Bayboos
Active member
This unusual construction - a mix of double layer flat panels and single layer rounded sections - is not meant to be particularly lightweight. It's a direct result of material conversion. The "classic" Depron construction methods are very similar to balsa/plywood constructions; and they do in fact allow building an extremely lightweight airframes. But the plans for this particular plane were designed for a very different material, and thus the "classic" approach was of no use here. And that in turn means it was time for what I love the most: improvisation.
The original material, a foam core board covered with paper on both sides, is much more stiff/rigid than the Depron foam itself, which makes it more suitable for large flat panels construction, but almost impossible to create tight curves. That's why all the "rounds" are created by multi-faceting (not bending); but also allows for large flat panels without additional support.
When building a plane like this, designed for a specific material that comes in only one variety (thickness), I always need to make a decision about what foam thickness I should use for each individual part. For all parts that are made of large, flat panels only, my go-to choice is 6mm Depron ("the thick one") - it's stiffness is similar to the DTFB (which is approx. 4,7mm thick) while being slightly lighter yet slightly weaker and much more brittle. For the rounded parts, it is much easier and more suitable to use 3mm Depron, which bends much more easily yet lacks both the strength and rigidity. The rigidity problem "solves by itself" since rounded shapes are much more stable and in most cases those parts are glued to the thicker material on all edges anyway, and the strength is not as crucial in this particular application.
The problems start where one part (as specified on the plans) consists of both large flat section and a rounded one. Theoretically I could split this part into two (the plans even provided a clear indication as to where flat section ends and rounded one begins), but that solution creates additional technological problem: it would require joining two parts of various thicknesses "by the edge". This type of joinery is always to be avoided at all costs, and the typical solutions are so complex I'd not even start to describe them. And please believe me, I'm doing you a favor
To avoid this problem, I decided to stick to the plans and make those parts in one piece. To accommodate the bending requirements, I make them with 3mm Depron, but double the thickness of the flat sections using additional layer of the same material. The doubled part is still a bit weaker (and heavier) than the same part made of 6mm foam with no glue; but it's good enough to achieve all the design goals.
And that's how I originally built the plane. The "rebuild" process introduced it's own challenge (namely: fitting the new parts to the existing structure) and thus additional change to the technology: instead of cutting out a single panel that would cover the entire fuselage section and doubling it in the flat sections, I decided to build it up in seven separate, single layer pieces. The assembly process started by glueing four 3mm flat would-be doublers (one for each side, top and bottom) to the core structure of the plane (reminder: at this point I still had to make them fit to the parts that were relatively undamaged and left in place after the post-crash clean-up), and only then I covered all of them with three outside facing panels that combined both rounded and flat sections. I decided to split this outside skin in the middle of flat sections, which gave me a very convenient way to join all of them together without (almost) any edge joinery.
The resulting structure is an unusual looking, but deliberate mix of materials and material uses with properties changing to cover a specific demands of each and every part of the plane. I am sure the description above is not very clear, but the result is good enough to make the plane strong and stiff enough, reasonably light and - as a happy accident - slightly better looking than the original one. At least until the next crash
The original material, a foam core board covered with paper on both sides, is much more stiff/rigid than the Depron foam itself, which makes it more suitable for large flat panels construction, but almost impossible to create tight curves. That's why all the "rounds" are created by multi-faceting (not bending); but also allows for large flat panels without additional support.
When building a plane like this, designed for a specific material that comes in only one variety (thickness), I always need to make a decision about what foam thickness I should use for each individual part. For all parts that are made of large, flat panels only, my go-to choice is 6mm Depron ("the thick one") - it's stiffness is similar to the DTFB (which is approx. 4,7mm thick) while being slightly lighter yet slightly weaker and much more brittle. For the rounded parts, it is much easier and more suitable to use 3mm Depron, which bends much more easily yet lacks both the strength and rigidity. The rigidity problem "solves by itself" since rounded shapes are much more stable and in most cases those parts are glued to the thicker material on all edges anyway, and the strength is not as crucial in this particular application.
The problems start where one part (as specified on the plans) consists of both large flat section and a rounded one. Theoretically I could split this part into two (the plans even provided a clear indication as to where flat section ends and rounded one begins), but that solution creates additional technological problem: it would require joining two parts of various thicknesses "by the edge". This type of joinery is always to be avoided at all costs, and the typical solutions are so complex I'd not even start to describe them. And please believe me, I'm doing you a favor
To avoid this problem, I decided to stick to the plans and make those parts in one piece. To accommodate the bending requirements, I make them with 3mm Depron, but double the thickness of the flat sections using additional layer of the same material. The doubled part is still a bit weaker (and heavier) than the same part made of 6mm foam with no glue; but it's good enough to achieve all the design goals.
And that's how I originally built the plane. The "rebuild" process introduced it's own challenge (namely: fitting the new parts to the existing structure) and thus additional change to the technology: instead of cutting out a single panel that would cover the entire fuselage section and doubling it in the flat sections, I decided to build it up in seven separate, single layer pieces. The assembly process started by glueing four 3mm flat would-be doublers (one for each side, top and bottom) to the core structure of the plane (reminder: at this point I still had to make them fit to the parts that were relatively undamaged and left in place after the post-crash clean-up), and only then I covered all of them with three outside facing panels that combined both rounded and flat sections. I decided to split this outside skin in the middle of flat sections, which gave me a very convenient way to join all of them together without (almost) any edge joinery.
The resulting structure is an unusual looking, but deliberate mix of materials and material uses with properties changing to cover a specific demands of each and every part of the plane. I am sure the description above is not very clear, but the result is good enough to make the plane strong and stiff enough, reasonably light and - as a happy accident - slightly better looking than the original one. At least until the next crash
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