RV-4 plans discrepancies

The inconsistencies in the fuselage measurements on the various drawings are beginning to affect my mental health!!

Right now I'm obsessing about the vertical position of the firewall in relation to the datum (top of the longerons, bottom as jigged).

Fig 7-25, which shows how to build the fuselage jig, shows the height of the firewall above the datum should be 23 1/8"


It also shows that the top (bottom as jigged) should be 1 7/8 above the datum but the drawing is kinda vague about where exactly the measurement is from/to; the bottom reference I can understand, but the top looks like its somewhere on the longeron.......

Drawing 26 shows the top of the firewall as 1 5/8 below datum (now we are the right way up. So what is it - 1 7/8 or 1 5/8??

The sketch with details for longeron bending suggests consistency with the distance of 1 5/8..... But the only way I can get the bottom of the firewall (top as jigged) the correct height above datum is if I use the distance of 1 7/8 from datum to the top of the firewall (bottom as jigged). Because of the way the firewall is constructed I don't see that somehow I might have made it a 1/4" too small.

Probably not the most vital thing to get exact but its nagging.

Next problem is that the F-404 spar box is a 1/4" too low and the only way to get that right will be to use big shims. Doesn't seem good to me. Again, because of the way its built I'm not sure how I could have made it the wrong size.

Then F-407 (back of the cockpit) is 1/4" too tall. This I can correct easily by trimming some metal off the it.

So - previous builders - any word on how to get these dimensions to work out or should I stop obsessing, make it fit as best I can and just build on?

Thanks

Chris

Some Clarification - Maybe!

Chris -

I built an early -4 (1985-1989) and may possibly be able to shed some light on your queries. Since then I have built an HR II, and am now working on a modified -3, so I am pretty conversant with the discrepancies in some of Van's dimensions. I kept my drawings, building manual - with notations added as necessary, and notes from that build to refer to from time to time. So, here goes:

My builders manual is way before there even was a Fig. 7-25. What I had (although I suspect it is pretty much the same today - except for revisions 1 & 2) was SK-39 Fuselage Jig. But SK-39 does indeed show the bottom of the firewall angle frame to be 23.125" above the datum set by the lower face of the main longerons. SK-39 doesn't show the 1.875" dimension, but by doing the math, and assuming (as I am pretty sure Van did) that the actual dimension for the 2x4's that provide cross support for the longerons was 3 1/2", that the dimension above the datum to the bottom of the FW frame is 1.625". SK-39 shows 23.125" for height above datum, 5.875" from the datum to the centerline of the motor mount hole, and from DR-22 .75" from the centerline of the motor mount hole to the firewall angle frame, outside edge. Given (assumption) that the 2x4 actually measures 3.5, then the height of the firewall frame above the datum is: 5.875 - .75 - 3.5 = 1.625". Also, given that the callout on DR-26 is also 1.625" - I would discount the 1.875 dimension given in Fig. 7-25.

DR-22 shows the finished vertical dimension of the angle frame for the FW as 21.5" - I just checked the one I finished for my current project and confirmed that dimension. Hopefully yours matches as well.

As regards the F-404 and F-407 - see my comment above about R1 & R2 for my fuselage jig sketch. Those two revisions (R1 - August 1986 & R2 - January 1987) changed the height of F-404 by + 1/8" up, and F-407 by - 1/8" down. F-404 was 25.813" and was revised to 25.938"; and, F-407 was 23.375" and was revised to 23.250". Van didn't change the size of the bulkheads to account for these changes - it was up to the builder to "make them fit". Probably still is - I still remember the big shim for the F-404.

I hope this helps a bit. If you need more info or would like to contact me directly you can e-mail me at [email protected] or call my on my cell at (559) 816-0435.

YMMV

David Howe

Thank you!

David, wow, thanks so much.... If you were nearby I'd take you to the nearest pub and buy you several beers for that!

I made my jig from 1 inch thick ply cut into 4 inch strips - so it never occurred to me to remember that a piece of sawn 4x2 isn't actually that size. I can now see that the measurement on my drawing 7-25 must indeed relate to the motor mount hole. So that's all nicely clear.

I'm concerned that I might need such a big shim under F-404 and the history of that measurement is interesting to read. I'll contact Vans and query the best thing to do.

Thanks again.

Chris