Sunday, April 24, 2011

Calibrating Board-Off Flex Profiling - Not too bad

I stumbled across the bag weighing scales that I had been looking for to calibrate the setting in the Board-Off Design Excel Workbook.

Here is a picture of the 'lab' and the sophisticated test equipment. A foam block with cm internals marked on it. The board clamped across the middle with a plank of wood and the bag weighing scales used to lift the tip of the board.

The scales are marked in kg but to get Newtons you just multiply by 9.8.

Testing the point load flex in the carefully controlled test environment in our car port!

Testing a 132x41cm full carbon Crazy Fly Raptor
I also put my Raptor on the test bed so that I can start building up some points of reference.

Here's the rest results for my first board and the predicted values. Drum roll please.........

Not too bad I have to say. I think the 20kg prediction /measured gap may be a confluence of experimental errors but at least the figure is in the ball park and if its within 10% then I think that is great given that there would be a large number of in situ effects that aren't taken into account in the model.

Most importantly the model can be used to determine the relative affects of different parameters and now I have a benchmark from my first board to have a real sense of what that stiffness feels like.

Centre and tip flex profiles (blue and pink respectively)
 One interesting thing in the analysis was the affect of the he direction of the plies. In board #1, one of the plies was laid off axis at about 30 degrees (rotated anticlockwise about 30 degrees). I was trying to lay it at 45 degrees but the cloth wasn't wide enough and I didn't want to join it cause I wasn't sure how to make sure it was strong enough. The 30degree offset has a disproportionate affect on the elastic modulus in the direction of the length of the board and with the standard weave it reduces the elastic modulus of that ply to about 60% of the axial value. Because the glass fibre's elastic modulus is about 6 times that of the resin the overall affect with all 6 layers of reinforcement is to reduce the 'average' elastic modulus by about 10-15%.

A valuable addition to Board Off will be some more granularity around the laminate properties given that there is so many permutations and combinations of lay-ups that possible.

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