Enlarged skylight...any safety concerns

[nellie2110]

Thanks to everyone on here, I post very little and use a lot. I have tried to stick to the script to keep build time lower, but I came across a thought for my first major(and hopefully only) deviation. I am using oratex and that is applied a little different with a top piece being last in a "V" shape from behind the pilot to in front of the vertical. My kids are young and will be with us and I enjoy a skylight, why wouldn't they, it always gives a more roomy feel.

Attached are 4 pics hopefully in order. The first one just blues out the usual skylight with the added area over the rear pax. The next pic is the outline with more of the frame showing with the third highlighting the under structure(some reinforcements need to be added back there) that all the skylight would be attached to. The last shows where the sky light is held down either by screws or rivets through strips or channel. The last pic color scheme is red for strips, yellow for channel, and the teal could be made to be either.

Does this impact safety in anyway?

Yes its heavier, yes it cost more, yes it will be hotter in the summer(but nicer on skis in the winter). I am aware of the cons, but don't want to miss anything that could relate to safety. If you can think of anything, let me know.

Thanks

[jaredyates]

One thought is that you'll want to have enough strength in the teal pieces to keep them from bowing outward when you shrink the fabric. Is the goal to do this without any welding? If you could weld or otherwise attach some tabs sticking up from the X-shaped fuselage tubes, you could get a lot more rigidity in the mid-range of the teal members. When I did a skylight I started the stringers at the back of the skylight. This is much simpler if you can weld standoffs to that lateral tube (parallel to the wingspan) under the aft edge of the skylight. Attach your teal members to the front of the standoff and attach your stringers to the back of it. You could weld the left-right running trailing edge of the forward skylight to that same post, and wrap your fabric around that piece. But I don't know how you could do this without some welding.
Once a friend with a skylight in a 65-hp light post-war airplane kept finding little scabs on his head. He eventually realized it was because when he was doing loops and getting low gs at the top (he didn't have the energy to stay positive all the way around) his head was hitting the protruding screws around the skylight.
When you select your steel profile, screws, nuts, and that type of thing, you might want to be mindful of what is protruding in a low-g turbulence situation or flip-over, if you aren't thinking about it already.

[nellie2110]

Thanks for the reply. The plan for the teal line would be for sure one if not two 1/4 4130 stand offs welded in. Be the same as used on all the stringers and the skylights for the newer kits. The newer kits use the stand offs every 11in. I would be less than 10in in one spot and looking for a second still.

I am glad you mentioned protruding items. That is something to really watch as the headroom shrinks back there.

[Battson]

Some of the risks which come to mind are the fabric letting go, and the skylight letting go / cracking and a part coming free.
Either one could interfere with the empennage and cause the aircraft to become uncontrollable. Let's think about each:

Fabric

The fabric on the top of the Bearhawk vibrates a lot, especially as it ages. I was amazed by the amplitude of this vibration.
Vibration is the enemy of glued joints in fabric, so you will want to ensure the fabric is attached very well (full wrap-around), and attached to a robust structure which will not vibrate excessively (the structure itself).

Jared's advice is good, the stringers will not be strong enough in their "sideways" axis (port to starboard in that location) to terminate the fabric without bending under normal tension. They will definitely not be strong enough to prevent vibration which could cause the fabric glue joints to fail eventually. I would be bolting some serious stiffeners to them, probably aluminium box section, U-channel, or similar. They may also need additional bracing - I would consider adle clamps to the fuselage tube structure and connecting aluminium angle which bolts onto the new structure.



Skylight

The internal corners ('acute' right angles) will be prone to cracking in the clear skylight material, which would be a risk of failure. You'd want to ensure there's a nice large radius there, say a couple of inches at least.

You also want to think about things hitting the skylight.
From time to time, in unexpected turbulence events, people's heads or objects in the cabin can hit the skylight. Its rare but it's a large sudden impact upon the plastic material, sometimes with sharp objects, which creates a risk of failure. You would want to ensure the material used for the skylight is tough enough and thick enough to withstand such impacts. A larger surface with more complex geometry will be weaker than a smaller, simpler, skylight. I have a large skylight, not quite as large as yours, and 3mm acrylic has been durable. In my case it is supported by stringers alone it's length in two places (continuing the line of the stringers from the back of the aircraft). I think this support is very important.

Finally, how you affix the skylight is important. You don't want "point loads", so having some design which distributes the force around the plastic is worth considering.

[Battson]

An afterthought, if you are terminating fabric to a structure which is suitable for mechanical fixtures (pins, rivets, bolts, even stitching if required) then those are good options to hold the fabric fast. This is a requirement in some covering system manuals for any lifting surface, for example.

[nellie2110]

Thanks for the reply. Was hoping to respond sooner, but I usually check the site from my phone quickly. Finally have time on a computer. I was going to weld and add support, but think I will pass on this mod. I thought one large sheet would be best so it stays together, but didn’t think about the two corners and the stress there. That could be the break point and just isn’t worth the extra work. Thanks again.