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Airhead

Active Member
I just finished with "phase 1" canopy frame bending (pre plexi fitting) and am waiting on some warmer weather to start cutting the canopy. I would like to use the sikaflex method of attachment, and have been reading all the references I can find. There is one step I have not found fully addressed;

If you attach using fastners (rivets/screws) you must make any final frame adjustments prior to drilling and attaching the canopy. This sequence insures that the spread of the canopy frame which occurs from attaching the plexi is compensated for prior to attachment, and the final fit (width-wise) should be correct.

Why would this same sequence not be the best when using sikaflex? The only documentation I can find of this step has been from builders who over-flex (using straps,ect) the canopy/frame combo just prior to sika application, hoping it will spring back to the proper width (some do, some don't). If the stars don't align and make that happen, you are left with the option of making bends to the now glued canopy.

After a couple of weeks of nervously bending on the frame (waiting for something to pop), I am extremely hesitant to consider this a viable option.

Input appreciated,
 
no worries bending a Sika'd canopy

Hi Bob - in an ideal world, you will get the canopy frame pre-bent to exactly the right width, so that it spreads out just right when the canopy is attached (using any attachment method).

Of course this is the real world. It won't be perfect unless you are very lucky. Mine was off maybe 1/8 to 1/4 inch after the canopy was glued on. No problem, as there was enough give in the system to easily place it on the rollers and it rolled OK too.

But the real strength (pun included) of the Sika method is the length of the bondline, so a little post attachment bending should be no problem. This very different from screws/rivets because the act of drilling holes in the frame weakens it.

After a week of sika dry time, I decided to test this theory. On a hot day, using a ratchet strap, I bent the canopy/frame assembly inwards about 2-3 inches to get my 1/4 inch of permanent bend. Now my canopy perfectly slides on the rollers.

No problem, don't worry about it.
 
You can bend the frame prior to canopy fit. Then fit and trim the canopy and attach temp via clamps or tape. Check the fit. If you don't like it, remove the canopy and rebend until you are satisfied.

Even with Sika, which is how I attached my canopy, I would be very reluctant to attempt any bending. If you don't crack the canopy outright during the bending evolution the chance of scratching, kicking, or dropping something on your nice new canopy is much greater.

When I did mine, I ensured everything was the way I wanted it. Once I applied the Sika and it was dried I installed the skirts, then removed the canopy and covered it in many blankets in a spare bedroom with the door locked util it was time to paint.

my .02

regards,

Ken
 
I did it

When I was fitting the skirt I discovered the edges bowed out too much, I but the canopy on the table and used a strap to pull the sides together with a 2x4 pinched to hold the main sides out, garage was about 68 degrees.

The cool part of the sika bead is that it does give, that is why it relieves the stress on the canopy that rives do not. I think the only question is how long the thing will hold up, so far no one has been flying with a canopy done this way for 15 or 20 years. I figure any de-bonding will be caught and repaired during annuals.

icrdyh.jpg
 
Thanks for the replys so far but keep em coming. I believe there is a little lack of detail in the archive posts regarding this, and it seems the sikaflex method has much to recommend over mechanical attachments.

Dave, did you mean on "a" hot day or on "the" hot day. What is hot in Iowa anyway. On a hot day here I can powder coat my parts at ambient garage temperatures.
 
Airhead said:
Dave, did you mean on "a" hot day or on "the" hot day. What is hot in Iowa anyway. On a hot day here I can powder coat my parts at ambient garage temperatures.
Click to expand...

LOL - Bob you should come visit in the summer. We often get every bit as hot and humid as FL... especially when the corn is nearly full grown. It releases moisture as it grows and can actually make fog!

I turned off the aircondiitoner in the garage for the canopy work, and it was 90+ every day. We can get a solid month of 90+ degrees with 90% humidity.

Oh - we also get the other side of the dial. It is 8 degrees outside now.. and so far this winter I've seen -18. It will break -20 at least once!

I agree with you about Sika - seems to be the way to go. The only hole in my canopy is for the release handle. I Sika'd the side and rear skirts on too.
 
Airhead

I found this to be the most pleasing aspects of using Sika to bond the canopy. You don't have to guess what the finished dimensions will be.
1. Measure the width of the fuselage at the front and at the rear of the canopy . These are your final (goal) dims. Write these numbers down on masking tape at the location where the measurements were taken.
2. Bend the frame to your dimensions anticipating some springback.
3. Install the canopy on the frame using spring clamps every three or four inches
4. Measure the assembly at your predetermined points comparing that to your numbers on the fuselage. Repeat this process until you are happy with your measurements. Now, you have totally eliminated the uncertainty of compensating for springback. It's already done.
Apply electrical tape to any non-Sika surface. Clean and prime your parts, clamp them up using spacers and apply an inch of Sika on tubing/canopy between the clamps. This just holds things together. Don't try to do too much at one time. Center tube, then one bow then the other. Let all this cure a few days then remove the clamps and spacers. Now, everything is in place.
Clean and prime again. Fill in the gaps with Sika and go for final shape. Don't be to anal on this. Sika sands easily after cure.
Good luck, have fun.
Bob
 
Thanks Bob. You have described the method that makes the most sense to me and so far no one has mentioned any reasons not to do it this way.

One follow up detail question. If you apply spacers (say 1/8") between plexi and frame, and you have your initial bend specs per plan (1/16" along side and rear bow and 3/8" at center of frront bow) the rear bow will have to be lowered 1/8 inch (easy if slider track is not yet anchored) to accomodate the spacers. Probably the front bow will not need to be lowered, assuming the windscreen will also be glued and will require the same spacers. This will make the side bows sit a little lower in the rear (mine are already about 1/16" lower at the aft end). Any problem with this when attaching side skirts?
 
This may seem to be a silly question but why is the space required between the frame and the plexi? If it requires a specific thickness for proper adhesion, isn't that provided on either side of the tube if the plexi touches the frame? I ask because my slider is already installed with rivets and I'd like to attach the windshield with Sika. If I must leave space the gap between them will be excessive.
 
Ability to flex is the key...

This product was designed for installing windows in boats. The thickness is to allow the plexi to move with respect to the frame due to thermal and mechcanical movement and allow the Sika to absorb that movement. The thinner the gap, the higher the stresses in the Sika.
Now, that being said, the Sika is tough stuff, and I have yet to hear of a failure, despite undoubtedly less than ideal applications (including mine). Good surface prep is essential (as it is on any bonded joint) and be sure to apply the sealant within an hour of applying the primer. Lack of joint thickness can be mitigated by increased bond area - like generous fillets on your windshield.
HOWEVER, we're diving in uncharted waters, so an abundance of caution is in order.
 
If the bonding qualities of Siak is that great, but expansion is a problem then why couldn't you not use spacers but just bond 1 inch every two or three inches. Kind of like riveting. The front of the windshield could be spaced because that is easy but the sides and over the back (tilt up) could be bonded without spacers and bonded 1inch for every two or three inches.

We need to do a test.............
 
I believe Carl has a good point about the space requirement being mitigated by the fillet produced, especially when bonding to a round tube. It is very likely that the guidance given by the manufacturer of Sikaflex concerning a gap does not speak directly to bonding between a flat and a round surface, where a considerable thickness is achieved either side of the tangent point.

Even when using spacers, the spacer thickness will decrease to near zero as you get near the outboard end of each the front and rear bows because the offset at the weld point to the side bow accomodates only the plexi thickness. Having said that, I will likely set mine up using a 1/8" spacer, but if there is any need to, I am prepared to go as little as 1/16" (or less if the length of the minimal thickness is fairly short).
 
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