Herbert wrote, "......my bellyboard turns down to about 80 deg. and after talking to some aerodynamicists and some practical trials I decided to let the board without holes. The tests I made are very simple.I took a foamsheet ( 1 meter x 1 meter x 5 mm) ,fixed 5 strings to the edges and the center with a weight (about 100 grams ) and used my "parachute" as testdummy. I dropped the dummy from my ten meter high rooftop (5 trials without holes and 5 trials with holes) and compared the times I measured. It was easy to recognice that the board without holes always gone down slower ! There was a big difference how they sank down.The board without holes tumbled much more than the Board with the holes (due to the holes,of course) Happy New Year again,Herbert, German Kr Bilder Von meinem"
The board without holes fell slower in your test. The problem is that at the speeds a belly brake is deployed, the holes in the belly board may generate vortices or turbulence, and hence, parasitic drag, that is more of a factor at the 100+ kph (62 mph+) that a belly brake is deployed. Note that the people who have flown both with and without holes report more drag with the holes. I would also think that there might be different interactions between the belly board and fuselage or tail when there are holes in it that contribute to the difference. That's why flight testing is so important, with the real world conditions.
