Small is difficult

6 February 2025

Just realised that for such a small fuselage, to construct the sides of the fuselage by building a frame over building from sheet balsa will only result in an absolute weight savings of less than half a gram. It is therefore more practical to build from 1/32" sheet balsa, tissued covered for prettiness.

• Tape down over the fuselage plan, with grain running lengthwise, shiny side up or down based on whether I want the images to be under or atop the tissue, my choice of white or coloured tissue.
• Draw, trace, colour.
• Cut roughly to shape, place on 1/32" balsa sheet.
• Brush on thinned EZE-Dope to attach the tissue to the balsa sheet.
• Cut and sand accurately the 'slab-sides' when dry.   

The wings though should still be a tissue covered balsa frame. It carries the flight load but can be lighter because it doesn't carry the rubber torque. The relative savings here is much greater, maybe 1-2 grams? 

• Superglue the ribs over the wings' plan on the packing taped building jig.
• Glue the 1/16"x1/16" square balsa leading edge and trailing edge. Alternative is 1/8"x1/32".
• Glue the paper tips to the wing tips.
• Tape the pre-decorated tissue, grain running spanwise, shiny side up over the balsa/paper frame.
• Brush on thinned EZE-Dope to attach the tissue to the balsa frame and paper tips.
• Leave to dry thoroughly, then pry out from the building jig, trim the tissue to shape.
• I'd prefer just to fracture the 1/16" square balsa edges for dihedral and patch the affected tissue. Then gussets (1/32" balsa or even just paper) can be added underneath at each juncture, this will strengthen the joint "tremendously".
• If using 1/8"x1/32" strips, make sure that at each dihedral joint, the matching ribs are angled correctly to provide the guidance when sanding the strips' ends.

The very small tail feathers can be made from tissue covered 1/32" balsa sheet. It can be built even lighter, but this will be slightly more work, simple work but work nonetheless. It will just be a pre-doped double or triple layer of tissue, with 1/8"x1/32" leading edges, and if still too weak, another 1/8"x1/32" central spar. Below is a triple layer tissue method.

• Draw the shapes on the shiny side of a piece of tissue with grain that goes spanwise, include the hinge/panel lines. Colour it if necessary.
• Tape a piece or pieces of tissue with grain that goes 45 degrees to the spanwise, shiny side down, on to a building board.
• Tape the pre-drawn tissue over it, shiny side down.
• Place the 1/8"x1/32" balsa leading edges/spar.
• Tape the 3rd piece of tissue with grain that goes perpendicular to the first layer over the balsa framed tissue, shiny side up.
• Apply thinned EZE-Dope, when dried, peel the entire tissue/balsa laminate.
• Cut to the shapes and we are done. The drawn lines and colour will show through, giving it a bit of blurriness, you can associate visual blurriness with distance, so that'd be a good visual effect. The triple cross-lamination of tissue grain will give it sufficient stiffness to maintain straightness or trim. The balsa bits will give it added spanwise strength (flatness) and that little bit of durability if at the leading edges.

The Business End

3"x4.5" transparent blister pack propeller with 1/8" square balsa hub (sanded to 45 degrees)

3" over a 10" wing is 30%, reasonable enough.

Paper clip for wire shaft is often not recommended, being too soft or too heavy and recommending piano wire instead. Bear in mind it is a very small model, spinning a 3" propeller with a single 160mm rubber band and suddenly, paper clip is the way to go. This is because it is stiff enough to withstand knocks, and isn't a weighty matter when noseweight is needed. As I see it, it is easy to bend and cut. I would be able to tweak the shaft, or if needed, to remove the entire prop shaft and propeller, and replace with new shaft and propeller.

5 February 2025

Ideas for fuselage construction for a 10" wingspan Ryan ST model airplane, which wouldn't exceed 7" in length.

The key components are 2 pieces of 1/32" thick balsa sheet that forms the planform of the fuselage, acting as horizontal keels. They are glued together at the rear, and that's simple enough. As they are flat over the planform, the edges would be parallel and straight, allowing a good aligned surface on which to glue the horizontal tail. The rear end will have sufficient length to glue on the vertical fin/rudder.

At the nose end, they are glued to the nose block, this allows a good transfer of force for the rubber motor and incase of nose dives.

2 small holes are made to accomodate the motor peg on the 2 fuselage's horizontal keels. To prevent the force from splitting the thin balsa keels, paper reinforcement will be glued to the inside at the hole location.

The wings are to be mounted a distance below the 2 horizontal keels, 2 formers of 1/32" balsa sheet will be inserted. These 2 formers will hold the keel square, and they are locally reinforced with short lengths of balsa and paper. The 2 formers will have large round holes for the motor to run through unobstructed. The 2 formers will accept 1/32" wing saddles, this will stiffen up the structure considerably and provide a strong flat surface for the wings to be glued to. The wing saddles can be cut to accept one piece lower wing.

At this stage, it looks nothing like the side view of the plane, that is fine, because the outline can be achieved by gluing paper and tissued patterned to show the fuselage outline below the top of the horizontal keel line. Paper and tissued pattern can be fashioned by curling to cover the fuselage outline above the top of the horizontal keel line. There won't be need for strength in the paper and tissued pieces, they are just there for show, they only need to stay there and pretty up the fuselage.

What happen to the bottom? You can like the way to cover the top of the fuselage, do the same for the nose's bottom, you can also leave it empty or mostly uncovered especially near the motor peg area as it provides good access when replacing broken rubber motors in the narrow fuselage.

4 February 2025

Flying small models is difficult to create. The smaller it is, the more difficult it will be to keep it light enough and yet capable of flying well enough.

Catapult Gliders

Catapult Gliders design has to cater to a unique challenge. Like the chuck glider, and perhaps to the extreme, it experiences both high and low speed and this requirement increases the difficulty in making it strong enough to withstand the high speed and yet maintain aerodynamic stability in both high and low speed.

A very small glider folded entirely from paper can glide stably, but it cannot handle the catapult stress and the high speed. Given the same wing area, a short wingspan handles the launching stress better but a long wingspan will glide better. Small gliders are especially susceptible to the many upsets it will encounter, it is vitally important that it will recover well from whatever attitude. In summary, a good catapult glider would be one that is strong and rigid enough so that it can be launched high, it shall recover well in the transition phase and have a slow descending glide path. A good design is one that is actually a compromise of the different requirements. It has to be a low drag design, strong enough and yet light, it has to be stable but not overly so. 

Many small catapult gliders for wingspans 12" or smaller, are made from 1/32" or 1/16" balsa sheets. They are adequate for average performance and ease of build. They glide fast, an indication that is heavy for its wing area.

If I were to design small catapult gliders for wingspans 6" or under:

• The 6" wing would be of a trapezoidal planform, constructed of a balsa frame made from 1/16"x1/4" balsa leading edge and tips, 1/32"x1/4" trailing edge, a few pieces of 1/32"x1/8" ribs (laid flat not upright and if necessary, placed diagonally). The construction would be directly over a single piece of tissue, shiny side down, so when the frame is constructed, the tissue covered frame can be cut free and sanded finely at the top, leading and trailing edges. Then it will either be a dihedral or a U-hedral or Polyhedral, which is easy because all it need is to score the top of the balsa frame and bend accordingly. The tissue being on the bottom is only slightly stretched by the bending up. Finish the wing with one thinned coat of EZE Dope.
• The <3" tail feathers would be 1/32" balsa sheet. It is just too small to even try the wing construction method.
• The fuselage could be a long piece of 1/16" diameter carbon fibre rod (preferably tube but that is more expensive and take some sourcing), some paperclip (acting also as some noseweight) for catapult hook and a folded card for tail grip. Or, if only balsa is available, from 1/8"x1/4" balsa sheet. 
• Catapult hook can be paperclip, acting as some noseweight, or a folded piece of card, or a short length of balsa or ply.  
• Tailgrip can be a folded piece of card/paper, short length of balsa.
• Glue the tailfeathers to the fuselage, then the wings, then the other none-aerodynamic components.
• Apply thinned EZE-Dope to balsa components, such as the fuselage, the tail feathers.  
• Use Blu-Tack at the nose for initial flight trimming. Once satisfactory, replace with sheet lead but still maintain a small pea sized Blu-Tack for future fine tuning. 

Thinned EZE-Dope over tissue (gives strength) and balsa protects the small glider from moisture, it will maintain its trim better, being more rigid and less susceptible to humidity changes.

How long the fuselage?

The wing is 6" span, I think perhaps 2" root chord, so the fuselage length is 1.5C+C+2.5C+C=6C=6*2=12"

Start with a 12" fuselage.

Flying Scale

A glider is easiest. If it is a gentle release type of gliding, it is easier than a Catapult Glider because the speed range is smaller. On the other hand, most gliders' wing are of higher aspect ratio, this translate to more engineering and if the chord is too small, a poor gliding performance. Some people would model jet planes and because the wings are of lower aspect ratio, they glide fine.

A rubber powered model is more challenging. Now, you have the weight of the power source inside your model and it makes your model heavy! The speed range is also small, but the fuselage must be able to withstand the compressive and torque forces. Further pieces of the aircraft such as landing gears add drag and weight. My priority would be : 1) flyable, 2) stable and easy to trim and fly, 3) easy to build,..... nth) looks like a scale model. 

For 10" wingspan models, it will be an amazing achievement if it flies stably. Still it is a good exercise and if I would to do it:

• The wing planform is a balsa frame made with 1/16"square or 1/8"x1/16" balsa leading edge, 1/16" square spars and tips, 1/16" square trailing edge, a few pieces of 1/16" ribs. Consider some form of dihedral brace with the fuselage if yours has a spar, in model of this size though, just gluing the root chord to the fuselage is strong enough. Sand and cover with single tissue on top, shiny side up. Finish the wing with one thinned coat of EZE Dope.
• The <4" tail feathers would be an outline made with 1/16" square balsa sticks, any curvature are made with a 2 piece 1/32"x1/16"  balsa strip laminate, at the centre would be a spar. Cover with single tissue and single thinned coat of EZE Dope. For this size, it is also possible to just butt glue the tail feathers to the fuselage, this can allow some ungluing and regluing to trim the incidences. See later alternative using paper patterns.
• The fuselage would be a basic balsa frame of 1/16" square longerons. Use only few formers, make sure that the rubber motor will not be obstructed. It is possible that top formers be just triangle shaped, stringers (and the glue) will add weight, remember that. Paper is also weighty. 
• The nose former can be made from cross-laminated balsa sheet, total 1/8" thick. In the interest of weight saving, it will not be removable and there will not be any free-wheel device. A short 1/4" length of plastic tube is inserted and glued to be the motor bearing. The motor shaft and propeller is installed on this motor bearing before the completed nose former is glued to the 4 main longerons. Rear motor hook will just be a short bamboo stick.
• Glue the tailfeathers to the fuselage, then the wings, then the other none-aerodynamic components, preferably none.
• Apply thinned EZE-Dope to balsa components, such as the fuselage, the tail feathers.  
• Use Blu-Tack at the nose for initial flight trimming, it will be at the nose. I will probably be satisfied with what looks like a steep glide, because it won't float, it will be too heavy for the wing area!. Mark or note the CG position and just try a few winds.
• It will be awfully difficult to trim.
• It will be an amazing accomplishment if it can be made to fly stably.
• It will be awfully difficult to replace a broken rubber band.
• It will look quite awful and angular.
• But hey, it is a <10" model that weighs only a few grammes!

Paper patterns

A hassle to laminate wing tips or tail feather's tips? A simple solution is to make paper patterns that matches the curved tip shapes, These can be glued to the wing/tail balsa structure before covering with tissue, or, if your tissue is pre-finished with trims etc, you could just apply the paper patterns to the underside of the tissue first, then you apply the tissue to the balsa structure. You get accurate shape, strength and rigidity to the tips.

The bigger it is the easier it will be. Compare this to an 8" Living Room Flyer (LRF). The LRF while of lesser wingspan (8"), has more area, has more stability (pylon wing, better moments, and dihedral), is of strong and light construction, has minimal drag. Yet, it is already difficult to fly. Note also that both rubber motor are very short.