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By Tony Bingelis (originally published in EAA Sport Aviation, February 1984)
THE BASIC ESSENTIALS for fitting and installing canopies were detailed in three of my earlier articles appearing in SPORT AVIATION. These were entitled The Installation of Sliding Canopies, Doors, Latches and Stuff, and appeared in the December 1974, January 1975 and February 1975 issues. Essentially the same information is contained in my book, The Sportplane Builder, in the section entitled Canopies/Windshields. (This information will not be repeated here.)
Almost ten years has since passed but the referenced information is as applicable today as it was then. This is not to infer that there have not been changes in canopies and canopy installations. There have been some, of course. But actually, the methods described for fabricating and installing canopy frames, canopies, doors, hatches and windshields were time proven even then and will probably remain unchanged for many, many years to come. However, as we all know, new canopy shapes, new materials and construction methods have come along giving builders greater flexibility and more options for making cockpit enclosures. Among these are refined construction practices that rely almost entirely on fiberglass and epoxy in the fabrication of unitized enclosures.
For the first time, materials such as Kevlar and Boron Fibers are becoming more readily available to homebuilders who are constantly striving to find better ways to build quicker and stronger.
Take weatherstripping materials, for example. All kinds and shapes are commonly available that permit solving almost any type of canopy or windshield wind noise problem.
You may not have been aware of it, but acrylic plastic (Plexiglas) bubbles and vacuum formed canopy shapes were not always readily available to builders. A few years back, many a builder didn't know where he could find or get Plexiglas formed to his needs. He often had to make his own canopy forms and molds, build a makeshift oven and then invest heavily in a number of expensive Plexiglas sheets before he finally succeeded in "blowing" a canopy or stretching a canopy door for himself. Even so, many builders had to put up with poorly formed canopies and windshields that were far from being optically perfect. Sometimes the distortion was so bad that it was hard to tell if it was a 747 or a buzzard crossing your line of flight.
Fortunately, a few of the more successful canopy making builders in an attempt of necessity to recoup part of their huge expenditures, branched out and began to make canopies and windshields for other homebuilders . . . a lucky break for the rest of us indeed.
As you would expect, a Plexiglas canopy produced especially for the design you are building will usually be the simplest to install with predictable results. However, since most homebuilts have similar cockpit dimensions, finding a good alternative to the original prototype canopy shape is generally no great problem. Even the builder who has an original design of his own can find a stock canopy (being currently produced) that will fit his expectations and needs. It would, therefore, make just about as much sense for a builder, today, to attempt to make his own wheels, tires, tubes and brakes as it would be for him to undertake the fabrication of his own canopy. Some will, of course, certainly not to save money but just to see if they can.
Well, since so many canopy designs are available, how is a guy to decide what is the best installation for his own project?
There is one sure way. Arrange to visit other builders who are building the airplane. Talk with them and examine their installations. Look for the good features and don't overlook the shortcomings. Here are a few things to think about and to resolve in your own way.
A lot will depend upon whether your airplane is a single seater or a two seater, won't it? If it is a two seater, is it a tandem or a side-by-side arrangement? Single seaters are the easiest to fit with enclosures (canopies) and offer the greatest number of installation options.
Your first concern, naturally, is headroom. . . not only vertical but lateral as well. If the canopy is too narrow, you will not be able to tilt your head to peek at the landscape below. Bumpy air can cause bumps on the head, too.
What else would you expect of a canopy? It must, of course, provide easy access for entry into the aircraft. It must be airworthy and well secured in flight. It should also be easy to open, shut and latch from both inside and outside the aircraft. And, certainly, it should have good optics and your vision, in all directions (except, perhaps, straight down), should be unobstructed.
Other attributes are also important. The canopy should be easy to seal against wind noise and rain . . . and it should be aerodynamically clean. This means that when the canopy is closed the airplane will present its sleekest image with smoothly flowing lines that are pleasing to the eye.
Have you ever speculated over what aesthetic degradation or transformation would take place before a canopy was opened? Would it raise up, tilt over, slide back or what? Maybe it would be a flip-over canopy or one that rises up and backward on spindly struts. (That kind often presents a flimsy and fragile "you ought to do something about that" appearance to me.)
With so many of the canopies being of the "high rise" variety when opened, what would you do in a strong wind? Wait until it subsides? Have food and water brought in? Maybe you could take a chance that it won't blow away and open it anyway. I'm only kidding! Some of those installations are well executed and stouter than they first appear.
The ones that can best cope with rather gusty wind conditions are those that are fitted with stay struts. These nitrogen cylinder lift struts are salvaged from a variety of foreign and U.S. manufactured used cars . . . hatchbacks mostly. Not only do the struts help in raising the heavy two seater canopies, they also stabilize them considerably while in the open position.
I guess I have to conclude that there is no single best canopy installation. Of all the airplanes you have seen and liked, I'll wager that many of them had cockpit enclosures that were "deficient" in one or more of the desirable features mentioned earlier.
You will notice that I have made no mention of the ease of fabrication and installation as being a desirable canopy feature. We all know that that has nothing to do with a canopy's ultimate appearance or with the aerodynamic qualities of the aircraft (sorry, Charlie, no sympathy here).
Although acrylic plastic weighs but 43% as much as aluminum, a Plexiglas canopy installation is heavy because of its 1/8" to 1/4" thickness. You should, therefore, attempt to get the maximum vision in all quadrants with the minimum use of the material. This merits some thinking and planning before you start cutting any material for installation.
Selecting a canopy that is wide enough for your aircraft is more important than concerning yourself with its length. Fiberglass or aluminum fairing skirts allow you to alter the canopy's shape remarkably.
A Plexiglas bubble that is much more than an inch or so too narrow (or too wide) could make the fitting more difficult and the final appearance something less than you had hoped for.
If you have to spread a too narrow canopy to make it fit, you will be reducing (however slightly that may be) the amount of shoulder room and headroom you could have otherwise had. Conversely, if you have to squeeze in the sides too much, it will cause the canopy to bulge and take on an odd waspish look when viewed from the front or rear of the airplane.
Let's confirm what you already may have suspected. "Free-blown" canopies are great for single seaters but they are lacking in headroom for side-by-side installations (see Figure 1).
Better than blown canopies for most of those two seaters are the vacuum formed type. These can be formed in a mold that can depart from the radial symmetry characteristic of a blown canopy. This vacuum forming permits designs like the somewhat flattened top of the typical T-18 canopy. This style of canopy provides the maximum of headroom for a side-by-side two seater and a minimum frontal area. Incidentally, the T-18 canopy has been beautifully adapted to a number of other aircraft designs . . . the recently completed crop of Mustang II's attest to this.
It has become quite apparent that the most popular of canopy installations, particularly in single seaters, is the side-hinged flip-over type. It may be installed in one piece (including the windshield) or the windshield may be mounted separately. There is good reason for this popularity; it is also the easiest of all types to install because little or no framework is needed to reinforce it.
Two seaters cannot indulge in the luxury of a frameless installation because a large unframed Plexiglas bubble is about as undisciplined as a large mound of Jello . . . and about as manageable, too. For these installations a canopy frame is essential.
Even though finding a suitable canopy is no longer difficult, the problem of canopy installation, unfortunately, still remains somewhat of an anxiety producing period in the life of many a homebuilder. Crack the canopy while drilling that last installation hole and there goes several hundred dollars down the drain. Tighten the fasteners too tight and sometime later . . . CRACK! The same thing might happen months later, in an installation where the screw holes were not large enough for the fasteners used.
Would you believe that the holes in acrylic plastic should be about 3/8" in diameter if a 3/16" screw (fastener) is used? You can expect Plexiglas to creep (expand) up to 1/16" per lineal foot with temperature changes.
It is no wonder that the builders of composite aircraft are smiling more than builders using aluminum and steel tubular frames for their canopies. The composite builders can eliminate the need for using steel screws and fasteners and rely, instead, on epoxy bonding of the "glass" to the frame. Very obviously this simplifies the installation and virtually eliminates the dreaded drilling of holes for fasteners. Naturally, this epoxy bonding method is most effective with wood and composite fiberglass frames.
All things considered, I would classify a sliding canopy as being the fussiest to install. It has a number of moving parts, is difficult to match to the windshield, is difficult to seal against wind noise and requires the most complex of latching mechanisms. It is a painfully fussy process as you will have to install, drill and remove the canopy and its fasteners numerous times during the fitting process, but all that shouldn't dissuade you from installing a sliding canopy. They are nice! We'll save the details for next month.