F770 Conical Bends- Now what to do.

Just as I messed up the bends on the elevators, I now have two F770 skins with cracked and flattened conical bends. It appears that when the bend got just above the hole for the steps, it flattened out and then cracked. I stop-drilled the cracks and tried to install the skins, but nothing is now lining up. I'm even entertaining an idea of cutting off the sections, fabricating bends out of .020 AL and riveting them on. Before I attempt this or order new skins, anyone got any ideas?

Thanks
Paul

AC 4130 will tell you to place a doubler one size thicker than the skin on the inside. My recommendation is to dimple the patch and skin and put a repair over that extends out far enough to have a double row of rivets. Make sure you smooth out the cracked area or it will crack past your repair. Or you can just buy a new skin if you can't live with looking at the repair for the rest of you life.

Mike Harris said:

AC 4130 will tell you to place a doubler one size thicker than the skin on the inside. My recommendation is to dimple the patch and skin and put a repair over that extends out far enough to have a double row of rivets. Make sure you smooth out the cracked area or it will crack past your repair. Or you can just buy a new skin if you can't live with looking at the repair for the rest of you life.

Click to expand...

Last night, like about 3:00 this morning, I started fabricating a forming block for making the patch. I measured the distance back from the start of the tight bend past the damaged section and the vertical distance the cone would form, then made that radius on the end of a 4X4 block of wood tapering down to a tighter radius distance. I also installed the step, tracing out where the doubler would meet. Sunday I have all day to work on it. Thankfully it will be on the bottom and partially covered by the step.

After looking at both skins and seeing that the damage started at the same place, the hole for the step may have been a contributing factor.

It happens...

Hey Paul,
The same thing happened to me this afternoon, mine was just from purely not paying attention to what I was doing. In my case I tore the skin toward the hole for the step almost halfway. I get it, the 43.13 says to install a doubler on the inside, there are a lot of things that come together in that little spot. My solution is to install a .025 doubler on the outside under the step that blends in with the curve of the skin but is covered mostly by the step, this way it will cover the nice crack I installed but also serves to help beef up the step area. Will I make one for the left hand side? I guess I will see if I crack that one too, stay tuned.
Hope that helps,
Karl

Karl:

Wow, and I only thought I did that. I'm still hoping the doublers will solve the problem. I hate to have to buy new skins at $76 each. I was really hoping to get to riveting over the holidays.

I have sent an email to Van's builder support hoping they also might have a solution.

I'm anxious to hear how you fair out on you attempts

Paul
N277PM
Fuselage
LAF

My Solution

Hey Again Paul,
Well, it appears that I am still unable to embed a photo here on these posts. Here are some links to the pics that I took, the first one is of the damage that I caused by, like I said, just pure not paying attention, I will hopefully learn my lesson for the other side. This first pic is pretty ugly, I still have some cleanup of the crack and de-stressing but I think it will be fine.

The next two are of the outside doubler that I fabricated from .020 Alclad, I did have to put a few extra rivets in the doubler but for the most part, it will pick up existing holes and will utilize the holes that I will have to drill in the step fitting.





I do feel like an amateur when I pull stuff like this but I guess it happens, and hopefully other builders can learn something from my mistake. Look at the picture in the builders manual and also look at how Bruce Swayze did it. http://www.europa.com/~swayze/RV-7A/Fuselage/20090404.html He did a very nice job on his bends.

I think the repair will work and mostly be hidden by the step assembly, beyond that, when the time comes, I will fair it in with Pro-seal and paint it. It's all good.

Best Regards,
Karl

....and finally

I did the conical bend on the opposite side, took me one try and about 15 minutes, it fit perfectly, no cracks and everything line up just like its sposeta. It's after one in the morning here in Montana, I guess I will call it a night.
Over and out.

Karl:

Amateur?!?! I hope my only turns out half as good as yours. That is excellent work. I didn't think about making the doubler to the outside dimensions of the step doubler. I hope you don't mind me stealing shamelessly of your fix. I'm sure others with RV's would appreciate seeing this.

Paul

Bruce's setup is really nice and would have made it easier for me; I did mine by the 'multiple converging small bends' method, which came out looking like one large smooth bend. But, while I can't comment on an appropriate fix (other than to say that stop drilling the crack is good but you should also go back and smooth the edges of the crack, which will probably mean using a cutoff wheel to turn the crack into a slot), in the spirit of helping others who come after, I want to point out something I saw in the first picture of this thread. The bend at the narrow end is too sharp. In fact, it is too sharp all the way to the end of the crack. Unless the geometry has changed from the -6 to the -7 (I'm too lazy to run upstairs for my plans), it should transition from the large radius to a minimum of 1/8" at the narrow end. I went slightly larger, using 3/16" drill stock as my bending guide. I found that I had to clamp my guide very firmly or it would tend to twist, especially at the narrow bend. Even so, there was enough slippage that my final bend actually was approximately 1/8" at the narrow end.

Thanks Paul,
My repair idea will work out well, except for the extra conical area of the patch, it will be hidden by the step, the two main reasons I did it this way was to hide that nasty looking crack and because there is just too much metal converging in that corner for an inside doubler, I did make it out of .025 Alclad so its the same thickness as the skin. It was about a 6 hour screwup that could have been used somewhere else. I will dimple it just like everything else when I pull the skin back off for the finish work. Another helpful thing that I do and I try to have my students do is to make a template of the patch, I used the cardboard back off of a steno pad but a manila folder works too. I hope my idea works for you and it saves you some money. Good luck Paul, let me know how it goes.
Karl

Karl:

It worked great. Frank Smidler stopped by while I was fabricating the doubler and helped me with the fit. I will upload a picture of the result soon. (I use photobucket to upload pictures, I think Frank uses picasa so he might be able to help you with your uploads)

Remember, the step doubler calls out AN470 rivets, so those holes won't need dimpling.

Thanks

Paul

Paul,
True story on the AN470 rivets, I think the step actually picks up new holes between the existing AN426 rivets, I will have to check again. My patch has a couple of new holes that I drilled on the curved area because of how I designed the repair. I'm glad you are headed down the road past this problem.
Karl

Conical Bend solution from Van's

One of mine turned out perfect, the other developed a little crack. I notified Van's and Joe B. offered the solution below. I stopped drilled it but haven't created a doubler yet. I moved on.

"Try stop drilling the hole with a #30 bit and then fabricating a doubler patch, and riveting it to the inside of the skin. That should beef it up and keep it from cracking further."

Doug
RV-7
Fuselage

It was nice to find this thread on how to bend the conical. I posted on the 5th of this month asking if there was a thread like this and it was closed the next day!?

I read all the postings and then went out to the garage and applied/blended a couples techniques. Both bends turned out very good with NO cracks! Very Pleased!

One thing I will add is that the angle iron has to be on the inside of the bend. I almost messed up the second one, the bend will start at the hole line, I was watching for it and caught it in time!

John A
RV7A