4 paper, cardboard
• Incense sticks, disposable chopsticks, ice-cream sticks
• Stapler, paper clips
• glue, CA, tape, double sided tape
• Disposable plastic cups
• CF rods, bamboo
• thread

Tools:

• Pencil, pens, markers, paint, brushes, tape, masking tape, double side tapes
• Tracing paper, saranwrap, rubber bands
• NT cutter
• Files, sanding blocks, sandpaper, plane
• HWC
• Ruler, compass, divider
• Pins, thumbtacks
• foam board, corrugated cardboard, wood
• Soldering iron and solder
• Cutting mat
• 
  

Structure

Tension, Compression, Torsion

Triangle frame has torsion resistant.

Rectangle frame has no torsion resistant.

Thick end frame gives a bit of torsion strength, so does making similar triangular gussets.

Catastrophic failures are:

Wing fold, at dihedral joint and outwards. Wing twist. Wing break due to impact.

Fuselage folds. Fuselage cracks.

EPS, XPS, depron, compressed foam. XPS can be prepared in sheets and sanded

Unit weight (gsm) comparison of material

• 1mm EPS: 20gsm
• 1mm XPS: 30gsm
• 1mm Depron: 40gsm
• 1 pc Paper: 70gsm (A4 is 5g per piece)
• 1mm Balsa: 160gsm (10lb/ft3, medium density)

Balsa

Use of 1/16" balsa wood: 

Small sections unless made of firm wood are fragile and does not have insufficient strength, maybe to use larger section, for example if it calls for a 1/8"x1/16" 9lb/ft3 density and I don't have that heavy a wood, increase the size to 3/16"x1/8", this will require some structure changes to the layout to accommodate the larger section.

Use of 1/32" balsa wood:

Even though it is convenient and easy to cut out the entire shape from sheet wood, bear in mind that a frame structure of 1/16" will be stronger and more resilient. This thickness is especially useful for making laminations for curve parts. As 0.8mm is very thin, go with 3 laminates of 1/32".

Windex

I read that windex and other glass cleaner makes the balsa more pliable then with plain water. I don't think I'd use saliva and I have also read that soaking the balsa strips in windex for a few hours is good.

Gussets

This could be from balsa sheet or even bond paper. 

Tweezers

I think a pair of tweezers is a useful tool for handling balsa. However, I also think that using tweezers made from metal might crush the balsa. I have read that crushed balsa may be recovered by dipping a bit of water on the affected wood. Maybe a better pair of tweezers may be made from bamboo chopsticks or a folded piece of plastic card?

Other tools

The biggest tool may be the building board. For my case, I think a piece of 1" blue foam is ok. 

A tool that may surpass the building board in size would be a frame for stretching tissue. I think there's no easy way to get the tool made, so I think I could just buy a frame from Art Friend. Might as well buy 2 at least, because there's bound to be more than 1 coloured tissue needed. I think I should also get a smaller frame for doping trimmings, A4 sized. To attach the tissue on a wet day, have the frame coated with dope and use thinner to attach the tissue. Any leftovers on the frame may be sanded off and re-doped.

Dope replacement, I think I'll just use thinner and EPS. It is not for doping the completed model, it is only for the attachment of tissue. For final protection, I think I'd just buy a rattlecan of clear lacquer.

Coloured trims will need to be firm enough to handle and crisp enough to cut neatly. I think a doped piece of coloured tissue is good and it can be attached with thinner. I will try this, it may be better than white glue or glue stick. I'll experiment with the dope replacement and the rattlecan. I think the rattle can is also thinner solvent.

Intricate photos may be printed, treated, cutout and applied in a similar manner. Hence the need for a small frame of A4 size.

Kitchen plastic wrap would protect the plan from CA and the underlying building board is also protected.

Pins will be needed, I think the regular tailors' pins are good enough. Other pins and thumb tacks are also useful. Locations requiring temporary anchors may be pinned, against the transferring load or to both sides of the transferring load. 

Right angle braces can be made from blue foam and pinned to the board to hold the sides together or to hold the former perpendicular. Is 10 degrees a good dihedral for free flight?

Glue applicator. I normally used excessive glue, I used offcuts of sticks but they lumped easily and seems not to be able to carry sufficient glue. I think I could use the eye of a largish needle to be a glue applicator. Maybe the eye should be cut/grounded as I have read? Or maybe I can just use a soft wire and make the eyelet, seems less 'dangerous'. A 'O' sized nylon brush should also work very well for water solvent glue, it might be ruined and have to be replaced frequently if using superglue.