Cadillac commercial accident?

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

FWIW - When my college club was actively doing auto tows (2-33 with a CG hook on a 3,000 foot paved runway with 1,000 feet of overrun), we would routinely do "repositioning" tows. For example after a simulated rope break with a straight ahead landing, we would turn around and do a tow to 100 feet to get back to the staging end of the runway (as long as the tailwind was not too strong). While this was usually on the same 1200 foot rope we used for the standard tows, we sometimes used a spare aerowtow rope (for example when uncoiling the rat's nest created by a release under tension with the long rope). I remember vividly John Campbell instructing me to make a VERY gradual climb starting with stick forward and slowly rotating to a much shallower climb angle. We did this dozens of times each winter without any problems. So, I'm agreeing with Bill that it's not inherently suicidal, but like anything else it needs to be thought through.

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...

I once caused a tow car (Jeep Grand Cherokee) with pulley on the bumper and
rope staked down at mid field to lose traction during taping of an episode
of "Secrets of Speed" for ESPN. We were launching my LS-6a for the opening
shot. Of course the upset is not shown in the aired program, but I have the
raw footage... Emerson Fitipaldi was riding shotgun in the Jeep and he
reached across and took control from the driver.


"Bill D" wrote in message
...
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 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 20, 9:24*am, Mike the Strike wrote:
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

Arguably, a better description is "trebuchet effect".

At 18:16 20 October 2011, Bill D wrote:

Arguably, a better description is "trebuchet effect".

I have often wondered whether a sort of trebuchet could be useful for
launching a glider off a ridge. One can imagine a short bit or tarmac 5m x
1m heading off the ridge and a large counter weight on a cable running down
it. One would then only need to wind up the weight, secure it, get in, have
some brave soul hook you up and release....

Jim

Acceleration due to gravity 32 ft/sec/sec = approx 20 knots/sec. If the
system were completely frictionless and the glider had zero drag that's
still 2 seconds to accelerate from 0 to 40 knots. At an average of speed
32 ft/sec, that's 64 feet or about 20 metres, not 5. Then you need to
figure the extra margin needed to overcome the frictional losses and the
glider's drag, quite likely doubling the run and you might want a bit of
margin above 40 knots which is around Vs on most modern gliders.

I believe something similar was used somewhere a long while ago, dropping
the weight down a disused mineshaft - don't recall the details.


At 07:52 21 October 2011, Jim White wrote:
At 18:16 20 October 2011, Bill D wrote:

Arguably, a better description is "trebuchet effect".

I have often wondered whether a sort of trebuchet could be useful for
launching a glider off a ridge. One can imagine a short bit or tarmac 5m
x
1m heading off the ridge and a large counter weight on a cable running
down
it. One would then only need to wind up the weight, secure it, get in,
have
some brave soul hook you up and release....

Jim