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prkaye

Well Known Member
Another canopy question. The main reason I'm not enjoying the canopy work is that I'm terrified every tiem i go out there to work. Terrified that it will break and cost me a fortune to replace. I've read all these warnings about how easily they can crack, so I feel like I'm working with an eggshell.
On the other hand, I tried to break a leftover piece that I had trimmed off, and it bent very far without breaking (was very hard to get it to break).
So, my question... how fragile is the canopy really? What's the biggest risk factor for cracking it?
 
How fragile.....

Having recently finished my tipup canopy (and it cracking during construction), I have come to think of the canopy material like clear shipping tape. It can be strong as all get out, but nick it in the wrong way and it tears like toilet paper!

Like you, I could hardly break a trimmed off scrap by folding it in half! But I accidently drilled a couple holes with a standard #30 drill instead of a plexiglass drill.... and BOTH of those holes developed a 4" crack about 10 min after drilling!!!!

Keep the canopy over 80 deg F when drilling/cutting,
ONLY use plexiglass drill bits,
Jig capture the parts so they don't move around when cutting,
Sand all cut edges so a crack doesn't propagate,
Use a 100 Deg grinding stone after cutting countersinks,
Take your time!!

It's not a difficult task... just tedious.

You can see my canopy pics at: http://picasaweb.google.com/mikerv9a/Canopy
 
yikes... what are you gonig to do about the cracks??

ONLY use plexiglass drill bits,
Click to expand...

I've read this only applies when enlarging holes, taht a regular twist drill can be used for the initial #40 hole. Did your cracks happen by enlarging, or initial drilling?
 
I've built quite a few RV canopies and have never used a plexi-drill bit. Not to say they aren't great. Just that they are not necessary. Be very careful drilling plexiglas. Use high speed and virtually no pressure. I've not had a canopy crack. Mine has been flying for over 15 years. Of course now that I've said that, I will probably go out and find a crack.
RV canopies are not that fragile IF HANDLED CAREFULLY.
 
one man's experience...

Phil,

Before I started my Tip up canopy... I used to think that I would have to do the following things to be successful:

- Read to the canopy at night time.
- Rub it with a Plexus soaked diaper while singing old Barry White tunes to it.
- Make a blood offering to the canopy from my already air-drilled, aluminum mangled fingers.
- Work on it only in a clean room environment with the temp set to at least 121.5 degrees F.
- Threaten the canopy by saying that you'll have Captain Avgas and GMC jetpilot over to enforce their supreme intellect upon it if it does not cooperate.

I worked on my -7a canopy in the middle of an Ohio summer so I had that going for me.... Nice and toasty back in 2004.

During some die grinder trimming (you'll do a bunch of it...) ... I accidently knocked the entire canopy off of the saw horses with the air hose and it bounced! Bounced that sucker right off of the concrete floor of the garage. No chips, or cracks. It was like watching a car crash in slow motion as I could not get to the canopy fast enough to grab it! Should I have been more careful?... Yes! Did it happen anyway?.... Yes!

The scream that was released by me during the event is still hanging over Lake Erie somewhere to this day.

Having done this, I would not try to reproduce it as it takes a few years off of your life to witness. However, it did demonstrate how tough these things are as a complete structure.

My opinion of what would produce the dreaded crack? What's the most dangerous thing to do around the canopy to tempt fate? To me it is more of a local problem than the complete structure.

#1...using non-plexiglass drill bits to drill your holes. Get the required size plexi bits to do your drilling. I practiced on scrap pieces with both styles of bits. The plexi bit was more forgiving with an operator induced bad drill angle and forcing the bit through the material. The std bits chipped the edge of the holes and "grabbed" the plexi producing cracks around the test holes at times. Go slow here with plexi bits and back the plexi up with a wood drill block as you drill.

#2...forcing the canopy to conform to a canopy frame shape that doesn't look quite right.... then screwing it down in this pre-stressed condition. If you have monster gaps between the frame and plexi... take a minute to re-evaluate the fit and go slow with the trimming.

If the above doesn't work and you end up cracking it, take a big glob of Pro Seal and give yourself a "Dirty Sanchez" Mustache with it! I guarantee that nothing worse will happen to you and your canopy while in the shop.

Jeff -7A 440 hrs. of RV flying bliss...
 
Like Mel, never used plexi drills, never cracked mine and still no cracks after 5 years. Don't do plexi work in cold temperatures, use light pressure.

I was paranoid also when I started mine but it proved to be no problem. Time consuming and probably the worst job on the airplane but like all other jobs, it eventually gets done.

I would be careful enlarging holes as the drill can climb in too fast.
 
plexi drills through steel?

For those initial holes they have to be match-drilled through the plexi and the canopy frame (right?). Do those plexiglass drill bits like going through steel??
 
I did all kinds of testing on the pieces that were initially trimmed off to find out what I could and couldn't do. I drilled several different sized holes, with plexi bits, regular bits, and a unibit. I used light pressure, hard pressure, FORCEFULLY drilling it slowly, quickly, and everything in between. I couldn't get the dang thing to crack by doing any of that. I did get some light chipping with the countersink two flute cutter.

I tried several different ways to get it to crack without using drill bits or the like. A hammer would not crack it while hitting a piece on a carpeted table. However, put it on the cement floor, and it cracked after three blows in the same spot (the first two blows did nothing but scratch it).

To the extreme, I took a pair of snips to an finished and unfinished edges, and that snapped the plexi like plate glass.

Just my experiences...
 
Canopy: How Fragile???

I approached my sliding canopy with a fair amount of respect. I did use the plexi drills as an added precaution. To me the varible is temperature. I built my canopy in an non air conditioned garage in south Louisiana. My canopy in the summer heat and humidity was a supple and pliable as a rubber mat. After I finished, I took left over pieces and dropped them on the concrete, bent them double, I tried to break them. Again in a cold temp, the plexi becomes very brittle.

Bottomline: get your shop as warm as possible. Work on the canopy when it is uncomfortable for you especially when you get those plexi crumbs down your shirt but the canopy loves it.

Steve Anderson
RV 7A H-6
Finished Waiting on Mike Stewart
Lafayette, La.
 
My Experience

Yesterday I finished all the drilling of my canopy, including the windshield. I drilled all the initial holes with regular drills into both the plexi and the steel tubes beneath, no problems at all. To enlarge the exsisting holes I used reamers with straight flutes. Absolutely a piece of cake with no tendency to grab at all. I would never use a regular drill bit to enlarge the hole, I would screw it up for sure. I tried some practice pieces and everything going fine, then that bit would grab and screw itself into the hole faster than I could react. instant crack. Mel, you must have the "touch". I've had the canopy clecoed on and off making adjustments, and am suprised by how durable it appears to be. Good time of year for it :)
 
cold

The plexi is naturally bowed a bit wider than the steel frame, and has to be pulled down against the frame (initially using spring clamps, then clecoes then screws I guess). This means that the canopy when its finished will be under slight tension. I'm building it in the summer, but eventually it will be stored in a hangar which will get very cold in the ottawa winter. Is tehre a risk that the finished canopy will oneday just spontaneously crack when it gets too cold, because the frame is keeping it under slight but constant stress?
 
I would not force the canopy down to the frame if it is a long ways off. Fit some spacers to make it stand off a bit. Mine didn't fit the frame very well at all nor the the frame to the airframe without major mods.
 
I would not force the canopy down to the frame if it is a long ways off.
Click to expand...

I don't mean localised spots that aren't sitting down (where spacers are appropriate). I mean the overal bow... the plexi naturally bows wider than the frame and has to be held in place. The natural tendancy is for the plexi bow to spring out a bit laterally, and gentle pressure is required to hold the overall bow to the place. It's this gentle, but continuous pressure that I'm wondering about.
 
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When my canopy is just resting on the frame, before I begin to cleco it down, I'd say there's 1-1 1/2" gap on both sides. Just a guess cause I'm not in the shop at the moment. When I put the clecoes in, I start at the center and work my way down both sides somewhat equally. I think once it's finished temp. isn't going to be an issue. There's many RV's, and other GA planes sitting out in very low temps.
 
The canpy must fit and plexiglass drills are cheap

The canopy frame must fit the fuselage as step one. Then the plexiglas must be fitted to the canopy frame. Pulling the "spreading wings" at the sides to attach to the frame is not a problem but force fitting the other sections to eliminate the gaps after you have worked to get the best fit you can get is. When you reach that point take your time and make aluminum shims to fit between the frame and the canopy at each location where a gap remains clecoeing them in as you work to get a proper fit at all locations at the same time. I have many custom shims at the attacnment holes around the bow at the front of the slider. After you are done it doesn't take a lot of ingenuity to conseal the shims. Another thing I did on my sliding canopy leading was to apply aluminum tape to the leading edge starting in line with the aft edge of the bow on the top of the plexiglas, forming it down around the front edge of the plexiglass and trimming it off there. I originally thought I would have to cut the thin aluminum but I found that it forms extremely well to the job - no relief cuts required.

Plexiglass drills are consistent in tip design, they are designed for the purpose and they are cheap compared to the expense of what you are working on. I bought a large one to drill the latch hole and I have had no problems. Cam Benton bought an RV-4 that he races but his whole season was wiped out this year because his canopy cracked.

Bob Axsom
 
working with plexi---

as someone else said, use high speed and don't let it "grab" the plexi. I used regular bits for drilling holes, and Unibits for enlarging holes(and it is important to enlarge holes so screws or rivets have no side tension). The unibits also work well for the large hole for latch handle, and caged countersink (also high speed) as required. The plexi is quite tough if it is allowed to "float" and isn't under strain at fasteners. After two sliders and no cracks, I guess i'm either good or lucky. ;)

I used the 3" cutoff wheel from Vans in a drill for most rough cutting, and a vixen file or disc sander to finish to size.

O.K.--I was lucky! :D
 
When fitting the canopy, leave it slightly long and fit the back to the rear frame and roll bar last, after you have trimmed it front to back. All your trimming should start in the front and work your way back. This is especially true for tip ups. Establish where the front will go and trim the front till it sets into the notches in the glare shield skin. Let it hang over the side rails and roll bar untill you have the front setting down the way should without forcing anything into position. Once you have it fitting nicely, trim up the sides and then finish the back. Don't expect the cut you made to separate the front from the back window to meet up. They probably won't. So if you leave the parts longer, you will have fudge factor to trim back for a good fit.

You don't want to do anything that will induce stress risers in the plexi. It will break just like glass, so treat it like such. Work in an warm environment, go slow, fit many times, remove and refit even more times. This is one area where patience really pays off big. Find the centerline of the canopy and use it for a reference while fitting so you always return the canopy to the proper place.

This is a pic of my canopy trimmed of salvage and set into place.

canopy2kz2.jpg


This is the canopy after the split and trimmed up in front and sides.

canopy5wh2.jpg


And this is the finished canopy ready for glassing in the front.

canopyfitshy8.jpg


Hope this helps,

Roberta
 
Heat and plexi

As others have mentioned, I believe that heat is the key. I just cut & drilled mine a couple weeks ago in 95+ heat and no problems. I recall reading a thread a few months back where someone built a box (perhaps could use the shipping box?) and heated the plexi to about 150 degrees for an hour(?), after cutting and setting in place. The idea was that the plexi would destress at about this temperature (poster seemed to know more about plexi than most of us) and therefore greatly decrease the likelihood of stress cracks. That same poster also cautioned not to go much higher temp or the plexi could start sagging.

greg
 
No....

prkaye said:
..........Do those plexiglass drill bits like going through steel??
Click to expand...

.....not at all. Just use the pin point mark they make on the canopy frame as the place to drill using a regular drill bit after you lift the plex off the frame.

Regards,
 
hair dryer?

What about using a hair dryer for heating the plexi locally just before drilling each hole?
 
I'm not a plexiglas expert by any stretch, but..

prkaye said:
What about using a hair dryer for heating the plexi locally just before drilling each hole?
Click to expand...
normally localized heating is not a good thing. It can cause stresses. If you're going to heat it, I would try to heat the whole thing as uniformly as possible.
 
oversize screw holes

The plans say to open the holes up with a 5/32 plexi drill. I've heard of other builders oversizing these holes to allow some wiggle room to prevent cracking, particularly when it gets very cold (as it does in Ottawa).
Any thougths on this? How much oversized should one go?
 
One guy's method

I have some info on my website (link below) about what I did with the plexiglass holes... basically I oversized all the holes and made little bushings out of silicone tubing. I stole the idea from another VAF member, I think. So far the canopy is holding up pretty well, in my spare bedroom anyway!

Since it sounds like you are struggling with sliding canopy questions - as we all did - you may find some other useful tips on there too:

http://www.rv7blog.com/category/fuselage/canopy/
http://www.rv7blog.com/docs/canopy-notes/

good luck,
mcb
 
I generally have treated my canopy as if it was nitro. Everytime I made a cut, I smoothed it out as if it was the final cut. I didn't touch it unless it was warm out... I put it in my mind that I would walk away from that part of the project (it's not like there aren't 1,000 other things to work on at this stage of the project) when the temperature dropped for the season, and I'd pick it up again the next season when temperatures warmed up.

Starting the fiberglass work now.

Overkill? Yeah, I'm sure it is. But I just didn't want to test the limits of plexi. What would have been the point?
 
Use a Dremel

For all of the holes in my canopy I used a Dremel to drill all of the holes in the plexi. I turned the speed way up and used regular drill bits, and light pressure, and the drills cut (or probably melted) their way through like a hot knife through butter.

For trhe canopy cutting I used the little steel cutting wheel from Dremel - I think it has an abrasive coating and it has 'interuuptions' in the edge, so it cuts its way through a bit like a saw blade. Can't remember the tool number, but it is about 1-1/4" in diameter and does a great job.

Allan
 
Colder shops

While searching for other plexiglass questions I found this thread and thought I could add some other insights.

Plexiglass is very notch sensative. You can bend a piece of scrap and not have it break, but put a small nick in it and then bend it and it will shatter like glass. I cut a 4" piece of scrap with a 2" cut, then bent the scrap, and it shattered before I felt like any load was placed on the material. So be carefull and support the trim as you go to prevent stressing the forward edge of the cut.

I also have done all of my trimming, not yet drilling, during the winter in Seattle in an unheated, uninsulated garage. I did bring in two propane heaters and rasied the garage temp from 45?F to between 65?F and 75?F with the heaters pointed at the area to be cut. So far this has worked out well. ONe thing I have noticed is when using Vans' cutitng disks, the plexy comming off the wheel is hot when it lands on the hand or arm, so the material is locally heating much higher than ambient anyway. I guess the trick is to not stress the crack tip.
 
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