i'll tell you everything i know based on my time with eta-biter, concordia, and talking to butler.

The blow holes on eta biter are driven by a naca duct. the naca duct has been modified to be closed via a switch in the cockpit that drives a servo. at a certain airspeed, the blowing is swithced of and the boundry layer is turbulated further back by....zig sag tape. there is a presentation by butler about this "blowing articulation" out there on the web. Due to manufacturing and maintainance complexity, concordia doesn't have blowholes. neither does the V3. that's kinda a schleicher thing.

When we refinished my 20C two years ago, the question was this: can we fill in the blowholes, and just put zig zag over the blow-hole location? the idea was to reduce the maintainance/not worry about filling them with wax/ no cracking of the paint over the longer term.
we wanted to make sure that the turbulator tape would provide the same benefits though, and not actually harm the performance. butler and waibel both said zig zig in the same location would offer the same performance benefits..

I personally wanted to keep the blowholes, and fantazised about installing blowhole articulation for a club class ringer. I was outvoted though, so we did zig zag tape. the glider flies really well. Ask erik nelson and his V2XA...

On Sunday, October 23, 2016 at 7:54:27 PM UTC-4, Jonathan St. Cloud wrote:
Just curious if there is research that compares blow holes versus tabulator tape as to effectiveness and drag? i.e., are the bow holes worth the maintenance or is tape effectively just as good? Does the new Ventus have tape or blow holes?