UNDERSTANDING ROTOR (OR NOT)

This yarn may require suspension of disbelief because it’s so unusual, but it really did happen and I wasn’t the only witness.  My analysis is admittedly suspect because of its source, so if you can provide a truer one please do.

We were working blue rotor between 7 and 8 thousand MSL, pointing south to windward above Crystal’s second ridge, basically straight and level at fifty knots.  This rotor was a biggie.  Smooth and delicious as the inside of a warty avocado, it felt like wave for a five hundred-foot gain, but now the climb was fading fast.  Don’t bite the seed!  As I dallied one beat too long before pushing over to penetrate, the glider pitched by itself majestically up and UP to a krazy angle of seventy degrees or more (seemed like 90).   Whaa?!   There we hung in half a G, long enough for me to point out that we still had forty something on the airspeed indicator – and long enough for the student to confirm.  Knowing nothing else, I jammed rudder and full forward stick.  Mighty sluggish at five knots over stall, the bird was just beginning to respond when everything stopped and CG tumbled us sidewise to fall away and recover with a net loss.  That’s the story.

But it’s not the whole story.  Several curious things occured in those eyeblinks, some accessible to reason, some to speculation.  (Yes it was a hammerhead, one not initiated by the pilot. But the hammerhead was a result, and its cause is what we want to know.)

I believe what pitched us up was a burst of sink hitting our tail and ‘weathervaning’ us skyward. What’s peculiar is the wing, even after rotating so radically, still had a positive angle of attack! Our vertical component of lift was zilch relative to the earth, but we did not stall while nosed UP into that downdraft. We stalled when we yawed out of it and the bird swapped ends. Thank Gaia for shoulder straps.

To make sense of this, picture a wheeling rotor viewed broadside.  Between the center and upper edge insert a glider facing the current.  Our glider has climbed in the rising side but is now drifting behind the rotor’s spinning core.  As air flow bends downward the craft’s innate stability keeps it facing UP – into the relative wind – until gravity, loss of inertia and who knows how many other factors all get tangled.

It’s tempting to assume we were thrown down by the wind, but don’t be so sure.  In that long pause before finally stalling, wouldn’t our actual movement relative to the ground have been simple freefall unimpeded by the air below?  If so, 32 ft/sec/sec will exceed sixty knots in three ticks of the clock.  (That’s tail-first by the way…)

The air’s velocity meanwhile would be our freefall plus that forty on the airspeed indicator… Adds up quick, doesn’t it?

If this crude analysis is realistic, look closer.  Wouldn’t the wind’s influence on our velocity during those heartbeats of suspension have equaled only our aerodynamic drag?  That plus the freefall, per second per second, would be our actual velocity – nearly straight down.  Airspeed?  Not applicable for now, though soon to become important indeed.

So, is this anything like what actually happened?  Too rich for a brain cell like mine.  Now again, if someone out there understands it better, please raise your hand.  We need to know what to do next time.

And never let anybody tell you something ‘can’t happen’ just because they haven’t seen it!