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Downsizing 1/3 to 1/6 scale plans

#1
Hi everyone.

I'm looking for a little bit of help and advice. I've read a number of good threads on this forum, so thought I'd join up.

I'm currently planning to build a balsa/ply/spruce 1/6th scale Bristol Scout biplane. I have got two sets of plans, one is 1/3rd scale and another 1/6thscale. I'm trying to build as close to realistic as possible and the 1/3rd plans are a lot closer to real than the 1/6th plans. I'm planning on downsizing the 1/3 plans to 1/6 and halving all dimensions and thicknesses, but I'm worried of any problems this may cause. As I understand it, getting smaller improves relative strength and rigidity but decreases lift and performance for the given weight. Therefore, should I be aiming to also reduce weight/strength as much as possible, so that overall weight is less than 50% of original? For the record, this is not going to be flown hard and stunted, so I don't mind a slow flyer. Also, I'm going for a glow engine to be as light as possible.

Any pointers would be greatly appreciated!

Thanks in advance,
Dan
 

Flitedesign 3d

Well-known member
#2
Hi everyone.

I'm looking for a little bit of help and advice. I've read a number of good threads on this forum, so thought I'd join up.

I'm currently planning to build a balsa/ply/spruce 1/6th scale Bristol Scout biplane. I have got two sets of plans, one is 1/3rd scale and another 1/6thscale. I'm trying to build as close to realistic as possible and the 1/3rd plans are a lot closer to real than the 1/6th plans. I'm planning on downsizing the 1/3 plans to 1/6 and halving all dimensions and thicknesses, but I'm worried of any problems this may cause. As I understand it, getting smaller improves relative strength and rigidity but decreases lift and performance for the given weight. Therefore, should I be aiming to also reduce weight/strength as much as possible, so that overall weight is less than 50% of original? For the record, this is not going to be flown hard and stunted, so I don't mind a slow flyer. Also, I'm going for a glow engine to be as light as possible.

Any pointers would be greatly appreciated!

Thanks in advance,
Dan
Generally speaking if you multiply the wingspan by 2 the voulume/mass multiplies by 2^3 or 8 So if you build it at 50% it must get a lot lighter than 50% of the 1/3rd scale. As a semi experienced balsa builder I advise you to aim to a weight that is 1/8 of the model douple the size. If Im correct the 1/6th scale wingspan is about 94cm so its a small plane and I would use 2-3 mm balsa as the main material
 

quorneng

Well-known member
#3
Although the relative strength/rigidity of the same material improves as things get smaller unfortunately the aerodynamic capability of the wing falls off just as fast due to a combination of reduced wing area and a reduction in the aerodynamic efficiency as the density and viscosity of the air remains constant regardless of size. To keep the power required to fly within bounds you actually have to build even lighter as you get smaller.
The geometric benefits of reduced size mean you can actually use a lighter materials and still retain the same effective strength.
Selecting the right grade and size of balsa is just important in small sizes to avoid the plane being over strength and thus over weight.
Done effectively (you need to become a bit of a structural engineer to judge what can be lighter!) the plane will still be just as strong aerodynamically but it will be more delicate as far a physical handling are concerned.

This picture of a Fokker Triplane under construction built to 1/17 scale shows how the benefits of a very small size allow 1.5 mm round balsa to satisfactorily match the size of the steel tube used in the original's fuselage.
Triplaneconstruction1.jpg

It flew well enough but needed very careful handling.
 
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