Water balloons? I was dropping Flour bombs yesterday, but I've filled them with water before. Just made with foam coffee cups hot glued together, the ones I used yesterday were 8 ouncers. Fill the first cup with flour, glue a paper disk on top to keep the flour up front to make it fall right. Invert a cup and glue it top to top. Glue the third cup bottom to bottom to make the bomb.
Drop mechanism is a servo on retract gear, wire goes out the side of the fuse, re-inforce that point. My plane is wood build up with an inner brace of light ply and covered in thin balsa, I just glued a popsickle stick to the side to reinfoce, use a disk of light ply on Foamcore OR, a popsickle stick. And in my plastic fuse planes where I normally double side tape the servos in place then re-inforce with a wire tie I slip a rubber band through the wire tie But this one I had a brass picture hanger I bent to slip on the fuse under the wing. Yesterdays skys were brilliant blue, sure makes dropping that white bomb from altitude look pretty. I've used bigger cups before but this plane sits too low and the 8 ouncers were all that would fit. They didn't have quite enough weight so I think scoring the front cup with a pocket knife would help disperse the flour better. One fell through the trees at the side of the field, opened up the cup really nice. Big cloud out of that one.
For water bombs with the same method, glue the first two cups together first, poke a hole in the top and fill it with water, you can add temper paint in first to color the water but that generally takes more paint then is worth it. then seal it in with the last cup. That's a pound of water by the way, 16 ounces. It makes quite a smack when it hits.
Weight with THIS aircraft, powered by a 46 size IC engine isn't a problem. Surprisngly DRAG is quite substantial, especially if the bomb hangs sideways. Should try to avoid that. I chain three rubberbands for the retainer, spread the middle rubberband around the seem between the first cups and it stays straight, unless you hit a big bump on takeoff.
