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RV4 Spacer used in center section

Sanch627

Member
What has been used in the center section , ( width dimension) and material used to maintain the space for the wing spars.

Chris RV 4 #4633
 
What I did..

Chris, seemingly 100 years ago (pre-internet era) when I built my fuselage I fabricated a laminated square that fit the carrythrough slot as well as the vertical frame sides. I wanted all this to be perfectly in alignment, because if this is off to the narrow side, you have to shave the spars down, on the wide side, it can be shimmed if need be (better choice). I had my wings done, and was able to measure the EXACT width of the stack-up, and engineer my spacer to match. I would advise you do the same, and not go by a "given" dimension. In my case, all the layers of my spar are primed individually and can change the plans given dimension. My spacer is a piece of kitchen cabinetry super dense particle board (you cant flex this piece at all) with formica type laminate on it , then built up with a thinner luan plywood and as I recall maybe even poster board in between to get the dimension I wanted. take your time and get your spacer to match YOUR spar. There have been many a -4 that had great difficulty in final assembly because this wasn't done correctly. I have attached a picture of a couple pictures in which you can barely see the spacer.
 

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What He Said ...

Bill's comments + 1.

Depending on where the spars came from, and how they were assembled, there can be differences in their thickness (in .000's of an inch).

In order to insure your spar will fit the bulkhead later, you need to determine their exact thickness, each one, as they may not be the same, then add (optional) a few thousandths, and make a "simulated" spar out of anything that is rigid & stable enough to maintain its dimension during its use - factor in where you live in relation to the change in moisture content during that time. I live in a very dry environment - so not much of an issue with me. I used kiln dried oak, sealed with some sort of "poly" finish.

One thing I learned from previous experience, is that the "spacer" can be difficult to remove when its job is done. What I did to offset that, was to build into the overall thickness required a .025 - 2024-T3 shim that had a "tab" on the outside long enough to act as a "handle" for ease of removal.

I also made two "half" sets of simulated spars (each 1/2 long - just like the real thing) instead of one spanning from one side to the other, to make the eventual removal easier.

When it came time to remove them, I took out the aluminum shim first, then removed the spacers, which because of the .025" clearance, were very easily pulled out.

I've seen some ugly results when the spacers were not made directly from measurements of the spars being used - not saying it will happen, just that can.

HFS
 
Pics

Pics of what I did on my -3:

HFS
 

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  • RV3 - Spar “Spacer” - Full View.jpg
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Re: spar installation

I recently observed a potential gotcha that worked out OK. My wings were built very well by a young engineer that now builds surgical instruments for brain surgery. The fuse was fixtured ready for skins when I bought it. I have not found the slightest deviation from plans or dimensions.

But, when the wings are installed, the orientation of the bulkhead in the stringers becomes critical. When assuring wings are not swept forward or back, AND, simultaneously an accurate triangle with the tail CL, the clearance for flaps at the fuselage becomes very tight. The manual suggests a dry run with spars and fuse skeleton in the jig, but I did not have that option.

So, while assuring good spar clearance in the forward and carry through bulkhead (alignment) , you may want to preview some alignment, triangulation and clearance requirements.
 
4 spar center section space.

Thanks for all the info, my spars were the first set of 4 spars made by Van’s around June of this year I had planned to mic them. On the 8 that I built I used machined alum spacers. Will let everybody know how thing work out.

Chris RV 4 #4633
RV 8 # 80881. Sold
 
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