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  #1  
Old 11-30-2008, 01:24 PM
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Jamie Jamie is offline
 
Join Date: Jan 2005
Location: Atlanta, GA
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Default Interesting in-flight oil-canning photo

This past Saturday Scott Will and I flew over to Augusta (Daniel Field) for some lunch. Scott has previously written about the place we ate at here. The line dude even drove us over to the gate in his golf cart and we didn't buy any fuel or anything. Nice guy.

Anyway, I asked Scott to get some air-to-air photos of my plane. In particular, I wanted him to do some of me rolling. Well...

That was successful! I wish the horizon was in the background for the full effect, but oh well.


Anyway...the purpose of this post was in one of the pics he captured of me as I was rolling.



Check out the zoomed in version:


Notice the oil canning happening in a substantial portion of the tailcone (please excuse the oily belly).

From the baggage wall (F-706) back you can see the skin wrapping around the spar/rib that runs to the next bulkhead (F-707). You can also see the oil canning running back to the F-708.

This was with a relatively light pull on the stick to pitch up for the roll -- about 1.5G I would guess. Anyway, I found it interesting and I think that with all the rolling I do if I build another -7 I would probably run a couple of j-stringers down those areas that are oil canning in this picture.

BTW: The pics make it look like we were a lot closer than we really were. He has a fancy SLR camera with a big lens and the photos were cropped.

Thanks, Scott for the pics. They were great.
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  #2  
Old 11-30-2008, 04:51 PM
Danny7 Danny7 is offline
 
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Location: central oregon
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Quote:
Originally Posted by Jamie View Post


From the baggage wall (F-706) back you can see the skin wrapping around the spar/rib that runs to the next bulkhead (F-707). You can also see the oil canning running back to the F-708.

This was with a relatively light pull on the stick to pitch up for the roll -- about 1.5G I would guess. Anyway, I found it interesting and I think that with all the rolling I do if I build another -7 I would probably run a couple of j-stringers down those areas that are oil canning in this picture.

why? do you think the oil canning will decrease the service life of a certain part? have you asked vans about this? I'm asking because i am building a 7 and i think it is good to get other peoples opinions on modifications, and the reasoning behind them.
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Old 11-30-2008, 05:14 PM
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Jamie Jamie is offline
 
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Quote:
Originally Posted by Danny7 View Post
why? do you think the oil canning will decrease the service life of a certain part? have you asked vans about this? I'm asking because i am building a 7 and i think it is good to get other peoples opinions on modifications, and the reasoning behind them.
Well, mostly just for jollies. Theoretically an oil-canning skin with fatigue more rapidly than one which does not oil-can. This particular case of oil-canning appears to only happen at high angle of attack and not during cruise flight, so I'm sure that it will not have long term effects.

I'm sure it's not really an issue at all -- I just thought it was an interesting photo.
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  #4  
Old 11-30-2008, 05:24 PM
alpinelakespilot2000 alpinelakespilot2000 is offline
 
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Quote:
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Well, mostly just for jollies. Theoretically an oil-canning skin with fatigue more rapidly than one which does not oil-can. .
Very interesting pictures Jamie!

Before anyone does anything, however, you might check with Van's. I'm almost sure I read from some official source, perhaps 24 Years of Rvator, that adding stiffeners was actually counterproductive and might lead to other problems. I distinctly remember reading this because it seems now (and when I read it) as being counterintuitive, but the explanation was logical.

Again, I could be wrong, but before anyone goes and adds stiffeners just to address a problem that might not be a problem, it might be worth checking with Van's.

Hope this helps.
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  #5  
Old 11-30-2008, 05:47 PM
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jsharkey jsharkey is offline
 
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Default Interesting Mechanical Test Data

Stick back for pull up ---> Down load on tail ---> Downward bend on on tail cone ---> Compression on lower skin ---> Compressive buckling of lower skin between #7 and #8 bulkheads ---> Some load shed outboard to J Stringers ---> Some of this load sheared through skin between #6 and #7 bulkheads into keel beam ---> Shear buckling of these skin panels - note ~ +/-45 deg angles of buckles on forward panels compared with panel to rear.

Any sign of the distortion when sitting on the ground after the flight?

Interestingly a friend's RV6 has a mid J Stringer on the big, bulkhead #7 to #8 skin. This plane hasn't flown yet but the extra stringer might stabilize the panel and help transfer load more evenly from back to front.



Has anyone built a finite element structural analysis model of an RV. It might help work out how to prevent the buckling and therefore any subsequent fatigue damage to these skins.

Jim Sharkey
RV6

Last edited by jsharkey : 11-30-2008 at 06:53 PM.
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  #6  
Old 11-30-2008, 05:50 PM
Sid Lambert Sid Lambert is offline
 
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http://www.rvproject.com/20060211.html

This seems to be fairly common in -7's.
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  #7  
Old 11-30-2008, 06:26 PM
terry.mortimore terry.mortimore is offline
 
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Location: Sault Ste Marie, Ontario Canada
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Default Oil canning

Hi gang:

Another word of caution, I do remember reading somewhere of a fellow who put siffeners in between the bulkheads, but did not tie them into the bulkheads. This resulted in cracks developing in the skins at the ends of the stiffeners.

cheers, terry.
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Old 11-30-2008, 06:46 PM
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jsharkey jsharkey is offline
 
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Originally Posted by terry.mortimore View Post
Hi gang:

Another word of caution, I do remember reading somewhere of a fellow who put siffeners in between the bulkheads, but did not tie them into the bulkheads. This resulted in cracks developing in the skins at the ends of the stiffeners.

cheers, terry.
Stiffeners certainly "attract" load that needs to be transfered in and out of them adequately.

By the way - Anyone noticed anything similar with opposite loads, e.g. when the tail wheel makes a hard contact putting the upper tail cone skin in compression. However I assume that the skin curvature here stabilizes it considerably.

Jim Sharkey
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Old 11-30-2008, 07:04 PM
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Default Corner Radius?

Out of interest - is the the lower corner radius of the tail cone on the RV7 tighter than on the RV6? The radius on my 6 looks much more generous than the ones on the pictures above.
Jim Sharkey
RV6
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  #10  
Old 11-30-2008, 07:08 PM
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Bubblehead Bubblehead is offline
 
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If you have stiffeners that do not tie into the bulkheads you could create a local flexible area between the stiffener and the bulkhead. Some people call it "hinging." My structural experience is almost exclusively with railroad cars, many of which have aluminum bodies. We always look for areas in our designs where two stiff areas are tied together using a less stiff piece. The same thing can happen with steel structures. You want to gradually transition from stiff to less stiff structures.

Obviously airplanes are very different from railroad cars but physics is physics and good practice usually holds true in different applications. In my day job I'd be concerned about transverse cracks in the aluminum skin alongside the bulkhead.
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