Motor Mount in Foamular 250
The Foamular 250 we are using is rated for compression of up to 25 psi (lbs/sq-inch). It is more stiff than other foam I have seen in model airplanes and a bit heavier. We cut out the foam where we don't need the strength, so we are not hurt by the weight. For motor mounts it seems like this extra strength could let us just gorilla glue some screws into the foam. We tried this and it seems to work fine.
For this experiment we used M2x20 screws. So 2 mm diameter and 20 mm long. The motor mount uses 4 screws but for this test we only used 2. We drilled two holes in the foam and filled them with water (poking small nail in to get all the air out). Then we covered the screws with gorilla glue and put them in.
First we tried just putting a weight on to hold them in:
But it was hard to get it to hold both screws down well. So we put a piece of plastic over and a clamp and let it cure overnight.
So for the test this is what it looked like:
And here is the video of the test:
It took 17 lbs of pull to break it. This was only 2 short screws. In the real plane we would use 4 longer screws. It seems strong enough.
After the holes looked like this: I had dug out the missing foam in the center so the mount could lay flat even with the string going behind the center.
And the short small screws after. It looks like the gorilla glue held to the screws and to the pink foam and the pink foam is what failed.
So I think this is a fine way to mount motors on planes made with Foamular 250. Not too heavy and strong enough.
Update Jun 9, 2021
It occurs to me that if the motor gets hot the screws could get hot, which could soften the foam, and cause it to fail. So there might be a problem. We will probably test a battery full on the ground before launching into the air.