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  #11  
Old 10-28-2017, 12:54 AM
rv8ch rv8ch is offline
 
Join Date: Feb 2005
Location: LSGG
Posts: 2,113
Default Offset Rivet set

Quote:
Originally Posted by keitht View Post
...I decided to reattach the ribs using Cherrymax rivets due to not feeling very proficient with an offset rivet set in a tight location despite setting a dozen or so rivets in a tightly spaced test piece with the offset tool in the rivet gun. Certainly a skill that I need to worked on.
I bought a bunch of cherry max rivets in both -4 and -5 in anticipation of doing this work, with the same fear. I ended up using all -4 normal rivets that Van's supplied with the SB kit.
What I found was that:
  1. with the HS flat on the table,
  2. the part I wanted to rivet on the bottom,
  3. good access to the area,
  4. putting the rivet gun in the larger area (manufactured head aft - yes I know!),
  5. thick cloth between the rivet gun and the skin,
  6. laying the rivet gun on the cloth directly (not trying to hold it in my hand),
  7. let the offset rivet set find it's natural location,
  8. take a lot of time to ensure the rivet gun is perpendicular to the work,
  9. take a deep breath,
  10. about a 1 sec blast (YMMV, depends on a lot of factors)
it went really well. I think I ended up with one small smiley, the rest were pretty good.

Strangely, the -4 rivets attaching the ribs were probably the most difficult, but I had almost full access. I have seen some rivet sets that look like giant crab claws out there somewhere - that would have been helpful for the last rivets.
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http://rv8.ch
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  #12  
Old 10-28-2017, 06:26 AM
keitht keitht is offline
 
Join Date: Jun 2016
Location: coupeville wa
Posts: 11
Default Generic versus specific - philosophy

I suppose it depends how well the design stress numbers match up with actual in-service stresses encountered in the design lifetime of the airframe. The confidence in the number of fatigue damage cycles that can be expected to be tolerated before cracks start to propagate is dependent on the quality of the system modeling performed and the fidelity with actual stress data from flight testing. Since Vans doesn't share any of the modelling analysis data or flight test data and I assume they are using a finite element modelling tool with an appropriate mesh and a representative number of flight /stress cycles then the edge distance numbers can be whatever the design / analysis will support and we have to assume in those critical cases they have done the actual analysis and its not the case of " it will be alright you will find".
The difficulty with which the manufacturing process variability can meet those numbers is the big question and reducing edge distances below 2:1 together with manufacturing variability will result in large variability in number of flight cycles before cracks start to propagate. Ability to drill round holes of exact diameter in exactly the correct place is critical to the process. Obviously we are not going to achieve that in the amateur build world so I much prefer to stick with the textbook design guidelines for that added margin. I feel less inclined to worry if it is all going to come apart when I get bounced around in turbulent conditions.
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  #13  
Old 10-28-2017, 09:17 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 6,982
Default

Quote:
Originally Posted by keitht View Post
I suppose it depends how well the design stress numbers match up with actual in-service stresses encountered in the design lifetime of the airframe. The confidence in the number of fatigue damage cycles that can be expected to be tolerated before cracks start to propagate is dependent on the quality of the system modeling performed and the fidelity with actual stress data from flight testing. Since Vans doesn't share any of the modelling analysis data or flight test data and I assume they are using a finite element modelling tool with an appropriate mesh and a representative number of flight /stress cycles then the edge distance numbers can be whatever the design / analysis will support and we have to assume in those critical cases they have done the actual analysis and its not the case of " it will be alright you will find".
The difficulty with which the manufacturing process variability can meet those numbers is the big question and reducing edge distances below 2:1 together with manufacturing variability will result in large variability in number of flight cycles before cracks start to propagate. Ability to drill round holes of exact diameter in exactly the correct place is critical to the process. Obviously we are not going to achieve that in the amateur build world so I much prefer to stick with the textbook design guidelines for that added margin. I feel less inclined to worry if it is all going to come apart when I get bounced around in turbulent conditions.
That is the great thing about the experimental category.... you can decide what is right for you.

For those that know they don't have the background knowledge to make structural evaluations on their own......
The SB was issued for an age related fatigue issue that no one had ever detected in the field until their attention was directed to it via the SB.

When the SB was issued, the fix was already engineered and analyzed via FEA and then evaluated further to approve corrective measures when some installers had rivet edge distance issues.
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Scott McDaniels
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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