Polycarbonate (Lexan) Skylight

[robcaldwell]

I drilled my 1/16" Lexan oversized with acrylic drill bits. I used these large head rivets to attach the Lexan to the stringers.

https://www.ebay.com/itm/330490032427

Under the rivets I used these neoprene washers as a cushion and a seal. No cracks in the Lexan yet.

https://www.mcmaster.com/catalog/90133a017

Here is my description for the skylight:

https://bearhawk4place.blogspot.com/2018/10/the-skylight.html

[Nev]

I used oversized holes with bushings made from fuel line on my second skylight, same as Glenn suggested. I’ve added nylon washers under the machine screws and the screws are not tightened. So far so good. I’m getting additional aluminium side strips fabricated currently, I’ll post pictures when it’s finished. It should result in a good way to capture the sides of the Lexan as well as a surface to mate the wing trim strips to.

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[robcaldwell]

Another look at my transition and gap cover from the wings to the skylight. The Lexan extends about 1.5" from the outside rivet line and the AL strip simply lays on top of the Lexan up to the rivet line and is secured to the wing edge with #6 PK screws. Before attaching the AL strip, I used foil tape to seal the gap, like Cessnas use. Forgot to get a pic of the foil tape. Very happy with the outcome.

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[svyolo]

When you say foil to seal the gap, do you mean you cover the entire wing root to fuselage with foil tape first?

[robcaldwell]

Yes. Again, I wish I had taken pictures... It's basically the gap between the inboard wing edge and the lexan skylight and the windscreen. The gap is about 2 inches wide, and the tape is 3 inches. Totally seals out water and air.

[svyolo]

OK. Thanks.

[jcowgar]

I have seen pop rivets on lexan covered doors and windows, with and without metal strips over them. I don't know if the rivets were AL or steel. I might try it on some scrap first.

Zenith uses AL pop rivets (body and stem). They make it very clear that these AL pop rivets should NOT be used elsewhere, only in the windows.

[Hewko]

Does Zenith use an avex rivet like Murphy? A 'pop rivet' and an avex rivet are two entirely different rivets. An avex rivet is an aircraft structural rivet designed in WWII for field repairs, while a hardware store pop rivet is not designed for aircraft structures. The steel head and mandrel design, and method of expansion to fill the hole makes the avex a structural rivet. The pop rivets will work on lexan windows though.

[jcowgar]

I clarified my comment. I was typing two thoughts initially and my editing was not good. Zenith does not use normal AL rivets for structural purposes. The *only* place a normal AL rivet is used is in the windows. Everywhere else, rivets from Zenith are used. I am unsure of their type, but there is a big difference between the "Zenith" rivet and the AL rivets they supply for the windows.

[Battson]

I used oversized holes with bushings made from fuel line on my second skylight, same as Glenn suggested. I’ve added nylon washers under the machine screws and the screws are not tightened. So far so good. I’m getting additional aluminium side strips fabricated currently, I’ll post pictures when it’s finished. It should result in a good way to capture the sides of the Lexan as well as a surface to mate the wing trim strips to.

Will you put rubbed strips under the aluminium strips, and will there be strips along the stringers (so to speak)? Just thinking about waterproofing for parking outside in the rain.

[Nev]

There are already foam strips under the Lexan along the stringers. I’ll also put foam strips on the outside aluminium pieces that capture the sides. The only place I’m unsure about are around the screws themselves. I might do a test once the windshield is on with a hose or bucket of water

[Battson]

The screws will seal up nicely if you use plastic washers.

[N3UW]

When installing fasteners into acrylic I have used aluminum spacers as used for circuit board spacers in electronics. They are made for different size screw screws. I then get the length equal to the acrylic thickness and make the hole in the a acrylic just big enough for the spacer to fit in. You can then stick in a machine screw and tighten it down without worrying about cracking acrylic. Adjust the size of the stand-off to adjust the fit .

[svyolo]

Did anyone try using "sealing washers" under the skylight fasteners, whether using rivets or screws? Basically a washer with a rubber washer bonded on the bottom. As you tighter the screw, the rubber expands and seals the fastener.

[AKKen07]

I looked into that briefly but could’t find anything with UV resistant rubber. I didn’t want to use something that would rapidly deteriorate.

[svyolo]

I thought of that, but the center rubber would be under the screw head. Only the outer edge would be exposed to UV. Maybe replace them on condition or every 5 years or something like that.

[svyolo]

"Relaxation" was a bit more of a concern. You would have to tighten them down every now and then.

[Nev]

Has anyone used acrylic for their skylight? Is there any reason not too ? I assume if the front windshield is made of acrylic then it’s plenty strong enough. I’ve had some success using acrylic for the rear windows and using heat to form a curve in it. Wondering if this is a better way to go with the skylight.

On another note, what are people using to seal around the edges of polycarbonate? Some sort of silicon ?

[Mark Goldberg]

Bob has spec'd Lexan because it is tougher than acrylic. Much less likely to depart the airframe if it cracks some. A skylight departing in flight would not be good. Mark

[Nev]

Bob has spec'd Lexan because it is tougher than acrylic.

Thanks Mark, makes sense too. But what I’m seeing with a combination of compound curves, attachment holes, chemical susceptibility etc is that the longer term result may not be as strong as we think. My first attempt resulted in cracks while trying to attach it securely.

I wondered if anyone has used polycarbonate successfully and if it avoids some of these issues and results in a more secure skylight.

[500AGL]

Long term Lexan is as good as it says it is.

YouTube has hints, tricks and advice, or talk to glass shops that work with it about best methods and practices.

I’ve formed curves in panels just be heating in the oven over a steel form and it worked well.

[500AGL]

Boot cowl windows needed a curve down around the bottom front.

[Battson]

Has anyone used acrylic for their skylight? Is there any reason not too ? I

Yes I have, and there have been no issues. It's survived a crash into river rocks and being walked on. Hardly a scratch.

There are pro's and con's for both Lexan and Acrylic, I don't see a clear winner.

[Nev]

Yes I have, and there have been no issues. It's survived a crash into river rocks and being walked on. Hardly a scratch.

There are pro's and con's for both Lexan and Acrylic, I don't see a clear winner.

Jon, I remember you removed the “hump” on yours. Did this result in your skylight having only the curve of the aerofoil ? In other words, no compound curves ? What I’m dealing with are two compound curves along the outside aft edges, and left and right edges mid “aerofoil”. It’s due to the middle two formers having a different curve to the outside formers. Not insurmountable, but somewhat challenging

[svyolo]

Nev, I would say it is a complicated answer. I removed as much of the hump as I could without having the fabric impinge on the fairlead for the trim cable. If I remember correctly, from the longeron to the stringer I think mine goes up maybe 1/2 or 3/4 of an inch at the aft edge of the skylight, located at the aft spar location. It fits great. I am not sure about fitment issues leaving the hump stock. It might be OK, but it concerned me.

On the other hand, leaving the hump stock puts more "shape" into the skylight, which stiffens it a lot. If it works, you might be able to use thinner lexan.

It was a judgement call on what to do.