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Old 10-30-2018, 06:54 PM
NorthernRV4's Avatar
NorthernRV4 NorthernRV4 is offline
Join Date: Mar 2013
Location: Prince Edward Island, Canada
Posts: 128
Default Flap Inboard end

Hi Gents

I'm making great progress on the flaps and have everything fit and drilled. Other than running into an edge distance issue on the port inboard rib which required making a new rib (read: making form blocks and 3 tries before I had a good final product). I'm wondering what I should do about trimming the inboard end of the upper and lower skins. I know I should leave a little extra for fitting upon assembling the wings to the fuselage but there is over an inch of extra material. I had a look at a friend's -4 and found the skins were a little different than what I've read about and interpreted from the plans. His don't have any extension of the bottom skin to overlap the fuselage bottom. Any suggestions on how to trim this now that won't cause me grief later?

Flap inboard end by James Soutar, on Flickr

Flap inboard end by James Soutar, on Flickr

His has the skin trimmed very close to the rib at the LE and angling inboard about 1/2" at the TE to match the fuselage side. Also it seems to somewhat match the angle that the rib leans in towards the top.

Flap inboard end by James Soutar, on Flickr

Flap inboard end by James Soutar, on Flickr
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Old 10-30-2018, 07:09 PM
131RB 131RB is offline
Join Date: May 2009
Location: Colorado
Posts: 301

Best to wait till the wings are fitted to the fuselage and trim as required at that point.
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Old 10-31-2018, 04:40 AM
fixnflyguy fixnflyguy is offline
Join Date: Jan 2006
Location: Winston-Salem, N.C.
Posts: 1,004
Default What I did..

Seems every -4 is a little different, and as mentioned, you will need to wait until final assembly to see exactly how your fit will be. I actually trimmed my overhangs off, and fabricated extensions that I slightly joggled to get flap/fuselage fair with flaps fully retracted, then riveted them on.I haven't seen 2 exactly alike, but have seen plenty that struggled to achieve a good fit.
Bill E.
8A7 / Advance NC
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Old 10-31-2018, 07:57 AM
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JonJay JonJay is offline
Join Date: Jan 2007
Location: Battleground
Posts: 4,190

Good advice given.
Ideally, you want the bottom skin of the flap to rest against the bottom of the fuselage when flaps are retracted. I got lucky and mine fit without any fuss.
As others, I have seen very few that didn't need a "joggle", and every one seems different.
If we think about it, the wing spar fits into the center section and there are no adjustments to its position. The rear wing spar is drilled in assembly to set wing incidence. I am not sure there is any tolerance, or enough tolerance, to adjust the flap trailing position. Wing incidence is far more important than bottom skin alignment of the flap. There may be more to this but I can't visualize it.

There is quite a bit of discussion on this buried in the forum so you might try a search.

I prefer a joggle over just cutting off the flap skin but I don't know if it really matters.
Smart People do Stupid things all the time. I know, I've seen me do'em.

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Old 10-31-2018, 09:41 AM
Brian Vickers Brian Vickers is offline
Join Date: Mar 2005
Location: Bainbridge Island, WA
Posts: 324
Default Inboard flap skins

The top skin has to be trimmed so the flap can retract, but the bottom skin is left long to cover the flap push rod hole when retracted. Plans page 37, bottom of page in the middle shows the flap pushrod assembly, and you can see the bottom skin extends past the rod end bearing. Also shown on plans page 40, "section B-B." Again, it's done that way so the elongated hole (hot dog shaped) in the bottom of the fuselage skin is covered when the flaps are fully retracted. Otherwise your passenger will get a lot of cold air from the push rod hole in the floor panel. I'm going to the airport on Friday and can take a photo or two to email if you want.
Brian Vickers
Bainbridge Island, WA
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Old 10-31-2018, 05:07 PM
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NorthernRV4 NorthernRV4 is offline
Join Date: Mar 2013
Location: Prince Edward Island, Canada
Posts: 128

Yes, I understand the top skin needs to be trimmed to fit just outboard of the fuselage and the bottom skin is left extending to overlap the fuselage bottom skin. now that you mention it, I've noticed the gap around the flap control rod opening while sitting in the back. Good point! Could be chilly in the winter months back there.
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Old 11-08-2018, 05:07 PM
FinnFlyer FinnFlyer is offline
Join Date: Aug 2018
Location: Bell, FL
Posts: 156
Default Don't drill and rivet piano hinge to flap

May I recommend that you don't drill and rivet the piano hinge to the flap until you have (test) mounted the wings, set the wing incidence and drilled the rear wing spar attach hole.

I got away with it on the left flap (just had to grid a bit off the upper part of the FL-406B flap bracket plate and install the top rivet flush). Not so lucky with the right wing. I just drilled the 48 rivets out that holds the piano hinge to the flap so I can move the flap 1/8" to 3/16" outboard. Probably will need new piano hinge because of old/new hole distance. Grrrr.....

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Old 11-13-2018, 09:53 AM
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vfrazier vfrazier is offline
Join Date: Jan 2007
Location: Mount Vernon, IN
Posts: 1,162

What everyone else said.... and I'll add one more reason to wait on riveting the piano hinge (or at least one half, either on the wing or the flap).

You can adjust the fore/aft position, within reasonable limits, by choosing a wider piano hinge. Wider, not longer, i.e. a P3 is narrower than a P4, etc...

This allows you to match the trailing edge line if the flap/ailerons aren't perfect.

For convenience:
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