Gravity-Powered Aerobatics

My brother sent me this cool video of Aaron Swepston doing aerobatic hang gliding at Dog Mountain in Washington State.

Wow.  It looks like there was no wind up top.  That was strictly a downhill ride; no soaring.

I asked my brother if still conditions are required for aerobatics.  Here is his response:

“Aerobatics are well outside the normal flying envelope, and could quickly go from a moderate ‘g’ level to breaking the glider if turbulence diverged the flight path.  So yes, still conditions are required for aerobatics.

“The ‘normal’ class in the certification standards tests the gliders at 60 mph positive (at the stall angle of attack) and a about 35 mph negative.  Air loads increase as the square of the velocity so at 91 mph you could (potentially) see double the flight loads if you went from a very low angle of attach (AoA) to the stalled AoA.

“There is also the phenomenon of ‘positive divergence’ due to the flexible nature of the wing.  At high positive loads the deformation of the wing tends to cause the AoA to increase with a force beyond the pilot’s ability to hold it down.  This increase in the AoA causes more deformation causing higher AoA, etc., in a positive feedback loop.

“Endgame is a glider blowing apart and the pilot (hopefully) deploying a parachute and descending under canopy.

“Probably TMI already, but in the mid-eighties Aaron blew a loop at an aerobatic competition and ended up with the glider stable, but upside down, and he was standing on the undersurface.  If he deployed his chute he would be disqualified, so he spent some time trying to yank on the control bar to get it to flip back over.  It didn’t work and finally he deployed.”

I had watched the video a couple of times trying to notice how much the wing was bending, but it was hard to detect any, perhaps because of the wide angle lens.  I did notice one of the control bar wires go slack at one point.

“I don’t think he pulls enough g’s to deform the glider in that manner, but did you notice when he pulls on the line at the base tube of the control bar?  That is the VG or variable geometry.  When the line is pulled, the crossbar moves further back and pushes the leading edges farther out, flattening the sail.  You probably could not notice that, either, given the camera position and wide angle lens.

“On that particular glider, the Nose Angle can change from 127 to 132 (the included angle between the leading edges).  Using my rusty trig, that would be an increase of about 0.658 feet (or almost 8 inches) in wingspan.  There is probably some flexing of the leading edges during this so it is likely less.

“My point is that given Aaron is keeping within a reasonable g range, you probably can’t see the deformation.  Here are a couple of videos with positive and negative load tests that show some deformation:  http://www.airborne.com.au/pages/hg_movies.html.

“On the ‘slack’ wires.  The T2 he is flying is a ‘topless’ glider – a cantilever wing with a composite crossbar.  The bottom side wires are only there to keep the control bar in the correct position, and are not actually structural to the positive loading of the glider.”

I had noticed he was messing with a loose line that seemed to be flapping along his body on the far side from the camera.  After glossing over a review of the T2 and spending five minutes back-tracking to try to figure out what VG means, Googling VG, and then, after finding a nice glossary on hgausa.com, realizing that my brother had already provided the definition, I now think that was the VG cord he was pulling.  Surprised it was so long; perhaps because a compound pulley system is used.  At first, I was concerned that a rinky-dink jam cleat is used, but after reading more realize that if it slips, the sail returns to its more forgiving trim.

Maybe tomorrow I’ll try to figure out what a “sprog” is.

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