Schleicher owners should note that the 29's (after the first few
serial numbers) have vent holes drilled in the leading edges of the
wing root ribs. These holes are about 7mm diameter and are filled with
metal, O-ringed plugs for ballasted flight. If your Schleicher doesn't
have these, I suspect it would be a straightforward retro-fit, but be
aware that the plugs can leak (I had to replace the factory O-rings on
mine with larger ones) and you can make a real mess if you forget to
put the plugs in before you rig and water up!