Canopy drilled - how to proceed?

It always happens at the last hole, doesn't it? I apologize in advance if this post goes long! I have 3 issues and looking for the best way to proceed. We'll start here:



There are two things, the crack and the bad edge distance in the plexi on the other holes. The crack happened when the plexi bit got stuck - tried to free it but to no avail.

And when it was all said and done, my perflectly flush canopy in the front lifted up:



So how to proceeed?

• Obviously the crack has to be removed - I assume just scallop it out.
• About the edge distance - after countersinking the edge of the countersink will be at the edge of the plexi or a tad beyond it in certain placed. I had to trim the plexi sides to make it lie flush with the joggle. What do I do about this?

1. How about using CS4-4 pop rivets instead of screws? The CS4-4 rivets are used on the RV-8 and use a much smaller countersink
2. How about drilling new holes between my existing holes? The new holes would be higher and allow for better edge distance PLUS at the same time perhaps we could ensure the front of the canopy is flush with the front while we re-drill.
3. Leave as is - countersink and maybe use some adhesive (ProSeal or Sikaflex) along the sides and front to take up most of the stresses?
4. Put clips on the front to hold the canopy down. It can be held down as it is with little pressure.

I'm not sure why Van's uses screws - probably because the rivets expand and put a radial stress on the canopy? But how does that explain the RV-8?

Thanks for the advice in advance!!

front gap

I drilled 2 small holes in front of the canopy where it needed pushed down the most and made small clips from scrap aluminum which were attached with screws. You could use more of these temporary clamps if needed to get the fit that you want.
I then mix my fiberglass with a lot of the white thickner (west system) until it was like putty. This I applied where the clips were not.
After it hardened I removed the clips and filled where they had been.
The thick putty was used to continue the plexi down to the frame as though I had cut it correctly. After that I proceeded with the cloth and resin as shown in the directions.

Two suggestion:

Don't push the putty into the crack with a lot of force, as you will end up just pushing it behind the plexi.

When you get close to your final curve on the front, sand down aggressively and then use only small amounts of filler. I ended up removing a lot of glass that I had put on.

Don't know about the crack. It didn't happen to me.

Kent

Don't panic!

We have a couple of holes that look like that one. We ground the front faces of a #40 drill to give it zero rake and used that to stop drill the ends of the cracks. That should stop the crack from spreading.

After the cracks we did a bit of reading up about drilling perspex - dont't press hard, lubricate the drill bit & try to keep the drill cool (stop frequently). After the cracked holes we stopped 2 or 3 times per hole & dipped the drill beit in water for cooling & lubrication. No more problems yet!!

I spoke to a guy I work with who is a composites structural engineer & he is going to check his data sheets & check out a suitable adhesive that we can try to run into the crack. He is pretty up to speed on perspex as he is doing a walk-through aquarium project. He is amazed that we attach the bubble with mechanical fasteners - he would just glue it! He suggested bedding the bubble down on a low modulus, neutral cure silicone sealant (Bostik Bond-Flex 300LMN would be his choice) that will do a lot to unload the screw holes. We were keen to use some sealant anyway to keep the water out.

Dave

PS I went to the PFA (UK) rally last weekend and as you can imagine was pretty interested in canopy cracks. One RV had a 6" diameter starburst crack on the canopy side (the painter dropped a scaffold pole on it). It's been stop drilled & cemented & is still fine 200 hours later. The builder has a new bubble waiting to fit, but finds it easier not to look in his 4 o clock high position than to take his plane out of service.

Gluing canopies and expansion issues

Just curious, have there been any problems with attaching canopies with glue, vice using mechanical fasteners (i.e., screws or rivets)? I understand that plexiglas expands to a greater degree than metal; expansion is accomodated by holes for the fasteners that are larger than normally used. Seems like using glue would not allow the plexiglas to expand, but I must be missing something, since a number of builders have successfully used glue.

Differential expansion - glues.

I've seen some aerospace structures where carbon fibre ribs were bonded to an aluminium skin. The differential expansion is pretty bad as the carbon fibre contracts when heated! It's all about choosing the right glue & techniques - in that case a low modulus (low stiffness) glue and a fairly thick glue layer (minimum 1 mm on that application).

I suspect that the biggest problem is the glue process. Cleanliness, appropriate surface preparation and temperature / humidity. Bonding processes often fail because the operator thinks that "clean enough" is good enough or that the surface prep is excessive. When bonding the carbon ribs to aluminium the aluminium surface was cleaned, abraded, cleaned, etched and then bonded...


Dave

Gluing canopy

I'm always happy to have a chance to tell everyone how happy I am with the
results of gluing my canopy! I'm really happy with the results!

http://www.rv8.ch/article.php?story=20050614184749626

Seriously, even Van's is coming around on this one. They sell glue for
the RV10 windows. I used Sikaflex 295 UV, but there are a lot of
options out there.

My fix

After not sleeping much the previous night - was it thinking about the plane or our baby who likes to play at 3AM? - I decided upon a fix.

Ken @ Vans recommended using AACQ blind rivets in the existing holes. I wasn't comfortable with that for a few reasons. One being the ashtetics and the other being the canopy being raised so much in the front.

I settled upon drilling NEW holes halfway between the 'bad' holes... this works out to be 1" spacing. On the backside (in the canopy) I'll bond a piece of decorative metal (about 0.5" high by 28" or so long) to cover the bad holes. You'll never see the bad holes. Once I drilled the pilot holes and thoroughly taped the front of the canopy down, I could immediately see where we went wrong. We drilled from the back and didn't secure the front.

Anyway, I am a happy camper now - the canopy sits nice and flush again <phew>. I just have to live with knowing there are a bunch of 1/8" lightening holes that we can't see. I'm also thinking of using some sort of adhesive to take some or all of the load off the screws. Who knows, maybe those holes will help help the glue!

Here are some pics: