Steve Leonard wrote:

Just looking to properly understand and maybe differently explain the dynamics of the situation, Chris. What you described before was:

"Third test: Terrier Tow-Plane, K 8b on C of G hook. I pitched the glider about 25 degrees nose up. The glider continued to pitch up fairly rapidly (as at the start of a winch launch) and substantial forward movement of the stick only slightly slowed the rate of pitch. The glider achieved about 45 degrees nose up, speed increased rapidly from 55 knots to about 75 knots and the glider was pulled back towards level flight (again as at the top of a winch launch). I released at that point. The entire sequence of events occupied a VERY short period of time (subsequently measured as 2 - 3 seconds). The Tow Pilot reported a marked deceleration and start of pitching down which he attempted to contain by moving the stick back; this was followed immediately by a very rapid pitch down accompanied by significant negative "G". The tow-plane finished up about 70 degrees nose down and took about 400 feet to recover to level flight. We both found the experience alarming, even undertaken
deliberately
at 4000 feet. Our conclusion was that the combination of the initial pitch down and the upward deflection of the elevator caused the horizontal stabilizer/elevator combination to stall and the abrupt removal of the down-force it provided caused the subsequent very rapid pitch-down and negative "G"."

Not to be to "chicken or egg" here, but, I don't think the horizontal tail of the towplane stalled. I think the glider provided an upward force, creating a nose down pitching moment far greater than the elevator could counter (obviously, right?). Once the glider has pitched the plane sufficiently nose down, the wing of the towplane is actually pushing DOWN (even though the pilot is pulling the nose up), hench the negative g felt in the towplane. The glider wing on its long moment arm can produce a far greater pitching moment than the horizontal tail on its short moment arm. The towplane transitions from steady, upright, one g flight, to negative g doward arcing flight because the glider changes the angle of attack of the wing of the towplane from positive to negative.

Your pictures taken later show this is likely. The glider has a large pitch change before it starts its vertical displacement. Similarly, the towplane will get a large pitch change nose down before it starts to deviate from its climbing or level flight path. And it will happen so rapidly, the pilot will not be able to tell if it happened because he pulled back and the tail stalled, or because the glider created such a large nose down pitching moment. I strongly suspect the results would be the same if the towpilot did nothing to try and keep the nose up in the CG hook kiting event. Glider pitches towplane, towplane responds due to lift vector changing magnitude initially, then direction.

We know that this event does not happen often, but when it does, it often has catastrophic results. I don't think there is a practical way to attach the rope to the CG of the towplane. So, we are left with training (which we have seen does not eliminate the problem) or some sort of automatic system (since it has been documented that it is unlikely a pilot can react fast enough to be able to save his or her own life if it goes really bad).

Time to start working on a secondary pull system (sorry, Kirk. No explosive bolts or missles. Would be more fun, though.) using attitude and elevator positon. Or maybe attitude and pitch rate. Need to keep it as simple (fewest inputs and software) as possible. I do like the idea of initial tests of set values to turn on a light in the cockpit for the pilot to see that the automatic system will have operated. Got any reasons not to start at 10 degrees nose down pitch attitude and half way to max nose up elevator travel?

Steve Leonard

Most people posting here do not understand the word "instantaneous".
Chris Rollings, and the CofG hook exponents, are the only people who
get it.