Tapping longerons for f-794 A

BGordon · #1 10-08-2017, 02:16 PM

Hey everyone,
Working on my rear fairings and DWG 44 says to tap longerons for a number 6 screw. Easy enough, my question is, do I counter sink the actual skin/longerons and dimple 794A, or do I C/S the 794A, meaning less will be removed from the longeron?
Thanks in advance
Brandon

sahrens · #2 10-15-2017, 05:17 PM

I countersunk mine, which means there is a slight knife edge on the fuselage skin. The countersink penetrates the longeron slightly.

AndyWW · #3 06-27-2020, 06:36 AM

I'm just about to rivet the longerons into the aft fuselage on my RV7 build, and stumbled across some comments about the inconsistency between DWG 44 (Empennage Fairing Attachment) and DWG 28 (Fuselage Skins and Rivets). The latter shows all the aft holes in the longerons riveted with flush AD3 rivets to the F-773 side skin, but the former shows a few of the holes drilled #36 and tapped for 6-32 screws to attach the F-794A empennage gap cover.

I know this isn't a new discovery from a google search of this site, but I haven't found any way of identifying precisely which holes should be drilled larger, countersunk and tapped. (So far all the skin holes are drilled #40 and dimpled for the AD3s, with corresponding countersinks in the longerons.) I figure this will be much easier to do now before the aft deck is riveted on, but I don't yet have the finishing kit which I guess contains the F-794A in it.

Can anyone help please with advice and preferably a precise count of which holes need to be drilled a bit larger and countersunk (presumably through the skin and into the longeron)? I've also read that some people just rivet this cover on. What are the pros and cons of that approach? I am worried that there isn't much depth left in the longeron for the tap to bite into (especially after I have already countersunk to accept skin dimples) and there must be a significant danger of stripped threads further down the line.

Thanks in advance. I will check with Vans on Monday but of course this issue came up on a Friday night!

Karetaker · #4 06-27-2020, 06:55 AM

Howdy everyone. Here is a link to a video that may help; I used nutplates. There are multiple videos in this series explaining my process.

https://m.youtube.com/watch?v=r-JvSmwcuMg

PilotjohnS · #5 06-27-2020, 08:38 AM

I had the same question on my 9. I called Vans and they sent me a picture showing the fairing from the finishing kit. This allowed me to not prep those holes for rivets and leave them blank. However, they told me that on the quick build kits, all holes are prep for rivets, and the ones with the fairing are just tapped during fitting of fairing. I dont know how this works out, since I actually identified the fairing holes when prepping the side skins to longeron attach.

I see no need for nutplates, the tapped longeron works fine.

wirejock · #6 06-27-2020, 08:43 AM

Has anyone tried attaching nuts to the inside using an adhesive?
I'm not a fan of tapping aluminum for such a small screw. Doesn't take much to strip the threads.
Does Van's approve nut plates? That would be my prefered solution.

rapid_ascent · #7 06-27-2020, 08:47 AM

I'm at exactly this same point.

I have removed my HS and am currently scratching my head. I think I have it figured out though. Most of my holes are still only drilled with a #40 drill. I have dimpled the trim piece for #6 screws.

Here is my plan:

1 - Countersink the skin and maybe a little of the longeron using my #40 countersink cutter to accept the dimpled trim piece.

2 - Drill the holes in the longeron with a #36 drill.

3- Tap the longerons with a 6-32 tap.

Does anyone see an issue with doing it this way?

Carl Froehlich · #8 06-27-2020, 09:20 AM

I used #6 nutplates instead of tapping the longeron on all there RVs. I put them on a diagonal so the holes are not in line.

I?m sure tapping the aluminum longeron is just fine - but I too do not like the idea.

Carl

PilotjohnS · #9 06-27-2020, 10:32 AM

Quote:

Originally Posted by rapid_ascent

I'm at exactly this same point.

I have removed my HS and am currently scratching my head. I think I have it figured out though. Most of my holes are still only drilled with a #40 drill. I have dimpled the trim piece for #6 screws.

Here is my plan:

1 - Countersink the skin and maybe a little of the longeron using my #40 countersink cutter to accept the dimpled trim piece.

2 - Drill the holes in the longeron with a #36 drill.

3- Tap the longerons with a 6-32 tap.

Does anyone see an issue with doing it this way?

I think this is what I did. I tried not to remove material from the longeron.

AndyWW · #10 06-27-2020, 12:24 PM

Has anyone tried riveting, either with AD3s or MK319BS? I guess it's fairly important the cover doesn't come loose in flight since it could foul the elevators or rudder.

Please would someone who has the F-794A cover fitted tell me the location of the holes that should be left open? Perhaps the best way would just be to count pre-punched skin holes from the aft end?