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Kevin Bonds

Joined: 09 Jan 2006
Posts: 171
Location: Nashville, Tn

Posted: Fri May 11, 2007 8:54 pm Post subject: Recent crashes

This is total speculation on my part, I may be wrong but, didn't all of

these accidents happen near the airport (in the circuit). Could all of these

planes have been using/deploying flaps at the time of the failure? The one

in the UK could have been making a "short field" take off with some amount

of flaps. The others seem to be on approach as best I can tell/remember. The

XL is the first from ZAC to use flaps, but the wing spar and rear attach

configuration hasn't changed. Again total speculation but these seem to be

common threads. 



How could so much twisting happen without the rear attachment coming

disconnected? The question is was the rear attachment the first point to

fail? If so, maybe it should be beefed up or maybe we should consider making

changes to the speed at which flaps can be deployed? 



As far as the load testing goes, could there be an aerodynamic affect

(possibly brought on by flap deployment) that could not be accounted for

with sandbags? If I were flying right now I would be extra careful about

this until it is figured out. I don't care what some of you guys say; I

would hope that even abrupt moves would not result in such catastrophic

failures. I would expect damage yes, bent wings etc. I've seen bent wings on

certified planes caught in very violent turbulence. I once landed at an

airport seconds after a Cessna that had fallen from 10,000ft to 5000ft in an

instant, then again down to 3000 a few seconds later, with the very

experienced pilot suddenly having no control at all. My friend and I had

come in from the other direction at about 3000ft. Everyone on the plane was

white as a ghost and in tears. The plane had noticeably bent and damaged

wings. It was probably totaled--but they stayed on. 



As far as the NTSB reports, they don't sound very concise to me. Through all

of the official mumbo jumbo speak; I get the impression that the

investigations are done hastily and not a lot is quantified.   



As they say "Asbestos suit on". 



Kevin Bonds



Nashville TN

601XL Plans building.

http://home.comcast.net/~kevinbonds



 



do not archive DO NOT ARCHIVE 



 

--

_________________
KevinBonds
Nashville, TN
Plans-building Zenith CH601XL w/Corvair Power
http://home.comcast.net/~kevinbonds

Kevin Bonds

Joined: 09 Jan 2006
Posts: 171
Location: Nashville, Tn

Posted: Fri May 11, 2007 9:54 pm Post subject: Recent crashes

I read CH's letter after the Cali crash. He states that hundreds of these

designs have been flying--some for more than 20 years. This is true to an

extent. I have the utmost respect for him, but my concern is with the XL

which I do not think has been around that long, and I'm not sure how many

examples of it are flying. The success of his designs, certainly speaks well

of his skills as a designer but doesn't guarantee that some gremlin can't

exist in one of his newest designs. 



Look, I'm not trying to find fault, I'm just trying to be diligent and

consider the worst case scenario in my planning. So worst case scenario;

there is a design flaw or a fault in a published number. How do we rule out

that possibility? In the event we can't, what changes do we make? 



Does the UL in the UK have flaps? Have there been any cases of damaged wings

during abrupt maneuvers that didn't come off? How abrupt were those

maneuvers? Or is it like coming to the edge of a cliff there is either no

damage or rapid, irreversible advance toward the catastrophic. Can we do

something to at least slow it down long enough so that someone might live to

tell about it? 



Kevin Bonds



Nashville TN



601XL Plans building.



http://home.comcast.net/~kevinbonds



 



do not archive DO NOT ARCHIVE

_________________
KevinBonds
Nashville, TN
Plans-building Zenith CH601XL w/Corvair Power
http://home.comcast.net/~kevinbonds

chris Sinfield

Joined: 28 Nov 2006
Posts: 270
Location: Sydney Australia

Posted: Sat May 12, 2007 1:40 am Post subject: Re: Recent crashes

Go read the reports people, and stop speculating...



The British one was a factory QB kit and a 601 UL not an XL.

He was doing a Low Fast pass and pulled up sharply to miss a wire when the wings failed.. He had been reported for doing beat ups before and had even recieved a letter..  the wing was not an XL wing but the older one with NO flaps.



Read about the damage of the rear attachment bolt holes being ripped Down and Outwards as it failed due to stress. (The bolt was in)



Now go and read the one doing the test flight after instalation of the wing and read.. Rear attachment point was undamaged and still attached to the Fuse. IE no Bolt damage as the wing folded up.



Now read what appears to be the latest VFR flight in IMC conditions.



Now go and read all about the other 601's and XL's flying with no ploblems. 

Chris.

Do not archive.

planecrazydld(at)yahoo.co
Guest

Posted: Sat May 12, 2007 4:10 am Post subject: Recent crashes

Kevin, sometyhing that has bothered me since I decided to build the XL is the "slight" forward sweep of the wings. As stated before this is a consequence of the canted spardesign coupled with dihedral effect.

   

  The problem (perhaps I should say the question) in my mind is this.



  There is a phenomenon called structural divergence - or something like that. Simply put, it is easy to design structure for a multi beam cantilever panel like a common straight wing that will twist during uniform loading to either maintian tip angle of attack or reduce the tip angle of attack. This results in the wing "auto-unloading" aerodynamically as it bends; the forces generating the bending are reduced by twisting the nose of the wing down as the panel flexes up and up as the panlel flexes down. In the case of a forward swept wing this phenomenon is reversed with conventional metal structure and it as the panel flexes upward the tip twists  to a greater angle of attack increasing the bending load,increasing the increase in angle of attack, increasing the bending load.....

   

  With the exception of the german WWII research planes and a german corporate jet in the  60s, forward swept wings have never been feasible until carbon composites came along - and the only reason that they made it possible without massive structure is that you can orient the fibers to result in bending/twist modes that are opposite the normal.



  In the case of a structurally divergent design, onset to failure can be virtually instantaneous - especialy if prior damage has been accumulated unseen.

   

  I have to admit, after a 30+ year career in Aerospace Materials Engineering at the biggest manufacturer, the series of catastrophic failures for what appear to be stupid triggers worries me some. 

   

  I think when I get my plane done and  have it licensed I will add equip or ballast to ensure tat I only operate in the forward half of the CG range. I keep reading that the XL has very powerful and sensitive elevator control - staying nose heavy will help me avoid PIOs and other upset/onset related issues to some degree. And, other than stalling faster and having to land a little faster due to the reduced stab/elevator authority, I see no issue with that plan. Anyone disagree?

   

  Thanks.

  

kevinbonds <kevinbonds(at)comcast.net> wrote:

  [quote]--> Zenith-List message posted by: "kevinbonds" 



I read CH's letter after the Cali crash. He states that hundreds of these

designs have been flying--some for more than 20 years. This is true to an

extent. I have the utmost respect for him, but my concern is with the  XL

which I do not think has been around that long, and I'm not sure how many

examples of it are flying. The success of his designs, certainly speaks well

of his skills as a designer but doesn't guarantee that some gremlin can't

exist in one of his newest designs. 



Look, I'm not trying to find fault, I'm just trying to be diligent and

consider the worst case scenario in my planning. So worst case scenario;

there is a design flaw or a fault in a published number. How do we rule out

that possibility? In the event we can't, what changes do we make? 



Does the UL in the UK have flaps? Have there been any cases of damaged wings

during abrupt maneuvers that didn't come off? How abrupt were those

maneuvers? Or is it like coming to the edge of a cliff there is either no

damage or rapid, irreversible advance toward the catastrophic. Can we do

something to at least slow it down long enough so that someone might live to

tell about it?       Take the Internet to Go: Yahoo!Go puts the [url=http://us.rd.yahoo.com/evt=48253/*http://mobile.yahoo.com/go?refer=1GNXIC]Internet in your pocket:[/url] mail, news, photos & more.  [quote][b]

robert.eli(at)comcast.net
Guest

Posted: Sat May 12, 2007 4:57 am Post subject: Recent crashes

David,

  

 I have been reading the discussions on the wing  failure topic for some time (without comment, until now).  I believe you  are on to something that I have also been thinking about.  One of the  crashes was associated with a ground observation of a severe wing flutter-like  phenomena an instant before the wing was observed to fold upwards.  Another  601 pilot reported a sudden severe vibration of the wings when over flying a  powerplant cooling tower, which he believed was so serious that he  felt lucky to have not sustained a structural failure.  I believe that  the structural divergence phenomena is something that needs to be investigated  immediately to see if the 601 wing does in fact have this behavior. If it does,  it would be a common thread that could explain all the failures. I don't believe  for a minute that all of these failures can be blamed on pilot or builder  errors.  It seems entirely plausible to me (as another aerospace  engineer) that a wing having the structural characteristics you  describe could be made to produce a divergent torsional oscillation that would  lead to destruction in less than a second or two under the right conditions. Any  sudden maneuver, that produces enough initial twist, under the right conditions,  could initiate the divergent oscillation that would be almost impossible to  predict, or to correct once it starts.

  

 Bob Eli



 [quote]   ---

ashontz

Joined: 27 Dec 2006
Posts: 723

Posted: Sat May 12, 2007 5:20 am Post subject: Re: Recent crashes

I didn't realize the wings on the XL were swept forward. Are you talking about the fact that the leading edge is straight and the trailing edge tapers producing a swept forward appearance.  I don't see how the wings could be swpt forward otherwise.



Good to hear and actual aeronautical engineer on here though. Something just doesn't sound right about all this stuff.



[quote="robert.eli(at)comcast.net"]David,

  

 I have been reading the discussions on the wing  failure topic for some time (without comment, until now).  I believe you  are on to something that I have also been thinking about.  One of the  crashes was associated with a ground observation of a severe wing flutter-like  phenomena an instant before the wing was observed to fold upwards.  Another  601 pilot reported a sudden severe vibration of the wings when over flying a  powerplant cooling tower, which he believed was so serious that he  felt lucky to have not sustained a structural failure.  I believe that  the structural divergence phenomena is something that needs to be investigated  immediately to see if the 601 wing does in fact have this behavior. If it does,  it would be a common thread that could explain all the failures. I don't believe  for a minute that all of these failures can be blamed on pilot or builder  errors.  It seems entirely plausible to me (as another aerospace  engineer) that a wing having the structural characteristics you  describe could be made to produce a divergent torsional oscillation that would  lead to destruction in less than a second or two under the right conditions. Any  sudden maneuver, that produces enough initial twist, under the right conditions,  could initiate the divergent oscillation that would be almost impossible to  predict, or to correct once it starts.

  

 Bob Eli



  	  Quote: 	
		     ---

planecrazydld(at)yahoo.co
Guest

Posted: Sat May 12, 2007 5:39 am Post subject: Recent crashes

Bob; You are thinking alongside me. The problem as I see it is that CH is extraordinarily qualified to have performed this design - if indeed HE did the complete design. When you look at the almost haphazard way that the tailwheel modifications are superimposed on the basic design, there is some indication that he did not do the whole thing.

   

  Another thing that bothers me a little is that the skin bearing/rivet shear/head pop balance may have been terrific for the 1604 rivet with the upset factory head when used with 0.016" thick skins but what happens when you increase the gauge of the skins to 0.020" or 0.025" as appears to have happened several places...



"Robert N. Eli" <robert.eli(at)comcast.net> wrote:

  [quote]          David,

   

  I have been reading the discussions on the wing failure topic for some time (without comment, until now).  I believe you are on to something that I have also been thinking about.  One of the crashes was associated with a ground observation of a severe wing flutter-like phenomena an instant before the wing was observed to fold upwards.  Another 601 pilot reported a sudden severe vibration of the wings when over flying a powerplant cooling tower, which he believed was so serious that he felt lucky to have not sustained a structural failure.  I believe that the structural divergence phenomena is something that needs to be investigated immediately to see if the 601 wing does in fact have this behavior. If it does, it would be a common thread that could explain all the failures. I don't believe for a minute that all of these  failures can be blamed on pilot or builder errors.  It seems entirely plausible to me (as another aerospace engineer) that a wing having the structural characteristics you describe could be made to produce a divergent torsional oscillation that would lead to destruction in less than a second or two under the right conditions. Any sudden maneuver, that produces enough initial twist, under the right conditions, could initiate the divergent oscillation that would be almost impossible to predict, or to correct once it starts.

   

  Bob Eli



  [quote]  ---

skyguynca

Joined: 05 Jun 2006
Posts: 128

Posted: Sat May 12, 2007 5:46 am Post subject: Recent crashes

I built a 601HD many years ago and will build a new one next year, great 

plane.  I also see tons of incidents with all kinds of planes and I still 

keep flying. I can understand all the people who seem to be getting 

emotional because some think there might be something wrong with the XL when 

3 clear incidents of wings failing........don't get emotional just be 

practical. 3 XL's lost structual integrity in the wings during flight, some 

in IMC, some in VFR. One in the pattern and one in a flyby, and one just 

flying. Just saying "he was flying faster than Va in turbulent weather" may 

explain one, but not the other two because only one was IMC conditions. I 

agree lets wait and see if the NTSB comes up with a answer, but I am sure 

they won't. Chris's designs are wonderfully strong and great flying planes. 

Maybe the failure is related to the building, maybe something in the plans 

are as clear as they need to be?



Lets keep emotions out of the discussions so they can be clear and to the 

point.



David Mikesell

23597 N. Hwy 99

Acampo, CA 95220

209-224-4485

skyguynca(at)skyguynca.com

www.skyguynca.com

---

planecrazydld(at)yahoo.co
Guest

Posted: Sat May 12, 2007 6:56 am Post subject: Recent crashes

Andy;

   

  Imagine that you have the outer wing panels and the centersection built as a single continuous element. When the spar web is oriented parallel to the Y axis or straight up and down, the wing leading edge would be straight when viewed from above. Take that same assembly and tip it forward the 12 or so degrees from the Y axis that it is in the carry through, add the constant chord leading edge ribs, and now when viewed from above the leading edge is swept forward a small amount - as is the outer spar.



ashontz <ashontz(at)nbme.org> wrote:

   	  Quote: 	
		  --> Zenith-List message posted by: "ashontz" 



I didn't realize the wings on the XL were swept forward. Are you talking about the fact that the leading edge is straight and the trailing edge tapers producing a swept forward appearance. I don't  see how the wings could be swpt forward otherwise.

	




  Dave Downey

  Harleysville (SE) PA

  Zodiac 601XL/Corvair?

   [quote][b]

ashontz

Joined: 27 Dec 2006
Posts: 723

Posted: Sat May 12, 2007 7:13 am Post subject: Re: Recent crashes

So you're saying because of the 9 degree forward incline on the main spar in conjunction with the dyhedral in effect creates a forward sweep? Yes, I can see that now. A few posts back I had wondered why the main spar was inclined forward 9 degrees. Now I know. It does in effect create a forward sweep slightly. With that arrangement the outer tip of the wing would in fact be maybe about 60m forward of the root, just a rough guess. So that was intentional to give it a slight forward sweep. Sounds a little hairy to me. How is the 601HD wing spar. Is that truly vertical, or does it also have a slight sweep forward due to a main spar that's inclined?



[quote="planecrazydld(at)yahoo.co"]Andy;

   

  Imagine that you have the outer wing panels and the centersection built as a single continuous element. When the spar web is oriented parallel to the Y axis or straight up and down, the wing leading edge would be straight when viewed from above. Take that same assembly and tip it forward the 12 or so degrees from the Y axis that it is in the carry through, add the constant chord leading edge ribs, and now when viewed from above the leading edge is swept forward a small amount - as is the outer spar.



ashontz <ashontz> wrote:

   	  Quote: 	
		  --> Zenith-List message posted by: "ashontz" 



I didn't realize the wings on the XL were swept forward. Are you talking about the fact that the leading edge is straight and the trailing edge tapers producing a swept forward appearance. I don't  see how the wings could be swpt forward otherwise.

	




  Dave Downey

  Harleysville (SE) PA

  Zodiac 601XL/Corvair?

    	  Quote: 	
		  [b]

planecrazydld(at)yahoo.co
Guest

Posted: Sat May 12, 2007 8:08 am Post subject: Recent crashes

Not having plans but having reviewed the sample stuff at Zenith, I believe that the spar is oriented vertically on the HD. Many of the builders that post on here will certainly know that beyond speculation.



ashontz <ashontz(at)nbme.org> wrote:  [quote]--> Zenith-List message posted by: "ashontz" 



So you're saying because of the 9 degree forward incline on the main spar in conjunction with the dyhedral in effect creates a forward sweep? Yes, I can see that now. A few posts back I had wondered why the main spar was inclined forward 9 degrees. Now I know. It does in effect create a forward sweep slightly. With that arrangement the outer tip of the wing would in fact be maybe about 60m forward of the root, just a rough guess. So that was intentional to give it a slight forward sweep. Sounds a little hairy to me. How is the 601HD wing  spar. Is that truly vertical, or does it also have a slight sweep forward?



[quote="planecrazydld(at)yahoo.co"]Andy;



Imagine that you have the outer wing panels and the centersection built as a single continuous element. When the spar web is oriented parallel to the Y axis or straight up and down, the wing leading edge would be straight when viewed from above. Take that same assembly and tip it forward the 12 or so degrees from the Y axis that it is in the carry through, add the constant chord leading edge ribs, and now when viewed from above the leading edge is swept forward a small amount - as is the outer spar.



ashontz wrote:



[quote] --> Zenith-List message posted by: "ashontz" 

 

 I didn't realize the wings on the XL were swept forward. Are you talking about the fact that the leading edge is straight and the trailing edge tapers producing a swept forward appearance. I don't see how the wings could be swpt forward       Take the Internet to Go: Yahoo!Go puts the [url=http://us.rd.yahoo.com/evt=48253/*http://mobile.yahoo.com/go?refer=1GNXIC]Internet in your pocket:[/url] mail, news, photos & more.  [quote][b]

leinad

Joined: 19 Sep 2006
Posts: 283
Location: Central Virginia

Posted: Sat May 12, 2007 8:09 am Post subject: Re: Recent crashes

I've always assumed the tilt in the spar was an artifact left over from the 601 HD wing, which had the landing gear mounted to the spar.  This gave the landing gear the tilt that put the wheels behind the center of rotation, just as the tilted firewall puts the front wheel forward of the firewall.   I think all 601 wings have had the tilted main spar, thus slightly forward sweep to the wing.



Dan (plans building XL)

_________________
Scratch building XL with Corvair Engine

planecrazydld(at)yahoo.co
Guest

Posted: Sat May 12, 2007 8:31 am Post subject: Recent crashes

Hi Dan;

   

  If you look at the attached files they show the main spar vertical and the aft spar canted slightly.



leinad <leinad(at)hughes.net> wrote: 

  [quote]--> Zenith-List message posted by: "leinad" 



I've always assumed the tilt in the spar was an artifact left over from the 601 HD wing, which had the landing gear mounted to the spar. This gave the landing gear the tilt that put the wheels behind the center of rotation, just as the tilted firewall puts the front wheel forward of the firewall. I think all 601 wings have had the tilted main spar, thus slightly forward sweep to the wing.



Dan (plans building XL)



--------

Scratch building XL with Corvair Engine





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ashontz

Joined: 27 Dec 2006
Posts: 723

Posted: Sat May 12, 2007 9:13 am Post subject: Re: Recent crashes

601HD, vertical spar http://www.601hd.com/wings.htm. Personally, I consider an inclined spar that gives forward sweep to the wing to be a design flaw. If it's meant to give better responsveness to the ailerons, I could care less about ailerons effectiveness if I don't even have wings left on the plane.



Had I know this I would have considered another design. Forward sweep is not how you create better aileron effectiveness; you design a better aileron, perhaps larger and lengthen the flaps to meet the lengthened aileron and reduce the flap deployment angle accordingly.



The concept makes me want to puke and throw the whole thing in the garbage and then demand that Zenith ship me completed 601HD wings and a set of palsn so I can finish the fuselage myself. Insane. Forward sweep? What, was he drunk when he drew up plans for that?



It's as simple as hold a board out a car window while doing 50mph. It's a hell of a lot easier to hang onto it with it pointed backwards rather than forwards. That probably why we haven't seen many forward swept wing planes. Forward sweep; that's some 1890s whirly gig flapping contraption concept that you jump off a belltower with.



 	  leinad wrote: 	
		  I've always assumed the tilt in the spar was an artifact left over from the 601 HD wing, which had the landing gear mounted to the spar.  This gave the landing gear the tilt that put the wheels behind the center of rotation, just as the tilted firewall puts the front wheel forward of the firewall.   I think all 601 wings have had the tilted main spar, thus slightly forward sweep to the wing.



Dan (plans building XL)

purplemoon99(at)bellsouth
Guest

Posted: Sat May 12, 2007 1:46 pm Post subject: Recent crashes

Ashontz, they are swept foward, We are getting what I consider great input, 

hope to get more...Joe N101HD   601 XL

---

MaxNr(at)aol.com
Guest

Posted: Sat May 12, 2007 3:10 pm Post subject: Recent crashes

I am uncertain if the swept forward 601XL wing lends it self to load testing with sand bags. How do you duplicate that twisting feature? I would like to see what if any load testing that ZAC has done. I posted some pictures of testing done on a relatively rare airframe after the builder decided to "kit" this plane. No. He never rebuilt it and put it back in the air as some suggest. Testing to failure was a responsible course to take when he offered it to the public. Has ZAC done this?

 

 A friend told me that when he was a Beechcraft test pilot in the early 60's, witnessed a load test on a Beech 23 Musketeer. The wing and fuselage were supported four feet above the floor. A great deal of weight was applied until the belly touched the floor. For about an hour. The weight was removed, belly came up and a straight edge used on the wing. Zero deformation. Nothing cracked. Although the 23 was not his project, (his was the big engine, armed T-34) he did ride with the project pilot through some wild aerobatics and observed the wing tips flex several feet under high G. Several feet. He was a WW2 fighter pilot that flew every US, Brit, German & Italian fighter that he could. The Be 23 has a laminar flow airfoil that looks somewhat like the 601XL. The skin forward of the spar is bonded with no rivets. I've never heard of a structural failure. 

 

 I've been doing loops since puberty and would like to do one on my birthday. (70th) The last thing I want to have to do is add Piper Pawnee lift struts or Fly-Baby flying wires. My wife has restricted me to no more than 2 G because of my age. One G when she is aboard. 

 Do not archive

 Bob Dingley

 Pace, FL

 XL

**************************************

 See what's free at http://www.aol.com.  [quote][b]

Kevin Bonds

Joined: 09 Jan 2006
Posts: 171
Location: Nashville, Tn

Posted: Sat May 12, 2007 7:55 pm Post subject: Recent crashes

_________________
KevinBonds
Nashville, TN
Plans-building Zenith CH601XL w/Corvair Power
http://home.comcast.net/~kevinbonds

robert.eli(at)comcast.net
Guest

Posted: Sun May 13, 2007 6:24 am Post subject: Recent crashes

David and other interested folks,

  

 After reading the latest flurry of Emails on the  601 wing structural failures, I will attempt to clarify what I was hypothesizing  about wing flutter. It is my understanding that flutter problems most  frequently involve control surfaces, for which designers have methods to  reduce the probability of its occurrence. It is also my understanding that  there are problems that occasionally occur involving divergent structural  vibrations (flutter) that cannot be predicted even by the best designers, of  which Chris Heintz is certainly one (I have the greatest respect and admiration  for Chris).

  

 I recall many years ago the difficulties the  turboprop Lockheed Electra had with a string of wing failures traced to a type  of torsional flutter introduced by the engines. The Electra wing was  extremely strong and withstood every load test that the engineers could throw at  it, and it took months, if not years, of work by 100's of the best engineers  Lockheed had to finally pinpoint the problem.  The point to be made is that  structural strength alone cannot save a wing if it can be induced to flutter in  a divergent (unstable) fashion. Even the strongest wing (or any other  structure) can be failed if it has a mode of vibration that is unstable (has a  vibration for which the amplitude is increasing with time). Another classic  example is the "Galloping Gertie" suspension bridge failure that I am sure  everyone has seen in the famous documentary film.

  

 The hypothesis is (and it is just that; a long  shot, out-of-the-box possibility) that the 601XL wing has a torsional  stiffness (resistance to twist) that would normally be sufficient for a safe  design; however, due to some combination of structural and aerodynamic  characteristics, it is insufficiently stiff to damp a particular type of  torsional flutter. This characteristic would not necessarily have any  relationship to the wing's strength as normally measured by load tests or design  calculations. A further feature of this hypothesis is that the center  of lift of the outer sections of the wing produces a twisting force moment  that tends to increase with the angle of twist (angle of attack). The process  would be initiated by some abrupt maneuver, wind shear, or whatever, that  produces some greater than typical twist in the outer section of the wing, which  in turn, introduces more twist due to the local increased angle of  attack. The whole process would depend on a natural torsional  frequency of vibration of the wing that is "in tune" with an oscillating  aerodynamic load that tends to produce progressively more twist with  each oscillation of the vibration. This process, if it were to occur, would  progress very quickly (a matter of a second or two), until finally the  structural strength of the wing skin and rivets would be exceeded. Using  some educated guessing, I would suspect that the primary mode of  failure would be the wing skin rivets where they attach to the ribs and spar (by  hole elongation and rivet shear). As this mode of failure progresses, the spars  would be twisted beyond the yield point (leaving a twisted spar) and the  failure would cascade on down to the fuselage attach points.

  

 All of this is probably, as someone suggested,  overdone hysterics; but I can't shake the two very strange incidents that were  reported here with regard to the observed wing flutter immediately prior to the  wings "folding back" in the one crash, and the observations of a pilot who  experienced the severe wing vibration over the powerplant cooling towers.   Maybe these two reports are not based in fact, but I took them to be reliable  accounts.

  

 Bob Eli

 [quote]   ---

ashontz

Joined: 27 Dec 2006
Posts: 723

Posted: Sun May 13, 2007 9:23 am Post subject: Re: Recent crashes

I was going to add this yesterday, swept forward is more a dynamic test than a static test. All the sandbags in the world won't tell you about the dynamic characteristics of a wing.



I just stopped at a small airport to take a look at a few production planes. The Grumman Traveller appears to have an ever so slightly forward swept wing as well. It also has ribs spaced every 15 inches, not 21 or 22 inches. Personally, I feel adding a ribs between 4 and 5, 5 and 6, 6 and 7, 7 and 8, and 8 and 9 on the XL would be highly advisable. I was going to put flase ribs (L angle) in between these ribs anyway because for some oil-canning I'm getting on the lower skin. Oil canning would be a great place for the wing to start to fail. The slightly swept forward wing may not be a problem, as long as the wing structure is beefy enough and has enough attach points. 21+ inch spacing for the center ribs is is probably pushing it.



[quote="MaxNr(at)aol.com"]I am uncertain if the swept forward 601XL wing lends it self to load testing with sand bags. How do you duplicate that twisting feature? I would like to see what if any load testing that ZAC has done. I posted some pictures of testing done on a relatively rare airframe after the builder decided to "kit" this plane. No. He never rebuilt it and put it back in the air as some suggest. Testing to failure was a responsible course to take when he offered it to the public. Has ZAC done this?

 

 A friend told me that when he was a Beechcraft test pilot in the early 60's, witnessed a load test on a Beech 23 Musketeer. The wing and fuselage were supported four feet above the floor. A great deal of weight was applied until the belly touched the floor. For about an hour. The weight was removed, belly came up and a straight edge used on the wing. Zero deformation. Nothing cracked. Although the 23 was not his project, (his was the big engine, armed T-34) he did ride with the project pilot through some wild aerobatics and observed the wing tips flex several feet under high G. Several feet. He was a WW2 fighter pilot that flew every US, Brit, German & Italian fighter that he could. The Be 23 has a laminar flow airfoil that looks somewhat like the 601XL. The skin forward of the spar is bonded with no rivets. I've never heard of a structural failure. 

 

 I've been doing loops since puberty and would like to do one on my birthday. (70th) The last thing I want to have to do is add Piper Pawnee lift struts or Fly-Baby flying wires. My wife has restricted me to no more than 2 G because of my age. One G when she is aboard. 

 Do not archive

 Bob Dingley

 Pace, FL

 XL

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 See what's free at http://www.aol.com.   	  Quote: 	
		  [b]

ashontz

Joined: 27 Dec 2006
Posts: 723

Posted: Sun May 13, 2007 9:28 am Post subject: Re: Recent crashes

I think it's definitely worth looking into.



[quote="robert.eli(at)comcast.net"]David and other interested folks,

  

 After reading the latest flurry of Emails on the  601 wing structural failures, I will attempt to clarify what I was hypothesizing  about wing flutter. It is my understanding that flutter problems most  frequently involve control surfaces, for which designers have methods to  reduce the probability of its occurrence. It is also my understanding that  there are problems that occasionally occur involving divergent structural  vibrations (flutter) that cannot be predicted even by the best designers, of  which Chris Heintz is certainly one (I have the greatest respect and admiration  for Chris).

  

 I recall many years ago the difficulties the  turboprop Lockheed Electra had with a string of wing failures traced to a type  of torsional flutter introduced by the engines. The Electra wing was  extremely strong and withstood every load test that the engineers could throw at  it, and it took months, if not years, of work by 100's of the best engineers  Lockheed had to finally pinpoint the problem.  The point to be made is that  structural strength alone cannot save a wing if it can be induced to flutter in  a divergent (unstable) fashion. Even the strongest wing (or any other  structure) can be failed if it has a mode of vibration that is unstable (has a  vibration for which the amplitude is increasing with time). Another classic  example is the "Galloping Gertie" suspension bridge failure that I am sure  everyone has seen in the famous documentary film.

  

 The hypothesis is (and it is just that; a long  shot, out-of-the-box possibility) that the 601XL wing has a torsional  stiffness (resistance to twist) that would normally be sufficient for a safe  design; however, due to some combination of structural and aerodynamic  characteristics, it is insufficiently stiff to damp a particular type of  torsional flutter. This characteristic would not necessarily have any  relationship to the wing's strength as normally measured by load tests or design  calculations. A further feature of this hypothesis is that the center  of lift of the outer sections of the wing produces a twisting force moment  that tends to increase with the angle of twist (angle of attack). The process  would be initiated by some abrupt maneuver, wind shear, or whatever, that  produces some greater than typical twist in the outer section of the wing, which  in turn, introduces more twist due to the local increased angle of  attack. The whole process would depend on a natural torsional  frequency of vibration of the wing that is "in tune" with an oscillating  aerodynamic load that tends to produce progressively more twist with  each oscillation of the vibration. This process, if it were to occur, would  progress very quickly (a matter of a second or two), until finally the  structural strength of the wing skin and rivets would be exceeded. Using  some educated guessing, I would suspect that the primary mode of  failure would be the wing skin rivets where they attach to the ribs and spar (by  hole elongation and rivet shear). As this mode of failure progresses, the spars  would be twisted beyond the yield point (leaving a twisted spar) and the  failure would cascade on down to the fuselage attach points.

  

 All of this is probably, as someone suggested,  overdone hysterics; but I can't shake the two very strange incidents that were  reported here with regard to the observed wing flutter immediately prior to the  wings "folding back" in the one crash, and the observations of a pilot who  experienced the severe wing vibration over the powerplant cooling towers.   Maybe these two reports are not based in fact, but I took them to be reliable  accounts.

  

 Bob Eli

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