Lexan Windscreen

[Skippydiesel]

More bad news - My Perspex cockpit lid has also developed a number of cracks. This time associated with screws/holes.

I did not install the cockpit perspex but did check all screw tightness and made sure they were only very lightly tightened.

I have now backed off all screws (except bottom on each side/front & back) to allow the Perspex to "assume" its natural curvature - this results in an approximate 2 mm gap, between "bow" & Perspex, at the top. The gap decreases down each side.

Seems to me that forcing the Perspex to conform to the bows, may be causing the cracks.

Please comment.

This Sonex has never flown

[sonex1374]

You may find SonexFlight Podcast Episode 63 (http://www.sonexflight.com/63/index.html) to be helpful in fitting a canopy without developing cracks.

Jeff

[13brv3]

Correct that forcing any of these canopy or windscreen materials to conform to a shape they don't want to be in is asking for trouble. Some redirection is always necessary, but if you have a big gap, and think it can just be pulled down with screws, there's a good chance it won't be a good idea in the long run.

Cracks will form anywhere there's an opportunity, so cut edges, or drilled holes have to be smooth. If there were no stress, it's unlikely they'd crack, but with stress, and any rough edge... For cut edges, smooth them very well with fine sandpaper to round and smooth the corners. For holes, do the best you can with a tool that will put a smooth chamfer, like a unibit.

The final bit of advice that I haven't seen mentioned is hole size. Always enlarge the holes over the size of the fasteners. Canopies have a good bit of thermal expansion and contraction, and that will cause it to push and pull on the holes. It's best to give them some room to allow movement. I personally didn't use countersunk holes on my canopy for that reason, since I HATE fitting canopies.

Sadly, no matter what you do, canopies will sometimes crack. The RV guys got all crazy with using a sealant to glue them on, so there are no holes for fasteners. Yep, they still crack. I hate canopies...

Rusty

[Bryan Cotton]

Probably a poor fabrication job, where the screws were used to pull the canopy down rather than straps. We pulled our canopy down tight and then drilled holes IIRC.

[Skippydiesel]

Spent a pleasant few hours on my Perspex canopy this afternoon -

Acquired a package of M3 x .8 mm & a package of M3 x.5 mm Nylon washers (electronics stores stock these ) in the previous few days.
I started on the rear "hoop"/frame, Perspex to hoop - To make sure everything remained reasonably aligned I left the centre screw loosely in place (the last to be packed). This allows the Perspex to conformed to its most "natural" curve.
I then used my .8 & .5 mm washers to pack any screw points, only where that point was not touching the hoop surface (took care not to force any washers in) replacing the screws as I went.
The washers are super fiddley - I found a dab of silicon grease on a piece of discarded cable (zip) tie worked a treat in getting the one to two washers in place.
Repeated the process on the front hoop.
All screws were just tightened enough to take up - "feel" through screwdriver then locked in place with a nut.

Hope this puts an end to unwanted cracking or at least reduces its occurrences.

(I tend to repeat this process on the rear hoop of the Lexan windscreen, just waiting for the Nylon M4 x .8 washers to arrive)

[Skippydiesel]

Spent a pleasant few hours on my Perspex canopy this afternoon -

Acquired a package of M3 x .8 mm & a package of M3 x.5 mm Nylon washers (electronics stores stock these ) in the previous few days.
I started on the rear "hoop"/frame, Perspex to hoop - To make sure everything remained reasonably aligned I left the centre screw loosely in place (the last to be packed). This allows the Perspex to conformed to its most "natural" curve.
I then used my .8 & .5 mm washers to pack any screw points, only where that point was not touching the hoop surface (took care not to force any washers in) replacing the screws as I went.
The washers are super fiddley - I found a dab of silicon grease on a piece of discarded cable (zip) tie worked a treat in getting the one to two washers in place.
Repeated the process on the front hoop.
All screws were just tightened enough to take up - "feel" through screwdriver then locked in place with a nut.

Hope this puts an end to unwanted cracking or at least reduces its occurrences.

(I tend to repeat this process on the rear hoop of the Lexan windscreen, just waiting for the Nylon M4 x .8 washers to arrive)

[DCASonex]

A bit late to mention this, but some folks have reported less tendency for canopies to crack if installed hot in direct sunlight.

David A.

[Skippydiesel]

A bit late to mention this, but some folks have reported less tendency for canopies to crack if installed hot in direct sunlight.

David A.

Has a certain logic but who can order in "hot in direct sunlight" just when you might be installing your Lexan/Perspex?.

[jerryhain]

Pretty easy if you live in Arizona.

[Skippydiesel]

Pretty easy if you live in Arizona.

I would have said something similar for NSW, Australia, but just had one of the wettest summers ever.