The aerodynamics of a towplane in a kiting glider situation

The idea of deliberately diving the tug when this situation is
recognised is interesting - maintain energy, maintain elevator
authority, get rid of the glider, avoid the stall - sounds good. But I
guess the upset has already happened when the tug pilot first
recognises the situation. And who'd want to push into an even
steeper dive at 200'?


At 15:04 18 May 2020, Tom BravoMike wrote:
On Monday, May 18, 2020 at 2:40:26 AM UTC-5, Marton KSz
wrote:
I found this diagram last week:

https://photos.app.goo.gl/nLpWwLHFmgwE5dEM9

Here's my interpretation:

The system of the connected towplane + glider has a center of
gravity,
somewhere along the towrope, closer the towplane.

When the kiting begins, the system of the two connected masses
start
rotating around this CG (just like groundlooping a tailwheel
aircraft).

However, since the glider is lighter, it rotates faster, which makes
the
impression that the towplane slingshots it.

Also, the glider has wings, and faster airspeed on the wings
means more
lift - the glider wants to go even higher.

All the energy for the extra lift + speed has to come from
somewhe the
supply is the kinetic energy of the towplane. As the kiting
aggravates, the
towplane drastically slows down. First it runs out of elevator
control,
then stalls.

That's quite an interesting picture, helping to understand the
process and
the discussion about the (initially) small angles.
The 800 ft minimum for the towplane to recover from a dive seems
a lot.
Could speak for using eg. touring motorgliders as tugs in hope
they could
recover faster.

The thing that seems to be missing from these discussions (except for the tow from the CG) is the fact that the problem is the up force of the towline exceeds the ability of the tow plane elevator to resist. I do not know how much downforce the elevator can create, but it is a LOONG way from the breaking strength of the rope or even any weak link you can reasonably tow with. Near the breaking strength of the rope, I think you will find the angle that will provide enough lift on the tail to "win" over the elevator, will be quite shallow. And of course, that angle at the tail is the same change in angle that might happen from the tow pilot reacting to turbulence. All the thought of angle detection, make me think the inventors have never towed in rotor, where we see lock to lock control movement fairly regularly.

My favorites in this thread, are pushbutton release, no effort, fast, accessible, and yes, in bad cases, too slow. And perhaps the integrating load cell to look for a sustained high load, but we have no data to know if that is a valid condition to look for. A few of our upsets (thankfully at altitude) we from a glider release, in a climbing turn, that resulted in an instantaneous nose down of the tow plane, not the sustained load that is thought to happen in a kiting situation.

I am afraid, without a means to tow from a point on the tow craft that does not influence pitch, we are stuck with better, and recurring training.

RR