Cadillac commercial accident?

On Tue, 18 Oct 2011 06:28:45 -0700, Andy wrote:


As I said in my first answer "if the climb profile was flown correctly".
Auto tow does not require an aggressive climb profile to reach maximum
altitude as long as the runway length is not limiting. Unlike winch
launching the rope length remains constant.

To me, auto-launching on a 200 ft rope sounds uncomfortably like aero-
towing on a CG hook except that now you're *trying* to get above the
'tug'. This sounds to me like a recipe for getting into the
uncontrollable sling-shot region that upsets tugs. Further, it seems to
me that if you do that to a rear-wheel drive vehicle your problems will
be compounded by a loss of acceleration due to loss of traction as the
rope tension reduces the weight on the driving wheels.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Oct 18, 6:28*pm, Martin Gregorie
wrote:
On Tue, 18 Oct 2011 06:28:45 -0700, Andy wrote:

As I said in my first answer "if the climb profile was flown correctly"..
*Auto tow does not require an aggressive climb profile to reach maximum
altitude as long as the runway length is not limiting. Unlike winch
launching the rope length remains constant.

To me, auto-launching on a 200 ft rope sounds uncomfortably like aero-
towing on a CG hook except that now you're *trying* to get above the
'tug'. This sounds to me like a recipe for getting into the
uncontrollable sling-shot region that upsets tugs. Further, it seems to
me that if you do that to a rear-wheel drive vehicle your problems will
be compounded by a loss of acceleration due to loss of traction as the
rope tension reduces the weight on the driving wheels.

--
martin@ * | Martin Gregorie
gregorie. | Essex, UK
org * * * |

Yep, but so far as I know, no one has yet upset a tow car. If the
rear end is light, the rear wheels can spin on low traction surfaces
but the solution is to fill the back end of the tow vehicle with
rocks.

On Oct 18, 10:53*pm, Bill D wrote:
On Oct 18, 6:28*pm, Martin Gregorie
wrote:









On Tue, 18 Oct 2011 06:28:45 -0700, Andy wrote:

As I said in my first answer "if the climb profile was flown correctly".
*Auto tow does not require an aggressive climb profile to reach maximum
altitude as long as the runway length is not limiting. Unlike winch
launching the rope length remains constant.

To me, auto-launching on a 200 ft rope sounds uncomfortably like aero-
towing on a CG hook except that now you're *trying* to get above the
'tug'. This sounds to me like a recipe for getting into the
uncontrollable sling-shot region that upsets tugs. Further, it seems to
me that if you do that to a rear-wheel drive vehicle your problems will
be compounded by a loss of acceleration due to loss of traction as the
rope tension reduces the weight on the driving wheels.

--
martin@ * | Martin Gregorie
gregorie. | Essex, UK
org * * * |


So, putting this thread together it seems we have another plausible
scenario: the glider does a "ground tow" using a short rope, but
following a climb profile, with the plan being to release and then
land straight ahead. The rope breaks or back-releases with the glider
still pointing up at about 175 feet. At this point it's nearly
impossible to recover. The glider stalls and spins, resulting in the
nose-down turn reported by the observers. That's a much more common
scenario than spoiler malfunction.

It would seem easy to use a 200 foot rope to just get up to speed,
getting to no more than 50 feet and then overflying the car. Using it
to get altitude, flying a regular climb profile but doing in 200 feet
what you normally do in 1000 feet, could easily lead to the surprise
rope break or back release while still climbing, as the moment to nose
over and release would come very fast and you can't see the car.

I presume those of you who have tried auto towing behind short ropes
(not me!) were basically just getting up to speed, say to do a modern
bungee launch from the top of a hill, not trying to get to the
standard 60 degrees or so maximum altitude release point.

At least it's more plausible than a plan to do a 180 turn from 200
feet!

Presumably at least the NTSB will get to see the video and we will
know what really happened.

John Cochrane

On Oct 19, 7:49*am, John Cochrane
wrote:
On Oct 18, 10:53*pm, Bill D wrote:









On Oct 18, 6:28*pm, Martin Gregorie
wrote:

On Tue, 18 Oct 2011 06:28:45 -0700, Andy wrote:

As I said in my first answer "if the climb profile was flown correctly".
*Auto tow does not require an aggressive climb profile to reach maximum
altitude as long as the runway length is not limiting. Unlike winch
launching the rope length remains constant.

To me, auto-launching on a 200 ft rope sounds uncomfortably like aero-
towing on a CG hook except that now you're *trying* to get above the
'tug'. This sounds to me like a recipe for getting into the
uncontrollable sling-shot region that upsets tugs. Further, it seems to
me that if you do that to a rear-wheel drive vehicle your problems will
be compounded by a loss of acceleration due to loss of traction as the
rope tension reduces the weight on the driving wheels.

--
martin@ * | Martin Gregorie
gregorie. | Essex, UK
org * * * |

So, putting this thread together it seems we have another plausible
scenario: the glider does a "ground tow" using a short rope, but
following a climb profile, with the plan being to release and then
land straight ahead. The rope breaks or back-releases with the glider
still pointing up at about 175 feet. *At this point it's nearly
impossible to recover. The glider stalls and spins, resulting in the
nose-down turn reported by the observers. That's a much more common
scenario than spoiler malfunction.

It would seem easy to use a 200 foot rope to just get up to speed,
getting to no more than 50 feet and then overflying the car. Using it
to get altitude, flying a regular climb profile but doing in 200 feet
what you normally do in 1000 feet, could easily lead to the surprise
rope break or back release while still climbing, as the moment to nose
over and release would come very fast and you can't see the car.

I presume those of you who have tried auto towing behind short ropes
(not me!) were basically just getting up to speed, say to do a modern
bungee launch from the top of a hill, not trying to get to the
standard 60 degrees or so maximum altitude release point.

At least it's more plausible than a plan to do a 180 turn from 200
feet!

Presumably at least the NTSB will get to see the video and we will
know what really happened.

John Cochrane

A steep climb on a short rope attached to the CG hook will likely
exceed the weak-link strength (1,000 pounds?) long before you get to
200 feet.

Mike

I believe the attainable launch height for auto tow is between 50% and
75% of the length of the rope.
A 200 foot rope is only useful for a hop and land ahead...
On the CG hook it would have a high propensity to kite - on the nose
hook, no back release and hard to pull up. Either way it is not a "good"
approach.

Presumably they were hoping to do something similar to the Michelin
advert with the glider overflying the launching/stopping car? That was a
BMW saloon on ice, also a shortish rope, but no pull up from the glider.

I can't see any way it would be possible on a short runway, with an
extremely short rope, to hope to do anything other than land ahead.

Just my .002 Euro...

On Oct 19, 11:28*am, Mike the Strike wrote:

A steep climb on a short rope attached to the CG hook will likely
exceed the weak-link strength (1,000 pounds?) long before you get to
200 feet.

Mike-


Mike, how is the load on the rope higher if the rope is short than it
is if the rope is long,as you propose? Answer: It isn't. Angle of
attack (lift) and speed determine line tension during the climb.

The difference in the long rope and a short rope is that if you assume
the same deck angle for the airplane, you get to the critical back
release angle at a much lower altitude on a short rope than you do on
a long rope. Same climb rate with the same plane at the same speed
gives the same line tension on a rope that is 200 feet long or 2000
feet long. You just hit max altitude much faster on the shorter rope.

Steve Leonard

On Oct 19, 7:38*pm, Steve Leonard wrote:
On Oct 19, 11:28*am, Mike the Strike wrote:



A steep climb on a short rope attached to the CG hook will likely
exceed the weak-link strength (1,000 pounds?) long before you get to
200 feet.

Mike-

Mike, how is the load on the rope higher if the rope is short than it
is if the rope is long,as you propose? *Answer: *It isn't. *Angle of
attack (lift) and speed determine line tension during the climb.

The difference in the long rope and a short rope is that if you assume
the same deck angle for the airplane, you get to the critical back
release angle at a much lower altitude on a short rope than you do on
a long rope. *Same climb rate with the same plane at the same speed
gives the same line tension on a rope that is 200 feet long or 2000
feet long. *You just hit max altitude much faster on the shorter rope.

There are clearly some differences in the dynamics of short vs. long
ropes. In particular, the shorter rope constrains the flight path to
smaller radius, which I assume causes a somewhat greater than normal
"water skier" effect once the glider pitches up to climb attitude. I
can imagine how a heavy SUV and short elastic rope, combined with a
slight over-rotation on takeoff, could easily degenerate into rapidly
increasing pitch angle, airspeed, lift, and rope tension...

Marc

On Oct 19, 8:33*pm, Marc wrote:
On Oct 19, 7:38*pm, Steve Leonard wrote:









On Oct 19, 11:28*am, Mike the Strike wrote:

A steep climb on a short rope attached to the CG hook will likely
exceed the weak-link strength (1,000 pounds?) long before you get to
200 feet.

Mike-

Mike, how is the load on the rope higher if the rope is short than it
is if the rope is long,as you propose? *Answer: *It isn't. *Angle of
attack (lift) and speed determine line tension during the climb.

The difference in the long rope and a short rope is that if you assume
the same deck angle for the airplane, you get to the critical back
release angle at a much lower altitude on a short rope than you do on
a long rope. *Same climb rate with the same plane at the same speed
gives the same line tension on a rope that is 200 feet long or 2000
feet long. *You just hit max altitude much faster on the shorter rope..

There are clearly some differences in the dynamics of short vs. long
ropes. *In particular, the shorter rope constrains the flight path to
smaller radius, which I assume causes a somewhat greater than normal
"water skier" effect once the glider pitches up to climb attitude. *I
can imagine how a heavy SUV and short elastic rope, combined with a
slight over-rotation on takeoff, could easily degenerate into rapidly
increasing pitch angle, airspeed, lift, and rope tension...

Marc


My back-of-envelope analysis suggested that the angle of the short
rope at the glider would increase more quickly than that of the long
rope and that this could result in a rapid increase of tension. This
is especially true if the pilot fails to control the angle of ascent
as this change occurs, it could create a slingshot effect that
accelerates the glider and rapidly increases line tension.

We'll have to wait for an analysis of the video to really know what
happened, of course.

Mike

On Oct 19, 8:49*am, John Cochrane
wrote:
On Oct 18, 10:53*pm, Bill D wrote:









On Oct 18, 6:28*pm, Martin Gregorie
wrote:

On Tue, 18 Oct 2011 06:28:45 -0700, Andy wrote:

As I said in my first answer "if the climb profile was flown correctly".
*Auto tow does not require an aggressive climb profile to reach maximum
altitude as long as the runway length is not limiting. Unlike winch
launching the rope length remains constant.

To me, auto-launching on a 200 ft rope sounds uncomfortably like aero-
towing on a CG hook except that now you're *trying* to get above the
'tug'. This sounds to me like a recipe for getting into the
uncontrollable sling-shot region that upsets tugs. Further, it seems to
me that if you do that to a rear-wheel drive vehicle your problems will
be compounded by a loss of acceleration due to loss of traction as the
rope tension reduces the weight on the driving wheels.

--
martin@ * | Martin Gregorie
gregorie. | Essex, UK
org * * * |

So, putting this thread together it seems we have another plausible
scenario: the glider does a "ground tow" using a short rope, but
following a climb profile, with the plan being to release and then
land straight ahead. The rope breaks or back-releases with the glider
still pointing up at about 175 feet. *At this point it's nearly
impossible to recover. The glider stalls and spins, resulting in the
nose-down turn reported by the observers. That's a much more common
scenario than spoiler malfunction.

It would seem easy to use a 200 foot rope to just get up to speed,
getting to no more than 50 feet and then overflying the car. Using it
to get altitude, flying a regular climb profile but doing in 200 feet
what you normally do in 1000 feet, could easily lead to the surprise
rope break or back release while still climbing, as the moment to nose
over and release would come very fast and you can't see the car.

I presume those of you who have tried auto towing behind short ropes
(not me!) were basically just getting up to speed, say to do a modern
bungee launch from the top of a hill, not trying to get to the
standard 60 degrees or so maximum altitude release point.

At least it's more plausible than a plan to do a 180 turn from 200
feet!

Presumably at least the NTSB will get to see the video and we will
know what really happened.

John Cochrane

John,

Not quite...

If he really got to 170' AGL before the rope break, the glider's
attitude would have been fairly level. It's only possible to get the
nose way up early in the launch. The "CG hook" is actually slightly
ahead of the actual CG so the rope's pull will bring the nose down as
the glider reached the top of the auto tow even with full-up
elevator. Also, if the rope angle is near 70 degrees, the automatic
"back release" function of the Tost CG hook will activate. Given
enough room and a knowledgeable pilot, there is nothing inherently
dangerous about an auto tow with a 200' rope.

It may have gone like this. The pilot rotates prematurely into a very
steep climb right after liftoff with the stick full back having been
trained that way launching old gliders with a nose hook or a
"combination hook". He may not have been prepared for how
aggressively a DG1000 with a true CG hook flown solo responds to full
up elevator. When the rope breaks, full-up elevator causes the nose
to "pop-up" so now the nose is up 60 degrees at low altitude. Only
airspeed and quick reactions could have saved him.

Calculations done by one of our winch engineering group show if the
glider had at least 60 knots at the break, and the pilot reacted by
pushing over at 0 G within .2 seconds, (Healthy human reaction time.)
he could have recovered straight ahead at Vs x 1.3 with a height loss
of only about 25 feet suggesting even this extreme event was
potentially recoverable if the recovery started above 25' AGL.

However, the pilot, surprised by the rope break, is unable to get the
nose down before the glider stalls. A wing drops in an incipient spin
but, being extremely spin resistant, the glider transitions into a
spiral dive almost immediately. The pilot reacts late to the
incipient spin applying spin recovery controls with the glider in a
spiral dive. The result is a turning dive into the ground.

On Oct 19, 9:49*am, John Cochrane
wrote:
On Oct 18, 10:53*pm, Bill D wrote:

So, putting this thread together it seems we have another plausible
scenario: the glider does a "ground tow" using a short rope, but
following a climb profile, with the plan being to release and then
land straight ahead. The rope breaks or back-releases with the glider
still pointing up at about 175 feet....
A nice thing, John, to bring the thread back to topic and
summarize.
There is one thing about this situaion that mystifies me: a witness
interviewed by local TV said "they had been flying about an hour"
before the accident happened. There's been no clue what actions that
hour of flying contained (or whether it existed). In any case, it
seemed to imply that the accident may not have happened on the first
go.
And, on a technical note, it *is* possible to do a safe 180 with
less than 175 ft altitude AGL, but from level flight safely above
stall speed, not from a steep slow climb.

Dan Johnson