03-08-2018, 04:57 PM
Hello Jamie,
I think it is important to understand what you intend your special to be used for; track, road, trial? Plywood has many positive attributes, and in wooden stress bearing structures it can be really useful, think of the DH Mosquito, In my experience, thin ply is exceptional good as a material for shear webs and making thin load bearing, but light skins, but, it does have some significant issues. Marine ply is a pretty generic term applied to all sorts of different products. Unless you buy properly attributed aircraft quality Plywood (usually Birch here in the UK), which is quite expensive, then you need to know what sort of wood it is made from, glued with what adhesive, direction of laminations would also be important. You also need to understand the different grades, which as you decend down the scale in a race to the bottom, allow for impurities in the source timber, areas without adhesive and knot replacement with plugs. It is also worth remembering that the plywoods outer veneers may be a different type of wood to that used in the underlying sandwich of laminations.
The issue assembly issue you will have with such formers is when attaching the skin, any pin or screw will be driven in-between the laminations, which often separate them, if you use screws, the accepted processes is to cross drill the plywood from face to face so to accept a glued in dowel, which the screw then goes into, but with pins, they will go where they go on the day, so not easy to plan for.
Frank Hernandez made a fascinating wooden body for his pale green Gordon England ish racer, which made use of plywood for the internal formers and body skin, it was strong and durable, but, he used only thin laminations of ply wood. If you look at the accompanying pictures (sorry for the poor quality), you will see that he achieved quite complex curvatures and that the internal formers are also of relatively thin section.
Franks car is of course a track car, so not subject to the rigours of pot holed roads, but, it must be reasonably torsionally rigid and quite flexible. I think the thing here that makes his very fast car work is the intelligent use of quality material, ply, adhesive and, when necessary straight grain woods and aluminium to produce a light, durable car for track use.
Something for road or trial will probably benefit from the use of Ash, or in the right place, even spruce (married to ply), both of which as structural materials having the ability to flex and absorb shock loads, which is why the key structural wood used in a Mosquito wing spar is laminated Spruce for the booms, which are laminated with thin ply webs. Often aircraft longhorns are made from Ash, but the gluing of thin birch ply to spruce structures with the correct adhesive makes for a very light and strong structure,
For my own wooden car, I made the tail using the Spruce and Birch Ply method, it is very strong, but also very light. However, for the cockpit area, I used Ash, which is covered in 1/16” Birch ply skin.
Thick sections of Ash steams and bends very well, but if using thin strips, say1/4” thick, withs a nice straight grain, it will take considerable bends before reaching breaking point, you could make some very strong, flexible and light formers using this method, and save the ply to make the tooling to bend it around.
If you like a chat about it, drop me a PM with numbers etc.
Regards,
Mark
I think it is important to understand what you intend your special to be used for; track, road, trial? Plywood has many positive attributes, and in wooden stress bearing structures it can be really useful, think of the DH Mosquito, In my experience, thin ply is exceptional good as a material for shear webs and making thin load bearing, but light skins, but, it does have some significant issues. Marine ply is a pretty generic term applied to all sorts of different products. Unless you buy properly attributed aircraft quality Plywood (usually Birch here in the UK), which is quite expensive, then you need to know what sort of wood it is made from, glued with what adhesive, direction of laminations would also be important. You also need to understand the different grades, which as you decend down the scale in a race to the bottom, allow for impurities in the source timber, areas without adhesive and knot replacement with plugs. It is also worth remembering that the plywoods outer veneers may be a different type of wood to that used in the underlying sandwich of laminations.
The issue assembly issue you will have with such formers is when attaching the skin, any pin or screw will be driven in-between the laminations, which often separate them, if you use screws, the accepted processes is to cross drill the plywood from face to face so to accept a glued in dowel, which the screw then goes into, but with pins, they will go where they go on the day, so not easy to plan for.
Frank Hernandez made a fascinating wooden body for his pale green Gordon England ish racer, which made use of plywood for the internal formers and body skin, it was strong and durable, but, he used only thin laminations of ply wood. If you look at the accompanying pictures (sorry for the poor quality), you will see that he achieved quite complex curvatures and that the internal formers are also of relatively thin section.
Franks car is of course a track car, so not subject to the rigours of pot holed roads, but, it must be reasonably torsionally rigid and quite flexible. I think the thing here that makes his very fast car work is the intelligent use of quality material, ply, adhesive and, when necessary straight grain woods and aluminium to produce a light, durable car for track use.
Something for road or trial will probably benefit from the use of Ash, or in the right place, even spruce (married to ply), both of which as structural materials having the ability to flex and absorb shock loads, which is why the key structural wood used in a Mosquito wing spar is laminated Spruce for the booms, which are laminated with thin ply webs. Often aircraft longhorns are made from Ash, but the gluing of thin birch ply to spruce structures with the correct adhesive makes for a very light and strong structure,
For my own wooden car, I made the tail using the Spruce and Birch Ply method, it is very strong, but also very light. However, for the cockpit area, I used Ash, which is covered in 1/16” Birch ply skin.
Thick sections of Ash steams and bends very well, but if using thin strips, say1/4” thick, withs a nice straight grain, it will take considerable bends before reaching breaking point, you could make some very strong, flexible and light formers using this method, and save the ply to make the tooling to bend it around.
If you like a chat about it, drop me a PM with numbers etc.
Regards,
Mark