On Friday, October 19, 2018 at 7:45:29 PM UTC-7, Steve Koerner wrote:
On Friday, October 19, 2018 at 4:12:53 PM UTC-7, 2G wrote:
On Friday, October 19, 2018 at 8:46:48 AM UTC-7, Bret Hess wrote:
Direct observation of a simultaneous break on both ends:
Three years ago I was flying in the front seat of a Grob 103 (nose tow hook) with an instructor for a spring checkout. He wanted me to try a slack rope recovery method that was very different from normal (and later we agreed was not the right approach). When I did the maneuver, we had a spectacular rope break...I could see the rope break simultaneously at the tow plane end and the glider end and fall away.
This has an aerodynamically interesting ending.
The instructor could hear the broken rope segment on our end making noise against the fuselage. We had plenty of height, so we let the tow plane land, and moved over the runway to drop our short length of rope so the tow ring, etc could be recovered easily. When I pulled the nose tow release to drop the rope, a second or two later I noticed something at the open canopy window to my left. There was a foot or less of rope flapping around just inside the cockpit, so I pulled gently on it, and the entire three feet or so of the broken rope segment with the tow ring on the end came in through the window. It was trapped in some low pressure region against the fuselage which was connected to the flow through the window!
I agree with the wave/propagation idea.
Before the rope breaks, it is stretched a considerable percentage, storing energy in the fibers. When the rope breaks at some point, this energy is released in the form of a travelling wave from the point of breakage in both directions with the long portion having the greatest energy. Upon reaching either end, it encounters an impedance discontinuity (the tow release) and is reflected backwards along the rope. This reflection can add to the still oncoming wave, amplifying its magnitude. If this exceeds the ultimate strength of the rope at this point, it will, too, break.
This travelling wave phenomena also occurs in electronics, and is exploited in antennas.
Tom
To invoke any theory involving waves there needs to be a reason to think that there is a wave created of some sort. Waves necessitate two forms of energy storage with oscillation between them. I'm not seeing how that works in the towrope. It seems like the energy stored in the elastic rope will get dissipated as heat as the rope progressively shortens after breaking. I can visualize that at the breaking impulse maybe a wee bit of energy would get spit out as transverse displacement wave but that would be second order and not significant; the main happening is longitudinal. What wave are we talking about?
First, before breaking, the rope is stretched considerably beyond its resting length - this undeniably stores energy in the rope (the same as a spring). Second, when the rope breaks this stored energy is released by accelerating the end of the rope, like the cracking of a whip or an earthquake starting a tsunami. This energy travels the rope until it reaches the terminated end, where it is reflected in the opposite direction.
Here is a video of a steel rope breaking; single-frame from 33 sec to see the wave-like action:
https://www.youtube.com/watch?v=W6gZawDnthI
Tom