Wires and stresses

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After an incident at the World Championships – German team pilot Markus Ebenfeld overloaded and folded up his Icaro Z9 over the landing field upon performing a poorly conducted loop – the usual discussion on the flying wires of hang gliders maybe not being strong enough broke out once again.

 

Davis Straub writes: “…Third, Markus was flying with two year old the thin thin wires, 2mm”. Now, what exactly are thin thin side wires? Are you referring to 7x7mm 2mm cables (the more flexible ones), or do you mean 1x19mm 2mm cables?

The difference is in the steel portion. 1×19 have a higher steel portion, and therefore have a higher breaking strength. Depending a little on the type of steel and the manufacturers brand, 7x7mm 2mm cables hold around 330kg, 1×19 2mm hold about 420kg. That is manufacturers specs for swaged cables like we use them.

Cables ALWAYS break at the Nico press exit, and they break by over stressing the strains on their skin first, and after that the core strains go.

I have tested many such 1×19 samples, as to see how close the manufacturers specs come to “reality”. The strongest samples would reach 450kg, the weakest were still slightly above 400kg. This was from the same roll of wire, so one can conclude the differences came really all from the swaging quality. Two swages do not hold more than one swage. The cable will still break where it goes form double to single. The two swage solution is just easier to do without the cable sticking out of the swage which would be hurting your fingers while rigging and de-rigging.

 

Jonny Durand. Getting it up on a tip.
Jonny Durand. Getting it up on a tip.

All Moyes gliders are currently certified with 1×19 2mm cables, so technically speaking, if you are using other cables, even the bigger 2.5mm dimensions that is, you would be out of certification. (One of the pitfalls of certification: you can’t improve on your own safety, if you want to comply with certification concepts!)

I would agree that in older days cross bars were the structural limits of the gliders. I would not necessarily say that having a harder time to bring a glider to speed would make flying aerobatics any saver, because pilots would then just try to squeeze their loops out of a slower wing, yanking them around harder in return – means pulling more G’s on their maneuvers.

I would also have do disagree that the weak point nowadays are the cables. I know the specs of all structural parts, and I ensure you, as far as load testing goes, the next weak link are now your nice sleek slipstream uprights. We never managed to break a side cable on truck load testing.

Flying faster through hard turbulence is happening more these days, so the perception of loading the side cables on turbulence shocks makes us feel the side cables are more in danger now. However, this is not when the cables fail!

The shocks during xc flying are all well absorbed by the flexibility of the wing, since even the stiffest flexwing is still a lot more flexible on his leading edges than it is on the side cables.

What kills side cables is, when they come under additional shock load while being already loaded to or past their elastic limit. The typical example is aerobatics in rough conditions. The loop itself doesn’t hurt the glider, the rough conditions on their own don’t do it either. But the combination of both can do the job quite nicely.

You think modern hang gliders are to fragile against overloading? Then think of this: Everyone of you has likely sat in a Jumbo while flying through nasty turbulence and you might have watched the wing violently swinging up and down with the load – and it looks scary, The jumbo flies at more than 800kph in this moment, and the hits of the turbulence are very noticeable in the fuselage. But the Jumbo flies on and it makes you think that Jumbo construction is really solid.

Why do the wings not over load and break, given the massive shocks they have to absorb? The reason is, that while the shock hits the wing, the wing is not statically loaded close to its strength limit, not even close to its elastic limit. Or in other more technical words, the maximum wing deflection and the shock load on the wing are not in phase. If the Jumbo would have to pull some 3G’s (which would then deflect his wings up about 3-4m), as it would be in a stationary steep bank turn while hitting said turbulence, the wings would fold in a heart beat.

Now, what is that elastic limit (Hooke limit)?

If you look at steel as a cable material you find that while its terminal load may by as high as 400kg, at about 200-250kg it reaches the limit of elastic deformation. Means, if you load steel cables past this point, there will be residual stretch. That stretch alone would not be so bad, as such stretch on a 2.5m swaged cable is only about 2mm. But what is bad is that with more residual stretch you loose more and more the ability to absorb energy (shock load situations). Just as a climbing rope that has been shock loaded past its elastic limit can’t absorb your next fall so well – the rope hardens with use.

This is where the “old cable” issue comes into play. An old cables will not hold any less STATIC load than a new cable, but since it might – over its lifetime – have endured its fair share of residual stretch meanwhile, the total energy it can absorb upon a DYNAMIC COMBINED load situation is less now!

I don’t want to go into details about pre-damage, since everyone understands that cables with pre-damage will be worse than good ones.

Uprights vs. side cables

Comparable spans (for roughly comparable areas):

  • Moyes RS 3.5: 10.3 m (33.7 ft), aspect ratio: 7.7
  • Moyes S 3.5: 10 m (32.8 ft), aspect ratio: 7.5
  • Wills T2C 144: 9.85 m (32.3 ft), aspect ratio: 7.3
  • Icaro Z9 13.2: 10.05 m (33 ft), aspect ratio: 7.63
  • Aeros 13.5 GT: 10.7 m (35.1 ft), aspect ratio: 8.5

I am sure the T2 144 would do just fine with 2mm cables. With a span of only 9.85m its tiny compared to all other competition wings, so the bending moment that loads the side cables is also proportionally less.

Uprights versus side cables really is an interesting subject: Therefore I wonder how “much longer” it could possibly be before the Wills uprights would fail. Maybe Steve can help us out with his load test data.

Testing a brand new 21.5mm aluminum upright to 6Gs is one thing – loading an upright that has been in the glider for a season or two is another. Lately pilots look hard for signs of side cable overload around their thimbles and nicos. How about also spending a minute on scrutinizing your uprights from time to time.

The occasional pile-in or simply a hard run over the landing field with your glider on the shoulders can easily bend your upright out of column.

Than the Euler load (that is the compression force you need to apply for spontaneous buckling) dramatically reduces. From memory, I recall that with 1/2 inch out of column on a standard length upright, the Euler load reduces from 380kg to only 250kg – that’s a shocker, isn’t it?

I have flown 2mm 1×19 cables now on all the Litespeed models since I joined Moyes, that is 13 years now all up. Usually I change the side cable set once a season, but on occasion have also kept them for 2 seasons. I did my share of wire slacks and wing overs, but I never even managed to over stretch a cable set (as could be seen on elongated thimbles).

On the other hand I have broken an upright in the air in turbulence at Monte Cucco 1998. A Slovenian carbon proto upright, 20mm thin, that was. Fortunately it was the right side, with the VG rope inside, so it would stay together. I managed to hold it straight with my hand while hovering down from 1200m in the valley wind

This is not to encourage pilots to be careless about their side cables, but there also is no reason to fret. The majority of comp pilots today are on 2mm 1×19 cables. I see no reason why they should be in danger all of a sudden, just because there are also 2.4mm 1×19 cables available now.

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