Lexan Windscreen
Posted: Thu Apr 21, 2022 6:28 pm
by Skippydiesel
Just wondering what might have caused the following symptom;
My recently installed Lexan windscreen has developed some small/short (3mm) stress cracks on one side of the front bow . The cracks are not full depth.
What think you??
Re: Lexan Windscreen
Posted: Thu Apr 21, 2022 8:00 pm
by Bryan Cotton
Did you polish the edges/corners? If not it can be prone to crack when you bend it.
Re: Lexan Windscreen
Posted: Thu Apr 21, 2022 11:31 pm
by wlarson861
Many solvents and especially auto gas will crack Lexan.
Re: Lexan Windscreen
Posted: Fri Apr 22, 2022 3:38 am
by Skippydiesel
wlarson861 wrote:Many solvents and especially auto gas will crack Lexan.
Definitely no fuel/hydrocarbons of any kind - engine has yet to be started.
Re: Lexan Windscreen
Posted: Fri Apr 22, 2022 3:43 am
by Skippydiesel
Bryan Cotton wrote:Did you polish the edges/corners? If not it can be prone to crack when you bend it.
Short answear - dont know for sure. I purchased the nearly completed Sonex from another builder. He is incredibly particular so I would think so. Will ask him.
In the mean time; Just in case the cracks were caused by uneven/over tightening, I have backed off all screws on the front bow and then tightened them just enough to stop the countersunk washer from turning.
Re: Lexan Windscreen
Posted: Fri Apr 22, 2022 7:12 am
by N43WX
Not quite to this point, but can someone clarify the polishing of lexan corners? I haven’t heard this before. Is it essentially just deburring? Scotchbrite or other? Not polishing as in using any chemicals?
Re: Lexan Windscreen
Posted: Fri Apr 22, 2022 8:05 am
by Mike53
N43WX wrote:Not quite to this point, but can someone clarify the polishing of lexan corners? I haven’t heard this before. Is it essentially just deburring? Scotchbrite or other? Not polishing as in using any chemicals?
Pretty sure he is talking about deburring or putting a slight chamfer on the edge of the hole.It is also a good idea to drill a small hole at the end of the crack (1/32 bit) as this will prevent the further expansion of the crack.
Re: Lexan Windscreen
Posted: Fri Apr 22, 2022 8:27 am
by DCASonex
If cracks are radiating from drilled holes, might be due to lubricant on drill bit. make sure drill bits are washed clean of any lubricant or solvent when drilling lexan.
David A.
Re: Lexan Windscreen
Posted: Fri Apr 22, 2022 12:48 pm
by Bryan Cotton
N43WX wrote:Not quite to this point, but can someone clarify the polishing of lexan corners? I haven’t heard this before. Is it essentially just deburring? Scotchbrite or other? Not polishing as in using any chemicals?
I sanded them smooth and also sanded the corners a little bit.
Re: Lexan Windscreen
Posted: Fri Apr 22, 2022 7:24 pm
by Skippydiesel
DCASonex wrote:If cracks are radiating from drilled holes, might be due to lubricant on drill bit. make sure drill bits are washed clean of any lubricant or solvent when drilling lexan.
David A.
In my (lexan) case they are very short (max 3mm) not full depth, located together, in the bottom left corner of the forward edge of the windscreen. They are probably only 6 cracks in total, appearing to come/radiate from the outer edge of the lexan.
As indicated above my initial analysis is thet the screws may have been over tight or unevenly tightened, causing a stress situation in one location. Hence my careful retightening of each screw, until I could not rotate the countersunk washer by finger pressure.
Re: Lexan Windscreen
Posted: Sun May 01, 2022 7:40 am
by 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
Re: Lexan Windscreen
Posted: Sun May 01, 2022 10:40 am
by 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
Re: Lexan Windscreen
Posted: Sun May 01, 2022 12:15 pm
by 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
Re: Lexan Windscreen
Posted: Sun May 01, 2022 3:54 pm
by 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.
Re: Lexan Windscreen
Posted: Fri May 13, 2022 6:24 am
by 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)
Re: Lexan Windscreen
Posted: Fri May 13, 2022 6:28 am
by 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)
Re: Lexan Windscreen
Posted: Fri May 13, 2022 8:41 am
by 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.
Re: Lexan Windscreen
Posted: Wed May 18, 2022 1:33 am
by Skippydiesel
DCASonex wrote: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?.
Re: Lexan Windscreen
Posted: Wed May 18, 2022 11:47 am
by jerryhain
Pretty easy if you live in Arizona.
Re: Lexan Windscreen
Posted: Wed May 18, 2022 10:06 pm
by Skippydiesel
jerryhain wrote: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.
Re: Lexan Windscreen
Posted: Wed May 18, 2022 10:22 pm
by Rynoth
I put a space heater under the plane. Kept the cockpit area toasty warm for the days/weeks I worked on the canopy/windscreen. I’d leave them in place overnight and let the heat soak in. Works best if the forward fuselage floor isn’t installed yet, but would also work if only the aft fuselage floor isn’t installed. Heat fills the high spots (i.e. the canopy/windscreen.)
Re: Lexan Windscreen
Posted: Thu Jun 02, 2022 6:46 pm
by Skippydiesel
In my yet to fly Sonex, the cockpit canopy (perspex) has developed a crack, in one fixing hole, close to the point where the “stay” connect to the rear frame/hoop.
Initially I thought this was due to the canopy curve & the frame not being perfectly symmetrical, resulting in stress on the perspex when the fixings are tightened down - so I “spaced”, using nylon washers of different thickness, the gap between frame & perspex.
I have come to the conclusion that the very slight asymmetry is, at best, a minor contributor.
Careful observation suggests the rear frame/hoop is flexing on each opening. Despite every care being taken, the crack has appeared. This does not bode well for the future, particularly when wind gusts might impact on the open canopy.
What mitigating strategies have other builders employed???
Re: Lexan Windscreen
Posted: Fri Jun 03, 2022 9:43 am
by DCASonex
I posted some info on a very simple and light weight brace for my canopy, that really stiffens the structure. if you cannot find that, PM me your direct email and i will send some photos.
David A.
Re: Lexan Windscreen
Posted: Sat Jun 04, 2022 7:54 am
by Skippydiesel
DCASonex wrote:I posted some info on a very simple and light weight brace for my canopy, that really stiffens the structure. if you cannot find that, PM me your direct email and i will send some photos.
David A.
Thanks David - found your old post & some very informative photos. Would seem we are on the same track - I have measured & planed for a similar “stay” on my canopy rear hoop.
Re: Lexan Windscreen
Posted: Sat Jun 04, 2022 3:21 pm
by NWade
For the original Sonex-type canopy (prior to the B-model changes), I followed a couple of other people’s suggestion and built additional bracing into the rear canopy bow. I found that it made a massive difference in the canopy stiffness. Here are a couple of photos of how I built it:
https://1drv.ms/f/s!ApMKXiyfUEPRhskCdhJeRQvrZ8j5-Q
—Noel
Sonex #1339
Re: Lexan Windscreen
Posted: Sun Jun 05, 2022 3:23 am
by Skippydiesel
NWade wrote:For the original Sonex-type canopy (prior to the B-model changes), I followed a couple of other people’s suggestion and built additional bracing into the rear canopy bow. I found that it made a massive difference in the canopy stiffness. Here are a couple of photos of how I built it:
https://1drv.ms/f/s!ApMKXiyfUEPRhskCdhJeRQvrZ8j5-Q
—Noel
Sonex #1339
Very nice Noel - if I was starting from scratch I would go with your concept but as I am retrofitting, I will stick with Davids single stay for the moment.
Re: Lexan Windscreen
Posted: Mon Jun 06, 2022 8:44 am
by DCASonex
I think you will find that extending the one diagonal brace up to top center of the canopy will make it even stiffer while allowing a slight bit of movement on the edge with the latch to make engagement easier.
David A.