M

Mustang

Well Known Member
Guys and Gals,
We picked up our QB wings last summer, but left them crated until recently. I removed the first (left) wing from the crate in January and we put it on the bench to inspect it. Of course it was oily from the WD-40 sprayed on in the Phillipines, and as I wiped the top surface down, I noticed general conformity to the NACA2013.5 airfoil everywhere except at the root. Here, there was a deformity. Laying a straight edge from the spar area, forward along the wing/tank root rivet line, there is a concavity where the ruler bridges three rivets before touching down on the convex curve to the leading edge.

This was upsetting! I thought there must be some easy explanation like the "Z" brackets not being tightened, etc, but we checked them and they were all tight (actually overtightened). We removed the tank and inspected the top skin which seemed to be slightly divergent on the trailing edge at the root. Also, it seemed that the Proseal had been layed in behind the aft tank skin seam to possibly cause interference with the spar when in place. BTW, the bottom skin exhibits a coresponding bulge (convexity) in sympathy with the top skin.

A call to Van's builder support was a bit surprising in that the initial listener's response seemed to be mildly incredulous that I was concerned?!?!? When I explained that I preferred my top wing skins to exhibit a convex conformity to the airfoil profile it was suggested that I remove the tank and open up the holes in the spar where the "Z" brackets are bolted to change the tanks vertical orientation on the spar for the fix. This was done, but the skin still exhibits some slight concavity in this root area. The tip end of the tank is perfect and the "Z" bracket holes in the spar were not changed here.

I have since removed the tank again and attempted to cautiously press on the skin overhang behind the aft tank seam to put some small convex shape in the overhanging skin. It would almost seem as there may be interference with the top skin/tank seam and the spar, that is pushing the tank out of position. Presently, the tank is still sitting on the workbench and I thought perhaps a note to the Forum would be in order before re-installing.

Would anyone have any comments or suggestions on this anomoly?

Thanks, Pete
(pic posted down a few posts)
 
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Pictures?

Perhaps because it's after margarita-thirty here, I can't quite visualize the problem. Do you have any pictures or a diagram?
 
Mustang said:
Guys and Gals,
Laying a straight edge from the spar area, forward along the wing/tank root rivet line, there is a concavity where the ruler bridges three rivets before touching down on the convex curve to the leading edge.

Thanks, Pete
Click to expand...

That sounds like you're talking about the inboard root edge. If so, that area would be covered by the wing root faring anyway.

Does that sound right?
 
Pete,
...Just went to the barn to check my QB wings for what I think you mean. I laid a straight edge (level) on the inboard rivet line from the spar forward and gently rocked it from the spar toward the leading edge, both upper and lower surfaces. It followed the convex contour of the airfoil with no gap anywhere along the rivet line.

Just curious, What was the max gap? Did it occur further outboard than the inboard rivet line? Do you think the wing root fairing will camoflage (sp) it?

Perhaps others can help more than I.

Good luck,

Don Gray
N17QB 7A in progress
Cortland, OH
 
Installed the tank again....

Guys,

This afternnon I re-installed the tank and jacked up on the root "Z" bracket while tightening it to the spar web. This seemed to fix the concavity on the top surface, but when I flipped the wing over and screwed the bottom tank skin to the spar, a huge bulge appeared, where the thickened part of the spar barstock interferes with the lower tank baffle seam/rivet line. I noticed that the top barstock of the spar has been factory bevelled in the same area.

Curious, I miked the barstock and found the top barstock to measure 0.876" and the lower barstock to come in at 0.750". However, the top bar is bevelled to allow clearance for the top tank baffle seam and the flat part of the spar is,.....0.750". Hmmm all very interesting, but why does the lower baffle seam run into the spar on the bottom if the tank is built properly. Something must have been amiss in the Phillipine factory before the tank was fitted to this wing spar.

Armed with this info I called builders assistance again and got Scott, the manager. I told him about the interference on the bottom spar barstock and asked if it would be OK to bevel the lower barstock as the top is, about an eighth of an inch in from the edge. He asked me to send some photos which he would forward to engineering to see if bevelling would be allowed on the bottom barstock. I went back to the shop and drew in the proposed bevel area right on the barstock with a felt pen, snapped some pics and fired them off to Scott. So that is where it sits right now and I will not hear back until Monday from engineering.

Surely something must have been built incorrectly for this to happen??? But how could they go wrong with the prepunched skins and parts?? If they will not allow the bevelled spar, then I will have to grind the seam joint right up to the rivet line to get the bottom skin to fit. That could mean possible fuel leaks? Either that or build a new tank.

Yes, I have some pics but I am not good at getting them on the forum so I may send one to Doug to put in for me.

Cheers, Pete
 
Answering questions

Yes,

The area in question can be found by locating the top corner rivet on the inboard end of the tank next to the spar. When we uncrated the wing, the upper surface bulged upwards from the screw (into the spar) to the corner rivet, so much that laying a straight edge from the bulge, forward on the rivet line, spanned 3 rivets before touching down on the tank skin. The concavity was more than a sixteenth deep.

I am not aware that this area will be covered by a fairing. If that is true then some of my concern may be diminshed, but there should be no contact between the baffle seam rivets and any part of the spar in my mind. The fact that the interfering part of the upper spar is bevelled would lead me to believe that this contact is not desireable from the designers point of view either. Why else would it be bevelled?

Cheers, Pete
 
Pic posted for Pete...

Left%20wing%20and%20spar%20024.jpg
 
One possible explanation might be that the flanges on the outside rib and/or the rear baffle plate were not bent at the correct angle thus causing the skin to deflect at an angle when it was riveted. Nevertheless, it's a relatively small area that is effected and I'd be inclined to screwn it down, which will take out some of it, and move on. This is without seeing it in person, maybe it's worse than it appears in the pic. These are not laminar flow wings and I'm not sure a fairly minor deviation in the airfoil shape will make any difference, especially right at the root end where the airflow is messed up anyway due to the intersection with the fuselage.

For comparison, I just took a look at my RV-3B QB wings and there is no sign of any concativity (is that a word?) in this area.
 
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Randy,

OK, the picture shows the bottom skin deflecting away from the spar where it is being pushed upwards as shown. (the wing is upside down) This is with the top surface now bulge free and with no concavity. Because the whole root end of the tank has been displaced upwards (or downwards in the picture) to match the upper tank skin to the desired airfoil shape, the lower skin seam/joint is riding on the bottom of the spar. If the tank skin was not doubled/rivetted to the rear baffle, the skin would lay perfectly flat and so the vertical spacing of the top and bottom skins appear to be correct. What seems to be wrong is the position of the rear baffle seam which appears to be too far aft so as to ride on the spar's barstock.

Thanks for publishing the picture and taking the time on this.

Cheers, Pete
 
An Email from Vans

Today I recieved an email from Scott at Van's and he told me that the engineering department had analysed my request and said that there was no problem with bevelling the lower spar barstock similar to the top bar which is already factory beveled. Scott told me to be sure and primer the new bevel which would remove the anodizing.

Scott thought the problem lay in having too much Proseal on the joint and I did ask him if there would be a problem with grinding the proseal off the aft end of the seam.

So, that where it stands now in case anyone is interested. I will update again if I hear back from Scott just to finish off this thread for reference purposes.

Cheers, pete
 
Scott's answer...

Scott emailed me again today and told me to just bevel the spar rather than grind the rear baffle flange/proseal aft of the rivet line. I think that is reassuring for them to allow spar bevelling in that area. There must be some extra strength built in to the spar for them to allow this.

We might just let this be anyway and "move on" as Randy suggested.

Thanks for the input.

Cheers, Pete
 
regarding my flying QB-9A

Once you get a fuel load into her, she will change again--and probably in the right direction, too. Also, once you fly, you will find many small convexities and concavities along the wings shape due to initial wing flexing and skin stretching. In my flying QB-9A and other previously owned certified flying machines, I just call these "personality markings". I would advise anyone worried about this to go to a flight museum and look over any airplane close-up. For that matter, go to your local airport, and look at any certified airplane. What seems smooth from 10 feet away, does not at 10 inches, but they still fly very well. If it were me, I would not work-off material on the main-spar just to make it look better. :eek: One very experienced RV builder told me before I got started that a "B" or "B+" looking airplane is better built than an "A" or "A+" looking airplane. I relate this to: "every airplane built has its own personality even within the same model". :)

John
 
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Let It be....

Yes Guys,

We have decided to let it be, as is, and be happy. And yes, every time I get depressed about workmanship, I go look at a Cessna!

Cheers, Pete
 
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