Yes, I remember the article written by Dick Kreidel. It was on canard.com. Here is the text. I might have the pictures somewhere if you need them.
by Dick Kreidel
I have been watching the various threads on canopies for quite a while now, and thought I'd throw in my experience, too. Surprisingly, I've not seen much discussion on latch/seal theory. One of the weakest design points on a Varieze/LongEZ (I don't know about Cozys) is the poor system of latching that requires the latches be pulled down hard against a seal. In general. you never want to do that, as the seal isn't operating as it was designed. What you do want is to draw the latches down against hard points, and control the amount of compression that the seal sees.
With Rutan's design for the canopy close out between the inner and outer skin with a flox corner, you are at the mercy of the varying gap between the longeron and canopy frame. A better way to control the gap is build it in. If you recess the foam about 3/16" along the bottom of the canopy frame for about 5/8", and then get a (1/4" min.) skin/skin bond on the inner/outer layups, you have a perfect place to cast in a seal valley that will allow you to precisely control the seal compression. Be sure to create flox hard points about 1/4"x5/8" opposite the three canopy latches. This is what you want to draw and adjust the latches to!
After you are satisfied with the way the canopy latches down, select your seal, and determine how much you want to compress it (usually no more than 33% of free state). Find some balsa wood strips the thickness of the seal in its compressed state, and tack bond the balsa onto the longeron where you want the seal to rest. If you made your "seal valley" correctly, you should be able to latch the canopy down hard with the balsa strips in place without touching the balsa at all. Now put one ply of gray tape over the balsa and longeron, fill the seal valley in the canopy frame with micro, and slam it shut with the latches locked down. When you are sure its dry, open it up, remove the balsa and tape, and voila, you have a perfect cast in place seal locator. I have been using a labyrinth type seal that is made from EPDM called ScandySuper self adhesive weather strip for 15 years. It is cellular EPDM rubber and stays resilient down to -40F. It is 3/16 thick by 3/8" wide and has 3 longitudinal seal chambers that are superb at eliminating air leaks. It is available in chocolate brown and white, in CA you can find it at Home Depot.
First of all, a couple of comments about modifications. I've built a fairly stock Long, N888EZ and a highly modified Long, a LimoEZ N26EZ. In spite of all sorts of good intentions and at least 1,000 hours of flying the first plane before starting on the second, the first plane is better. I am really impressed with the development Dick Rutan mandated that Burt do when the Long was being developed. It is the only one of Burt's designs that has multiple iterations and subsequent improvements - and a well built Long is a delight to fly! Keeping all that in mind, recall that Rutan's Mojave location is as temperate as it gets. Even in the winter, it doesn't get all that cold and air leaks are almost impossible to find in the summer (about 9 months long here!) Therefore, canopy sealing and additional structural integrity of the canopy was low on Burt's priority. Those of you who have known him for a while surely know whatta I'm talkin 'bout. The stock canopy hinging, latching and structure is a compromise, but a pretty good one. One of the biggest problems is the relatively high coefficient of linear expansion of the acrylic bubble in relation to the foam and fiberglass/epoxy structure. Basically, the acrylic expands/contracts a whole bunch more than the foam/glass structure. How much is a whole bunch? I have measured the expansion of the canopy at .105" on a hot day (115F), and estimate (not measure) the contraction at about the same amount at 20F. So we're talking about a range of 3/16" to 1/4" in round numbers.
This wouldn't be so bad if the canopy was not held in place by the two hinges, which in essence places uneven stresses on the canopy frame/acrylic bubble. The challenge is to CONTROL the canopy movement, not prevent it - fish gotta swim! The canopy will have a tendency to bend itself like a banana as it heats and cools. When it gets real hot, the gap between the front edge of the canopy frame and the fairing above the instrument panel is reduced, and mid span of the longitudinal axis of the canopy frame has a tendency to rise (a higher WL.), and the canopy width will increase. When it gets cold, like at FL200, the opposite occurs; the gap at the front of the canopy increases, the banana shape is reversed with the front and rear ends of the canopy frame bowing away from the longerons, and the canopy frame wants to narrow. On first thought, I was going to deal with these problems with a typical mechanical engineers sledgehammer approach - muscle!!
On further reflection, it became apparent that control of the canopy frame was paramount, not precise positioning. I tackled the problem in a couple of different ways. First, I made a saw cut on the acrylic bubble about mid length that ran into a 1/8" hole that was drilled so that it would be buried inside the canopy frame. I filled this 1/8" wide kerf with RTV silicone to allow some of the thermal expansion and contraction to dissipate. This is similar to what Dave Ronneberg has done on the Berkut, except he kept going and sawed the canopy into 2 pieces. The proto Berkut that I flew did not seem to have any canopy thermal issues. I'm not sure if this did me any good, but I'm sure that it hasn't hurt.
Next, since I knew that if I could not hold the canopy fixed in the longitudinal axis, I at least needed to control it transversely. I machined 6 aluminum blocks, about 1"x.30x.40 with a tapered groove running the 1" length on the .30" face. If you were to take a cross-section of the block, it would have a "V" cut in it with max width of .080" with 15 degree per side taper. Then I made 6 aluminum"blades" with a 15 degree taper. The blades are about .75 long and have to counter sunk holes that allow them to be screwed to the vertical inside face of the longerons with SS flat-head wood screws. The blades are located in the front , middle and aft longeron locations so that they engage the aforementioned aluminum blocks that were floxed in flush with the inner canopy frame skin. The effect is 6 tapered grooves on the canopy frame that engage the six blades that sit up about 3/16" above the longeron surface. Because of their location, the black anodized blades do not get in the way at all. The sharp part of the blade is not really sharp, as it is about .050" at the top, and was fab'd from .080 sheet. Of course, you mount the blades first, then flox the receptacles in place while everything is lined up nominally and canopy latched down hard (see previous posting about hard points to latch against) This solves the problem of the canopy frame rails moving in and out, but still allows the canopy to move fore and aft with thermal loads. CONTROL, remember? No muscle, just a little finesse. I built this into our 2 birds from the git go, and retrofit 3 others with great success. This also eliminates the problem some have had of latching the canopy when the airframe has cold soaked and the canopy has pulled in enough that the latches don't line up properly and the %&^*$ canopy won't close and you can't take off!! For those who are thinking about conical receptacles with conical blades to position the canopy, I suggest you don't . As one enterprising individual found out about 10 years ago, the thermal loads aren't small and a cracked canopy makes a cool ant farm for kids.
The toughest problem to think through, and the easiest to solve was what to do to keep the right front corner of the canopy down in cold air. I designed all sorts of extra latches, some of which were separate from the latching system and some that were integrated with all sorts of kinematic mechanisms that would have made Rube Goldberg proud. I eventually rejected the independent latch as undesirable because I knew that someday I would forget to unlatch it, and throw the canopy open, thereby tearing out the RH latch, structure and really p$%%ing me off. The other alternatives seemed too complex and seemed to all have an unacceptable failure mode. What I came up with is a really simple PASSIVE (no moving parts, no thinking, no action required by the pilot) auxiliary latch that is difficult to describe, but is nothing more than a piece of .125 Al that is shaped something like an eyebrow and is bonded into the inner canopy skin so that the curved surface engages the LOWER horizontal face of the RH longeron. The shape is such that as the canopy closes, the eyebrow pulls the canopy frame down to the longeron, and it can't move because it is captured. After about 2 years, the eyebrow wore through the glass over the longeron, so I had to make a little "L" shaped strike from .020" SS that the eyebrow rides on. I know that this may not make any sense, and a sketch or picture would explain it instantly [see the photos below]. You have to fool around for a while with the shape to get it to cam correctly, but if you make a drawing showing the hinge location, longeron and canopy frame you will see what the parts need to look like.
I've helped at least a dozen people make this mod to their birds who had given up with the cold air coming in through the RF corner of the canopy. You may recall that I was not a big proponent of exotic cabin heat schemes; fanatical attention to air leaks will make it cozy inside (sorry about that!) w/o heat and also make your plane faster. When I was racing N888EZ a lot at Wendover and Jackpot, a lot of speed was found by managing air in the fuselage. 233.2 MPH over a120 mile closed course was obtained with really careful sealing of air leaks (and a good prop!)
View from front seat; note canopy latch mechanism to the right. The removable instrument panel was machined from a piece of 1.75" thick 6061-T6 aluminum tooling plate that weighed 57# after rough bandsawing to shape. The back side has .060" x 1.6 ribs between all of the instruments, around the bottom, top etc. This also gave me a place to mount the avionics trays w/o scabbing on a lot of little pieces of metal. The panel face is about .085" thick; the painted panel weighs 17 ounces after a zillion years of turning the handles on the Bridgeport.
Shot of canopy seal, seal valley, passive latch and front seal scheme. The original passive latch was .125" thick aluminum, and it wore a groove in the longeron after about 400 hours. Being the smart guy that I am, I then made a .015" stainless steel strike to protect the longeron wear point. It's not nice to fool mother nature - the passive latch then wore on the stainless strike!! So I added a .090" stainless steel doubler by riveting it onto the aluminum passive latch! When this wears out, I'm selling the plane.
How power gets to lights and wet compass. I made a plunger base from a piece of phenolic I had laying around, machined 3 brass plungers (contacts) and cannibalized 3 ball point pens to get the springs. The wires are soldered into tiny holes drilled into the underside of the plungers. The plunger body is mounted so that the top of the plungers are below the rollover structure surface by about 1/16" to prevent stray wrenches, etc from smoking the system if the power is on - of course they are tied to a breaker on the panel. One wire is common ground, one wire is +12vdc for the wet compass light (non-dimming) and the third is variable DC for the two eyeball lights. Panel lighting is soft red light and needs dimming as it hardly takes any light at all to see on a dark night. This is one of the things I kept the same on the second plane - it works great! 2,000 + hours and zero maintenance. The canopy side contacts are small brass phillips head screws I used phillips thinking the screwdriver relief would scratch up the brass plungers and be self cleaning as everything vibrated nicely together - it seems to work as I've never touched them in 15 years.
