Length of rubber motor determines the fuselage length

7 February 2025

Using the length of rubber motor to determine the approximate length, I found that the 10" Ryan Trainer is too small, the 9" Chipmunk is also too small. The Chipmunk is cuter though, so instead of printing it at actual size on A4 paper, I enlarge it to print on A3 paper and found that the wingspan is just under 13" and the fuselage length appears appropriate for the rubber motor length. This will then be a "Peanut" sized model. Instead of using proportionally increase in sizes of the balsa, what if I continue to use the same thickness? This will result in a lighter wing loading and I think it is good. Next thought was whether I should continue to use 1/32" balsa sheet as the fuselage. I used Gemini and it told me that a 9 square inch of 1/32" medium density balsa is 0.645 grams; a 9" long of 2 pieces of 1/16"x1/16" is 0.102 grams. That means if I construct both sides of the fuselage (approximately 18 square inches), The framed structure could save up to 1.083 grams. I think this is doable.

 

6 February 2025

On AliExpress, affordable large rubber bands are in abundance, and I've bought some:

The idea is to use these rubber bands 160x1.4x1.4mm for rubber powered free flight models. The motor run of one rubber band will be short; 2 bands in parallel will be even shorter but it will be more powerful; 2 bands in series will extend the motor run but is weaker; 4 bands in total of parallel and series will extend the motor run and is more powerful. More bands equal more weight, but the model will be larger and easier to build and fly. It will be an amazing feat to use only one rubber band and fly stably, but I like the deceitful simplicity, even though I know it is the absolute opposite. Using only one rubber band is my first choice, failing which, adding rubber bands in parallel is still possible. To be clear, this mean that the model will be very small, smaller than a peanut scale model (admittedly, getting a peanut scale model to fly is an amazing accomplishment). 

Should the propeller for 1 rubber band be built up? Ideally, it should be, to minimise the overall weight of the model, but if the nose moment is short and nose weight will be required, a stronger, more accurate plastic propeller is weight at the nose tip. My first choice is to use plastic propeller, if one rubber band does not have enough power, then I will add another rubber band. Or, in the case where tail weight is actually required, I will make a built up propeller as replacement. Let's start with a <5" plastic propeller with soft paper clip wire as the wire shaft. Oh wait, that's not realistic, perhaps a 3" paddle propeller? As suggested by Gemini:

"Think in terms of fractions of the rubber band length. A propeller diameter of roughly 1/6th to 1/4th of the rubber band length (160mm) would be a more appropriate starting point. This translates to about 26mm to 40mm, or roughly 1" to 1.5". Even a sub-2" prop is not out of the question for such a tiny model."

Requirements:

• uses single or double 160x1.4x1.4mm rubber bands, do not require braiding or tying.
• <3" propeller, paper clip as wire shaft
• a motor peg for replacement of rubber bands if needed.

The length of rubber motor determines the hook to peg distance. Allowing for a loose fit, this length will be 150mm. Allowing 20mm for the distance between the rear of the propeller and the end of the hook, we get 170mm. 

If 1/3 of this 170mm is apportioned to the front of the model's CG, the distance is only 57mm, while CG to motor peg will be 113 mm in distance. This gives an idea of sizing the fuselage to suit the length of the rubber motor. Draw the profile outline of the fuselage, with nose to CG at approximately 57mm. Draw suitably proportioned wings and tails. Draw in the key elements and we now have a plan that suits this rubber band.