Fuselage extra stiffener

Some builders have reported oil canning of the bottom skin between F-707 and F-708, so I am thinking of back-riveting a J-stiffener down the centre there, before aft fuselage assembly. Does anyone know of any reasons not to do that?

Conor
RV-9A 90990 fuselage

Shouldn't be any problem with that. I noticed Dan C. just RTV'd a channel on his to take out the oil canning. Just make sure that you put the ends of the channel very near to the flanges of the bulkhead ahead and behind it... that way you don't create a point where the skin bends and eventually cracks.

I did what you suggest on my RV6 back when I built it in 1997. I joggled the ends of a piece of angle and riveted it on. No more oilcanning and it was in perfect shape when I sold the plane almost 900 hours later.

sf3543 said:

I did what you suggest on my RV6 back when I built it in 1997. I joggled the ends of a piece of angle and riveted it on. No more oilcanning and it was in perfect shape when I sold the plane almost 900 hours later.

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By "joggle" do you mean that on one end you attached it on top of one of the bulkhead flanges? (On the -9, at least, I couldn't do it on both ends since the bulkhead flanges all face forward.)

Also, is this a common enough problem that I should do it now before I put the center and aft sections together, or should I just wait to see if it's a problem? There is a little looseness there now, but I haven't added the top skins yet.

Thanks.

I asked Tom Green (Van's GM) about this and he was quite definitely against adding stiffeners to the fuselage skins. He said it was not needed and only added weight. He also said that if I ignored his advice to be sure to rivet the stiffeners to the bulkheads on either end, otherwise the skin was sure to crack at the end of the stiffeners. I took his advice and built according to the plans, ie: no extra parts added. Steve.

I agree with the above - Don't do it. I've never heard of this "problem" until now. Chances are you'll create a sress point.

Stephen Lindberg said:

otherwise the skin was sure to crack at the end of the stiffeners.

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Same reason the control surfaces crack at the end of the stiffeners.

I agree with the comments regarding bending stresses. However, how much does the fuselage floor actually bend while in flight....don't know. Maybe it's a concern, maybe not.
If you do add a stiffener, bend the ends up into a 90, or whatever and rivet to the bulkhead, or at least over the bulkhead flange on both ends. I have also seen where folks have added stiffeners simply by bonding them on with JB Weld or pro Seal,
You'll never hear it oil can while you are flying around, but every time you clean it.......

Reviving old thread due to issue stated

Hello,

I purchased a 2005 -6A last year and have flown 200 hours since. During this annual we have found 4 cracks on the bottom skin of the tail cone at the first rivet away from the bulkheads on 2 fuselage stiffeners. The cracks are definetely from a focused stress point due to the stiffeners being installed a little way off of the bulkheads. They look to be about 1/2" distance between the bulkhead and the end of the stiffener.

Now, short of new skin how do I fix this issue? FAA 4313 finger stiffener across the bulkhead and joining the stiffeners is how I am leaning, but would like to hear if others have experienced the same issue and resolved.

Thanks,

Kevin

I put the j-channel stiffeners in my -6 but fortunately the cracking problem had surfaced by then so I fastened them to the formers. No cracking in 800 hours. As for Van's thoughts on the subject, I asked them about .020 skins for the elevators and rudder (a long time ago) and they asked "Why do you want to do that?" Now .020 is standard...