Reshaping top stringer profile for skylight

[Matt K.]

Matt, are you flying yet? Seems like it's been a while since you updated your build blog.

I did want to do my skylight in such a way that the ability to revert to a cable elevator trim is possible. The cable run is impeded by the crossbar, though I guess a guy could fabricate a fairlead into it to allow pass through. I'm pretty dedicated to electric trim but in the event that I sell one day I didn't want to turn anyone away on that detail.

Not flying yet. Still building... Slowly. Had kids. Moved houses, moved the project, etc. Life happens. I'll update the build log, eventually. Here are a few pics. I did lower 1 set of stringer standoffs, over the passenger area (just low enough to be higher than the fairlead). Attached are a few pics. I need to take some more. I pretty much cut the front tubes that accept the stringers and put the front of the skylight just over the trim wheel, the top of the trim wheel clears the bottom of the forward skylight support tube by maybe 1/8".

[Collin Campbell]

OK, I'll share my 2 cents, what I did on N370CC...

Collin Campbell
Bolivar, MO

[Mark Moyle]

Mark, are "nutcerts" the same as rivnuts?

. Yeah....though I called them out correctly... I do use.....special words...like gookumpucky, thingamabob's and describe special people as being somehow.


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[Zzz]

Wow, Colin that looks great. Did you also use rivnuts in your formers for anchoring the plexi? I'm trying to figure out why the holes drilled in the bottoms of those.

[Collin Campbell]

On fastening the plexi to the formers... I originally used rivnuts (or nutserts) to anchor the glass. I drilled the holes all the way through the former thinking I could just back drill the holes into the glass. That worked fine, but didn't like the holes left in the bottom. The pictures I posted were early ones before I came to a final solution. Main problem was the nutserts don't have a "locking" feature. Not good because you don't want the screws to be tight so as to allow for expansion/contraction of the glass...and there is a LOT of that going on with a piece of glass that big! Common practice is to tighten the screws until they bottom out and then back them off a quarter turn. Problem is that leaves them loose and they will quickly vibrate out...duh!! So what I ended up doing was using screws long enough to go through the entire former with AN364's on the bottom. That way I could loosen them a bit. Problem solved. The screws around the perimeter all went into anchor nuts so no problem there.

Collin

[Mark Moyle]

I'm using automotive urethane window sealant between the formers and the polycarbonate. Will run the screws in until the window fits the formers and squishes the urethane sealant. The sealant will act like loctite. Plus it glues the window to the structure...the screws become a secondary fastener...Between the polycarbonate and the external aluminum strips I'm using permagum. Permagum is water tight, handles like clay putty, is used on submerged electrical connections. Never hardens


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[Collin Campbell]

One more thing on the fasteners...Holes in the plexi should be enlarged to allow for movement. It's that expannsion/ contraction thing again. I used 3/16" for a #6 screw. Used nylon oversize washers under the heads. First time you pull your new bird outside in the hot sun you'll see what I mean. Don't even ask how I know this, seems like I learn things the hard way sometimes..lol. I don't know about the sealant to formers thing. I'm thinking glueing it down will not allow the glass to slide/move on the formers like it needs to. Just something to think about. Another thing, (I'm confessing all my sins here...lol) I tried using locktite on the screws, (you know the ones I had to leave a little loose to let the glass move) ...well it seems locktite and plexiglass don't like one another much...go figure. Had all these little "starry" cracks start to develop at the holes where I used the loctite. Got to make a new skylite...least I had the old one for a pattern. Joys of homebuilding!

Collin

[Zzz]

On fastening the plexi to the formers... I originally used rivnuts (or nutserts) to anchor the glass. I drilled the holes all the way through the former thinking I could just back drill the holes into the glass. That worked fine, but didn't like the holes left in the bottom. The pictures I posted were early ones before I came to a final solution. Main problem was the nutserts don't have a "locking" feature. Not good because you don't want the screws to be tight so as to allow for expansion/contraction of the glass...and there is a LOT of that going on with a piece of glass that big! Common practice is to tighten the screws until they bottom out and then back them off a quarter turn. Problem is that leaves them loose and they will quickly vibrate out...duh!! So what I ended up doing was using screws long enough to go through the entire former with AN364's on the bottom. That way I could loosen them a bit. Problem solved. The screws around the perimeter all went into anchor nuts so no problem there.

Collin

That reinforces my decision to use channel instead of tubing for the formers. It will nestle the nylocks in there so you can bump your head on them, but still allow you to use an actual locking nut instead of torquing on a rivnut.

[whee]

Bob told me I could thread the holes in the .035 tubing which is why I used it. Sure doesn't seem thick enough to me but I'm going to try it. Red loctite should keep the screws in place.

[marcusofcotton]

Bob told me I could thread the holes in the .035 tubing which is why I used it. Sure doesn't seem thick enough to me but I'm going to try it. Red loctite should keep the screws in place.

Loctite doesn't play well with polycarbonate either, same cracking issue. Generally good to have two full threads of engagement of the screws.

This has been an excellent thread!

[Zzz]

OK, I'll share my 2 cents, what I did on N370CC...

Collin Campbell
Bolivar, MO

Colin, did you adjust the height of your stringer posts at all?

[David Weber]

I left the kit original and used heat gun to contour the lexan.

[Zzz]

FOLLOW-UP:

I added mine to a quickbuild kit so I had to remove the "bearhawk bump" formers. I added an arched crossbar in the rear to help the straight-back lines of the aircraft. I used 2 formers instead of just one like Whee, but this is personal preference. The posts were there so I used both sets.

Welding is not quite complete, though it is more than tacked in. Even my pro TIG welder buddy was having a hard time welding the underside welds in this orientations. We'll finish it up when I can roll the fuselage.

Thanks to Matt Kunkel, Colin Campbell, and Gavin Chester for providing the examples photos.

[rforsythe]

Hi all,

First post here but quite seriously considering acquiring a BH QB kit in the next 12-24 months, so I'm lurking and soaking it all in. I had a thought on this thread though with respect to skylight fastening - what about motorcycle windshield well nuts? These are the ones with the long rubber bushing that squishes up into the structure as you tighten the screw, forming a compressive but non-rigid point of attachment that doesn't involve leaving screws loose. I've seen 4 of these hold a piece of lexan to a bike head on at 200mph (streamlined but still enough force to bend it inward in motion), I should think a number higher than 4 would hold a skylight to the top of a plane. They attach+insert through the screw side but are the nut itself and simply cinch up as you thread in the screw, so they also don't require any access underneath - you could put them into top holes on the formers but have the underside (interior) remain sealed. It might be a totally wrong application for them here, but maybe not...

BTW, nice mod there Zane. I may have to steal that one when the time comes.

[Bcone1381]

I had never heard of Well Nuts before! These look like they have a lot of handy applications!

[Bjorn]

Could you post a picture or a link to the " motorcycle windshield well nuts" ?
I have allready attached my skylight with 6# screws and looknuts but I am curious about the screws you are talking about.

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