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  #581  
Old 02-20-2014, 10:05 AM
Bavafa Bavafa is offline
 
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Quote:
Originally Posted by Walt View Post
Helps to relieve the stress concentration in the corner of the web where the cracking is occuring.
Thanks Walt. I do realize I had not explained my question right but you got it.
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  #582  
Old 02-20-2014, 10:29 AM
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SilverEagle2 SilverEagle2 is offline
 
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Quote:
Originally Posted by Walt View Post
Helps to relieve the stress concentration in the corner of the web where the cracking is occuring.
I was just about to ask this question as it seemed silly to cut them off. Your explanation clears it up. Thanks!
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  #583  
Old 02-20-2014, 11:21 AM
jayjabour jayjabour is offline
 
Join Date: Sep 2009
Location: Beavercreek Ohio
Posts: 5
Default SB 14-01-31

HS Spar inspection showed no cracks, took some time to remove the Paint overspray and Primer, but none seen. Relief notches per drawings.
RV-7, 200 hrs, some Acro, O-360, no cracks
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  #584  
Old 02-20-2014, 11:32 PM
Achelis Achelis is offline
 
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Posts: 14
Default RV6-A: No Cracks

RV6-A
First Flight 1996
980 Hours TT
No cracks in HS or Elevator Spars
Relief per plans
Heavy Acrobatics first 25 hours / very little thereafter
98% paved / 2% grass or dirt / 10% very poor landings
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  #585  
Old 02-21-2014, 03:25 AM
SvingenB SvingenB is offline
 
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Location: Norway, Stj?rdal
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Quote:
Originally Posted by Walt View Post
Helps to relieve the stress concentration in the corner of the web where the cracking is occuring.
Maybe, but in this case I can't understand how that would matter at all. The main problem is clearly an original design choice where the flange (on a structural beam) changes from being on the fwd side to the aft side with no overlap. Thus leaving a small section of the structural beam void of flange material which causes the shear web to carry too much compression/tension load. In fact, so much that it ends up cracking.

Since the main problem has not been addressed by the SB, the forces carried by the shear web is exactly the same amount as it was before. However, the added web material will of course make the stresses in the web much less than they were.

Fatigue in metals is a function of the main stress value and the stress amplitudes together with the number of cycles. Stress "peeks" as function of design means nothing as long as the critical values are controlled. In the SB, the single only thing that decreases stress is the added material on the web. Trimming of the flanges certainly help smoothing out the stresses, but has little or no effect on the max stress amplitudes. You cannot fool the basic laws how structural beams work.
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  #586  
Old 02-21-2014, 07:13 AM
gear1 gear1 is offline
 
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Location: Pagosa Springs CO
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Default Inspection report

RV8
IO360
Hartzell CS prop
1650 hours
Lots of acrobatics up to 5 G
Some unpaved runways
No relief notches
Tiny (1/8") cracks on top right and bottom left (blended out by creating notches).

Installing modification per SB

Questions for Walt:
1. one rivet hole measures .147" on the bottom angle (HS 714) and I slightly damaged (oval) one hole in the upper angle (HS 714). Should these holes be drilled out to 5/32" and AN470AD5-7 rivets installed?

2. The two rivets just inside the bend line (labeled "staggered rivet is oriented up" and its counterpart) seem very close to the notch adding to the stress riser. Would it be appropriate to move this rivet down 3/8"?
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Last edited by gear1 : 02-21-2014 at 07:19 AM. Reason: misspelling
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  #587  
Old 02-21-2014, 07:23 AM
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Walt Walt is offline
 
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Quote:
Originally Posted by gear1 View Post
Installing modification per SB

Questions for Walt:
1. one rivet hole measures .147" on the bottom angle (HS 714) and I slightly damaged (oval) one hole in the upper angle (HS 714). Should these holes be drilled out to 5/32" and AN470AD5-7 rivets installed?

2. The two rivets just inside the bend line (labeled "staggered rivets is oriented up" and its counterpart) seem very close to the notch adding to the stress riser. Would it be appropriate to move this rivet down 3/8"?
I appreciate your confidence in me, but, I know my limitations... when it comes to deviating from an approved spar repair I think the engineer would be the best place to go for that info.

It's "probably" ok as most engineers will build in some buffer for mistakes, but I don't think probably is good enough in this case, get the answer from "the man".
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Last edited by Walt : 02-21-2014 at 07:25 AM.
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  #588  
Old 02-21-2014, 10:56 AM
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rvbuilder2002 rvbuilder2002 is offline
 
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Location: Hubbard Oregon
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Quote:
Originally Posted by SvingenB View Post

Since the main problem has not been addressed by the SB, the forces carried by the shear web is exactly the same amount as it was before. However, the added web material will of course make the stresses in the web much less than they were.
???
Not sure how forces/stresses can be the same and be less at the same time...

Quote:
Originally Posted by SvingenB View Post
Fatigue in metals is a function of the main stress value and the stress amplitudes together with the number of cycles. Stress "peeks" as function of design means nothing as long as the critical values are controlled. In the SB, the single only thing that decreases stress is the added material on the web. Trimming of the flanges certainly help smoothing out the stresses, but has little or no effect on the max stress amplitudes. You cannot fool the basic laws how structural beams work.
As mentioned previously in the thread, this mod is not for making the structure stronger. Previous static testing has already proven that it meets the design requirements for limit and ultimate loads.
The SB is to a localized stress area that after lots of load cycles can possibly develop a crack. Tapering the flange of the fwd. spar, and confirming that there is a tapper in the top flange of the top angle does help further distribute the load through this area. This was substantiated with FEA and is also a fundamental engineering practice.

Please refrain from comments that might make people take shortcuts while doing the SB modification, without having personally done a full analysis of the structure.
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  #589  
Old 02-21-2014, 11:48 AM
SvingenB SvingenB is offline
 
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Originally Posted by rvbuilder2002 View Post
Not sure how forces/stresses can be the same and be less at the same time...
Stress = Force/Area

The forces are the same. Your airplane weighs the same, you pull the same amount of G etc. Adding more material obviously decreases the stress in the material. ABC in engineering.

Tapering the flanges weakens the structure further. It does however, smooth out the stress distribution. But to what gain? Absolutely none stress wise. The stress at the weakest point does not change if you weaken the area around it. The strains could benefit from the tapering however, making the HS flex in somewhat the same way as before. This could in fact be important regarding vibration and also for the other SB, the cracks in the elevator.

Static loads and fatigue loads are different things. But stress and strain does not differentiate between the name of the load. To decrease stress, you increase material dimensions for a given load (force). This is the same for static loads and dynamic loads.
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  #590  
Old 02-21-2014, 12:55 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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I strongly recommend that people installing the doubler parts prescribed in the SB do not take short cuts and skip any of the steps.
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