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RV-6 Fuselage Bulkhead Alignment

Randall Henderson, RV-6
Editor, Home Wing Newsletter
http://www.edt.com/homewing
randall@edt.com

“When I check alignment of my rear bulkheads by running strings between F-606 and F-612, I cannot get F-610 to align with the other bulkheads.”

This is an issue that I came up against, and although the answer is still somewhat ambiguous, I went through an interesting process, and found out some things about this very issue that you probably want to consider.

On the bulkhead fabrication drawing (my old one, I didn’t get a new one either), at some point there was a revision to widen the F-607 and F-608 bulkheads by 1/8” per side, or 1/4” overall. Local lore had it that this was done on purpose “so the skins would lie better”. When I got my fuselage all jigged up I found that there was a bulge in the longerons because of this widening. I decided to call Vans and confirm that this was indeed supposed to be there, since nothing was said about it in the manual, and in fact the longeron curve profile on DWG #22 specifically shows the longeron being “straight aft of this point”. Tom and Bill both told me no, the longerons are supposed to be straight, the revision was because previously the bulkheads were too narrow. Not being satisfied with this, I did the geometry and found that, mathematically, using the dimensions prior to the revision, and at the station lines called out, the bulkhead sides would all have formed a straight line. The extra 1/8” per side added in the revision would add a bulge, and you’d have to move the station lines to make them straight again. I pointed this out to Vans (they really hate it when you do that) but they were still not convinced, and told me if I had to change the station lines to make them straight, then do so. This seemed really weird to me. If I had to move the station lines because of the revision, then why the revision in the first place, and why not move the station lines at the same time as the widening of the bulkheads?

As a reality check I looked at four other RV-6s under construction in the area, and ALL of them had this bulge.  So I decided to track down the lore that said they were supposed to bulge.  I finally found that it originally came from Art Chard who, as we all know, used to build prototypes for Vans.  Art told me that yes, a side effect of the way the tailcone skins are rolled is that there is a slight curve to the skins perpendicular to the bottom “curl”, and that the bulge was added so as to offset the tendency of the skins to oil-can.  He also said that aerodynamically, an 1/8” bulge should be insignificant.

So according to Art Chard, there is supposed to be a 1/8” bulge per side between the F-606 and F-610.

I went back to Van's with this, and they finally conceded that there may have been a revision such as this that no one remembered the reason for, but they also said that they went out and laid a straightedge up against the factory demonstrators, which were all built by Art Chard, and they couldn’t detect any bulge.  Go figure.

So I left the bulge in.  I did end up widening the next aft bulkhead by 1/16” (F-610) to remove a slight “fishtail” flaring at that point.  Now when you sight down it you can see a slight curve, but otherwise it’s undetectable.  My skins lie nice and smooth, and there appears to be no oil canning.

Bottom line is, I really don’t know if it helped or not, but at least it didn’t hurt anything.
 


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