Aerotow ropes: short or long, breakable or unbreakable?

IF the glider is flown perfectly balanced in the perfect position behind
the tow
plane and NEVER gets out of station or forms slack in the rope.


Tom



At 00:22 14 November 2013, Bob Cook wrote:
yes, I underestimated....

Say a 800 lb glider at 40:1 L/D so the drag is about 20 lbs...but
the
tension component in the rope due to the climb angle (say 10:1) is going
to be 80lbs....so you've got 100 lb tension on the rope.

But a rope "in spec" would offer a safety factor of 6x up to 16x.

Some tow planes can climb better than that...so more tension...

Cookie







At 23:46 13 November 2013, Martin Gregorie wrote:
On Tue, 12 Nov 2013 18:43:41 -0800, Cookie wrote:

My rough calculations show that the "normal" tension on a tow rope in
smooth tow is very low...It is basically the drag on the glider, plus
a
tiny bit extra due to the climb vector...so maybe 60 lbs tension...

I think its a bit more than that, but not much. Some time ago I did a
fairly detailed spreadsheet calculation for my Libelle being towed at 60

kts in a 600 fpm climb and got a total rope tension of 37.62 kg, which
was more or less what I'd guestimated from a mental round number
calculation.

The makeup of the rope tension did surprise me though: like you I
thought

drag would be the major part of it, but the spread sheet gave the
glider's drag as 9.97 kg and the force needed to haul my 280 kg Libelle
up a 5.67 degree slope at 60kts as 27.65 kg.

Yeah, I know, I should have only kept 1 decimal point: 2 decimals is
spurious accuracy.

Of course, the other interesting number would be what fraction of the
total drag of a climbing tug+glider combination was due to the glider. I

suspect the glider is contributing less than Wg/Wt where Wg is the
glider

weight and Wt is the tow planes's weight. I'd expect the straight ratio
to over-estimate the glider's contribution because it is likely to fly
rather more efficiently than the tug.


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

Of course!


Dynamic forces could be many times the static forces...in fact all the way
up to the limit of the rope!

Again good reason for an upper limit on the rope.

Cookie

At 01:34 14 November 2013, Tom Claffey wrote:
IF the glider is flown perfectly balanced in the perfect position behin
the tow
plane and NEVER gets out of station or forms slack in the rope.


Tom



At 00:22 14 November 2013, Bob Cook wrote:
yes, I underestimated....

Say a 800 lb glider at 40:1 L/D so the drag is about 20 lbs...bu
the
tension component in the rope due to the climb angle (say 10:1) is
going
to be 80lbs....so you've got 100 lb tension on the rope.

But a rope "in spec" would offer a safety factor of 6x up to 16x.

Some tow planes can climb better than that...so more tension...

Cookie







At 23:46 13 November 2013, Martin Gregorie wrote:
On Tue, 12 Nov 2013 18:43:41 -0800, Cookie wrote:

My rough calculations show that the "normal" tension on a tow rope in
smooth tow is very low...It is basically the drag on the glider, plu
a
tiny bit extra due to the climb vector...so maybe 60 lbs tension...

I think its a bit more than that, but not much. Some time ago I did a
fairly detailed spreadsheet calculation for my Libelle being towed at 6

kts in a 600 fpm climb and got a total rope tension of 37.62 kg, which
was more or less what I'd guestimated from a mental round number
calculation.

The makeup of the rope tension did surprise me though: like you I
thought

drag would be the major part of it, but the spread sheet gave the
glider's drag as 9.97 kg and the force needed to haul my 280 kg Libelle

up a 5.67 degree slope at 60kts as 27.65 kg.

Yeah, I know, I should have only kept 1 decimal point: 2 decimals is
spurious accuracy.

Of course, the other interesting number would be what fraction of the
total drag of a climbing tug+glider combination was due to the glider.

suspect the glider is contributing less than Wg/Wt where Wg is th
glider

weight and Wt is the tow planes's weight. I'd expect the straight ratio

to over-estimate the glider's contribution because it is likely to fly
rather more efficiently than the tug.


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

On Tuesday, November 12, 2013 11:22:14 AM UTC-5, son_of_flubber wrote:
With the dewpoint at 15d F. today, I'm ready to heat things up.



Regulations on aerotow ropes vary by region. What is the evidence to support the various approaches?



PTT (Premature Termination of Tow)(aka rope break) correlates with a number of fatal accidents. How often has a hard_to_break towrope led to a fatal accident?



Options to land after a PTT vary by airport and can be non-existent at certain altitudes. Why is an extra strong towrope not an option where the straight-ahead landing option is poor or non-existent?



In what year did the FAA set the FAR for towropes and what was the quality of towropes at that time? (obscure, but interesting question).



At the airport where I am towed, the same towrope satisfies the FAR for both the frequently towed heavier two-seater, and my less frequently towed lighter weight single place glider. Does this mean that the two-place is more likely to break the towrope?

The 80-200% rule is in the FARs but it DOES NOT APPLY to ANY glider which - at the time of US-certification (standard or experimental) - had a manufacturer's issued and approved POH, which spelled out the breaking strength of a weak-link for the CG and aero-tow hook! In some cases, these may be the same; in most cases, the aero-tow weak-link strength is lower.
In Europe, typically very strong tow ropes (mooring lines, as one US visitor to Europe called them) are used but there is a proper weak-link attached on the glider end.
A short while ago, I did launch a quick survey on the German forum to find out how many tow rope failure anyone had. The response: zero tow rope failures!

It's rare, but I have twice seen ropes break when they snagged something solid while the tug was on final. Something to consider before going to an "unbreakable" rope with a weak link only at glider end.

T8

I've seen the tow rope snag the fence across the street form the airport
several times...sometimes the rope breaks, sometimes the fence
breaks....once I actually saw the rope wind up once around the top pole of
the fence , then unwind without breaking anything.

Another time the tow rope wound up around the outside rear view mirror of a
passing car. It neatly removed the mirror from the car. The driver
arrived a the glider shack, mirror in hand. He was not too mad...he had
just taken a glider ride...and enjoyed it...He said it did scare him a bit
though...insurance payed for the car repair.


BTW...If you read the FAR...two weak links are required. The FAR is
worded funny...they use the singular twice, instead of the plural. "a week
link at the glider" further down, "a weak link at the tow plane"

Students almost always get my question about this wrong...they stop at "a
week link"..

Cookie




At 11:50 13 November 2013, Evan Ludeman wrote:
It's rare, but I have twice seen ropes break when they snagged something
solid while the tug was on final. Something to consider before going to
an
"unbreakable" rope with a weak link only at glider end.

T8

On Tuesday, November 12, 2013 9:16:19 PM UTC-7, GM wrote:
On Tuesday, November 12, 2013 11:22:14 AM UTC-5, son_of_flubber wrote:

With the dewpoint at 15d F. today, I'm ready to heat things up.







Regulations on aerotow ropes vary by region. What is the evidence to support the various approaches?







PTT (Premature Termination of Tow)(aka rope break) correlates with a number of fatal accidents. How often has a hard_to_break towrope led to a fatal accident?







Options to land after a PTT vary by airport and can be non-existent at certain altitudes. Why is an extra strong towrope not an option where the straight-ahead landing option is poor or non-existent?







In what year did the FAA set the FAR for towropes and what was the quality of towropes at that time? (obscure, but interesting question).







At the airport where I am towed, the same towrope satisfies the FAR for both the frequently towed heavier two-seater, and my less frequently towed lighter weight single place glider. Does this mean that the two-place is more likely to break the towrope?



The 80-200% rule is in the FARs but it DOES NOT APPLY to ANY glider which - at the time of US-certification (standard or experimental) - had a manufacturer's issued and approved POH, which spelled out the breaking strength of a weak-link for the CG and aero-tow hook! In some cases, these may be the same; in most cases, the aero-tow weak-link strength is lower.

In Europe, typically very strong tow ropes (mooring lines, as one US visitor to Europe called them) are used but there is a proper weak-link attached on the glider end.

A short while ago, I did launch a quick survey on the German forum to find out how many tow rope failure anyone had. The response: zero tow rope failures!

Precisely!

FAR 91.309 (the 80 - 200% rule) is for gliders whose POH didn't specify aero tow weak links (Think Schweizer). FAR 91.9 is for those which do. (Think any glider certificated under JAR-22/CS-22) If your manual specifies an aero tow weak link, 91.9 requires you to use it. 91.9 trumps 91.309 - that's the law.

The combination of 91.309 and 91.9 pretty much requires a weak links on each end of the rope with the one on the tow plane end 25% stronger than the one on the glider end. With two weak-links why would any sane person want to use a weak towrope?

We should use massively strong ropes and let the weak-links handle overloads. That way there should never, ever be a rope break. It's not what we do - and we pay the price.

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 load on the rope is relatively light and can be figured using the L/D of the glider.

The "L" is the lift required to make the glider fly, which equals the weight of the glider.

Dividing the glide ratio into the weight equals the drag, or load on the tow rope.

A gllder with an L/D of 30, weighing 900 pounds would have a load on the tow rope of only 30 pounds.

Since the glider is being towed uphill, the load is slightly more. (Towed by the space shuttle, straight up, the load would equal the weight of the glider, disregarding acceleration forces.)

Anyway, the load on the tow rope is small. This load can be simulated by pulling the glider by the rope by hand on a smooth surface.

Typically, we use a tow rope with a breaking strength of 1,200 pounds, (regulated by the rules - weight of the loaded glider) or a much stronger, more durable rope with a much higher breaking strength with complying weak links on both ends of the tow rope.

The weak links can be the mechanical types made by Tost. These are expensive and weigh enough to cause the tow rope to fly very low behind the descending towplane.

The weak links can also be made from smaller tow ropes that comply with the strength requirements. The rope manufacturer must be trusted to conform to the requirments.

Bottom line is weak links are an important, regulated safety requirement. They rarely are needed, but there are occasions when they can save lives.

Finally, pilots and line crews need to be vigilant about the condition of launch equipment, glider and towplane.

Tom Knauff

Previous posters have written
- "...plus a tiny bit extra due to the climb vector", and
- "Since the glider is being towed uphill, the load is slightly more."

Actually the load due to being pulled uphill is typically 10% of the
weight of the glider (thinking of a typical climb of 6kts at 60kts
airspeed).
This is significantly larger than the drag component, which is in the
range 2.5% to 2% for a decent performance glider (say L/D of 40
to 50), and maybe 3.5% for an old 2-seater (L/D of 28).

Pedantry?

I guess I used kinda vague terms like "tiny bit", and "slightly".

You did the math and put some actual numbers on it. ( Large compared to the
drag...small compared to the rope strength)

So if we say the tension on the rope is smooth tow is 12% ~ 15% of the
glider's weight, and the rope strength is 80% to 200% of the glider's
weight, we have a pretty good safety factor there.

Yet a rope that should break if things get "hairy"..

Cookie




At 15:24 13 November 2013, James Metcalfe wrote:
Previous posters have written
- "...plus a tiny bit extra due to the climb vector", and
- "Since the glider is being towed uphill, the load is slightly more."

Actually the load due to being pulled uphill is typically 10% of the
weight of the glider (thinking of a typical climb of 6kts at 60kts
airspeed).
This is significantly larger than the drag component, which is in the
range 2.5% to 2% for a decent performance glider (say L/D of 40
to 50), and maybe 3.5% for an old 2-seater (L/D of 28).

Pedantry?

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?