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Foam Board Strength - A Scientific Test!

DutchRoll

Well-known member
#1
With the advent of the new FAA rules requiring transponders for all drones/rc airplanes over 250g/0.55lbs, I have seen a significant amount of interest on this forum on foam board choices and how to shave off grams off the foam board to get a design under the legal limit, while still having the resulting structure be strong enough to take the static and dynamic loads of flight.

I'm endeavoring to approach this problem scientifically, creating a testing program that will build a 1.5" x 20" straight box spar from each type of foam board I can get my hands on, and test the box spar to destruction.
 
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DutchRoll

Well-known member
#2
Reserved For Methodology:

I will be simulating the loads on a box spar fuselage, like an Ugly Stick, without a taper towards the tail.
  • Testing Square Hollow Spar: 2" x 2" x 20" (50.8mm x 50.8mm x 508mm)
  • Fold Options: Crush Fold, Spiral Fold, A-B Fold, Tri Fold
  • Remove paper from Interior, Exterior, Totally, and Partially (circles, braced, etc)
  • Attach 2" of one end to a static 2"x 2" mount, hang a tray from the other end, and progressively add weight to tray until failure in box structure occurs.
  • Note any deflection or torsion in structure leading up to failure point
  • Location and method of failure, include pictures.
  • Additional test - Moisture test, both wet and dried
  • Additive structural supports will not be tested. While Balsa and Newsprint have been suggested as additions to the testing process, I am going to restrict myself to strictly subtractive techniques for the following reasons:
    • Keep with in the original design parameters as presented by Flitetest - Availability, Accessibility, and Affordability.
    • Reduce the amount of additional materials needed for purchase.
    • Adding additional support structures takes a measure of skill that I or other newbies might not have.
    • I suffer from essential tremor, and this limits the dexterity of my hands.
      • I will sometime have episodes of uncommanded spasms in my hands, especially dealing with fine detail work. That's how I ripped the tail off of my Simple Soarer while trying to install a control rod on a control tab.

test_rig01.jpg

Test Rig (Mounted to the back of my 3d mill)

  • Fiskars Cutting mat to see deflection
  • Cup can hold 1 Kg of steel washers ( 9.6 g per washer)
  • Pail to catch cup after beam failure
  • Naked Styrofoam Test beam



Edit - 03/12/21: Beam size changed from 1.5" to 2" to make testing measurements easier. Photo of Test rig and description added.
 
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DutchRoll

Well-known member
#3
Reserved For Tested Materials:

Board Manufacturer

Color

Cost

Board Size: inch (mm)

Thickness: inch (mm)

Mass: oz (gram)

Density

Readi-Board

Green

$1.00

20"x28" (508x712)

0.1732" (4.40)

(110)

(0.069 g/cm3)

Readi-Board

Black

$1.00

20"x30" (508x762)

0.1988" (5.05)

(116)

(0.059 g/cm3)

Readi-Board

White

$1.00

20"x30"

0.2016" (5.12)

(113)

(0.057 g/cm3)

Ross (Walmart)

White

$0.88

20"x30"

0.2087" (5.30)

(125)

(0.061 g/cm3)

Elmers (Walmart)

 

$2.97

    

Hobby Lobby

White

$1.77

20"x30"

0.1772" (4.5)

(256)

(0.147 g/cm3)

Maker Board

 

$2.32

20"x30"

   

Depron

      

Card Board

Brown

Cheap

20"x30"

   

DS Card Stock

White

$0.68

22"x28"

 

(120)

 


Adhesive Materials options:
  • Clear Hot Glue
  • 3 minute Slow Set Hot Glue
  • Gorilla Clear Glue
  • Packing Tape - Clear
  • Packing Tape - Fiber reinforced
  • Scotch Tape
 
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DutchRoll

Well-known member
#4
Reserved For Results:

Board Test

Deflection Weight

Failure Weight

Comments

Readi-Board (White Box - Full Paper)

   

Readi-Board (White Box - No Interior Paper)

   

Readi-Board (White Box - No Exterior Paper)

   

Readi-Board (White Box - No Paper)

   

Readi-Board (White Tri - Full Paper)

   

Ross (White - Full Paper)

   

Ross (White - No Interior Paper)

   

Ross (White - No Exterior Paper)

   

Ross (White - No Paper)

   

Hobby Lobby (White - Full Paper)

   

Hobby Lobby (White - No Interior Paper)

   

Hobby Lobby (White - No Exterior Paper)

   

Hobby Lobby (White - No Paper)

   

Test 2x4 Naked Styrofoam Beam

Empty Cup - 52g

No Failure / Deflection: 203mm @ 1025g

PD: 32mm

 
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DutchRoll

Well-known member
#8
I’m watching this thread with much interest. :) Will you be testing brown flitetest foam board as well?
If I can find a local source, I will. I'm loathe to order an entire box just for testing a sheet, considering that I have 50 sheets of Readi-Board and Ross already. That being said, Makerboard is made by Adams, just like Readi-Board, but with higher quality control. The formulation for the foam should be the same, I would think (Please correct me if I'm wrong!). It's just that the paper changed to brown at first to be more water resistant, and now they've changed back to white. The fact that it's a double sheet folded over doesn't affect this particular testing process.
 

Wildthing

Well-known member
#10
If I can find a local source, I will. I'm loathe to order an entire box just for testing a sheet, considering that I have 50 sheets of Readi-Board and Ross already. That being said, Makerboard is made by Adams, just like Readi-Board, but with higher quality control. The formulation for the foam should be the same, I would think (Please correct me if I'm wrong!). It's just that the paper changed to brown at first to be more water resistant, and now they've changed back to white. The fact that it's a double sheet folded over doesn't affect this particular testing process.
Why don't send flite test an email and see if they would send you out some samples for testing.
 

Hai-Lee

Old and Bold RC PILOT
#16
Should you wish to really aid in the building in FB and very light you could also consider a second round of strength testing covering methods to strengthen FB spars and the weight advantage or penalty in doing so.

I lighten the FB structure in the appropriate areas and reinforce the FB where needed. I can build an FT design, (in heavy FB), and still have my resultant plane lighter and stronger than the FT original. In addition there are many other FB builds that are reinforced with FB that are so very heavy in original form.

Minimal structure and minimum weight with maximum strength and maximum performance would allow it to be easier for some budding designers IF they had a series of lightening and strengthening methods available, to keep their design weights below the magic 500 grams..

Just a suggestion!

Have fun!
 

DutchRoll

Well-known member
#17
Should you wish to really aid in the building in FB and very light you could also consider a second round of strength testing covering methods to strengthen FB spars and the weight advantage or penalty in doing so.

I lighten the FB structure in the appropriate areas and reinforce the FB where needed. I can build an FT design, (in heavy FB), and still have my resultant plane lighter and stronger than the FT original. In addition there are many other FB builds that are reinforced with FB that are so very heavy in original form.

Minimal structure and minimum weight with maximum strength and maximum performance would allow it to be easier for some budding designers IF they had a series of lightening and strengthening methods available, to keep their design weights below the magic 500 grams..

Just a suggestion!

Have fun!
LOL - It's less than that! It's 250 total grams, not 500, that the FAA is madating for us as the limit before a transponder is needed. As for the strength vs weight loss testing, I was going to look at removing only portions of the paper, either as circular or triangular cutouts, trying to keep in mind how stress propagates through the structure. The paper functions as a stressed skin, strong in tension, but weak in compression, where as the foam is weak in tension but stronger in compression. <- but I must test this assertion!
 

Hai-Lee

Old and Bold RC PILOT
#18
LOL - It's less than that! It's 250 total grams, not 500, that the FAA is madating for us as the limit before a transponder is needed. As for the strength vs weight loss testing, I was going to look at removing only portions of the paper, either as circular or triangular cutouts, trying to keep in mind how stress propagates through the structure. The paper functions as a stressed skin, strong in tension, but weak in compression, where as the foam is weak in tension but stronger in compression. <- but I must test this assertion!
You will find that the paper to foam junction provides a greater compression resistance than foam alone. I did a hybrid build a fair while ago where paper was removed and some foam as well and thin balsa was laminated onto the foam. The result was lighter and far far stronger. It is actually still flying under the control of its second owner.

Just remember if the bird can be built lighter it can use a lighter motor and electronics setup.

Have fun!
 

Bricks

Well-known member
#19
What I think would be a good add on to your testing is DTF with the original paper pulled off and covered with newspaper glued on. I have done a few elevators and rudders it was lighter but way stiffer and water proof. I used Elmers Wood glue mixed with a little water.