Aerotow ropes: short or long, breakable or unbreakable?

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

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

I know of at least one Ka-8 breaking up in mid-air after an aerotow upset at an altitude sufficient for the tug to pull out of the dive. The weak link at the tug side was the black one (900kg), put there to protect the tug in case of catching an obstacle with the rope while coming in to land. The weak link broke too late to save the Ka-8. So yes, too strong towing ropes do kill people.

And I personnaly experienced three real rope breaks, 2 at height during instruction flights (broken weak link when taking out the slack of the rope after a slightly botched exercise), one at 150 ft resulting in an outlanding (that rope was heavily frayed inside the retracting system and broke without real cause).

So my advice is: strong rope, correct strength of weak link at sailplane end, and check frequently.

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?

I would like to think that in such a situation, I would have pulled the
release before the situation got that bad. Maybe not always as easy as it
seems?

So I agree with the notion of a rope on the strong side, but within specs
for breaking before something more important breaks.

I guess what really worries me, is not so much the actual strength of a
"new" rope, but how quickly that degrades due to use, wear, exposure, and
actual damage.


Cookie






At 14:23 13 November 2013, wrote:
I know of at least one Ka-8 breaking up in mid-air after an aerotow upset
a=
t an altitude sufficient for the tug to pull out of the dive. The weak
link=
at the tug side was the black one (900kg), put there to protect the tug
in=
case of catching an obstacle with the rope while coming in to land. The
we=
ak link broke too late to save the Ka-8. So yes, too strong towing ropes
do=
kill people.=20

And I personnaly experienced three real rope breaks, 2 at height during
ins=
truction flights (broken weak link when taking out the slack of the rope
af=
ter a slightly botched exercise), one at 150 ft resulting in an
outlanding
=
(that rope was heavily frayed inside the retracting system and broke
withou=
t real cause).

So my advice is: strong rope, correct strength of weak link at sailplane
en=
d, and check frequently.

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 |

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 |

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 |