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Static Wing Load Testing of Old Wooden Glider Wings
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LongJourney
Dec 18, 2016, 8:51:39 PM12/18/16
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I'm restoring a 1963 Std Austria and trying to decide the risks and merits of load testing the wings. I can't find anything definitive on the web, at least nothing that doesn't get contradicted by some other source.
My concern is that load testing may damage the wings in an undetected way. OTOH, I don't want to find out that the spar already has some undetected damage by spotting one wing departing the aircraft while I'm flying it.
Opinions are welcome, but if anyone knows of an AC or other documentation that may shed light on this topic, I would greatly appreciate that information.
Thanks!
Jeff
My concern is that load testing may damage the wings in an undetected way. OTOH, I don't want to find out that the spar already has some undetected damage by spotting one wing departing the aircraft while I'm flying it.
Opinions are welcome, but if anyone knows of an AC or other documentation that may shed light on this topic, I would greatly appreciate that information.
Thanks!
Jeff
jamesrc...@gmail.com
Dec 18, 2016, 9:17:21 PM12/18/16
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Scott Williams
Dec 18, 2016, 9:24:51 PM12/18/16
to
I do not think there is any way to perform a meaningful non destructive load test on potentially airworthy wings.
Several years ago I refinished an shk-1, and used tap testing, harmonic frequency evaluation, I attempted to camera the interior, carefully examining the exterior surfaces. I and My I.A. concluded that the wings were airworthy and I flew it including winch launching.
Best of luck,
Scott
Several years ago I refinished an shk-1, and used tap testing, harmonic frequency evaluation, I attempted to camera the interior, carefully examining the exterior surfaces. I and My I.A. concluded that the wings were airworthy and I flew it including winch launching.
Best of luck,
Scott
nealpf...@sbcglobal.net
Dec 18, 2016, 9:52:02 PM12/18/16
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Jeff,
This past summer, the Vintage Sailplane Association published an English translation of the German book by Hans Jacobs, Werkstattpraxis (Workshop Practice). It is available directly from the VSA or Cumulus Soaring in the US and Eqip in Europe. This book details the construction, inspection, and maintenance of wood gliders.
There are also a number of articles in recent quarterly issues of the Bungee Cord by the VSA and the VGC News published by the Vintage Glider Club that describe the various glues used in the construction of wooden German gliders.
I would strongly suggest getting a copy of the Workshop Practice book and joining one or both of the vintage organizations. You will make valuable contacts with either group.
..... Neal
On Sunday, December 18, 2016 at 7:51:39 PM UTC-6, LongJourney wrote:
This past summer, the Vintage Sailplane Association published an English translation of the German book by Hans Jacobs, Werkstattpraxis (Workshop Practice). It is available directly from the VSA or Cumulus Soaring in the US and Eqip in Europe. This book details the construction, inspection, and maintenance of wood gliders.
There are also a number of articles in recent quarterly issues of the Bungee Cord by the VSA and the VGC News published by the Vintage Glider Club that describe the various glues used in the construction of wooden German gliders.
I would strongly suggest getting a copy of the Workshop Practice book and joining one or both of the vintage organizations. You will make valuable contacts with either group.
..... Neal
On Sunday, December 18, 2016 at 7:51:39 PM UTC-6, LongJourney wrote:
LongJourney
Dec 19, 2016, 8:30:25 AM12/19/16
to
Neal, I joined the VSA this past summer, but haven't had a chance to travel to any of the meets yet. I hope to this coming year. I should have put Workshop Practices on my Christmas list, but failed to do so. I'll order a copy in the next couple of months.
Thanks, again, for the responses.
Jeff
johnsin...@yahoo.com
Dec 19, 2016, 9:26:39 AM12/19/16
to
Jeff,
I have static loaded several wings, over the years. It is something not to be taken lightly. First off you need a test stand that is high enough to hold the inverted wing, under load allowing for something like 2 foot wing bending and strong enough to hold the wing and a good 2000 pounds of sand with provisions for the drag spar(to prevent wing twisting). Then you need the weight, 90# concrete bags are nice, but you'll have $300 invested. Loading is done simotaniously with two people adding a bag to each wing, watching for any side leaning. As the last bags are loaded,the wing may bend a good foot with each new bag! If one bag should slide off, the other wing will slam down, resulting in more load shifting................you get the picture? One can destroy a wing, just seeing if it is strong enough! The wings I have tested had extensive repairs. If your wing has not been damaged and repaired, I'd recommend a thorough tap test and frequency testing, then get a fresh re-pack on your parachute and go fly! Restricting G load and VME is always a good idea with old wooden birds.
Cheers,
JJ
I have static loaded several wings, over the years. It is something not to be taken lightly. First off you need a test stand that is high enough to hold the inverted wing, under load allowing for something like 2 foot wing bending and strong enough to hold the wing and a good 2000 pounds of sand with provisions for the drag spar(to prevent wing twisting). Then you need the weight, 90# concrete bags are nice, but you'll have $300 invested. Loading is done simotaniously with two people adding a bag to each wing, watching for any side leaning. As the last bags are loaded,the wing may bend a good foot with each new bag! If one bag should slide off, the other wing will slam down, resulting in more load shifting................you get the picture? One can destroy a wing, just seeing if it is strong enough! The wings I have tested had extensive repairs. If your wing has not been damaged and repaired, I'd recommend a thorough tap test and frequency testing, then get a fresh re-pack on your parachute and go fly! Restricting G load and VME is always a good idea with old wooden birds.
Cheers,
JJ
Bob Kuykendall
Dec 19, 2016, 11:27:22 AM12/19/16
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On Sunday, December 18, 2016 at 6:24:51 PM UTC-8, Scott Williams wrote:
> I do not think there is any way to perform a meaningful non destructive load test on potentially airworthy wings.
When we static tested the first set of HP-24 wings, we had two major objectives:
> I do not think there is any way to perform a meaningful non destructive load test on potentially airworthy wings.
* Establish that the wings would safely support at least utility-category limit load.
* Establish that our calculations of wing stiffness, and their underlying assumptions, were valid.
To that end, we did a static test to 4.4g while measuring wing deflection. The test was observed by three experienced aeronautic engineers, and none of them thought it out of the ordinary. No anomalies were noted, and the deflection was within about 0.5" of that predicted. Those wings now have about 600 hours of flight time on them.
Thanks, Bob K.
Scott Williams
Dec 19, 2016, 12:42:34 PM12/19/16
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On Monday, December 19, 2016 at 10:27:22 AM UTC-6, Bob Kuykendall wrote:
> On Sunday, December 18, 2016 at 6:24:51 PM UTC-8, Scott Williams wrote:
>
> > I do not think there is any way to perform a meaningful non destructive load test on potentially airworthy wings.
I posted an incomplete opinion, I stand corrected.
> On Sunday, December 18, 2016 at 6:24:51 PM UTC-8, Scott Williams wrote:
>
> > I do not think there is any way to perform a meaningful non destructive load test on potentially airworthy wings.
I do however doubt that an average 'Joe" in his garage or driveway could duplicate a professional calculated evaluation of 1963 Austria wings and end up with useful results and/or useful wings at the end of the exercise. However, with the correct application of money and expertise, load testing of the wings in question is more than easily done.
With respect, Bob would you entertain offering a reasonable cost/expense estimate of duplicating your exercise? including the engineer expertise? Please consider that the original poster seemed to indicate no prior knowledge, test fixture, data, or test area/facility.
I think my humble opinion was simply that such a procedure was beyond the average skill ability of the typical sailplane Do It Yourselfer, or even A.I.
The process of actual load testing of aerodynamic surfaces is of course a routine and usually mundane exercise for design development and production.
Thank you in advance,
Respectfully,
Scott W.
bob.kuy...@elastica.co
Dec 19, 2016, 3:41:47 PM12/19/16
to
Thanks for asking! Some semi-random commentary to put the issue in context:
* First off, you and JJ are probably right that, assuming a rigorous inspection shows the wing to be in good condition, a static test is probably not the best use of resources.
* You are also probably right that this isn’t something that the “Average Joe” is going to undertake with full confidence of success. But that’s what I find so completely enthralling about messing about with amateur-built and vintage gliders: The people who get into it are pretty uniformly extraordinarily resourceful.
* That said, I am always alert for the words “always” and “never,” because they often offer an opportunity to contribute an enlightening (or at least valid) counterexample.
* The HP-24 static test is described, with photos, on this old web page: http://www.hpaircraft.com/hp-24/update_10_dec_11.htm
* I insisted that we do some sort of static test on the first HP-24 wings. Me being a relapsed Liberal Arts dropout, I wanted some assurance that we had enough carbon in enough of the right places to make the wings safe to fly. Jim Marske had reviewed the basic main spar design, and we had done coupon tests on the lift pin anchors, so I had high confidence we were in the ballpark. But I wanted a bit more assurance and insisted on a static test to at least check the deflection. I originally wanted to go to 5.3g, but settled on 4.4g as a reasonable compromise between that and no test at all.
* My primary motivation was the cautionary tale of the American Spirit kit glider, of which two or three had demonstrated primary structural failures below (and on one case well below) the published limit load.
* All told, the static test cost us about $3k, but most of that was travel expenses to bring the wings south from Seattle and the crew north from the SF Bay Area. We borrowed a hangar and 2300 lbs of lead shot from a friend in central Oregon, and the test stand, built as JJ describes, cost about $300 to put together out of plywood and lumber and a couple of welded bits.
* We basically self-insured the test by promising to make a set of replacement wings in case the ones we tested broke. Assuming that risk was probably the most expensive part of the adventure.
* The engineers were all volunteers. When you set up to push things to the limits, they pretty much come out of the woodwork to kibitz and see if things to boom. We started out with just Steve Smith, but engineer and serial builder Bruce Patton chipped in to help build the test stand in his garage, and designers Ken Kruger and Dick VanGrunsven came by to watch things bend.
* My original proposed load distribution was basically a Schrenck’s (sp?) approximation that I whipped up in an hour with Excel. That’s basically where you take your area distribution and an elliptical lift distribution and split the difference between them. For our wings Steve Smith added a bit of load at the outboard end to approximate a winglet plus a deflected flaperon.
Thanks, Bob K.
https://www.facebook.com/HP-24-Sailplane-Project-200931354951/
* First off, you and JJ are probably right that, assuming a rigorous inspection shows the wing to be in good condition, a static test is probably not the best use of resources.
* You are also probably right that this isn’t something that the “Average Joe” is going to undertake with full confidence of success. But that’s what I find so completely enthralling about messing about with amateur-built and vintage gliders: The people who get into it are pretty uniformly extraordinarily resourceful.
* That said, I am always alert for the words “always” and “never,” because they often offer an opportunity to contribute an enlightening (or at least valid) counterexample.
* The HP-24 static test is described, with photos, on this old web page: http://www.hpaircraft.com/hp-24/update_10_dec_11.htm
* I insisted that we do some sort of static test on the first HP-24 wings. Me being a relapsed Liberal Arts dropout, I wanted some assurance that we had enough carbon in enough of the right places to make the wings safe to fly. Jim Marske had reviewed the basic main spar design, and we had done coupon tests on the lift pin anchors, so I had high confidence we were in the ballpark. But I wanted a bit more assurance and insisted on a static test to at least check the deflection. I originally wanted to go to 5.3g, but settled on 4.4g as a reasonable compromise between that and no test at all.
* My primary motivation was the cautionary tale of the American Spirit kit glider, of which two or three had demonstrated primary structural failures below (and on one case well below) the published limit load.
* All told, the static test cost us about $3k, but most of that was travel expenses to bring the wings south from Seattle and the crew north from the SF Bay Area. We borrowed a hangar and 2300 lbs of lead shot from a friend in central Oregon, and the test stand, built as JJ describes, cost about $300 to put together out of plywood and lumber and a couple of welded bits.
* We basically self-insured the test by promising to make a set of replacement wings in case the ones we tested broke. Assuming that risk was probably the most expensive part of the adventure.
* The engineers were all volunteers. When you set up to push things to the limits, they pretty much come out of the woodwork to kibitz and see if things to boom. We started out with just Steve Smith, but engineer and serial builder Bruce Patton chipped in to help build the test stand in his garage, and designers Ken Kruger and Dick VanGrunsven came by to watch things bend.
* My original proposed load distribution was basically a Schrenck’s (sp?) approximation that I whipped up in an hour with Excel. That’s basically where you take your area distribution and an elliptical lift distribution and split the difference between them. For our wings Steve Smith added a bit of load at the outboard end to approximate a winglet plus a deflected flaperon.
Thanks, Bob K.
https://www.facebook.com/HP-24-Sailplane-Project-200931354951/
Tango Eight
Dec 19, 2016, 6:23:31 PM12/19/16
to
On Monday, December 19, 2016 at 3:41:47 PM UTC-5, bob.kuy...@elastica.co wrote:
> Thanks for asking!
That was a really interesting read.
Thanks Bob.
-Evan Ludeman / T8
> Thanks for asking!
That was a really interesting read.
Thanks Bob.
-Evan Ludeman / T8
Bob Gibbons
Dec 19, 2016, 9:04:54 PM12/19/16
to
Jeff, Austrailian Alan Patching is an world renowed expert on the
effect of aging on both wood and fiberglass glider structures.
Alan has been performing long term fatique testing for fiberglass
gliders since the early 70's. My recollection is that they had not
discovered any finite lifetime limits to date for fiberglass
construction.
Alan received the 2006 Lilienthal Gliding Medal for, among many other
contributions, his long study of lifetime limits in FGRP structures.
If you have access to Technical Soaring you can find a number of
Alan's articles. You may find something useful there on wood
structures.
[BTW, we do have access at TSA to past issues of Technical Soaring.]
=================
Volume I, Number 2, October, 1971
C.A. Patching, Establishing the Structure Integrity of Aging Gliders
[Construction], page 17
Volume VI, Number 3, March, 1981
JG.P. Esson and C.A. Patching, Fatigue Life Considerations for Gliders
Operated in Australia, page 10
Volume 15, Number 4, October, 1991
C.A. Patching and L.A. Wood, Fatigue Testing of A GFRP Glider
[Construction], page 100
Volume 22, Number 1, January, 1998
C.A. Patching and L.A. Wood, Further Fatigue Testing of a Glass Fiber
Reinforced Plastic Glider Wing [Aerodynamics\Testing], page 11
=============================
effect of aging on both wood and fiberglass glider structures.
Alan has been performing long term fatique testing for fiberglass
gliders since the early 70's. My recollection is that they had not
discovered any finite lifetime limits to date for fiberglass
construction.
Alan received the 2006 Lilienthal Gliding Medal for, among many other
contributions, his long study of lifetime limits in FGRP structures.
If you have access to Technical Soaring you can find a number of
Alan's articles. You may find something useful there on wood
structures.
[BTW, we do have access at TSA to past issues of Technical Soaring.]
=================
Volume I, Number 2, October, 1971
C.A. Patching, Establishing the Structure Integrity of Aging Gliders
[Construction], page 17
Volume VI, Number 3, March, 1981
JG.P. Esson and C.A. Patching, Fatigue Life Considerations for Gliders
Operated in Australia, page 10
Volume 15, Number 4, October, 1991
C.A. Patching and L.A. Wood, Fatigue Testing of A GFRP Glider
[Construction], page 100
Volume 22, Number 1, January, 1998
C.A. Patching and L.A. Wood, Further Fatigue Testing of a Glass Fiber
Reinforced Plastic Glider Wing [Aerodynamics\Testing], page 11
=============================
LongJourney
Dec 20, 2016, 9:05:37 AM12/20/16
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On Sunday, December 18, 2016 at 7:51:39 PM UTC-6, LongJourney wrote:
Jeff
Scott Williams
Dec 20, 2016, 10:49:22 AM12/20/16
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On Tuesday, December 20, 2016
On one last note you might find useful,
The wing construction of the Austria including my SHK-1 did not use a conventional spar design. they employ a typical main spar to about two thirds out from the root, transitioning loads to multiple 'skin' longerons with rib structures almost every 4-6 inches resulting in a structure which may not respond to Bob K's "ladder" device protecting the wing from point loads. I.e. No doubt a wing blueprint would reveal a tactic to safely load the wing.
Good luck,
Scott, N8915, formerly restored N1521
On one last note you might find useful,
The wing construction of the Austria including my SHK-1 did not use a conventional spar design. they employ a typical main spar to about two thirds out from the root, transitioning loads to multiple 'skin' longerons with rib structures almost every 4-6 inches resulting in a structure which may not respond to Bob K's "ladder" device protecting the wing from point loads. I.e. No doubt a wing blueprint would reveal a tactic to safely load the wing.
Good luck,
Scott, N8915, formerly restored N1521
LongJourney
Dec 21, 2016, 9:29:30 PM12/21/16
to
Frank Whiteley
Dec 22, 2016, 1:02:49 AM12/22/16
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Frank Whiteley
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