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Instrument access in fuselage

DaveO

Well Known Member
I am considering making three entry access points on the top of the forward fuselage, in order to gain an easier way to install wiring and instruments.

I need to know exactly how to do this and also a way to waterproof so water does not leak into the fuselage. Does anyone know of a video or instructions?

Thanks
Dave
 
Not really needed with standard EFIS installs. Pull the EFIS display and you have a lot of access. Put a few connectors on the rest of the panel and just pull it when needed. I can have the whole panel out and on the bench in 15 minutes.

Carl
 
Several have done that, including me, and

so glad I did it! I have done many panel upgrades to my -6A, tip up, and have never had to get on my back, under the panel. I can't post pics, but if you will send me an email, I can provide a few. FYI, I seal mine with heavy silicon GREASE, like that used for swimming pool O-rings, and such.

Several others have done the access panels --- search the forum.

Ron
 
Access panels.

I put three access panels in the front skin of my 9 tip up. It’s time consuming to do it. I asked a friend that put them in his 7 if he would do it again and he said yes. So I went for it. They made it very easy for riveting the skin. I’m assuming you have not installed the skin. Important. When laying out the holes,the two that are on the sides run parallel to the longeron. They are trapezoids not rectangles.

Layout the locations on the skin and carefully cut them out, file and clean the edges. Layout and drill #40 holes around the perimeter using proper edge distance. I used a rivet fan to space the holes. Cleco on the skin to the airframe to fit the backer plate. Rough cut the plate oversize as it will change as it curves. Match drill the backer plate and cleco all around. Trace with a sharpie around the hole onto the backer plate. Remove the backer plate and create an outside line 5/8” from traced line, then do the same 5/8” inside the traced line. Cut both inside and outside line, clean and polish edges. You now have your backer plate.

Next make the cover plates. These must be made with the skin cleco’d to the fuse. Rough cut larger than needed. Match drill and cleco. You will need a helper to climb in and push the plate to conform while you match drill. Trace and cut 1/8” outside the line. The plate now needs to be finely tuned to fit the hole. Cut and file to fit. Cleco on the backer plate for support while fitting. When your happy with the fit, if not make another one, layout #40 holes, match drill cover plate into backer plate.

The rest is straight forward. Dimples for rivets and #6 screws in cover plate. Install nut plates in backer plate. Last is to rivet the backer plate if your satisfied with the fit and finish. Otherwise do it all again. It’s only a small amount of 032 sheet skin.

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Keep in mind that the skin of the aircraft is part of the primary structure, cutting big holes in it without sufficient reinforcement may lead to problems.
 
I did it, but wouldn't today!

I did it on my 7a because there was so much stuff that I needed to mount fwd of the sub panel. That was back in 2007 when we were still installing analog gages. I have gotten back in there a few times but now that my plane is painted I'm trying to not have to open them up.

With the new glass panel stuff, it is a lot easier to mount that stuff between the sub panel and the instrument panel. If I was building today I would be doing that and not cutting the holes. But one thing I did and would do again is use a modular panel, mine is called the "affordable panel" But no longer being sold, I think. That way you can remove just the side you need to work on or behind. You can see my panel and my upgrades on my web site listed in my tag line.
 
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Danger Will Robinson! Danger!

Keep in mind that the skin of the aircraft is part of the primary structure, cutting big holes in it without sufficient reinforcement may lead to problems.

+1

Aircraft structure is semi-monocoque, which means the skin is loaded as well as stringers and frames. The skin takes shear load which allows this type of structure to be very light for its load capability.

If you cut a portion of the skin away, that load has to go somewhere else. Generally that means going into the stringers and frames/bulkheads. I'm sure there is some extra margin built into said structure, but without knowing the loads you're rolling the dice a bit. I doubt Van's will share this information with you.

If you don't know the loads, a way around this issue is to do a capability analysis of the existing structure to determine how much load carrying capability is there. Then you design a reinforcing doubler that can take at least as much or preferably a little more load than the existing structure. Do not count on the access panel to take any load. Removable fasteners by design have loose tolerances so they can't be counted on to load up the panel.

If the cutout reinforcement is undersized, you most likely will see cracking around the panel and surrounding structure. Worst case scenario is you could buckle or cripple the stringers and/or frames.
 
No argument on the idea that skins are involved in load carrying in a semi-monocoque a/c structure. Having said that, follow the load paths (or get an engineer friend to do it for you). What loads does the skin above the top longerons carry? Bonus questions: What loads does the stainless firewall carry? What loads does the (screwed-on) boot cowl on an RV-4 carry?

(Hint: Look at the boot cowl skin on an RV-8.)

Disclaimer: I'm definitely NOT suggesting doing something potentially structurally stupid without properly evaluating (or having a qualified person evaluate) what you're considering.

Charlie

edit: If one does elect to do this kind of mod (I did it), then I'd consider using #8 screws if you expect to actually remove/replace the panel. IIRC, most, if not all of the access panels that are designed-in use #8 screws. I've found the #6s to be a bit less than durable, especially when used with nut plates. Hard to get enough 'bite' with the screwdriver, especially if the skin can flex a bit while you're driving or removing the screw.
 
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Thanks for all the responses. They give me much to think about. I was thinking about using fiberglass for the panels, but maybe aluminum would be better if I do them.
Dave
 
Thanks for all the responses. They give me much to think about. I was thinking about using fiberglass for the panels, but maybe aluminum would be better if I do them.
Dave

Unless you're really good with 'glass, aluminum will be easier. Fiberglass, especially hand layup, will be thicker (requires spacing the backing to compensate) and inconsistent thickness, and weaker at the point loads of the screws. It's not difficult at all to make the cover plate in aluminum. Just cut a bit oversize as others have said, and work it with files/scotchbrite wheels to make it fit the hole (remember to make allowance for paint).

Charlie
 
As already mentioned, if you make openings in a structural skin, you should at least be using AC 43.13 as a guide for maintaining the the same strength as what the uninterrupted skin had.
This would be a lot easier to do by using the same materials vs switching to fiberglass laminate.
 
Here is one solution. I used these on my RV-7, it was faster than fabricating my own.

http://www.mykitairplane.com/product_pages/rvak/index.htm

Oh, I like this pre-made kit for access plates. I'm just finishing wiring on my 7A and asking myself the same question, "How will I maintain the things behind the subpanel?" From underneath is my answer, but that is beginning to seem impossible. It takes good skill to cut and make straight lines. Plus the extra material is expensive, especially if you scrap some. This kit really simplifies make the access. You only have to cut the access hole.
 
As already mentioned, if you make openings in a structural skin, you should at least be using AC 43.13 as a guide for maintaining the the same strength as what the uninterrupted skin had.
This would be a lot easier to do by using the same materials vs switching to fiberglass laminate.

Exactly. Follow AC 43.13 as Scott says. The strength can be maintained but only the weight will go up due to reinforcing and fasteners.

But I have to admit. Once you have one of those 10" + glass displays, when removed it yields a huge access panel and makes life great when working on the backside of the panel. That alone is a big plus for those big square TV looking thingys.

Jim
 
I put 2 access panels in my -7 Tip-Up during construction and after 3 years of flying would absolutely do it again. I have the VP-X, remote transponder, and other avionics equipment just aft of the firewall and it is a bit difficult to get to when the interior is in. he access panels make it SO easy. I made doublers for the forward skin with two rows of rivets around the perimeter and brought them over to Van's. One of the engineers unofficially blessed it so I moved ahead with it.
 
I smeared a very thin coat of the FlameMaster (makers of ProSeal) access panel sealant before securing the covers. I can't remember the model number of the sealant off the top of my head. Water has never been an issue.
 
Access panels double as GPS antenna mounts

In my access panels I used fiber glass instead of aluminum. The access panels incorporate molded in mounts for GPS antennas. There are three total in my system and all are working great in this location with an excellent view of the sky. I find this location to be superior to the engine compartment or on top of the cowling.

I did not do an FEA on the air frame after I cut out the panels (I seriously doubt Vans did one either for the RV7) ;) I kept the opening size as small as was reasonable that would still accommodate the antennas and I used the same spacing for the screws as was used for the rivets bonding the skin to the air frame. The screws were #6.

I have my VPX Sport mounted directly behind the panels and I've had to take that out twice in two years. I've also added another GPS antenna in that time. I think I've opened the panels 4 or 5 times in the two years and I am very happy that I took the time out to build them.

I neglected to take any pictures with the GPS antennas installed, but here is a photo that shows the sizing.

EN9lDTGGhiY44z0JHXjuaYAK4sB9LpIRWGnz78efHgCulP06EBa2JgbFUefKsKubsCpP66oj4NIwMZCPZaaRzUI1DtEHmbxurihuFX7tYsNNf_rbP-TmKd-nG45F7IgJMt7ECa3F4TEmNmLz5ZZ8eKsdCZiTwpi8LATh8i4EKKSzjhnW7bzTPkTWrXSb7gHOFQBixciUAaSwiEQxn5KG8UgIxvn7TjSkrT8ZFyHoVwRFmuLqT1lO_j2ZqV1iKwUH6ipXfMXwLHKUGlE89nrya_15b852TD_yhCDs4tD3DoOX5kocHFEyew-SvPQX4lFzwGPI7B1HyslhUqIY1knXP3Y1_WP-yhRG73oZ0rspTEH-XxPw5qezSQxKCnHmvsu2jtXfdcQSGfYn_E2MirDcaz2K1qlKkergDupoC5lCSGtwo4T2Taj_V23OaPDI3wPF8KEXMs5zrYUlYiKsW2saln56GQohMorPIgsOduEN0d9DxqXWkXfkt13y97VWlnVv4FgpweSFn6vx-CCqZIQqfKoCRToht_QDVt6UpgE2PLltLPtlIPydZgT7HvG5xxMVE4pHIJ_qB5y5Khni8Q9TwPeemoaYR-fFS2_3IF47IppJgBtevhQCNMun9bMR7g3s8QwXv_xx2tlFI7fjuH2L6cimj_B__Lvc=w640-h480-no
 
Not so sure

I would argue that cut-outs in the skin, like the OP made, with a frame riveted around the opening and the 'doors' screwed into the frame nutplates would be stronger than the original un-opened skin. This is similar to putting a sun roof into a car. Despite removing all that metal, the sun roof frame actually stiffens and strengthens the roof.

Nice job Al.
 
As already mentioned, if you make openings in a structural skin, you should at least be using AC 43.13 as a guide for maintaining the the same strength as what the uninterrupted skin had.
This would be a lot easier to do by using the same materials vs switching to fiberglass laminate.

THIS !
So glad I added two access panels on my RV-7. Even on a tip up, it saves SO MANY panel dives. Plus, it's a good 'skin patch' practice for A&P training later on.
 
With most of the large format EFIS displays, you have a built in quick access that is faster to get open than 24 screw heads. Perhaps even a larger opening also.
 
With most of the large format EFIS displays, you have a built in quick access that is faster to get open than 24 screw heads. Perhaps even a larger opening also.

Suggest rereading posts and reasons by those who have done it.
 
This is what I did for access: built a pallet that sits under the skin between teh FW and subpanel, and plopped most of the boxes on that. It's attached using piano hinges to the fore/aft ribs, and drops down easily for maintenance or removal

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Access panels

What do you do to prevent water intrusion?

On the 2 planes that I have added acess panels there is an additional .032? aluminum spacer ring between the nutplate flange and the front deck skin. This extra stiffener allows for a .032? thick molded in place pro-seal ?gasket?. I made a jig and used a router with a spiral upcut bit. Cutout looked like it was done with a laser. Still have the jig and could duplicate on any 7 or 9 slider front deck skin.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
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