Solutions to reduce canopy cracking problems

I'm interested to know if any builders have deviated from Vans specs on the canopy installation in order to reduce point stresses on the plexi caused by differential expansion between the frame/fasteners and plexi canopy.

I'm building a slider and that's what I'm particularly interested in.

I'm not interested in using Sikaflex. I've researched that extensively and I'm just not comfortable with it.

At this stage I'm inclined to attempt an improvement of the existing system using mechanical fasteners.

Personally I believe that virtually all in-service cracks initiate at drilled holes. That makes sense because the Vans fastener is approx 1/8 and the holes are only 5/32. That leaves a margin of only 1/64 in any direction for the plexi to move before it hits the fastener.

In many installations, due to very slight fabrication inaccuracies, fasteners may already be touching the plexi. Differential thermal expansion between the steel frame and the plexi canopy will therefore cause very high point stresses on the plexi, often resulting in cracks.

Has anybody tried drilling the holes in the plexi to a slightly larger size (say 3/16) and using neoprene o-rings to centralize the fastener in the hole.

Any other successful experiments out there.

One of our local guys spent an extra month or two doing exactly what you said. Made larger holes, put in little spacers made from surgical tubing and used screws, etc.. to hold the thing on. Another guy in my shop right now just glued his canopy on andit looks great.

That being said, my old RV6 has been flying almost 5 years with the standard Van's attach method. Temperatures experienced here in good ole MN test the extremes, as this time of year we get to use both the heat and air-conditioning in the same day. My canopy has seen temps from -30 degrees to +110 degrees and hasn't broken/cracked yet (probably just jinxed myself though).

I know a LOT of slider guys who've done the same thing and rarely hear of problems. My take...."if it ain't broke don't fix it". Van's seems to have gotten it right the first time

Just my 2 cents as usual.

Cheers,
Stein.

Numbers

My maths is a bit rusty, but...

Plexi has a coeff. of expansion of about 70 x 10-6 in/in/F
Steel is about 7 x 10-6 in/in/F

Assuming a 100 F temperature change, this give a difference of 63 x 100 x 10-4 in/in

For a 50 inch canopy length (my guess) this is about a 0.315 inch delta.

Even though a 100 F change from the temperature you built it at might be high, this gives you an idea of the dimensions the materials want to go to with a temperature change...

Enlarging the last 25% of the holes at each end of the canopy track like you suggest might be a good idea. Keep the center located and let the ends move..

Any time the plexi can't move, it's under tension or compression.. I just don't know how much differential it can take before cracking...

gil in Tucson

I'm not interested in using Sikaflex. I've researched that extensively and I'm just not comfortable with it.

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Hey Bob, I was thinking I would use Sikaflex. What does your research show and what makes you uncomfortable?

Thanks,

tomcostanza said:

Hey Bob, I was thinking I would use Sikaflex. What does your research show and what makes you uncomfortable?

Thanks,

Click to expand...

Hi Tom, the answer(s) to your question would involve a quite lengthy document discussing technical issues. I'm not sure this is the thread for that.

Let's just say that the manufacturer of the sealant (Sikaflex 295UV) does not advise using their product in this application without mechanical fasteners.

In addition the manufacturer of the RV (Vans) also does not recommend that builders bond their canopies to the frames without the use of mechanical fasteners.

These people might know what they're talking about.

I'm just finishing my canopy up and did it Van's way. I've done enough plex work over the years to feel comforatable with the way it is done and the fasteners used. If you can get it all mounted with no cracks you probably won't have any trouble down the road. Don

I used a combination of fastners

I started off planning to use #6 screws on the front, rear and center tubes, but my plexi seemed to have a mind of it's own. I had the tubes and plexi drilled and cleo'd, but when I started tapping the tubes for screws, it seemed that half the holes did line up . I enlarged the holes quite a bit more than the plans called for and still no joy.

In the end, I pop riveted the center and front and all the holes have a lot of clearance around the fastners. The center strip covers the oversize holes down the center. I made a matching aluminum strip for the leading edge of the canopy - the same width as the center strip. I considered tinnerman's but felt the matching strip was worth the effort. I thought alot about using a sealant in the holes where I used pop rivets, but decided it was not necessary.

I drilled through the rear tube and used screws with tinnermans on the plexi every other hole. I made my rear skirt from fiberglass and screwed through the glass using the holes I left open when I attached the plexi to the rear tube.

I did all the work when the temps were around 100 and the canopy survived it's first temperature test last winter with temps in the 20's.

manufacturers recommendations

Be aware that once most manufacturers find out you're going to use their "fill in the blank" in an aircraft application, they'll tell you not to do it. People using things like flap motors, fuel pumps, valves, even gauges have all experienced this situation. I can't say that I blame them given our lawsuit happy society. A good illustration of this is that Van's doesn't condone making Rockets out of RV's...

If you don't believe me, just start making a few phone calls to companies you see named on parts and components in your airplane and ask them if their widget is approved for use in an "aerobatic airplane". Take your alternator to a rebuild shop and tell them how they recommend wiring or installing it on an aircraft...

As always, do some real world testing and make the best decision you can from that...they're "EXPERIMENTAL."

We know canopies crack, we see it all the time. It may not have happened to YOU yet, and it may never happen. The fact is, it does happen, and fairly regularly. At every fly-in where there are canopies similar to these being installed similarly, see if you can spot any cracking...there's a valid reason why this question is being asked.

Regarding the use of adhesives, including SIKAFLEX, be aware there are people at Van's who think it's a good idea...and there are people who think its not. I'd be very interested to monitor any failures of canopy installations using that adhesive method, where prep work was done according to all manufacturers instruction. As far as I'm aware, there have been no indications of failures over the (6 years?) people have been doing it. Can we say the same of rivets/screws?

Built and attached mine per Van's plans. After 14 years, no cracks.

exactly

You couldn't be more right Mel. And there are people who've installed Van's alternators and have never had an issue with them...others have.

My flap motor gave up the ghost after 55 hrs...others have 15 years on theirs...

Ya makes your choices for your own reasons, and ya go with 'em...

Canopy...mechanical mounting

Bob...

I am the guy Stein referred to in his post. Yes, I came up with a nylon or PVC grommet, and #6 screw to hold my plexi canopy and windscreen in place. It took considerable labor but I also had acquired epoxy poisoning so all my plexi on my RV6 has aluminum skirts as well as windshield trim out of necessity. It looks well and works well with no sign of cracks or even signs of stress so I say it worked but at a cost about double the labor. I did some aircraft rebuilds and always saw plexi held in between two layers of aluminum so that is what I did. Each plexi hole was drilled to about 1/4" and was machine countersunk so the overlying aluminum, which was dimpled, would center in the countersunk plexi hole. Inside the plexi 1/4" hole was a PVC plastic bushing made from leftover brake line material. This bushing was cut slightly high so the screw, which was dipped in proseal and tighted into tapped holes in the canopy frame, would clamp on the bushing and not the plexiglas. The busing inside diameter is considerably larger than the screw diameter allowing the plexi expansion room. Since I was always good at forming aluminum and I had some 5052-4H aluminum from other projects, I formed my windscreen fairings with a borrowed shrinker-stretcher. No pop rivets were used but proseal was used under all aluminum to seal against water. The RV6 has operated in high summer heat down to around -10 degrees and no problems. I do know of one RV6 and two RV4's with poprivets that have cracked canopies but I am sure my labor was doubled by this method. If you are still interested I have drawings and actual measurements. Email me at diesel at rconnect.com for that. Also, thread lockers for the screws could not be used as it will cause crazing in plexiglass and the only safe material I knew of was proseal.

Dick DeCramer
Rv6 N500DD
Northfield, MN
200 Hours

Bob Brown said:

Regarding the use of adhesives, including SIKAFLEX, be aware there are people at Van's who think it's a good idea...and there are people who think its not.

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Bob, it's not that I doubt you, or that I'm sceptical....but could you please advise us who at Vans thinks it's a "good idea".

Bob Brown said:

I'd be very interested to monitor any failures of canopy installations using that adhesive method, where prep work was done according to all manufacturers instruction. As far as I'm aware, there have been no indications of failures over the (6 years?) people have been doing it. Can we say the same of rivets/screws?

Click to expand...

I think you'll find that, regardless of whether they think it's for an aircraft or not, Sika's engineers will recommend that mechanical fasteners need to be used on any critical application which bonds such a large area of plexi to a steel frame. In such circumstances the "manufacturers instructions" will be to NOT rely on the Sikaflex alone.

No problem with Sika...

I did the Sikaflex route since: 1) I did not want to drill any holes in plexi 2) It looks a LOT better! Wieght is probably the same as the pile of fasteners, so that is not a factor.

The strength of the bond was a concern for me. So what does any good engineer do? He tests the bond! I glued together a couple scraps of the plexi and let them cure. I then did my best to rip them apart manually (no I did not use a calibrated test rig) and was not sucessful. I then had varioius A&P's and other builders try the same without success. The only way we could fail the approx. 1.5" square blob was by twisting and shearing them. It is very strong in tension and remains flexible to this day.

Another factor that made me comfortable was the way both sides of the tip-up canopy is attached. The plexi is sandwiched between the frame and the outer skirt. By using the sika 'glue' and clamping the sandwich it is extremely tight seal and very strong. My gut says it is better than fasteners since there is no point loading, only a uniform seal.

I was not the pioneer, but I convinced myself that Sika was not only feasible but superior. If I had to do it again...I would do it again!

More Sikaflex testimony

I read all the threads on cracked canopies and did the same as Kellyp - I conducted my own testing with the copious amount of leftover plexi. I found the Sika bond to be extremely tough. I followed the Sika instructions exactly. I bonded my RV-8 canopy and skirt with no pop rivets. 112.6 hours in 106 - 20 degree F conditions and no indications that that plexi will ever let go as well as no cracks. I have at least .125 bond thickness which allows for any expansion/contraction. I also drilled holes initially as Van suggested, but ended up just squirting Sikaflex through the plexi holes and into the canopy frame holes.
As always, my opinion only - but I wanted to provide a (flying) data point.

FM
RV-8, 391FM
Flying, 100hr inspection done

sikaflex

I just recently had a mishap while working on my F1 and am likely the first to have to completely remove a glued-on canopy to install a new one. I glued it on well over a year ago. To make a long story short, in my experience there is absolutely no way on earth the canopy will come off glued on with Sikaflex 295. As an experiment I tried to see if I could cause the bond to fail. I would have destroyed the canopy frame if I went any further with it. I even experimented with pulling the canopy off after starting the cut with a knife and ended up breaking the plexi. I also pulled, pushed, twisted in a few different directions and couldn't get the bond to fail, the plexi always broke first.

Regards,
Bob Japundza
RV-6 850 hrs. F1 under const.

Just a note on the need for mechanical fasteners on the sika canopy..

On the 7, the canopy latch would hold the canopy on if it were to delaminate. The process would be very slow, and you would have plenty of time to get on the ground.

That all being said, sikaflex is spec'd as a structural adhesive for direct glasing of polycarbonate. And if you ever see a canopy which has been properly bonded, I think your opinion may change, this is tuff and very very strong stuff.

There are many production planes which have windows bonded in with pro-seal, much weaker.

to each their own.

CYA

The manufacturer of Sika... does not recommend its use in "amatuer" building. To meet their full specifications, the filet size has to be a certain thickness and consistency, and the prep work and prime done properly. I think they are just CYA. If you prep it right, there is no way it is coming apart, and the radius of the canopy frame will almost guarantee that at some point the proper filet size will be achieved even by accident.
I used it as a sealer for my windscreen under my metal fairings. I did not prime or prep it on purpose. It works great as a sealer but it does come apart easily if the surface is not prepared and primed properly.
I built my canopy before Sika was being experimented with, so mine is traditional construction. No cracks, yet, but if I had to do over, I would use Sika. Besides, the pressures on the canopy in flight will hold the canopy on with our without any fasteners, at least until you land