On Monday, October 24, 2016 at 3:26:10 PM UTC+1, Jonathan St. Cloud wrote:
Thanks for the reply. I seem to remember that Dick Butler had throttled the blow holes on his ETA biter, not sure how he did that. seems like tape would have more drag but the manufacturing must be very time consuming (expensive).

On Sunday, October 23, 2016 at 5:45:04 PM UTC-7, Michael Opitz wrote:
At 23:54 23 October 2016, 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?

That subject goes back over 30 years. As I recall, the blowholes
would be better performance-wise if they could be regulated to blow
really strongly at low speeds, and then blow only a little at higher
speeds. To do this, one needs a large inlet to supply enough air to
the ducts at thermalling speeds. Then, you have to shut that
supply down to a trickle when cruising at higher speeds. This goes
backwards to just having a fixed size inlet that is not adjustable in
flight. One would have to have an extendable scoop on each wing
that the pilot could extend and retract depending on airspeed to
achieve this.

Anyway, the Z-tape works pretty well, is cheap to install, and is
easy to maintain. The blowholes are labor intensive both in
construction ($) and in maintenance.

I had DG-300 serial number 2, which was one of the first gliders
with this technology. Back in 1983-1985, I did side by side
comparison tests with the blowhole system working on the regular
fixed size inlets. Then, I repeated the (side by side) tests with the
inlets removed (to nullify the blowholes), and used Z-tape for the
turbulation instead.

I could not see any measurable performance difference between
blowholes (with fixed size inlet) and Z-tape. There were arguments
about the Standard Class rules to the point that there would have
been protests filed if I had come up with and used a variable inlet
scoop, so I left it alone.

So, long story short. The Z-tape is a lot cheaper and easier, and
yields good results. If you are in a class which would allow inlet
size adjustment in flight, and you have lots of money to pay for the
installation, and lots of time to spend maintaining all of of those tiny
holes, then you might see a marginal improvement over Z-tape.

That's how I remember it, although it was a long time ago, and the
blowhole technology may have gotten better.....FWIW

RO

Other than a little care during polishing blowholes don't need much attention but zig-zag tape keeps catching on grass, clothing, under finger-nails and on rigging trestle tops resulting in little sharp and draggy lifted corners that won't stick back down easily.