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Attachment fitting at top of lift struts.
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kenharrison(at)comporium.
Guest

Posted: Thu Oct 18, 2007 2:07 pm Post subject: Attachment fitting at top of lift struts.

Hello all,  

   

  I have read a good deal in the archives regarding the connection to the wing at the top of the lift strut.  Is there a definitive method to determine whether those fittings are on far enough.  Mine do not have a witness hole and I’m very nervous about not having enough threads.  I also see a small amount of surface rust on the threads.  That whole connection just makes me nervous.  I would think a connection that important, and under that much stress would have all the bolts in sheer, not tension.  If those 8 or ten threads fail, your cooked.  

   

Has anyone heard of a Kitfox failing at that point…other than the load testing that was done at the factory?  

   

Does anyone have a secondary or beefed up system on his plane?  

   

Ken  

Kitfox III  

   

   

       [quote][b]

john(at)leptron.com
Guest

Posted: Thu Oct 18, 2007 6:49 pm Post subject: Attachment fitting at top of lift struts.

Ken,  

I have never heard of a failure at this point. I also believe the factory never was able to cause a failure at this point. I did witness many of the test that were done one the classic 4 and was impressed with the results.  

   

John Oakley  

         

  

From: owner-kitfox-list-server(at)matronics.com [mailto:owner-kitfox-list-server(at)matronics.com] On Behalf Of Ken Harrison

 Sent: Thursday, October 18, 2007 4:06 PM

 To: kitfox-list(at)matronics.com

 Subject: Attachment fitting at top of lift struts.  

  

   

Hello all,  

   

  I have read a good deal in the archives regarding the connection to the wing at the top of the lift strut.  Is there a definitive method to determine whether those fittings are on far enough.  Mine do not have a witness hole and I’m very nervous about not having enough threads.  I also see a small amount of surface rust on the threads.  That whole connection just makes me nervous.  I would think a connection that important, and under that much stress would have all the bolts in sheer, not tension.  If those 8 or ten threads fail, your cooked.  

   

Has anyone heard of a Kitfox failing at that point…other than the load testing that was done at the factory?  

   

Does anyone have a secondary or beefed up system on his plane?  

   

Ken  

Kitfox III  

   

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dave

Joined: 22 Sep 2006
Posts: 1382

Posted: Fri Oct 19, 2007 1:50 am Post subject: Re: Attachment fitting at top of lift struts.

Ken, 



I think you will find  that there to my knowledge has ever been a inflight breakup of a Kitfox from the wing strut attach fitting.   I think  the testing showed  in excess of +14G and still no failure.  But I stand to be corrected.  



I will say that AVIDs do have what seem to me to be a stronger wing attach fitting by eliminating the rod end. 



Kitfox used rod ends  I think to compensate for variances in build tolerances but  I don't think it anything to worry about.  I would check with John McBean to find out the exact procedure of what you are asking.

You only need one  thread to fail ,so no matter how far it screwed in the rod end  the one that will fail will be out side of it .   Does anyone know the actual shear strength of the threaded part of the wing strut attach point ?   I would bet each of the 4 attach points are 4000 to 6000 pound tensile and we fly at 1200  or so lbs  divided by  4 = 300  each ?  MAke sense ?  If I  am correct  that means at 10 gs you are pulling 3000 pound force and I guarantee you that you won't see 10gs.    I am done talking outta my ass now.   <smile>





 	  Quote: 	
		   I would think a connection that important, and under that much stress would have all the bolts in sheer, not tension. If those 8 or ten threads fail, your cooked.

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Fox5flyer
Guest

Posted: Fri Oct 19, 2007 2:49 am Post subject: Attachment fitting at top of lift struts.

One thing that needs to be remembered is that AN fittings have "rolled" 

threads, not cut threads.  This makes them much stronger than the normal 

non-AN threaded fitting and much less susceptible to stress cracking at the 

threads.  Personally, if there was anything to worry about I believe it 

would be the fittings at the horizontal stab where there is a history of 

breakage in the IV.

For what it's worth.

Deke

S5, NE Michigan

dave

Joined: 22 Sep 2006
Posts: 1382

Posted: Fri Oct 19, 2007 3:09 am Post subject: Re: Attachment fitting at top of lift struts.

 	  Quote: 	
		  I believe it 

would be the fittings at the horizontal stab where there is a history of 

breakage in the IV. 

	




I think that perhaps the breakages was due to  rough ground handling using the horiz.stab braces for handle rather than using the Fuselage handle.

Would this be the likely culprit  ?



He have rolled thread on the strut atttach point? 

SO what is the tensile of them and the rod ends ?

kenharrison(at)comporium.
Guest

Posted: Fri Oct 19, 2007 3:21 am Post subject: Attachment fitting at top of lift struts.

OK, that makes sense.  Except I wonder if one of the lurking engineers on

the list could calculate the in-flight load on one of those fittings.

Because I think you have to account for the fact that the strut is not

attached to the wing at 90 degrees.  The in-flight vertical load per fitting

is 300 pounds, but the strut is at about 60 degrees from vertical (or more,

I didn't actually measure it).  Just for example, if the strut were at 45

degrees to the vertical, and had a 300 pound vertical load from the wing,

the tension felt by the strut would be 600 pounds, and 600 pounds of

compression felt by the inboard wing spar.  (Now I'm showing my ignorance.)

Is that how it would work?  The same as the vertical and horizontal

components of lift felt by a wing in turning flight?



Anyway, I think I'm just worrying for no reason.  It sounds like the

connection is plenty strong.  I'll go back to worrying about ground-looping.



Thanks for the info.



--

dave

Joined: 22 Sep 2006
Posts: 1382

Posted: Fri Oct 19, 2007 3:36 am Post subject: Re: Attachment fitting at top of lift struts.

 	  Quote: 	
		  Anyway, I think I'm just worrying for no reason. It sounds like the 

connection is plenty strong. I'll go back to worrying about ground-looping 	


Ken, I am no engineer but I can tell you that past history does speak personally and from the other 5000 plus Kitfox and Avids out there. 

If these were going to be over stressed we would be seeing reports of breakage.  I think the most likely reason for Kitfox accidents is likely form pilot incompetence from lack of training. 



For example if you look at another popular design  the Challenger  there have been in lfight break ups  at the strut to longeron fitting called a RONY bracket.  They recommend to change them ever so many hours.  Now that is a RED FLAG to me . 

Hope this helps

Lynn Matteson

Joined: 10 Jan 2006
Posts: 2778
Location: Grass Lake, Michigan

Posted: Fri Oct 19, 2007 4:04 am Post subject: Attachment fitting at top of lift struts.

Is that "history of breakage in the IV" when they were 3/16" rod end  

bearings? I know there's an SB on changing these to 1/4", which is  

what I did during my building process. I got the AN490 threaded rod  

ends (1/4"), welded them myself, and got the necessary 1/4" x 3/16"  

rod end bearings, and have been avoiding the grim reaper ever  

since. : ) (knock on wood)

Lynn Matteson

Grass Lake, Michigan

Kitfox IV Speedster  w/Jabiru 2200

flying w/420+ hrs





On Oct 19, 2007, at 7:46 AM, fox5flyer wrote:



 	  Quote: 	
		   

 <fox5flyer(at)idealwifi.net>



 One thing that needs to be remembered is that AN fittings have  

 "rolled" threads, not cut threads.  This makes them much stronger  

 than the normal non-AN threaded fitting and much less susceptible  

 to stress cracking at the threads.  Personally, if there was  

 anything to worry about I believe it would be the fittings at the  

 horizontal stab where there is a history of breakage in the IV.

 For what it's worth.

 Deke

 S5, NE Michigan

_________________
Lynn
Kitfox IV-Jabiru 2200
N369LM

MichaelGibbs(at)cox.net
Guest

Posted: Fri Oct 19, 2007 7:37 am Post subject: Attachment fitting at top of lift struts.

Ken sez:



 	  Quote: 	
		  ...I wonder if one of the lurking engineers on the list could 

calculate the in-flight load on one of those fittings.


	


I don't want to discourage the lurking engineers from having some 

fun, but that calculation was performed at the factory 20 years ago 

when the design was developed.  Since there has never been an 

inflight failure, I'd say their numbers have been fully validated 

through operational experience.



There are things to worry about when building and flying your Kitfox 

but this is probably not one of them.



Mike G.

N728KF

Phoenix, AZ

n61kf

Joined: 27 Aug 2006
Posts: 23
Location: Waynesville Ohio

Posted: Fri Oct 19, 2007 8:45 am Post subject: Re: Attachment fitting at top of lift struts.

Ken.



There should be 1 1/16" of threads on the end of the lift strut. The witness hole on my rod ends are 5/16" from the open end. I too have had some concern about the strength of the threaded portion of the lift strut. This stems from the fact that both of the threaded ends on the left wing of the Mod IV that I am repairing, broke off, just below the rod end bearing. This was due to a severe compression load on the strut due to a pancake landing which collapsed the landing gear and bent the fuselage. I think in this case some thing had to give, and  it was the threaded end that gave. I this case you are already on the ground and having a bad day. so the rod ends breaking is not the worst of it.



Keith Schneider

Mod IV 912,almost done

Waynesville, Ohio

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Keith Schneider
Red Stewart Airfield
Waynesville Ohio
KF IV 912ul

Guy Buchanan

Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA

Posted: Fri Oct 19, 2007 4:10 pm Post subject: Attachment fitting at top of lift struts.

At 04:20 AM 10/19/2007, you wrote:

  	  Quote: 	
		  OK, that makes sense.  Except I wonder if one of the lurking engineers on

 the list could calculate the in-flight load on one of those fittings. 	


 Sigh. I know it's fun to hash these things out verbally every time they arise, but one of the "lurking engineers" has calculated in-flight loads on these fittings in the past two years. Indeed this entire subject was re-hashed and therefore appears in the archives. Since I know no one will look I will quickly summarize my findings:



 Using the rod end fittings found on my IV-1200 there was NO WAY you were going to get to 14g's. (Personally I think that's an urban legend, since I've not seen any NUMBERS to justify it.) Indeed my rod ends would fail at something closer to 6g, as they should for this design. (Utility category limit loads of +3.8g, -1.5g, break at +5.7, -2.25g.).



 Though it's not the best way to accommodate a load, there's nothing terribly wrong with the rod ends when loaded in tension, indeed they allow for all sorts of manufacturing defects. (There are stress concentration factors associated with loading threads in tension, but you design for that. Look around and you'll see all sorts of designs with treads loaded in tension; like cylinder and head bolts.) Unfortunately the rod ends are less desirable in compression, since any misalignment can cause the exposed threaded rod to buckle prematurely.



 The bottom line is that there has never been an in-flight break-up of a Kitfox to our common knowledge, and I personally know of several who have tried, and probably still are trying. Over 15(?) years that's a pretty good record.



 

 Guy Buchanan

 San Diego, CA

 K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar.    [quote][b]

_________________
Guy Buchanan
Deceased K-IV 1200
A glider pilot too.

MichaelGibbs(at)cox.net
Guest

Posted: Fri Oct 19, 2007 5:25 pm Post subject: Attachment fitting at top of lift struts.

Guy sez:



 	  Quote: 	
		  Using the rod end fittings found on my IV-1200 there was NO WAY you 

were going to get to 14g's. (Personally I think that's an urban 

legend, since I've not seen any NUMBERS to justify it.)


	


It's more than a legend, Guy.  It was documented in a builder 

newsletter from the factory in the late '80s or early '90s, complete 

with photos of the physical testing.  I have it around here 

somewhere...  The caption on one of the photos specifically stated 

that the sandbags piled on the inverted wing/lift strut assembly 

represented 14g.



Because it was a general interest publication, no engineering data 

was included.



Mike G.

N728KF

Phoenix, AZ

dave

Joined: 22 Sep 2006
Posts: 1382

Posted: Fri Oct 19, 2007 5:52 pm Post subject: Re: Attachment fitting at top of lift struts.

Guy,  how do you justify this statement ? 

 	  Quote: 	
		  Using the rod end fittings found on my IV-1200 there was NO WAY you were going to get to 14g's. (Personally I think that's an urban legend, since I've not seen any NUMBERS to justify it.) Indeed my rod ends would fail at something closer to 6g, as they should for this design. (Utility category limit loads of +3.8g, -1.5g, break at +5.7, -2.25g.).

dave

Joined: 22 Sep 2006
Posts: 1382

Posted: Fri Oct 19, 2007 6:00 pm Post subject: Re: Attachment fitting at top of lift struts.

I have pics and video here as well but I think you will find this satisfying info on John Mc Bean's Site  

http://www.kitfoxaircraft.com/Testing%20information.htm



Nextd tim you see an AVID look close at the wing strit attach to wings points.  There are no rod ends as Dean Wilson designed it to be strong. Not to say the Kitfox is not strong but I think with a Rod end that just adds one more part that could fail. 



Does anyone have tensiles of a rod end used ?  



Also what is tensile of the wrapped end on the Avids ?

Guy Buchanan

Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA

Posted: Sat Oct 20, 2007 5:18 am Post subject: Attachment fitting at top of lift struts.

At 06:52 PM 10/19/2007, you wrote:

 	  Quote: 	
		  Guy,  how do you justify this statement ?


	


Which statement? Do you mean you would like me to provide the math? 

Or do you mean the catagory limit loads?

Guy Buchanan

San Diego, CA

K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar.

_________________
Guy Buchanan
Deceased K-IV 1200
A glider pilot too.

dave

Joined: 22 Sep 2006
Posts: 1382

Posted: Sat Oct 20, 2007 5:48 am Post subject: Re: Attachment fitting at top of lift struts.

yes,  please show us how you can claim your rod ends will fail at 5.7g s  and what what weight of aircraft? And document your testing to make these statements. Since you know how to figure this tell us when the outboard section of spar from wing attach point outwards will actually bend and stay distorted. 



Thanks in advance

 	  Quote: 	
		  Using the rod end fittings found on my IV-1200 there was NO WAY you were going to get to 14g's. (Personally I think that's an urban legend, since I've not seen any NUMBERS to justify it.) Indeed my rod ends would fail at something closer to 6g, as they should for this design. (Utility category limit loads of +3.8g, -1.5g, break at +5.7, -2.25g.).

Guy Buchanan

Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA

Posted: Sat Oct 20, 2007 9:40 am Post subject: Attachment fitting at top of lift struts.

At 06:48 AM 10/20/2007, you wrote:

 	  Quote: 	
		  yes,  please show us how you can claim your rod ends will fail at 

5.7g s  and what what weight of aircraft? And document your testing 

to make these statements. Since you know how to figure this tell us 

when the outboard section of spar from wing attach point outwards 

will actually bend and stay distorted.


	


I don't know, Dave. It sounds like you may have a chip on your 

shoulder and that I'd be pretty much wasting my time. You know very 

well I haven't done any testing and that all the information publicly 

available on Kitfox structural testing, (I've looked,) is contained 

in those few pictures plus an anecdotal value of 14g's. Perhaps John 

will come forth with details of the structural testing, including 

methodology, drawings, detail photo's, and results.



When I find the time I will, however, provide the math and rod end 

specifications for your use. Don't hold your breath but you may 

remind me from time to time if I dally.



And you needn't be alarmed by my results. I DID note that they were 

consistent with the stated design loads.

Guy Buchanan

San Diego, CA

K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar.

_________________
Guy Buchanan
Deceased K-IV 1200
A glider pilot too.

Guy Buchanan

Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA

Posted: Sat Oct 20, 2007 9:40 am Post subject: Attachment fitting at top of lift struts.

At 06:23 PM 10/19/2007, you wrote:

 	  Quote: 	
		  It's more than a legend, Guy.  It was documented in a builder 

newsletter from the factory in the late '80s or early '90s, complete 

with photos of the physical testing.  I have it around here 

somewhere...  The caption on one of the photos specifically stated 

that the sandbags piled on the inverted wing/lift strut assembly 

represented 14g.


	


OK Mike. I know your word is good. I'll see if I can reconcile my 

results with those in the pictures on John's site. If nothing else is 

available John might be able to send enlargements of the pictures so 

I can learn more about the testing. If anybody has the newsletter I'd 

be grateful if I could get a copy.

Guy Buchanan

San Diego, CA

K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar.

_________________
Guy Buchanan
Deceased K-IV 1200
A glider pilot too.

jdmcbean(at)kitfoxaircraf
Guest

Posted: Sat Oct 20, 2007 11:25 am Post subject: Attachment fitting at top of lift struts.

We do have the documentation on the structural testing done to the Fox.  It

was extensive and done with FAR part 23 in mind for certification purposes.



I can tell you that when testing the wing section at 5.7g's positive... the

rod end did not fail.

Fly Safe !!

www.kitfoxaircraft.com

"It's not how Fast... It's how Fun!"



5:41 PM

temco(at)telusplanet.net
Guest

Posted: Sat Oct 20, 2007 11:42 am Post subject: Attachment fitting at top of lift struts.

Guy



I do have a copy of that test and I will forward a copy to you as soon as I

find it.



Ted Palamarek

K-IV-1200/912UL

Edmonton, Alberta

OK Mike. I know your word is good. I'll see if I can reconcile my 

results with those in the pictures on John's site. If nothing else is 

available John might be able to send enlargements of the pictures so 

I can learn more about the testing. If anybody has the newsletter I'd 

be grateful if I could get a copy.

Guy Buchanan

San Diego, CA

K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar.

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