Post by thevon on Nov 16, 2007 17:29:20 GMT 10
Some of you technical guys might be interested in this. I got into a bit of an argument with some Yanks about whether carbon ribbon spars should be flat laid or embedded vertically. The original discussion is here:
www.rcgroups.com/forums/showthread.php?t=770003&page=8#post8559168
As a result I ran a small experiment which I've posted in a new RCG thread: www.rcgroups.com/forums/showthread.php?t=772801#post8559251 but I'll repeat it here too.
THE EXPERIMENT!
I took a thick slab of high density EPP from a wing bed and cut 2 identical pieces from it. Size 105 x 175mm and it was tapered width-wise, but approx 30mm thick where I placed the spars.
I got some new carbon ribbon of 10mm x 0.5mm size (that’s all I have, so it’s wider but thinner than the commonly available 0.8 x 8mm ribbon) and cut 4 pieces of the same length as the EPP pieces.
On one piece I laid the ribbons flat, and on the other one I embedded them vertically. I marked lines along both sides of both pieces. The lines weren’t in the middle – they were placed a bit off centre to put them more towards the thicker side of the slab. But on both pieces, both ribbons were directly above/ below each other.
CA glue was used to glue all the ribbons. On one piece I cut 10mm deep vertical cuts into the foam along the lines. Then bent the EPP to open up the slit a bit stuck the CA tip in and ran a thick bead in. Then quickly jammed the ribbon in and pushed it down flush with the surface. Then repeated for the other side.
On the other piece I zigzagged CA thickly over one side of a ribbon and pressed it onto the EPP line. Repeated on the other side. I was worried that it wouldn’t bond well so I put it in my huge vise and lightly pressed the 2 ribbons to the surface. I left both pieces for about 30mins.
I got a couple of paving bricks stood on their ends on the bench and laid the first EPP piece across, so only 8 - 10mm was perched on each brick.
I scratched my head about how to work out a load test and this is what I came up with. I got the platform scales we use for work and set them up with the platform on the floor next to the bench, and the digital reader (attached by a long lead) sitting next to the bricks. These scales have 4 corner load cells and currently are set to a resolution of +/- 50gms. Every day we use them they're checked with a series of standard weights that we own, and they’re dead accurate.
I stood on the platform without touching anything and recorded my weight (exactly 87.00kg) then put the heel of my hand in the centre of the EPP and started pressing down. The weight on the reader dropped and my son and I kept watching it as I increased pressure. We watched carefully for the lowest weight reading just when the EPP/spar system failed.
With the flat lay, we got the weight down to 67kg (ie 20kg downward pressure) when it began to bow a lot. But it didn’t really fail – it just began to bow as the ribbons began to squash the foam. I could push some more kilos onto it but it began to go banana shaped, however when let go it returned to its normal shape. I pushed it harder beyond the initial 67kg bend, to see what happened and it just bent more. This shot is while releasing it:
When I tested the the vertical lay, I have to that on first impression it felt stiffer than the flat lay. I got to 66kg (ie 21kg downward pressure) then it totally snapped. Both spars broke thru completely and so did the EPP.
When I retried the flat laid piece again after doing this, I was able to again get nearly 20kg of pressure onto it before it started to bend. It’s still useable.
I won’t try to make any interpretation here but I think it was a very interesting result.
www.rcgroups.com/forums/showthread.php?t=770003&page=8#post8559168
As a result I ran a small experiment which I've posted in a new RCG thread: www.rcgroups.com/forums/showthread.php?t=772801#post8559251 but I'll repeat it here too.
THE EXPERIMENT!
I took a thick slab of high density EPP from a wing bed and cut 2 identical pieces from it. Size 105 x 175mm and it was tapered width-wise, but approx 30mm thick where I placed the spars.
I got some new carbon ribbon of 10mm x 0.5mm size (that’s all I have, so it’s wider but thinner than the commonly available 0.8 x 8mm ribbon) and cut 4 pieces of the same length as the EPP pieces.
On one piece I laid the ribbons flat, and on the other one I embedded them vertically. I marked lines along both sides of both pieces. The lines weren’t in the middle – they were placed a bit off centre to put them more towards the thicker side of the slab. But on both pieces, both ribbons were directly above/ below each other.
CA glue was used to glue all the ribbons. On one piece I cut 10mm deep vertical cuts into the foam along the lines. Then bent the EPP to open up the slit a bit stuck the CA tip in and ran a thick bead in. Then quickly jammed the ribbon in and pushed it down flush with the surface. Then repeated for the other side.
On the other piece I zigzagged CA thickly over one side of a ribbon and pressed it onto the EPP line. Repeated on the other side. I was worried that it wouldn’t bond well so I put it in my huge vise and lightly pressed the 2 ribbons to the surface. I left both pieces for about 30mins.
I got a couple of paving bricks stood on their ends on the bench and laid the first EPP piece across, so only 8 - 10mm was perched on each brick.
I scratched my head about how to work out a load test and this is what I came up with. I got the platform scales we use for work and set them up with the platform on the floor next to the bench, and the digital reader (attached by a long lead) sitting next to the bricks. These scales have 4 corner load cells and currently are set to a resolution of +/- 50gms. Every day we use them they're checked with a series of standard weights that we own, and they’re dead accurate.
I stood on the platform without touching anything and recorded my weight (exactly 87.00kg) then put the heel of my hand in the centre of the EPP and started pressing down. The weight on the reader dropped and my son and I kept watching it as I increased pressure. We watched carefully for the lowest weight reading just when the EPP/spar system failed.
With the flat lay, we got the weight down to 67kg (ie 20kg downward pressure) when it began to bow a lot. But it didn’t really fail – it just began to bow as the ribbons began to squash the foam. I could push some more kilos onto it but it began to go banana shaped, however when let go it returned to its normal shape. I pushed it harder beyond the initial 67kg bend, to see what happened and it just bent more. This shot is while releasing it:
When I tested the the vertical lay, I have to that on first impression it felt stiffer than the flat lay. I got to 66kg (ie 21kg downward pressure) then it totally snapped. Both spars broke thru completely and so did the EPP.
When I retried the flat laid piece again after doing this, I was able to again get nearly 20kg of pressure onto it before it started to bend. It’s still useable.
I won’t try to make any interpretation here but I think it was a very interesting result.
