Sudden storm causing severe damage to multiple gliders at Narromine

[David S]

For those that don't follow r/gliding on Reddit.

https://www.reddit.com/r/Gliding/comments/1p73ihj/sudden_storm_causing_severe_damage_to_multiple/

   ...david

[n5...@comcast.net]

Looking at some of the broken tiedown straps reminds me to change out my tiedown straps / rope every 3 years or so. They do weather and weaken from UV damage. They may look OK, but better safe than sorry.

[Steven Leonard]

If using rope or straps, change every year.  Maybe more often.  The sun is VERY destructive to plastics.

[Charles Mampe]

We use chains as primaries on the wings and tow hook. We use hollowbraid PE (old tow ropes) as secondaries.

In general, we're good, but, years ago we were reminded to check chains at ground level when a big storm hit and broke chains at ground level where they had rusted enough to reduce strength.

Also, I harp on people to get chains tight, as in, NO wing rock when chained. If you get gusty winds and chains have some slack, you get higher loads on the chain making breakage more likely. Check out max load vs. shock load, max is higher. This is sorta like a planes max G vs maneuvering load.

[Eric Greenwell]

I've often wondered about the loads on tiedown straps, and how they are affected by wind direction, spoilers out, wing covers, and flap position. Years ago, I asked Prof. Fred Thomas, author of "Fundamentals of Sailplane Design", if there were any studies. "Not that I know of," he replied, "but it'd be a great thesis subject for a post-grad!". So, anyone know of studies of the forces?

In practice, I use 1" nylon straps on my 850 lb, 18M ASH26E with the cam style (not ratchet) buckle. These are typically rated at 400 lbs working load, and 1200 lbs breaking load. I park it on the main and tail wheel with flaps in negative setting, often with wing covers. No problems in 50 years, but I've never had winds that exceeded 50 kt gusts. With a nominal stall speed in negative flap of about 45 kts, I think it'd be fine with winds up to  60 kts, where each strap would have about 400 lbs tension in the worst case.(facing directly into the wind).

Eric

[Matthew Scutter]

The weather stations near Narromine indicated 60kt gusts, but that is probably the peak reading they support. The forecast (non-storm) gusts for the day were already 40kts, so it was already a dangerous day to have your glider outside.

I was taught, assuming you only have your carry-on tiedown kit;

Flaps negative, elevator up (less likely to fly)

Airbrakes closed (won't fill with rain)

Tail first quartering into prevailing wind, upwind wing tied down (no rudder flapping, no lifting surface pointed at the storm)

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[Ian Molesworth]

How old are your straps? 

After a year of UV exposure they may well be down to 50% of their load capacity. If they are a few years old ...... 

[Ian Molesworth]

UV Exposure and straps

[Eric Greenwell]

Specifically: what is a year of UV exposure? Summer, fall, winter, spring? Is the exposure to straps laid flat on the ground, vertical straps like ones used on tied down aircraft? Also, elevation would make a major difference.