Might be moisture. Was the sand sticky enough to stick all over your hands when you molded it up? I hate to beat the same drum all the time but is that a fairly sharp edge? Between surface tension of the metal not wanting to let the edge fill in and steam pushing against it trying to escape you might have been fighting an uphill battle. At least on top of the casting the steam has somewhere to go. Oh, now I see why your gates look like that, they're engines!
Yeah...fairly sharp edge. But it only failed in some areas, but not all. I could use some bondo on my wing edges to thicken them up on my positive maybe and try again? yep...four engines! I built the positive model from wood.
Looks like a couple of things may have happened. The sand looks like it may need some adjusting...maybe less moisture or higher clay for better hot strength. May have sand laying in the mold as well if you bumped or moved the mold prior to filling it or could have just been washed out in the filling. The casting does not look that heavy and you may benefit in slowing down the entry of the metal a little/ smaller ingates. I would try a thin ingate the length of the leading edge of the wing and use riser vents on each of the engines...big enough to feed but small enough to remove easily. Add small vents in corners of the tip furthest from the ingate. Good luck and cool casting! I have a few...around a hundred... RC airplanes and have thought about casting some micro support structures and engine mounts just for fun but can buy them too cheap.LOL
