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Big foam board structural questions

#1
So I was looking at those big motors in the radial series and I was thinking about building a 200% scout, but I was questioning the structural integrity of the big foamboard pieces after I saw josh putting an aluminum spar in it. Is the paper thicker on both sides? Does the foam end up compressing permanently at that thickness? Also, are reinforcements on the fuselage still necessary? I know they probably are on the wing, but I've seen videos of higher scale models having two layers of foamboard (meaning there is paper in the middle) almost all the way down the sides of the fuselage.
 

Chuppster

Active member
#2
So I was looking at those big motors in the radial series and I was thinking about building a 200% scout, but I was questioning the structural integrity of the big foamboard pieces after I saw josh putting an aluminum spar in it. Is the paper thicker on both sides? Does the foam end up compressing permanently at that thickness? Also, are reinforcements on the fuselage still necessary? I know they probably are on the wing, but I've seen videos of higher scale models having two layers of foamboard (meaning there is paper in the middle) almost all the way down the sides of the fuselage.
I think these questions may call for some testing to be sure, and I'll admit I've never even seen the new foamboard. However, in theory, scaling everything up proportionally should make things stronger in general. Due to the "Box" nature of the fuselage I would not be concerned about rigidity length-wise, however, I would say an aluminum spar would be a wise precaution in the wing, as it's a fairly cheap way to add a ton of rigidity to your wing. That's just my 2 cents, hopefully someone with more experience can chime in.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#3
I also have not used any of the thicker FB but I have had a large amount of experience in using the thinner FB, (3mm), and this was in scaling down FT designs and building at full scale in the thinner FB. There are significant differences when you vary the thickness of the foam centre layer.

The largest issue you may confront is that the thicker foam will make the FB more flexible as the foam itself adds a layer of flexibility or compressability to the material. When scaling down, to the thinner FB, the result was actually a stiffer and less resilient material.

The thinner FB also weighed more per unit volume and so increased thickness FB will have less weight per unit of used volume.

If you find your builds a little flexible I would recommend the use of a few FB stringers along the inside of the fuselage to reduce the tail boom flex, (if encountered).

For wings you will almost definitely require some form of rigid spar structure if you are to avoid a wing that is too flexible and could even appear to slightly flap in flight. Without a rigid spar structure the wings could even flex/twist in flight and the plane could become almost uncontrollable with issues such as control surface reversal becoming a problem as the flight speed increases.

Laminating the FB will improve the rigidity but the large amount of flexibilty of the foam will remain and you could quickly find that the weight is increasing faster than the strength!

Just a few things to ponder!

Have fun!
 
#4
For the wing I've pondered the idea of designing a more classic rib and spar structure and then wrapping it in foam board. I know others have done similar. With bigger models, a bolt on removable wing is nice. At some point rubber bands get a little sketchy (for me anyway) and it's nice to get the wing on exactly the same way each time without any slop or guessing about alignment. All that takes a bit of engineering and design though. One of these days I'd love to build a 200% or 300% storch, but I want it to last more than one flight!
 

DamoRC

Well-known member
Mentor
#5
For a 200% Scout, I wonder if you even need to go to the thicker foamboard. With the right supports and cross-bracing using regular foamboard I reckon you could make it work, particularly if you are going to make the rib-spar-cover type of wing.

Has anyone bought any of the thicker foamboard and have some specs / thoughts on how it stacks up against regular?