Aerotow ropes: short or long, breakable or unbreakable?

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

as different to many above statements,
we in central Europe are mostly connecting weak link of tow rope to towing plane release end, not to the glider end.
Two good reasons:
- there is usually practised towing plane landings with tow rope still connected due to economical and practical reasons. In this case, the rope end with weak link is not get dirty by contact with soft ground and not subjected by direct hit to possibly hard surface, then damage probability of weak link assy is lowered. And yes, we are mostly using mounting climber type braided nylon ropes for our aerotows (dia 7-12mm), on which the damage usually starts by weared rope braid on glider end (mostly from hit to ground during landings), which indicates that there should be some corrective action taken (rope shortening during saison, replacing after saison).
- when towing plane hits some obstruction via towing rope during landing, there is safety weak link very useful to avoid damage to towing plane and pilot.
- exception: tow planes with retractable rope device by electric drum winch (still not so common, but slowly grows) are equipped with weak link on glider end. But they do not land with rope extracted, then above two points do not apply to them.

On Friday, November 15, 2013 2:37:32 PM UTC-5, StaPo wrote:

we in central Europe

How long are your tow ropes?

At 16:02 15 November 2013, Steve Leonard wrote:
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.

No confusion in my mind ... when I mentioned doubling I was just
pointing out that the difference was substantial. I could have been
less conservative and said 'tripled' or 'quadrupled'!

If you look at a typical graph of lift coefficient vs AoA you will see
that to double the lift you can expect to much more than double the
AoA. (Although the curve is substantially linear for most of the AoA
range, it doesn't pass through the origin). Not that I said anything
at all about doubling the lift!
J.

Dne pátek, 15. listopadu 2013 23:00:04 UTC+1 son_of_flubber napsal(a):
On Friday, November 15, 2013 2:37:32 PM UTC-5, StaPo wrote:



we in central Europe



How long are your tow ropes?

40m standard length, 15m special version for off-field aerotows (quite rare nowadays). I remember that 31 years ago (when I was learning to fly) there was rule for 30m ropes without weak link, but I do not remember when it was changed to current 40m. Many years ago...

On Friday, November 15, 2013 6:53:35 PM UTC-5, StaPo wrote:

How long are your tow ropes?

40m standard length, 15m special version for off-field aerotows (quite rare nowadays). I remember that 31 years ago (when I was learning to fly) there was rule for 30m ropes without weak link, but I do not remember when it was changed to current 40m. Many years ago...

For our USA friends... 40m = 131 feet. The usual North American tow rope is 200 feet (say 61 meters). That seems like a big difference (some of the physics is not linear).

It seems odd that the experts inside a country can agree on tow rope length, while the experts in different countries see things so differently. I had (naively) thought that tow rope length would be a matter decided by physics and engineering, but it appears to be decided by cultural perspective and tradition.

What other operational parameters and procedures in soaring are matters of culture and tradition?

Would my Polish SZD glider fly better behind a 131 foot tow rope?

Regular use of shorter tow ropes probably prepares pilots better for tows in turbulent air. This is how I understand this tradition here in C. Europe. Also, I think it would be harder to tow on a long rope while tightly (perhaps up to 45 deg bank) circling in a thermal, which is practiced by some tow pilots here. Sure such tows are not boring.

One case is not a proof of anything, but this summer I had seen whole bunch of fairly early (about 20-30h) solo students dealing ok with tight circling tows in strong (turbulence) thermals, while a MUCH more experienced foreign pilot had a rope break. I would guess being (not)used to following a bit shorter rope was a factor, but it is just my guess. I do not question this pilot skills, just the issue of being familiar with this setup.

So I would guess the SZDs do like to fly on a shorter rope :-)

On Saturday, November 16, 2013 4:27:57 AM UTC+1, son_of_flubber wrote:
On Friday, November 15, 2013 6:53:35 PM UTC-5, StaPo wrote:



How long are your tow ropes?



40m standard length, 15m special version for off-field aerotows (quite rare nowadays). I remember that 31 years ago (when I was learning to fly) there was rule for 30m ropes without weak link, but I do not remember when it was changed to current 40m. Many years ago...



For our USA friends... 40m = 131 feet. The usual North American tow rope is 200 feet (say 61 meters). That seems like a big difference (some of the physics is not linear).



It seems odd that the experts inside a country can agree on tow rope length, while the experts in different countries see things so differently. I had (naively) thought that tow rope length would be a matter decided by physics and engineering, but it appears to be decided by cultural perspective and tradition.



What other operational parameters and procedures in soaring are matters of culture and tradition?



Would my Polish SZD glider fly better behind a 131 foot tow rope?

On Saturday, November 16, 2013 12:42:24 AM UTC+1, James Metcalfe wrote:
At 16:02 15 November 2013, Steve Leonard wrote:

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.



No confusion in my mind ... when I mentioned doubling I was just

pointing out that the difference was substantial. I could have been

less conservative and said 'tripled' or 'quadrupled'!



If you look at a typical graph of lift coefficient vs AoA you will see

that to double the lift you can expect to much more than double the

AoA. (Although the curve is substantially linear for most of the AoA

range, it doesn't pass through the origin). Not that I said anything

at all about doubling the lift!

J.

James, Steve

not wanting to hi-jack this thread but let me re-state what I meant to say more precisely: the AoA during the winch launch is no where near the the stall AoA. We did fly several winch launches with a side-string attached on the outside of the canopy. Since the line pull forces during the steepest part of the climb are nowhere near the forces shortly before the release and since we are using Spectra, which weighs a few pounds for the 4,500ft we can lay out, the loading on the glider is well below 2g. So the AoA should be that of a glider loaded at 1.5 - 2g.

Uli

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.