Does anyone use a tug tow hook that releases automatically whenglider kites?

Lots of good ideas, but some of the dynamics of a tug upset has been
missed. Chris Rollings mentioned the loads in the rope rapidly increasing
from c50lbs to over 700lbs as the glider assumes a winch launch attitude.
This load on the tug will RAPIDLY slow it down, thus losing flying speed.
So not only does the glider climb, the tug descends with rapidly falling
airspeed, and the elevator becoming ineffective. As the ug loses speed it
will pitch nose down anyway.

Chris also mentioned a very strong deceleration, and I can vouch for this
when a K6E kited behind me on a tow at Aboyne. The K6 pilot missed the
release toggle due to wearing gloves, and had already started his climbing
turn. He realised his mistake and releassed the rope, by which time I was
steeply nose down with little airspeed, albeit at 4000 feet! I was
effectively a passenger during the upset, but interestingly did not lose
sight of the glider in the tug mirror, showing the tail was NOT pulled up.
I have always been very wary of tugging gliders on belly hooks since!

Terry Slater

At 13:16 21 February 2014, kirk.stant wrote:
On Thursday, February 20, 2014 1:31:56 PM UTC-7, Martin Gregorie wrote:

Back to some sort of bridle from the wingtips so any pulling vector
is

through the CG (or close).



I'm sure that would work well in the air, but isn't there a potential

problem during the landing roll-out? With no tension on the tow rope it


seems to me that you've got a U-shaped loop of rope dragging on the

ground behind the tow plane which would provide considerable drag on a

grass field if it isn't recently mown. Would this also be a problem at

the start of the take-off roll?

Well, you could make the bridle in two pieces that connect at the
release,
and come apart when the glider is released; you would now have a long
towrope trailing one wingtip and a short one trailing the other.

Starting to get more complicated, of course...

Kirk

Terry, the dynamic of the towplane slowing was not lost on me. The only source for the kiting energy of the glider is the tug. Any climb rate for the glider above that being provided by the engine of the tug will come out of the tug in the form of lost airspeed. That is probably the first thing the towpilot will notice. But, the same loss of speed could occur as a result of the glider's powerful dive brakes openning, so that cannot be used as a trigger for an auto system. Also, slowing the tug will not make it pitch down, unless the slowing is to below the stall speed of the tug. It will make it start to drop, as lift is now less than weight. And, Terry, if you were nose down, your tail likely was pulled up. In an amount that kept the glider in your mirror. Your airpseed loss was the transfer of your kinetic energy to the increased kinetic and potential energy of the glider. The nose down was likely a combo of the glider pulling your tail up and possibly stalling of the wing of the tug. You effectively became a moving anchor point for the glider.

GC, stalled tail and accelerated stalls are not at all the same. Maybe you need some training? :-) My belief is that a kiting glider with a wing that is 3-5 times the size of the tugs tail can provide far greater tug pitch authority than the tail on the tug. You don't have to stall the tug's tail in order to produce more force with something else than it can produce.

You can test the stalled tail theory by flying the tug sans glider at tow speed. Push the stick rapidly forward to create the nose down pitch rate, then pull rapidly back to arrest it. If the stick back pull does not stop the nose down pitch, the tail was stalled. If it does, the tail is not stalled. This may not be the case for an aerobatic plane intentionally doing an outside snap.

As to "well trained glider pilots", ask Terry about the "well trained pilot" he was tugging who thought he pulled the release, so he started his climbing turn away. Well trained or not these things do happen. Not often, but they do happen. And when they happen at low altitude, the result is usually fatal. First step in training is to burn it into the heads of all glider pilots that on air tow, you don't even consider turning away from the towplane until you have seen the rope going away. Amazingly simple but forgotten more than most would imagine.

Kirk, I don't think we want to consider a bridle from the tips. This would require HUGE spar changes, as you could easily impart 500 lbs aft force at both wing tips. This will pretty likely fold the wings aft and make for a very high speed, although only vertical and down capable, tug. Maybe the missle you were hoping to use? :-)

Mark, fairleads over the tail have issues, too, as boxing the wake will quickly show. But, maybe the air load and the pull of the rope in the other direction will sort of cancel, and not risk bending the fin? However, if there is restriction to up or down pull, you have lost the benefit.

As to other failure modes of an automatic system and the "need to train for them", well, don't we already train for rope breaks? If the system is designed well, it won't make a disconnect any more likely than it is now. And, it is not a change to the airframe, but only an additional means of operating the release. I tend to think that this would be much easier to certify, once you have shown sufficient reliability of the system by design, and that it does not substantially increase the risk of an unplanned seperation of the tug and the glider in practice.

So, with what we have available to us today, why are we afraid to try and design a system to reduce this risk?

Steve Leonard

On Friday, February 21, 2014 11:41:20 AM UTC-6, Steve Leonard wrote:

Kirk, I don't think we want to consider a bridle from the tips. This would require HUGE spar changes, as you could easily impart 500 lbs aft force at both wing tips. This will pretty likely fold the wings aft and make for a very high speed, although only vertical and down capable, tug. Maybe the missle you were hoping to use? :-)

I know that - I was really just throwing out some hare-brained ideas to see what would stick.

My solution is a lot simpler:

1. Near term: Replace ALL Schweizer tow hooks on tow planes with Tost hooks (so you can at least release if you get upset high enough), then provide very specific training to ALL glider pilots on the cause an effect of a tug upset, to include a demo ride in a towplane at the receiving end. And treat all aerotows of gliders with CG hooks as potential kiting accidents and brief accordingly.

2. Long term: some sort of sensor controlled automatic release on tug to eliminate problem, assuming our respective government aviation agencies will agree to their installation. Make it mandatory so the numbers are high enough to drop the price down to something affordable. And remember, it has to not only apply to fixed tow hooks, but also to retractable tow ropes (now how do you measure the forces?).

I still prefer the Bearcat towplane option. Kite away!

Kirk
66

I just had a flash (brain fart) of an automatic release system. You
mechanical guys could probably make something like this work. Please bear
with me...

How about simply inverting a Schweizer release and mounting it on a sturdy
hinge. Then, in addition to the normal release rope, which the tuggie
should be able to release in any attutude, add a second release cable fixed
at the under side of the release, and forward of the hinge, such that, if
the hinge is rotated upward (as in kiting) the fixed cable draws tight and
releases the rope? The length of the fixed cable could be set to open the
release at a preset angle.

Seems simple enough. Please show me the error of my ways.

"kirk.stant" wrote in message
...
On Friday, February 21, 2014 11:41:20 AM UTC-6, Steve Leonard wrote:

Kirk, I don't think we want to consider a bridle from the tips. This
would require HUGE spar changes, as you could easily impart 500 lbs aft
force at both wing tips. This will pretty likely fold the wings aft and
make for a very high speed, although only vertical and down capable, tug.
Maybe the missle you were hoping to use? :-)

I know that - I was really just throwing out some hare-brained ideas to see
what would stick.

My solution is a lot simpler:

1. Near term: Replace ALL Schweizer tow hooks on tow planes with Tost hooks
(so you can at least release if you get upset high enough), then provide
very specific training to ALL glider pilots on the cause an effect of a tug
upset, to include a demo ride in a towplane at the receiving end. And treat
all aerotows of gliders with CG hooks as potential kiting accidents and
brief accordingly.

2. Long term: some sort of sensor controlled automatic release on tug to
eliminate problem, assuming our respective government aviation agencies will
agree to their installation. Make it mandatory so the numbers are high
enough to drop the price down to something affordable. And remember, it has
to not only apply to fixed tow hooks, but also to retractable tow ropes (now
how do you measure the forces?).

I still prefer the Bearcat towplane option. Kite away!

Kirk
66

At 18:10 21 February 2014, kirk.stant wrote:
On Friday, February 21, 2014 11:41:20 AM UTC-6, Steve Leonard wrote:

Kirk, I don't think we want to consider a bridle from the tips. This
wou=
ld require HUGE spar changes, as you could easily impart 500 lbs aft
force
=
at both wing tips. This will pretty likely fold the wings aft and make
for=
a very high speed, although only vertical and down capable, tug. Maybe
th=
e missle you were hoping to use? :-)

I know that - I was really just throwing out some hare-brained ideas to
see=
what would stick. =20

My solution is a lot simpler:

1. Near term: Replace ALL Schweizer tow hooks on tow planes with Tost
hooks=
(so you can at least release if you get upset high enough), then provide
v=
ery specific training to ALL glider pilots on the cause an effect of a
tug
=
upset, to include a demo ride in a towplane at the receiving end. And
trea=
t all aerotows of gliders with CG hooks as potential kiting accidents and
b=
rief accordingly.






2. Long term: some sort of sensor controlled automatic release on tug to
el=
iminate problem, assuming our respective government aviation agencies
will
=
agree to their installation. Make it mandatory so the numbers are high
enou=
gh to drop the price down to something affordable. And remember, it has
to=
not only apply to fixed tow hooks, but also to retractable tow ropes
(now
=
how do you measure the forces?).

I still prefer the Bearcat towplane option. Kite away!

Kirk
66



I see no way to do this ,but,
put a pylon on the main spar and tow from the cofg ,shorten the vertical
stabiliser and add some area below the fus then protect the tail feathers
with
struts.
If I remember correctly ski boats went from a rope on the back to center
poles to give more maneuverability .
Try getting that past EASA or the FAA .

On 22/02/2014 05:10, kirk.stant wrote:

My solution is a lot simpler:
....snips
I still prefer the Bearcat towplane option. Kite away!

I liked the Bearcat idea - in fact I like Kirk's whole approach to the
discussion. I put the idea to some old colleagues who flew Sea Furies
who also liked it.

Unfortunately, there is a simpler solution which has been implemented
and proved effective at zero cost but doesn't involve Bearcat
endorsements for tuggies. You don't get kiting if you fly low tow.

Australia's move to low tow was to prevent kiting in an era of low wing
loading gliders in a country with sharp-edged thermals. It's been an
effective solution and it's globally applicable even today. If kiting
is the problem Steve and others make it out to be, low tow will be
adopted immediately. If it's not, this whole discussion could just look
like a giant circle w..k.

Steve?

GC


Kirk 66