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Old 08-13-2021, 07:51 AM
keithaloft keithaloft is offline
Join Date: Aug 2021
Location: Mt. Rainier, MD
Posts: 6
Default Wing spar channel size through fusilage RV-4

Hello: I'm the new owner of a storied (1986? #552) RV-4. I just got it, empennage finished and fuselage on wheels. I got it at Oshkosh for anyone who may have seen it there at the Aeromart (sorry if I bought it out from under you: lots of people were looking). It is supposed to include all parts: I haven't quite looked through all the boxes. Wings are not started.
I have been told that it's a good idea to build a plywood spar mock-up to put in the channel through the fuselage: this could also help me move/store it (I'm not going to be able to start on it for a few months).
Does anyone know what the dimensions of the spar channel are? I'm assuming it's a simple rectilinear thing. I will sift through the plans, but I'm building a house right now so I'm sort of busy with other things mostly... It's close to 1 1/4 x 8, I think it was, but I measured quickly on my way out the door this morning and didn't write it down. Are all measurements in SAE?
Thanks! I'm sure I'll be back for lots of guidance.
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Old 08-13-2021, 09:44 AM
vfrazier's Avatar
vfrazier vfrazier is offline
Join Date: Jan 2007
Location: Mount Vernon, IN
Posts: 1,428

I have a thick board that was planed down to 1.25", which is the thickness of the spar.

The spar opening in the #4 bulkhead is slightly larger, so you can get the spar to slide in. This dimension varies depending on the builder... remember, these are hand built, not pre-punched.

If you're building the #4 bulkhead, you should follow whatever Van's plans and text say. Building the #4 on the actual spars is safest, with some spacer material between the spar and the #4 parts so you don't make the fit too tight. (Tape, paper, etc.)

How thick your spacer material needs to be depends on whether you actually plan to install the wings later. (Joking, but not joking!) Leave space for primer/paint, if you're doing that later.

Anyway, you'll want a final clearance size of about +/- 0.020" on each side of the spar. You can build it tighter, just be careful you don't make it so tight that the spar won't slide into the #4 due to burrs, misalignment, paint, dirt, etc.

There is no one way to do this. There are many. Just make sure the spar still slip fits into the #4 after you rivet it together!
Vince Frazier
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Old 08-13-2021, 12:13 PM
keithaloft keithaloft is offline
Join Date: Aug 2021
Location: Mt. Rainier, MD
Posts: 6

Thanks Vince! I think I met you at Oshkosh, we talked a bit.
I'll do exactly that: plane something down to 1.25", and see if it fits. I think all that work is finished on the fuselage, but I'm still deep in the "I don't know what I don't know" part of the process.
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Old 08-13-2021, 04:44 PM
rv468 rv468 is offline
Join Date: Jan 2005
Posts: 92
Default F-404 spacer


It seems if the fuselage is on wheels, the F-404 bulkhead is already riveted in place. Having this spacer bolted in place is most needed when riveting the F-404 bulkhead in place. The front and rear portions of this bulkhead can shift during riveting if proper precautions aren’t taken. If your F-404 is already riveted in, you’re not going to have any adjustment on this width – it is what it is. It’s also wise to have a spacer in place to finish out the floorboards and associated angles and brackets. So definitely build a spacer if there is more work to do in the area.

I also purchased a partially completed RV-4 back in 2001. The fuselage had been started but the previous builder did not have a spacer in place during riveting. It shifted – a lot. I ended up buying all new F-404 parts and completely replacing the entire bulkhead assembly. It was a big undertaking, the parts for that bulkhead must be match drilled to the wing spar and splice plates.

If yours is riveted in, I would suggest that before you go too far, try fitting the wing spars into the fuselage. Put all the bolts in. Check the splice plates also. There will be an upper and lower splice plate for both the front and rear. They will most likely not be interchangeable. I would check this before starting the wing build.

I made my spacer out of MDF. I used two pieces with a piece of linoleum between them to get the thickness just right (I don’t remember what it is). I then drilled all the holes simulating the bolt holes for the spars. I had a bolt in each hole when I riveted the new assembly in place. The final result ended up good, but as mentioned above, it was a lot of work.

Good luck with your build. The 4 is such a fun airplane to fly.

Cameron Smith
RV-4 serial #68

Last edited by rv468 : 08-13-2021 at 04:49 PM.
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Old 08-13-2021, 06:33 PM
MS1095 MS1095 is offline
Join Date: Jun 2016
Location: West Chester, PA
Posts: 149

The plans call for the board to be 1 7/32". 1.25" will probably work, but I just wanted to point out what's in the plans.
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Old 08-16-2021, 09:01 AM
keithaloft keithaloft is offline
Join Date: Aug 2021
Location: Mt. Rainier, MD
Posts: 6

Thanks everyone. I've built a spacer, just because I needed to hang the fuselage from the ceiling of my shop to get it out of the way. In any case, it looks like I'm in trouble on this point, because the channel varies from 1 3/8 barely at the top and bottom to about 1 1/4 in the middle, it doesn't look like precise workmanship at all. Furthermore the two internal supports in the center of the front floor (sorry I don't know the terminology) encroach in to that space a bit, which is to say that it is not a straight line across that strut opening, which means that I can't imagine being able to put the struts in there. I do notice that there is also dihedral to the wings as reflected in that strut opening I hope that's correct. I haven't really sat down with the plans yet, I just ordered the USB stick, so I'll start familiarizing myself with those kinds of details soon. I am a little disappointed that there are more problems of this form that I've already noticed than I expected, but I guess that's the process, and I'll become a better builder for it hopefully. My impression is that the empanage was built well, but there seem to be a number of problems with the fuselage.
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