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Major NACA vent screw up....ideas?

Seansull

Seansull

Active Member
Well, I have just made the most upsetting error in my build to date. I was in the process of installing NACA vents in my tailcone when I kept thinking to myself that something just doesn't look right.

I finished cutting out the vent template and deburred. After looking at it for a few minutes, it hit me like a ton of bricks. I cut the entire vent perimeter instead of just the leading 1/2. I am now the proud owner of a huge idiot mark on the side of my plane. The "easy" fix is to cut a piece of aluminum and rivet it on the surface of my skin and feather in. The thought of this makes me cringe.

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My next thought is to try to build something up out of fiberglass to keep my skin surface flush while covering the trailing 1/2 of the vent. I am not sure how I would support the structure, and future cracking worries me.

The best looking alternative is an entire new side skin for my tail cone. Honestly, the thought of drilling out everything and replacing this makes me want to throw up.

I am hoping someone here might be able to offer an acceptable solution. Having another piece of skin material scabbed on top just seems like wearing a big red letter "A" on my shirt. I am still dumbfounded that I have done this.

Thanks!
 
Could you cut a rectangle bigger than the vent, put a doubler ring (for lack of a better term) on the inside. Then re-cut the vent.

I guess think along the lines of the stall warning access panel in the wing...but a little larger.

I bet with some epoxy/micro, you could fill in the patch line and it would only be visible from the inside.

Just my initial thought.

-Jim
 
That is a pretty good idea. It would basically be a large access plate with a vent attached to the back of it. If i filled the seam with micro on the surface, I wonder if I am going to have problems with hairline cracks (similar to some control surface tips as they meet the aluminum) in the future.

I guess I could rivet plate nuts on the doubler and make the new panel a functional access plate. It might look like a thought out plan to bypass removing the tailone access plate :rolleyes: for checking on things.

Thanks for the thought Jim. This has given me a little more to think about.
 
Sean,
It looks like you cut a clean hole after all. Could you possibly split the NACA duct and space it out a little and then glass it back together, making it fit the hole you cut instead? Just a thought.
Jim
 
Easy fix that will require some patch work, flush rivets, and a little body filler prior to paint. You'll never see it from the outside.

Buddy of mine patched the wing root vents in his C-180 and you can't tell they were ever there.

No worries, you just gave yourself some more work.
 
While you are noodling over the fix, can you incorporate a door that can open / close off the air flow to the cabin. If it looks snazzy, everyone will want to incorporate this goof.
 
The hard answer is to do a new skin but, like you, it's not something I'd want to do. Sometimes you have no choice. My HS skins got dented and I ended up drilling them off and putting on new ones. The damage wasn't structural but was greater than I felt comfortable filling.

That said, I think there's a way out. You have two objectives here: fill the hole and not disturb the workings of the vent. For the former, a plate is obvious. For the latter, you really don't want either a doubler plate or rivet shop heads in the repair area. The answer may horrify you but ... make the hole bigger. Mark across the hole you currently have where the vent cutout should have ended and extend those lines a bit, until they are about 1/2" to 5/8" past the edge of the vent (not the opening but the whole part). Trace around the rear of the vent, maintaining this 1/2" to 5/8" distance. Cut out the hole you just made and smooth the edges.

The hard part here will be to fabricate the patch out of material the same thickness as the skin. If you are very careful here, it will fit the hole with only a fine join line. While you are at it, make a doubler piece that fits around the aft edge of the vent and is 1" to 1 1/4" wide all around.

Now, cleco or clamp the vent onto the skin, use it as a guide to position the doubler, and drill the doubler to the skin. Then position the patch, and drill it to the doubler. Now all that's left is to rivet skin, doubler and patch together. You should have a very thin joining line between the patch and skin to fill, plus a few rivets, to hide the evidence.

There is one caveat here. I use rivets as well as glue to attach my vents; I would put a rivet on each side of the joint where it crosses the vent, just to prevent any cracking along the join line there (I don't mean a crack in the skin - the repair should be sound - I mean cracking in the paint if the patch flexes slightly relative to the skin there). If you only glue the vent on, I am not sure the paint over the joint will never crack. When I riveted the vents on my -6A, I made little 'washers' of scrap aluminum which sandwiched the vent material with the skin. I used CS4-4 rivets on the -6A but could have used solid rivets.

The point of the repair, if it wasn't clear, was to keep both the doubler and the join line outside the perimeter of the vent so that under the vent, and especially in the vent opening, there is only a single metal thickness and no fasteners to disturb airflow.
 
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No problem

The fix is pretty easy in my opinion. And you now have the opportunity to make the scoop more efficient; As said earlier, the design of a NACA scoop is very specific; sharp edges, ramp slope - and one element that is missing from the standard Vans installation - a "leading edge" on the skin. So I would simply put a scab patch of .016 over the area that you mistakenly removed and make sure to roll the leading edge of the overhang a little. Sure, you will have to feather in the repair with filler, but that will be a lot less work and be lighter than trying to do a "dollar/dime" type patch to make it flush.
 
I'd add a doubler on the inside of the skin that returns the opening to the proper shape. Then I'd build up the recessed area with microballoons until you get it flush with skin.
 
The proper repair is a nickle/dime patch. I think thats what Patrick (flion) was trying to describe. To keep the explanation simple its basically an internal doubler that is larger than your current opening after you square it up. The next step is a piece of skin cut to match the opening and the same thickness as the damaged skin riveted to the internal doubler. After riveting both skins you can cut your duct opening again. The seam can later be filled with micro balloon and sanded for paint.

Piece of cake.

Pat
 
Thanks everyone for the great ideas and instruction. I will post a few photos of the finished product very soon.

I really appreciate this group and the wealth of knowledge that is here!
 
Instead of resin and micro I would recommend using tank sealer to fill any gaps. I work for a large aircraft plant in Wichita, KS and we use ProSeal to seal gaps on panels and fairings on the aircraft after installation. It remains flexible and it is paintable.
 
ELT access

Perfect opportunity to make an access hole. Just tell everyone its for ELT access. Oops, too late you already posted.
 
Here is what I came up with......

Thanks again for all of the great ideas! You guys are awesome.

I basically took a little bit of everyones advise along with a few of my own and came up with a fix that I think will look just fine. I still need to sand my micro balloons then prime and sand a few times. After that, I am pretty confident that I will be the only that knows this is a fix. Thanks again!
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I've said this many times - the real art of building is knowing how to turn your own mistakes around into a success. Anyone can buy a new part when they mess one up - a true builder can fix most mistakes to established standards. Good job!

Paul
 
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