OK - easiest thing in the world is to lose a machine screw when you open a model up to replace the battery (especially if it's vanished in the grass). Sure, you can hunt for it with a magnet, but try this instead
First of all, wrap some cotton round the shank of the screw when it's fully inserted through the wing (keep it close to where it will sit normally when tightened up so that the head is flush with the outer washer). Make sure the cotton is as close to the inner face and the wrapping is wider than the hole as you can and carefully dab some cyano on it to lock it in place.
Next thing is to ream out very slightly the plate where the screw enters (so that the cyaonoed thread doesn't stand proud but fits inside - 1/16 or 3/32 inch should be enough). Let it dry then assemble the model - what shopuld happen is that, when you unscrew the wing, the thread holds the screw in place on the wing and it doesn't fall out