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An aerodynamic experience



 
 
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  #1  
Old March 26th 17, 05:39 AM posted to rec.aviation.soaring
Bret Hess
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Default An aerodynamic experience

Of course all of our flights are aerodynamic experiences, but we can usually make some sense of what we experience. Here's one that was a mystery from the beginning, and I’m still surprised by when I think about it.

A couple of 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. 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 tow release to drop the rope, a second or two later (don’t really know how long) 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 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.

That defied all my expectations. Why wasn’t the rope with ring immediately swept away? How did the end find its way to the window? I can start thinking about it with some aerodynamics principles, but I’d rather hear ideas from others.

Bret
  #2  
Old March 26th 17, 06:58 AM posted to rec.aviation.soaring
bumper[_4_]
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Default An aerodynamic experience

On Saturday, March 25, 2017 at 9:39:24 PM UTC-7, Bret Hess wrote: but I’d rather hear ideas from others.

Bret


Bret,

What was the rope made of?

Last year, after about 7 years of making the MKIV "high tech" yaw string, I made prototypes for what I thought would be the MKV. I am happy with the MKIV, but always looking to improve things. In this case, the advantage was eliminating the labor intensive "sandwich" construction of the MKIV, the V would be plastic welded to the base instead, as it used a slick round dacron cord rather than the more traditional yarn of he MKIV. I was unable to flight test the MKV due to a tractor accident, so sent a flight test sample to a friend as his MKIV was worn to a nub.

He sent back a short video showing the disappointing test results. The new yaw string wagged wildly like a happy dog's tail in about an 80 degree arc for 5 or 6 seconds, then froze to the canopy in a random position for maybe 3 or 4 seconds. FAIL!

My assumption is due to the "polished" smooth surface of the thin cord, there was minimal damping as there would be with "draggier" yarn. The cord set up a sort of positive feedback flutter while building static electricity that, when strong enough, was causing the freeze mode. Then when the static bled off, the cycle would repeat, over and over.

Wonder if that short length of rope would be doing something like that?

bumper
zz Minden
MKIV & QV



  #3  
Old March 26th 17, 07:23 AM posted to rec.aviation.soaring
Bret Hess
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Posts: 121
Default An aerodynamic experience

Yes, I've heard of your (in)famous yaw strings. That's good physics you had going on there with static electricity and cycling!

It was made of the normal yellow polypropylene. I wish I had the foresight to look closely at where the rest of the rope was when I was pulling it in.. My vague impression was that it was aft of the window, but the whole experience was a bit dreamlike as I pulled it in because I was astonished. The ring or rope didn't make noise on the canopy that I remember.
  #4  
Old March 26th 17, 12:03 PM posted to rec.aviation.soaring
Martin Gregorie[_5_]
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Default An aerodynamic experience

On Sat, 25 Mar 2017 23:23:39 -0700, Bret Hess wrote:

Yes, I've heard of your (in)famous yaw strings. That's good physics you
had going on there with static electricity and cycling!

It was made of the normal yellow polypropylene. I wish I had the
foresight to look closely at where the rest of the rope was when I was
pulling it in. My vague impression was that it was aft of the window,
but the whole experience was a bit dreamlike as I pulled it in because I
was astonished. The ring or rope didn't make noise on the canopy that I
remember.


IIRC from a lecture by Prof. Boermans that I attended, there's a fairly
pronounced flow UP in front of the wing root on most gliders and there's
often a separation bubble on top of the root as well. So, was the loose
flapping end of the tow rope above the wing rather than under it as you
seem to have assumed? If so, its likely that the ring end followed it up
when you released it.

As to how the end got inside: is the airflow through the clear view
inward or outward in a G.103? I've flown them a fair bit, but never with
that panel open in flight.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |
  #5  
Old March 26th 17, 09:33 PM posted to rec.aviation.soaring
son_of_flubber
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Posts: 1,550
Default An aerodynamic experience

On Sunday, March 26, 2017 at 12:39:24 AM UTC-4, Bret Hess wrote:

... 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.


So what did you do to break the tow rope when you were just trying to take out the slack?
 




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