Hai-Lee
Old and Bold RC PILOT
Whilst the title of this thread is broad I intend to discuss matters or problems you might strike when scaling up a FB design. For this discussion I will assume a simple doubling of linear dimensions, (length, width, and height).
As most persons just enlarge the original plans each of the parts will have twice the surface area and as the material, (the FB), is not being scaled in volume the weight of the bare airframe will be 4 times that of the original design. Sounds simple!
There are a few advantages of building larger and basically they are that the larger mass of the plane should provide greater inertia and therefore a more docile handling craft from the point of aerodynamic upset forces, (turbulence etc). Another added bonus is that the larger wings will provide a far greater gain in lift than the simple multiplication of the wing area. The air does not scale and as the wing size increases its Reynolds number changes making it more efficient.
Having pointed out a few advantages there are a serious number of structural issues that you may need to address, (model design dependent), which if ignored could turn your dream build into a truly horrific nightmare. Most of them are due to material properties and then structural properties of the design.
Material properties:
FB has physical properties to consider on each of its 3 axis's. They are elasticity, compression, stretching and torsion. over short lengths and low weights the FB is actually more than capable to withstand the forces encountered in flying the models BUT if you double the linear dimensions of the FB piece it suddenly alters its performance significantly.
The elements of consideration in a typical FB are the two paper surfaces, the foam core, and the 2 paper to foam bonds. The paper coatings have only one real dimension of strength and that is in resistance to being stretched, under compression it will crumble almost immediately, a sideways or folding force again will meet little resistance and the paper has very little real resistance to torsional forces unless those forces can be applied as stretching forces. The foam has
some resistance to compression, stretching, bending and torsion but by itself the resistances are minimal. The paper to foam bonds allow the transfer of forces and properties between the various elements of the FB as well as converting forces from one plane of application to another. The result of the bonds is to give FB properties greatly in excess of the sum of the individual elements.
To demonstrate the possible issues attributable to properties of FB in just enlarging the pieces I will consider just the tail boom segment of the fuselage. At twice the length and applying twice the forces from the double size tail elements the forces applied to the tail boom may be considered as having increased by MORE than 4 times! (remember aerodynamic surfaces are more efficient at larger sizes). So the forces trying to push the tail up, down, sideways or even twist the tail
boom can be seen as being over 8 times the same forces applied to the original design, (original scale), when applied to where the tail boom affixes to the remainder of the fuselage.
Now measure the amount of "Flex" you get from the original design. If you can get a tail movement of 1mm under load from the original design this translates to a design tolerance of 2 mm in the scaled up design BUT you are having to deal with over eight times the forces and sadly the flexibility of the FB actually increases with the sheet size or linear dimensions. Now try twisting the tail boom and see what effects you can expect in the larger model if just using larger pieces of FB.
Now here comes the good news! If you now add some "Bulkheads" or "stringers" on the larger lengths of the FB the strength is improved by a factor far in excess of the added weight. As most FT/FB designs use a box construction a few diagonal braces again can provide far more additional strength and resistance to twisting than the additional weight would suggest.
Sure you could just sandwich a double layer of FB for every piece and realize the same strength goals, (possibly excepting twisting forces), the weight would not increase by a linear factor of 4 but actually the "CUBE" of the linear dimension change, (8 times).
What is an airplane? Basically it is a machine made to lift or carry weight in a controlled manner through the air. Individual elements are the wing, (the main lifting device and a compulsory element of the design), the tail or control surfaces, (the main aerodynamic control mechanism and therefore considered as compulsory in the design), the fuselage, (a somewhat parasitic element that connects the wing, control surfaces, the carried load, the propelling force, and the fuel to drive the
propelling force. Of all of the elements the one which offers the least aerodynamically to the design is the fuselage. When building in FB the tendency is to use sheet, (slab), FB as both the structural support and the surface covering, (the surface covering is for aerodynamic efficiency only.
When building larger designs it is advisable for you to consider what part of the fuselage design is structural and what is just covering. If the covering is replaced with something far lighter significant balance issues that are common in enlarged designs will suddenly seem to disappear or become more easily managed. The weight savings can be extreme where FB is replaced by alternate materials in providing structural support.
Where FB is to be the structural material of choice then you should consider altering the mechanical properties of the FB to improve its strength. A simple method I have used in the past is to paint the FB in white glue and let it dry before finishing, The white glue when dry adds some compression and torsion resistance for a small weight penalty and an increase in the brittleness of the structure.
Why does any of the above ramble matter really? Well consider history! A twisting wing can cause control reversal at speed with the obvious impacts! The same can apply to a set of tail feathers on a flexible tail boom, again there will be impacts!
Do not be discouraged though as I and many others have built some real monsters out of FB and they perform quite well but if you ignore the rules of structural design in your build you had better add a garbage bag to your flight box!
Please feel free to add your own comments or ask questions!
From my personal experience!
Have fun!
As most persons just enlarge the original plans each of the parts will have twice the surface area and as the material, (the FB), is not being scaled in volume the weight of the bare airframe will be 4 times that of the original design. Sounds simple!
There are a few advantages of building larger and basically they are that the larger mass of the plane should provide greater inertia and therefore a more docile handling craft from the point of aerodynamic upset forces, (turbulence etc). Another added bonus is that the larger wings will provide a far greater gain in lift than the simple multiplication of the wing area. The air does not scale and as the wing size increases its Reynolds number changes making it more efficient.
Having pointed out a few advantages there are a serious number of structural issues that you may need to address, (model design dependent), which if ignored could turn your dream build into a truly horrific nightmare. Most of them are due to material properties and then structural properties of the design.
Material properties:
FB has physical properties to consider on each of its 3 axis's. They are elasticity, compression, stretching and torsion. over short lengths and low weights the FB is actually more than capable to withstand the forces encountered in flying the models BUT if you double the linear dimensions of the FB piece it suddenly alters its performance significantly.
The elements of consideration in a typical FB are the two paper surfaces, the foam core, and the 2 paper to foam bonds. The paper coatings have only one real dimension of strength and that is in resistance to being stretched, under compression it will crumble almost immediately, a sideways or folding force again will meet little resistance and the paper has very little real resistance to torsional forces unless those forces can be applied as stretching forces. The foam has
some resistance to compression, stretching, bending and torsion but by itself the resistances are minimal. The paper to foam bonds allow the transfer of forces and properties between the various elements of the FB as well as converting forces from one plane of application to another. The result of the bonds is to give FB properties greatly in excess of the sum of the individual elements.
To demonstrate the possible issues attributable to properties of FB in just enlarging the pieces I will consider just the tail boom segment of the fuselage. At twice the length and applying twice the forces from the double size tail elements the forces applied to the tail boom may be considered as having increased by MORE than 4 times! (remember aerodynamic surfaces are more efficient at larger sizes). So the forces trying to push the tail up, down, sideways or even twist the tail
boom can be seen as being over 8 times the same forces applied to the original design, (original scale), when applied to where the tail boom affixes to the remainder of the fuselage.
Now measure the amount of "Flex" you get from the original design. If you can get a tail movement of 1mm under load from the original design this translates to a design tolerance of 2 mm in the scaled up design BUT you are having to deal with over eight times the forces and sadly the flexibility of the FB actually increases with the sheet size or linear dimensions. Now try twisting the tail boom and see what effects you can expect in the larger model if just using larger pieces of FB.
Now here comes the good news! If you now add some "Bulkheads" or "stringers" on the larger lengths of the FB the strength is improved by a factor far in excess of the added weight. As most FT/FB designs use a box construction a few diagonal braces again can provide far more additional strength and resistance to twisting than the additional weight would suggest.
Sure you could just sandwich a double layer of FB for every piece and realize the same strength goals, (possibly excepting twisting forces), the weight would not increase by a linear factor of 4 but actually the "CUBE" of the linear dimension change, (8 times).
What is an airplane? Basically it is a machine made to lift or carry weight in a controlled manner through the air. Individual elements are the wing, (the main lifting device and a compulsory element of the design), the tail or control surfaces, (the main aerodynamic control mechanism and therefore considered as compulsory in the design), the fuselage, (a somewhat parasitic element that connects the wing, control surfaces, the carried load, the propelling force, and the fuel to drive the
propelling force. Of all of the elements the one which offers the least aerodynamically to the design is the fuselage. When building in FB the tendency is to use sheet, (slab), FB as both the structural support and the surface covering, (the surface covering is for aerodynamic efficiency only.
When building larger designs it is advisable for you to consider what part of the fuselage design is structural and what is just covering. If the covering is replaced with something far lighter significant balance issues that are common in enlarged designs will suddenly seem to disappear or become more easily managed. The weight savings can be extreme where FB is replaced by alternate materials in providing structural support.
Where FB is to be the structural material of choice then you should consider altering the mechanical properties of the FB to improve its strength. A simple method I have used in the past is to paint the FB in white glue and let it dry before finishing, The white glue when dry adds some compression and torsion resistance for a small weight penalty and an increase in the brittleness of the structure.
Why does any of the above ramble matter really? Well consider history! A twisting wing can cause control reversal at speed with the obvious impacts! The same can apply to a set of tail feathers on a flexible tail boom, again there will be impacts!
Do not be discouraged though as I and many others have built some real monsters out of FB and they perform quite well but if you ignore the rules of structural design in your build you had better add a garbage bag to your flight box!
Please feel free to add your own comments or ask questions!
From my personal experience!
Have fun!
