Aerotow ropes: short or long, breakable or unbreakable?

Are we gonna fly today or will the eight hour test and evaluation session
use up all available daylight?


"son_of_flubber" wrote in message
...
On Wednesday, November 13, 2013 8:17:22 AM UTC-5, Tom wrote:
The reality is there have indeed been fatalities due to over-strong tow
ropes, and one of the preventative measures is to use a rope designed to
break in case of over-stress.

The problem is that rope degradation is not recognized and defective ropes
are used until they fail prematurely.

Is it a problem that we use tow ropes of a type that is subject to rapid
degradation? The structural part of the rope is exposed to abrasion and UV.
The open weave of the rope allows grit to penetrate and destroy the rope
from the inside out. We use ropes that are dirty and show signs of
"acceptable" wear. We test our ropes by towing the next glider.

Rock climbers approach this problem differently. Their ropes are designed
to minimize the penetration of grit. The function of the outermost wrapping
of the rope is to protect the structural core from grit. When the outermost
protective wrapping is worn, the rope is discarded. In the old days, rock
climbers would wash their ropes in the washing machine to remove micro grit.

Three questions:
1)Would a simple visual inspection find 99.9% of bad tow ropes before they
break (Prematurely)? Do "good" towropes ever break?
2)Are we using the right type of rope?
3)Is there a way to test a tow rope on the ground without destroying it?

If you applied a known load to a tow rope (on the ground) and measured the
elongation, would this not give an objective estimation of the condition of
the rope? (As fibers break or weaken, the rope will elongate more under a
known load). If the elongation is outside the limits, the rope is retired.

Would a magnifying glass improve the visual inspection process?

On Thursday, November 14, 2013 10:50:46 AM UTC-5, Dan Marotta wrote:
Are we gonna fly today or will the eight hour test and evaluation session

use up all available daylight?

Good point. Time is limited.

If an elongation test would detect weakening ropes, you would only need to do it once a week or so because many forms of degradation is gradual. If you had a permanent jig for testing elongation, it would take 10 minutes. Or you could test the ropes at the beginning of wave season when you know they are gonna get stressed (or even better, you could replace your ropes at the beginning of wave season).

Ever have a rope break at Moriarty? I know that you have a lot of grit.

Our ropes are 200 ft long so the jig would either have to be very large or
the rope would need to mounted and dismounted numerous times.

I've had three rope breaks in the past three years while flying the tug at
Moriarty - never before in the previous 20+ years. Two were the fault of
the glider pilot horribly mishandling the glider and one of those resulted
in the rope being wrapped around the wing of the HP-14. The wing was cut
back to the spar before the rope broke. The third rope break had one of
our instructors flying his Libelle and the rope broke at about 300' AGL on
tow. He and I were both surprised and he handled the emergency perfectly.
The rope broke about 10 feet in front of the glider, probably in an area of
high wear.

We inspect our ropes daily in the morning and during operations throughout
the day. If sufficient wear is noted, the rope is replaced. More often
it's the weak link which gets replaced due to abrasion with the pavement.
We use the top half of drinking water bottles, slipped over the weak link,
and wrapped heavily with duct tape. These work well, but, if the tug lands
such that the protector hits the end of the runway, it's ripped off. We
then replace the weak link.

(Whew!)


"son_of_flubber" wrote in message
...
On Thursday, November 14, 2013 10:50:46 AM UTC-5, Dan Marotta wrote:
Are we gonna fly today or will the eight hour test and evaluation session

use up all available daylight?

Good point. Time is limited.

If an elongation test would detect weakening ropes, you would only need to
do it once a week or so because many forms of degradation is gradual. If
you had a permanent jig for testing elongation, it would take 10 minutes.
Or you could test the ropes at the beginning of wave season when you know
they are gonna get stressed (or even better, you could replace your ropes at
the beginning of wave season).

Ever have a rope break at Moriarty? I know that you have a lot of grit.

Is there a difference between a Winch launch Weak Link, and an Aerotow Weak
Link?

I would think the Winch launch requires a much stronger link because of the
higher angle of attack.


At 00:46 15 November 2013, Dan Marotta wrote:
Our ropes are 200 ft long so the jig would either have to be very large or

the rope would need to mounted and dismounted numerous times.

I've had three rope breaks in the past three years while flying the tug at

Moriarty - never before in the previous 20+ years. Two were the fault of

the glider pilot horribly mishandling the glider and one of those resulted

in the rope being wrapped around the wing of the HP-14. The wing was cut

back to the spar before the rope broke. The third rope break had one of

our instructors flying his Libelle and the rope broke at about 300' AGL on

tow. He and I were both surprised and he handled the emergency perfectly.

The rope broke about 10 feet in front of the glider, probably in an area
of

high wear.

We inspect our ropes daily in the morning and during operations throughout

the day. If sufficient wear is noted, the rope is replaced. More often
it's the weak link which gets replaced due to abrasion with the pavement.

We use the top half of drinking water bottles, slipped over the weak link,

and wrapped heavily with duct tape. These work well, but, if the tug
lands

such that the protector hits the end of the runway, it's ripped off. We
then replace the weak link.

(Whew!)

On Friday, November 15, 2013 7:04:22 AM UTC-5, Peter Higgs wrote:
Is there a difference between a Winch launch Weak Link, and an Aerotow Weak

Link?



I would think the Winch launch requires a much stronger link because of the

higher angle of attack.



Peter - you seem to confuse angle of attack with deck angle. The angle of attack of the glider during a winch launch is no different than if the glider was in free flight at the same speed! During a winch launch, the glider is flying up a trajectory with a deck angle of up to 45degr.
And yes, the weak link strength spelled out in the plane's POH typically call for a stronger one for the CG hook vs. the nose hook.
With our winch, we measured the line forces and the results show an interesting force distribution - not at all what everybody expected.

Uli Neumann
'GM'

At 12:39 15 November 2013, GM wrote:
... The angle of attack of the glider during a winch launch is no
different than if the glider was in free flight at the same speed!

Actually, it is significantly different: the wing has to deal with the
tension in the cable (including the cable self-weight) as well as
supporting the weight of the glider. This could easily double the
AoA required in steady flight & smoth conditions towards the end of
a winch launch.

On Friday, November 15, 2013 7:41:22 AM UTC-6, James Metcalfe wrote:
At 12:39 15 November 2013, GM wrote: ... The angle of attack of the glider during a winch launch is no different than if the glider was in free flight at the same speed! Actually, it is significantly different: the wing has to deal with the tension in the cable (including the cable self-weight) as well as supporting the weight of the glider. This could easily double the AoA required in steady flight & smoth conditions towards the end of a winch launch.

Well, James, now you have gone and confused AoA with lift coefficient. You do not necessarily double AOA to double lift coefficient. Lift coefficient generally goes up about. 1 per degree of AoA while you are below separation of flow on the wing surface. At some point on the curve, doubling AoA doubles Cl, but only at that one point. And I would guess that point is no where near where you operate on a winch launch.

Steve