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Stub spar socket fitment

bsbarnes10

Active Member
Hi,
I've got some questions about how the stub spar socket (F-1206C) is supposed to fit on to the end of the spar (W-1208B-R & L & W-1207B). Looking at step 8 on page 20iS/U-03 it states:

"File the rear stub spar rounded ends of both wings to closely fit the recessed machined curve of the F-1206C. See Figure 4. The small flat on F-1206C locates the top of the part."

What orientation should F-1206C be "clocked" to when filing the end of the spars? If I had to guess I would think it should be as shown in figure 1, with the flat on the bottom of the spar aligning with the flat on the socket. However, figure 2 & 3 show the socket rotated further and I didn't see anywhere in the plans where it is clearly described which is correct. I could also see how the orientation in figure 3 could make sense.

I did search and find an earlier post on this but the image was broken so I couldn't see what the forum members agreed was correct. Since I don't have a completed airplane handy I can't check and see the orientation of the wings vs. the fuselage socket...

Figure 1 (this is how I think it should be):

spar%20socket%202.png


Figure 2 (rotated about midway):

spar%20socket%203.png


Figure 3: (rotated all the way the other direction):

spar%20socket%201.png



Any help would be greatly appreciated!
 
The position reference should be where it fits the best before you do anything to the parts (middle photo)
Keep the material removal to the bare minimum possible.
 
Excellent, thanks for the quick reply Scott. You're always there with guidance and it is immensely appreciated!

Bruce
 
Scott,

My 12 is coming up on 10 years and a little over 1100 hours. I noticed a slight “click” every time I got onto the engine after a couple of years, and I shimmed the spar tongues with a .025” shim traced from the spar tab with the bottom of the shim bent under the spar tab. Problem fixed. I am wondering how critical this issue is. Isn’t the stub spar tab basically an anti- rotation device to keep the wing from twisting? If so, is a slight “slop” in fit really a big issue?

Rich
 
Scott,

My 12 is coming up on 10 years and a little over 1100 hours. I noticed a slight “click” every time I got onto the engine after a couple of years, and I shimmed the spar tongues with a .025” shim traced from the spar tab with the bottom of the shim bent under the spar tab. Problem fixed. I am wondering how critical this issue is. Isn’t the stub spar tab basically an anti- rotation device to keep the wing from twisting? If so, is a slight “slop” in fit really a big issue?

Rich

The fwd. and aft stubs are there to take twisting/torsional loads but that is not the primary.
The highest loads are an inward compression load at the fwd. stub. That is why there is some pretty heavy structure on that bulkhead member.

A slight amount of fore/aft play in a wing is not a major concern but there is a limit. We have a document that has instructions on how to measure the amount of play and what is the expectable limit. It is not publicly available because we prefer to have direct communication with someone looking at a loose wing issue.

As for adding a .025 shim? I would consider that extreme, and would be way outside of what our acceptable limits are.

Of a bigger concern is that adding a shim in that manor has probably compromised the fit of the stub in the socket.
With the way the parts fit, adding a constant thickness shim will have a negative impact on the proper nesting of the two parts.

For the lack of a better illustration, I have attached a photo of a solar eclipse to mimic the two round parts. Note that as the two radiused edges become displaced, the amount of displacement varies along the edge, with the most difference being at the mid point and no difference at the top and bottom.

This should also be considered if someone decides to remove material at initial wing fitting because they can't quite get the wing inserted fully. This depiction shows that most material should be removed at the mid point and then varying the amount along the edge to where no material is removed at the top and bottom.

Another point related to a .025 thick shim is that if we evaluate the ratio of wing panel length/span to the distance of the shimmed stub from the hinge point of the movement (the main wing spar). If the .025 shim were installed at the rear stub, the dimension ratio is approx. 5:1, so that .025" shim thickness would be equivalent to a fore/aft wing movement of 1/8" out at the tip. We would consider that excessive.
 

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Scott, what’s the difference between using a W-1207B-25 spar stub vs. a 0.025” shim? Don’t they both perform the same function?
 
Don’t they both perform the same function?

No

Using the W-1207B-25 would extend the dimension of the stub in the same way as depicted in the eclipse illustration in my other post.

If a .025 shim is used, it is adding .025 to the entire radius of the end.

Look closely at the sliver of the sun that is visible. Its visible width is changing along the entire length of the radius. This is important so as to maintain contact between the spar stub and the socket within the fuselage, around the stubs entire circumference.
 
Is this assembly specific to the RV-12 and not the RV-12iS? I couldn't find that illustration in the plans I have for the 12iS.
 
Is this assembly specific to the RV-12 and not the RV-12iS? I couldn't find that illustration in the plans I have for the 12iS.

It applies to both. I haven’t seen a specific drawing that shows the spars fitting into the receptacles, but read Step 8 on Page 21iS/U-03: File the rear stub spar rounded ends of both wings to closely fit the recessed machined curve of the F-1206C. See Figure 4. The small flat on F-1206C locates the top of the part.

Also see Step 6 on Page 21iS/U-13: File the curved recess of both F-1203C (the bevel locates the top of the part) to closely fit the rounded mating ends of the Stub Spar Assemblies of both wings then mark the parts to ensure they can be assembled on the side to which they were fitted.
 
It applies to both. I haven’t seen a specific drawing that shows the spars fitting into the receptacles, but read Step 8 on Page 21iS/U-03: File the rear stub spar rounded ends of both wings to closely fit the recessed machined curve of the F-1206C. See Figure 4. The small flat on F-1206C locates the top of the part.

Ah. The OP stated page 20iS/U-03, not 21iS/U-03. Hence my confusion.
 
Scott –

I've been thinking about this all day.... You say the highest loads on the spar stubs are an inward compression load at the forward stub. I’m trying to figure out how the forward stub sees a compression load. I’m assuming a compression load to be spar stub being forced into spar socket. In-flight loads would sweep the wings in trail which would put the front stub into tension, so-to-speak.

If not too much trouble, can you help with understanding…
 
Scott –

I've been thinking about this all day.... You say the highest loads on the spar stubs are an inward compression load at the forward stub. I’m trying to figure out how the forward stub sees a compression load. I’m assuming a compression load to be spar stub being forced into spar socket. In-flight loads would sweep the wings in trail which would put the front stub into tension, so-to-speak.

If not too much trouble, can you help with understanding…

Not intuitive I know, but when a wing is producing lift, the load vector is not always straight up entirely loaded on the main spar(s) vertically. It can also be angled forward (applying a load that is trying to sweep the wings fwd)....

In high speed flight at low angles of attack this load is typically the lowest I believe, because it is (as you mentioned) being countered by the drag load on the wing, which would try to pivot the wing aft.

The higher the angle of attack, the more fwd the load vector will be. Worst case will be critical angle of attack at max rated G load.

Because of this, when wings are statically tested (at least with the method we use by utilizing ballast weight strategically placed on the wing), the wing is positioned at a number of different load conditions (angles) to simulate varying angles of attack.

The RV-12 wings for example, were tested upside down while attached to a simulated fuselage section that could be re-positioned to different pitch angles. (It is done upside down so that gravity can be used to simulate a positive G load)

The worse case condition for the inward compression load has the wing tilted nose down at such a steep angle, that the stacks of 25 lb shot bags have to be taped in place to keep them from sliding off the wing. It is this steep angle that is inducing/simulating the fwd load vector because the downward load induced by gravity, is in part applying a fwd load to the wings as well.

Next time you have the seat pans out of your fuselage for inspection, take a close look at the bulkhead with the control stick cut-outs. There is a lot of extra structure on that bulkhead that would not be there if it wasn't necessary, and for the most part it is there to take inward compression loads.

I hope that explains it well enough, or perhaps someone with an actual aero engineering diploma hanging on their wall can fill in the gaps or correct errors.
 
Very interesting. Thank you Scott. As always, we appreciate your time and thorough explanations...
 
I just did the fitment for both of these over the last few months and put them in the fuselage. Reading this thread makes me worried I could Have removed too much material or fit them incorrectly and will have too much movement in the wings. There doesn’t appear to be an easy way to fix this if that happens to be the case now that they are riveted in the fuselage.
 
Stack tolerances and build technique can produce fore/aft wing tip movement noticed during final assembly or early-on in the flight program. It’s not too uncommon to remove rear stub spar and replace with longer version sourced from Van’s. My left wing is solid fit to the fuselage and my right wing has movement. I have measured and purchased 0.055” longer rear spar stub and will be installing during next annual condition inspection when wings are removed.
 
Scott,

I don’t think you understood what I did with the shim. Because the wing moved slightly when I stepped on the wing walk and because there was about 1/16” fore and aft movement at the wingtip I made a D shaped shim to basically increase the tongue thickness, and I wrapped the bottom of the D-shaped shim under the straight edge of the tongue to eliminate the up and down movement. I did nothing to shim the end of the tongue.I’ve got about 800 hours and a number of years on the repair with no issues.

Rich
 
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