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Interesting in-flight oil-canning photo

Jamie

Well Known Member
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.
2008-11-29.2227.jpeg


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

2008-11-29.2228.jpeg


Check out the zoomed in version:
2008-11-29.2229.jpeg


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.
 
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.
 
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.
 
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.
 
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.

billret0.jpg


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
 
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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.
 
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
 
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
 
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.
 
Does a small elevator deflection really cause that much downforce? Enough to crumple the tailcone? If you look close at the photo, it's a very small deflection, and besides, the oil canning is apparent well into the roll and presumably after the elevator has been neutralized.
 
I'm a mechanic for a major airline. Recently in one of the hangars at our facility we have been doing gear changes on 747's. The entire airplane is put on jacks and the gear is removed. With the nose gear gone the aircraft is a bit tail heavy and in the afternoon when the sun shines in at a low angle you would not believe how it looks, you'd think it was made of tin foil with major oil canning all along the aft section of the fuselage. You can actually see a few large wrinkles in the structure.

As for adding ribs or stiffeners to prevent this sort of thing - not a good idea. You're simply transmitting the forces that cause the oil-canning to another point and probably concentrating them there as well. I recently rebuilt the horizontal stab on my Luscombe. Over it's sixty odd years of life the bottom of the stab had been reskinned several times. The last guy thought it would be a good idea to reskin it in .025 rather than the original .020. The result was a bunch of cracked ribs.
 
Mine too

My 9A exhibits a thump on the flare, only with full flaps and if I am a bit too fast. Narrowed that down to the tailcone skin oil canning in that situation. Now that it's cold outside, I cant make it happen. Thought about bonding in a stiffener, so I called Vans a few weeks ago, and they advised against it. Keep in mind, the 9A is a non aerobatic bird, so stresses are not as big an issue. The plane will never see the G's you guys give them. I was told that this is a common issue with these planes. My thoughts are...Seems there are a bit more problems to be had with the fix than there is with the issue. At least as far as the 9 is concerned.

Info only...HTH
Chris
 
If I recall correctly, local buckling (oil canning) is not so much an issue unless that local instability reaches such a point where the main geometry's beam properties are lost.

To delay that phenomenon stiffeners are usually installed. If you do so, try to taper their ends by sloping the stringer's web down to the flange at say 30? and, make the end rivets one size smaller. The ends of a joint (including the rivets) are always more loaded than away from it (structures 101). Those two strategies tend to make a more even load transfer distribution along the joint of two parts and minimzes the amount of bending loads at the end ofthe joint (indeed, at the end you get a skin on a skin).
 
Does a small elevator deflection really cause that much downforce? Enough to crumple the tailcone? If you look close at the photo, it's a very small deflection, and besides, the oil canning is apparent well into the roll and presumably after the elevator has been neutralized.

I've flown a bit of formation and had quite a few people tell me that they don't see those areas oil canning during straight and level flight. The only area people have noticed any oil canning during normal operations is on the right side, aft of the baggage wall, between the lower j-stringer and where the side skin and floor skins meet.
 
I'm a mechanic for a major airline. Recently in one of the hangars at our facility we have been doing gear changes on 747's. The entire airplane is put on jacks and the gear is removed. With the nose gear gone the aircraft is a bit tail heavy and in the afternoon when the sun shines in at a low angle you would not believe how it looks, you'd think it was made of tin foil with major oil canning all along the aft section of the fuselage. You can actually see a few large wrinkles in the structure.

Boy I really did not want to know that.
 
That may have been me!

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.
During build, I added a stiffener and only tied it to the bulkhead at the forward end. After about 10 years, a small skin crack developed at the aft end of the stiffener. I tied it to the bulkhead with a piece of angle and it's been fine ever since.
 
I'm a mechanic for a major airline. Recently in one of the hangars at our facility we have been doing gear changes on 747's. The entire airplane is put on jacks and the gear is removed. With the nose gear gone the aircraft is a bit tail heavy and in the afternoon when the sun shines in at a low angle you would not believe how it looks, you'd think it was made of tin foil with major oil canning all along the aft section of the fuselage. You can actually see a few large wrinkles in the structure.

As for adding ribs or stiffeners to prevent this sort of thing - not a good idea. You're simply transmitting the forces that cause the oil-canning to another point and probably concentrating them there as well. I recently rebuilt the horizontal stab on my Luscombe. Over it's sixty odd years of life the bottom of the stab had been reskinned several times. The last guy thought it would be a good idea to reskin it in .025 rather than the original .020. The result was a bunch of cracked ribs.


I'm sure the wrinkles smooth out when the fuselage is pressurized at 30,000ft:)
Jim Sharkey
 
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

I checked out the -7 preview plans and sure enough the lower radius of the tail cone is tighter than on the -6. Plus the -6 has two lower J-Stringers, one at the top and bottom tangent points of the corner curve. The -7 only has one at the top edge of the corner. The effective size of the panels that buckle or "oil can" are therefore larger and less stable on the -7.

Jim Sharkey
 
I'm a mechanic for a major airline. Recently in one of the hangars at our facility we have been doing gear changes on 747's. The entire airplane is put on jacks and the gear is removed. With the nose gear gone the aircraft is a bit tail heavy and in the afternoon when the sun shines in at a low angle you would not believe how it looks, you'd think it was made of tin foil with major oil canning all along the aft section of the fuselage. You can actually see a few large wrinkles in the structure.

Just take a look at the aft lower fuselage of a 757 sitting on the ground, the wrinkles in the fuselage are plainly visable between the stringers and frames. We noticed that 15+ years ago when they were first delivered, Boeing said don't worry about it... they are still there and have never been a problem.
 
Just take a look at the aft lower fuselage of a 757 sitting on the ground, the wrinkles in the fuselage are plainly visable between the stringers and frames. We noticed that 15+ years ago when they were first delivered, Boeing said don't worry about it... they are still there and have never been a problem.


I witnessed a 767 being incorrectly jacked (major airline) and the oil canning we saw spanning the top of the fuselage was terrible. Our Engineering department spoke with Boeing Engineers about it. They said "Let it down off the jacks and see if the skins straighten out. If so, you haven't exceeded the Ultimate Yield Point of the sheetmetal so don't worry about it." As bad as it looked on jacks it was fine after being lowered. Aircraft is still out there flying.

Ken
 
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One builder's solution...

Jamie's post was very timely for me since I'm currently building my fuselage, so I decided to add some stiffeners. Here are some photos that show the arrangement:

First was to locate and drill two additional J-channel stiffeners. About 4 or 5 inches in from the edge of the bottom tailcone skin, you can find rivet holes that form a straight line between the F-706, F-707, and F-708 bulkheads. I used roughly 1" rivet spacing between the bulkheads.
20090326-01-tn.jpg


I extended the stiffeners about 6" aft of the F-708 bulkhead and tapered them down over that 6" length so that I wouldn't end up with a stress riser at the end of the stiffener.
20090328-01-tn.jpg


At the forward end, I fabricated some little brackets and tied the stiffener into the F-706 bulkhead where one of the baggage ribs attach.
20090328-07-tn.jpg


Since I was going to have to cut out a section of the lower flange in two places on the F-707 and F-708 bulkheads, I made up some doublers which
will also serve to tie the flange of the J-channel stiffeners to the bulkhead flanges.
20090107-02-tn.jpg


Here's what the stiffeners look like as they pass through the F-708 bulkhead. Notice how the doublers tie everything together. The F-707 is similar.
20090328-12-tn.jpg


And here's a full view of the stiffeners. All ready for disassembly, deburring, dimpling, priming, and riveting.
20090328-13-tn.jpg


Hope someone else finds this post useful.
 
Oil Canning

I used to live down the street from the Air Force Museum in Dayton, Ohio. The amount of money put into engineering and construction of these aircraft is far beyond that available to us mere homebuilders. If you have never had the opportunity to visit, you will be amazed....and you will see more oil canning and wrinkles than you thought possible on planes that go super-sonic.

Adding additional structure to "eliminate" local oil canning is only adding extra weight. Unless the structure uses techniques of manufacturing far beyond our capabilty, and much heavier aluminum, you'll have oil canning in the light .025 2024-T4 aluminum. And even with those advance techiques you'll still end up with some wrinkles.

If you want to build a plane with no "oil canning" then build a compost one, not an aluminum one. That is one of the purported advantages of a Lancair, e.g. Personally, I don't think a lot of those airplanes, but then I love metal airplanes.

Tom Hunter
 
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At the forward end, I fabricated some little brackets and tied the stiffener into the F-706 bulkhead where one of the baggage ribs attach.
20090328-07-tn.jpg

snipped

Hope someone else finds this post useful.

Mike
The piece you made to tie your stiffener to the bulkhead will not help. You need to connect the VERTICAL portion of the stiffener to the bulkhead to archive your desired goal. You need to fabricate a doubler that is triangular in shape. Make it come out farther onto the doubler, to spread the stress. The doubler needs to be connected to the bulkheads at BOTH ends. If it is free at either end, all it does is concentrate the stress into a very small area. This will INCREASE, rather than decrease the chances of stress cracking.

Added-Stiffener-front-tie-in-detail-1.jpg
[/url][/IMG]

Forward-tailcone-wiring-run-2.jpg
[/url][/IMG]

Charlie Kuss
PS I FINALLY figured out how to attach images. Image Shack is a lot easier to use than Photo Bucket.
 
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Unless you are an engineer and in the aircraft industry I would not be try to fix something that is not broken. You will end up transferring loads to where they were not designed to go thus possibly causing more problems and maybe bringing an end to your life prematurely.
 
Mike
The piece you made to tie your stiffener to the bulkhead will not help. You need to connect the VERTICAL portion of the stiffener to the bulkhead to archive your desired goal.
<snip>
addedstiffenerfronttiei.jpg


Charlie Kuss

Charlie,

I actually considered this approach originally, but chose to make the attachment an "L" bracket since this is very similar to what Van's does when they tie the stiffener into one little tab on a bulkhead ... and the "L" bracket was easier to fabricate.

That said, I think you've swayed my opinion and I'll probably make up some triangle brackets tomorrow. :)

Thank you for sharing your thoughts.

Unless you are an engineer and in the aircraft industry I would not be try to fix something that is not broken. You will end up transferring loads to where they were not designed to go thus possibly causing more problems and maybe bringing an end to your life prematurely.

Norman,

Thank you for your concern. Actually I am and engineer (although not in the aircraft industry). The photos clearly show there is a problem (IMO). Here's why: in monocoque construction (like the Van's RVs), the skin is intended to carry the majority of the loads. However, when the skin buckles like this it's not going to be able to do that. By preventing the skin from buckling the structure will retain it's shape and it's ability to carry load as designed.

Essentially these stiffeners will keep the loads where they are designed to go instead of shedding the loads to other portions of the structure that are less suited to carry them.

If you're also an engineer, yet have a different analysis, please share it. Safety is job 1 in my aircraft factory, after all.
 
No I am not an mechanical engineer. But I have seen many planes where someone has thought of added pieces to in their opinion make things better only to end up in grief. I know of no aircraft designed by Van's to have broken up because of his design and 100's of thousands of flying hours. When I see pictures of design changes such as the stringers with a small piece of AL bent to form an angle to support the stringer I get worried. The gusset attachment of the stinger is a major improvement which will carry the load and transfer the loads. I have never seen an AL aircraft that didn't have some oil canning on it somewhere. Be safe.
 
Making changes to STRUCTURE

Mike,

When you make changes to structure and then promote your changes as needed to the design, you open yourself and anyone who follows your advice to possible litigation.

When you are building a kit and make non kit manufacturer endorsed changes to STRUCTURE, your insurance can become invalid. And if you sell the airplane and the new owner is in a fatal accident....well, you fill in the blanks.

Lastly, there is no proven problem that your "fix" is fixing.
Also, it is misleading to claim that the skin is "buckling". Oil canning is common in all light metal aircraft. "Buckling" is not.


Tom Hunter
 
I have always found VANS very helpfull. Send them a email with pics and explanation and let them endorse the alterations. [if they will] Then print their reply and keep it in your build file. As all of you said, the factory are the only ones allowed to make structural changes.Why cant a engineer endorse it ?
Then again, having said that , the rocket is a "modified" RV and insurance is available.
You guys worry too much about litigation. The unauthorised alteration had to be the cause of the accident for the litigation to take place.
 
Post buckled structure

The oilcanning is simply post-buckled structure and its residual strength, post buckled, is figured into the strength of the structure. I looked briefly, but could not find any photos of Vans structural testing, but if memory serves me, there is signifiacnt oil canned or post buckled structure at those loads. With such a thin skin structure and large bays, buckling is hard to avoid. Just about every large helicopter structure is designed for post buckled conditions. You can get a lighter structure to handle the same loads if one can design for this condition.
 
talk to Van's

I'm sure everyone and their dog will jump in here with the same advice, but before making changes of this type I strongly recommend talking to Van's, who will almost certainly recommend against it.

If you've ever looked out the window when flying a long winged aircraft, you will see a lot of deflection - it's normal. Same for the tail - there will be deflection, it's part of the design. Trying to "fix" that can only be done with a comprehensive analysis of the structure, and adding a lot of unnecessary weight.

Accept the deflection. Deflection is good, deflection is right.
 
Isn't it EXPERIMENTAL?

Mike,

When you make changes to structure and then promote your changes as needed to the design, you open yourself and anyone who follows your advice to possible litigation.

When you are building a kit and make non kit manufacturer endorsed changes to STRUCTURE, your insurance can become invalid. And if you sell the airplane and the new owner is in a fatal accident....well, you fill in the blanks.

Lastly, there is no proven problem that your "fix" is fixing.
Also, it is misleading to claim that the skin is "buckling". Oil canning is common in all light metal aircraft. "Buckling" is not.


Tom Hunter


Tom,

While I agree that modifications should be made with reasonable engineering analysis, remember that these are EXPERIMENTAL airplanes. The builder is the manufacturer and therefore ultimately responsible for any modifications. I have made many modifications to my airplane and not once did I consider the legal implications of doing so. My only interest was to consider the implications of making those modifications to my safety and the safety of my passenger. Kinda like when I was flying for the airline: I never really consciously considered the safety of the hundreds of passengers in the cabin behind me. I just did my level best to keep my own butt safe and figured that they were bound to benefit from that level of care.

I know a former RV builder who gave his plane to a museum when he was through with it rather than expose himself to future legal action if some manufacturing defect might cause harm to a second owner. Maybe he sleeps better at night, but I suspect that his paranoia has merely concentrated on something else, like mad cow disease or global warming. I know, I'm the eternal optimist. Flame me if you must, but I do sleep quite well. :D
 
Switching to Liability Concerns

"When you are building a kit and make non kit manufacturer endorsed changes to STRUCTURE, your insurance can become invalid. And if you sell the airplane and the new owner is in a fatal accident....well, you fill in the blanks."

[Maybe the moderators will switch this topic because it is now going off of "structural modifications" - which should always have an engineering analysis or review by Vans.]

Most folks don't maintain insurance after they sell an airplane and coverage is only for the term of the policy (unless your are buying some special form of policy).

If I were ever to sell my airplane, my method to avoid future claims against me would be to obtain the following provisions in a 'Purchase Agreement' or a 'Bill of Sale' (signed by the purchaser): (1) Release of Liability (by the purchaser, heirs and anyone else claiming by or through purchaser); (2) Hold Harmless; (3) Indemnification; (4) Assumption of Risk after thorough Inspection and Acceptance; and (5) Insurance Requirement (where the purchaser is contractually obligated to maintain insurance naming me as an additional insured and specifically insuring the indemnification provision contained in the Purchase Agreement or Bill of Sale).

Whenever I allow anyone to fly in my aircraft I also have that person sign a release, hold harmless and indemnity agreement. Call me overly cautious, but the protection is for my family because if something happens and I'm not around to protect them there will be people coming after the money my family needs to live on. It would be the same for many others that frequent this board. This is also the reason I do not fly younger folks in my airplane (as much as I would like to). If anyone has a solution for that, please PM me.

Now the big disclaimer - this is not legal advice and may not be applicable in your specific situation. If you think you need help, please see a trusted adviser or attorney.
 
No I am not an mechanical engineer. But I have seen many planes where someone has thought of added pieces to in their opinion make things better only to end up in grief. I know of no aircraft designed by Van's to have broken up because of his design and 100's of thousands of flying hours. When I see pictures of design changes such as the stringers with a small piece of AL bent to form an angle to support the stringer I get worried. The gusset attachment of the stinger is a major improvement which will carry the load and transfer the loads. I have never seen an AL aircraft that didn't have some oil canning on it somewhere. Be safe.

I don't think this "extra" stringer thing is a biggie, and it's getting thrown out of proportion, when me make conjectures of problems with no proof! I worry much more when builders make massive modifications to the rear bulkhead behind the baggage compartment to install doors for ski's, etc. Same goes for the support bar behind the seats. IMO, the stringer is just stiffening up the skin a bit to make it more wrinkle free. The stringers wouldn't need heavy duty attachments to the bulkheads either, since the purpose is keeping the flat metal from tin canning & not supporting loads. At worst, it's a bit of extra weight.

I'm not an engineer either. I've just been building all kinds of sheet metal ducts & contraptions for nearly forty years. I've seen what flimsy sheet metal can do when it's built into particular 'box" structures which can support an amazing amount of weight without twist. That's why the baggage bulkhead modifications bother me much more!

Now, when a structural/ aeronautical engineer wants to prove me wrong, with a tested flying example for poof, I'll change my mind! Afterall, everything Van's does, doesn't mean it's the only solution, or the only correct way. Besides, I've got some additional stringers back there, that support my ELT & Strobe light package.

L.Adamson --- RV6A
 
Wow

My goodness! Lots of replies in this thread. And to think I had just posted these pictures in case someone else would find them useful. Let me address some of these posts:

  1. While there are obviously many capable people at Van's, I'm certain they don't have the time (or inclination) to evaluate the soundness of the large number of modifications that builders have made and will make on their airplanes.
  2. If I use the -A nosewheel design as an example, even when there is a known design deficiency (i.e. flipovers have occurred), a solution may not appear for some time. Van is well aware that good design is a balance of many factors (such as strength, cost of manufacture, ease of assembly, weight). The addition of these stiffeners in my airplane is an only an indication that either:
    One of us has information that the other doesn't (I don't have the original calculations and Van may not have seen these photos) or...​
    One of us has prioritized these factors somewhat differently than the other does. This isn't necessarily good nor bad...just different.​
  3. As far as liability goes, while I have no intentions of ever selling the airplane (she's my baby! :D), I wouldn't think twice about this modification impacting my liability. Frankly, I think we (as builders and pilots) concern ourselves too much about liability. Shouldn't we be responsible for our own actions?
  4. Other builders are going to have to decide for themselves whether they think the structure needs additional stiffeners. If they decide it does, I've shown one way of doing that. As I stated in my first post, I hope someone finds the information useful.

Thanks everyone for taking the time to share your thoughts.
 
Oil Canning

It's been 20 years since I flew the Douglas DC9, but they oil canned regularly and to my knowledge never caused a problem, Roger Moore
 
Torsion ?

Could this deformation be being caused by torsion rather than a bending force ? The wing is inducing the roll, the tail is following. Inertia and air resistance to the axial rotation of the empannage create the forces twisting the fuselage. It also strikes me that the RV tailcone is designed to resist positive G directed bending forces rather than negative G and it should not surprise me that the bottom of this area might wrinkle some.

My example of flying planes with wrinkles is the B-52 ! This plane has huge wrinkles on the side of the fuselage when at rest.

Kerry Stevens
 
Exactly what thought of....

......
My example of flying planes with wrinkles is the B-52 ! This plane has huge wrinkles on the side of the fuselage when at rest.

Kerry Stevens

This is a local Tucson B-52 at the Pima Air Museum, a special one that carried the X-15s.

Note the diagonal wrinkles in the side skin forward of the leading edge of the wing. Similar ones are aft of the wing. The photo does not really do the winkled effect justice.

b52_x15.jpg
 
Schweizer 1-26

Well here is another one. It is very interesting watching a metal wing Schweizer 1-26 winch launch from directly behind. It is an auditory and visual feast.

As the glider pulls up to, roughly, a 45 degree angle, you hear this big WHANG then you look at the top of the wings that are a mass of skin wrinkles from one end to another; deep wrinkles.

Now this whole deal is done at barely over one g but since the tow rope is pulling near straight down, it is as if the glider weighs much more than normal. I'm guessing that typically, depending on speed, the loading on the airframe is equivalent to 4 gs or so.

Kind of scary to watch, but I have heard of no structural problems.

By the way, my RV has a lot of black, and all I have to do to get some whanging noises from it is pull it into the Sun. It doesn't worry me.
 
Torsion vs. compression

Could this deformation be being caused by torsion rather than a bending force ?

<snip>

Kerry Stevens

Kerry,

If the deformation was caused by torsion, I would have expected the diagonal-ness of the deformation to be in the same direction in both bay areas between F-706 and F-707. Since it's not (the deformation is symmetrical around the centerline), I believe this belly skin is telling us it's in compression.

2008-11-29.2229.jpeg
 
Correct :)

Kerry,

If the deformation was caused by torsion, I would have expected the diagonal-ness of the deformation to be in the same direction in both bay areas between F-706 and F-707. Since it's not (the deformation is symmetrical around the centerline), I believe this belly skin is telling us it's in compression.

2008-11-29.2229.jpeg
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.

Jim Sharkey
 
Follow the plans!

I built my plane adhering to the plans as much as possible. I had previouly seen entries regarding stiffeners and also saw a very knowledgable builder in the Mineral Wells, TX area show me the stiffeners he was adding to his -7 in the aft belly area. I elected to forego those. The only experience I've had with oil-canning is that after landing and taxiing to my hangar, just before engine shutdown, I occasionally hear a "tomp!"; (less than a THUMP!) and I assume it's the aft belly skin relaxing after the compression it experiences from the flair of a landing. Every preflight I carefully take a look-a-round for any loose (smoking) rivets or deformed skin. So far, nothing out of the ordinary. (100+ hours) JMHO (Don't over-engineer something that isn't broken) BTW, great thread!
 
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I agree with both sides. I don't think the extra stringers will hurt anything, BUT it's probably overkill. Anyone pulled the tail off one yet?
 
what kind of loading?

That belly shot looks like it's being subjected to air loads, not compression. There would be some visible deformation of the side skins too if the compression loads caused the deformation in that shot.

If I am correct (airloads causing the deformation), the panel is a bit too large for the loads -- it should be made into smaller sections -- using ribs, or stringers/stiffeners. Be sure to fasten the stringers to the bulkheads as noted earlier.

Rule of thumb: the stringers/stiffeners should be made of material at least as thick as the skin.

We use belly skin stiffeners to reduce the 'bonking' noise when we taxi over grass surfaces (tailwheel acting like a needle on a record, with the aft fuse acting like the megaphone). My C-140 did this with a vengence!

Carry on!
Mark
 
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