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Installing a Canopy Window... Why and How...
By Tony Bingelis (originally published in EAA Sport Aviation, July 1995)
Beechcraft, Mooney, Piper and other producers of sophisticated personal-use aircraft do it on purpose . . . that is, they install a small vent window in the pilot’s side of the canopy (Beechcraft Bonanza folks call it a storm window).
They do this not to jack-up an already high price for their top-of-the-line aircraft, but rather to offer it as a safety feature. Such a vent window built into the cabin canopy is really a very useful feature and can be a life saver in the event of an emergency.
Consider these situations (there are others, of course):
- The engine blows its crankshaft seal and the fine oil spray mist quickly covers the windshield effectively limiting vision. By opening and peeking through the vent window, a calm pilot should be able to see enough to make a fairly decent landing.
- The cockpit fills with smoke or fumes from smoldering electrical wiring. Open the vent window and, thanks to the extra ventilation, it may help keep the pilot and passengers from becoming totally incapacitated.
The window vent also provides a convenient means for disposing chewing gum in flight rather than sticking it to some part of the aircraft structure.
Unfortunately, the window can also create a temptation for some inconsiderate pilots who will use the window to dispose of larger cockpit clutter… candy wrappers, paper cups, WW II sectional charts, old copies of the Wall Street Journal, and the like.
In short, this window vent thing may be well and good for those fancy certificated aircraft but what about the homebuilts? Why should anyone with a nice sporty homebuilt even consider installing a similar window vent in his aircraft? After all, it does entail more work.
It is well to note that window vents are a popular trend in European sport aircraft. The GROB motorglider, for example, has a nifty sliding window vent. A number of other newcomers to the sport aircraft scene also feature similar sliding window vents. Some are very cleverly executed and are often completely fabricated of clear acrylic strips.
Actually, I can see where the window vent feature could be even more appealing for the U.S. sport pilot. In addition to the already described emerg- ency-use benefits, think of its strictly sporting potential.
Remember the old Piper Cubs? They and a few other "cabin class" vintage airplanes have windows that open. Unfortunately, most low wing homebuilt aircraft are deprived of this convenient luxury. Their slinky bubble canopies simply cannot be safely opened in flight. This means, of course, their emergency options are severely limited. Furthermore, the pilots may even be deprived of the pleasure of participating in sporting activities like "bomb drop" contests, and toilet-paper ribbon cutting capers (frowned on by some as unsafe sporting activities).
Ah, but you free spirits can nevertheless increase the utility of your low winger homebuilt by installing a canopy vent window to change all this.
"O.K., sounds good," you say, "but, isn’t all this a bit of propaganda for installing a window in a perfectly good canopy?"
True, to a degree. But, there is a story behind the idea…and in spite of that guy "Murphy," it has a happy ending thanks to the ingenuity of one builder (me).
The Tale Unfolds
It was one of those long winter nights with a lot of time on my hands and with very little left to do on my latest (10th) project. That big heavy PLEXIGLAS™ bubble for my RV-6A was just sitting there on the bench and seemed to be begging for attention.
Everyone knows you shouldn’t do any heavy plastic work when the weather is cold…the stuff becomes brittle and is unforgiving of the slightest abuse.
You’d think that risk potential would have sunk into my cranium after all these years, especially when the $500 (more or less) invested in that big bubble was at stake.
Well, I honestly didn’t intend to do any heavy work on it, really. I simply planned to cut away its bulky base so it would be ready, later, for trimming and installation.
Cutting away that heavy PLEXIGLAS base was simple with the air drill and the abrasive disc I had chucked in it. The cutting went quickly until I reached a point where I had to turn the canopy around to better get at the line of cut.
I had already cut away the excess PLEXIGLAS base about half way around the bubble. It didn’t occur to me then, but I had neglected to leave a generous radius when I cut away the trimmed strip with a 90 degree cut. This obviously created a classical stress point.
Well, suh, the next dumb move was to try to handle that floppy, heavy canopy alone. Remember, I mentioned that the shop was cold and obviously I had no business messing with the canopy. Anyway, as I raised one corner to turn it around on the bench, I caused it to flex more than expected and I heard a sickening c-c-crack! A crack appeared, without warning, at the point where I notched out the base flange. The crack shot straight up about 6". Oh, no…not me!
Oh, yes, it happened to me…in spite of all I have written and preached on the subject of working with and handling PLEXIGLAS windshields and canopies.
Naturally, all I could think of was to try to "stop-drill" it. I did so feverishly, with a 1/8" drill, but in my haste did not notice that I didn’t drill the hole at the very end of the crack.
You guessed it, the crack soon progressed another 3/4" before a second stop-drill hole worked as it should have.
This time I used a magnifying glass to make sure I did drill the hole at the very end of the crack. This, thankfully, stopped its further progression.
In spite of a heavy heart, and much embarrassment, I finally did complete a very nice canopy installation (except for that ugly crack).
It is little consolation to know of other builders who are flying with cracked canopies. Usually their cracks are tiny and are located around the canopy frame. However, my vertical 6 incher seemed to be larger and more visible than any other I have seen.
I guess all these builders ran through a few agonizing options and wild schemes for hiding or attempting to hide the crack(s). I did too, without any good solution for cloaking my embarrassment. The most common fix, apparently, is to stick a decorative decal or an EAA emblem over it.
Unfortunately, most cracks are located where a fix like that, while better than nothing, is still too obvious.
Well, I have been flying that RV-6A with its cracked canopy for 3 years and the crack is no bigger now than when it first made its sickening debut. No, I made no attempt to hide it or to disguise it.
In spite of the passage of time, in my eyes, that crack was becoming more and more offensive. Besides being ugly to look at, guess what happens to the value of an otherwise nice airplane with a cracked canopy?
Well, I finally decided to do something about it. No, I wasn’t about to replace the canopy…not this penny-pincher. Fortunately for me, the location was such that I could cut out the offending crack and replace it with a nice canopy window vent…just like one of those you’d expect to find in many a fancy certificated aircraft.
Here’s how I went about it.
Making the Window Layout
Obviously, my window vent dimensions would have to include the entire area containing the crack. This meant my vent window would have to be approximately 7" x 8" with a generous radius at each corner.
To assure myself that the layout would be functional and of practical use in flight, I located the opening as far forward as I could get it. Then, I outlined the proposed vent window dimensions using black electrical tape. After a few flights, I was convinced that the size and shape I worked out would be practical and quite effective. The next step was to cut out that huge opening in my canopy.
Talk about anxiety, if I goofed again it would cost me another $500 bucks to replace the entire canopy installation.
The Risk of Drilling Holes In Acrylics
My greatest concern during any canopy installation is in drilling holes in the PLEXIGLAS. Drilling holes for this modification was no less a concern. I reminded myself not to risk using a standard high speed metal drill, or a wood cutting bit, because of the uncontrollable tendency they have to dig in or grab.
When this happens, the backside of the hole will, at the very least, become chipped. At the very worst it will develop tiny cracks around the edges of the drilled hole. Of course, if the drill really grabs and lurches uncontrollably, it could cause a large crack in the plastic.
It is important, therefore, to use a bit that has been modified so that the sharp cutting edge is honed flat. When a drill bit is sharpened in this manner, it will scrape a clean hole through the PLEXIGLAS without any tendency to dig in, or abruptly break through leaving a hole that is marred by chipped edges.
After you sharpen your drill bit as suggested, try it in a scrap piece of PLEXIGLAS to see how well it works.
As a rule, any hole in PLEXIGLAS that is to receive a fastener (a screw or pop rivet) should be drilled slightly oversized. This will allow for the expansion and contraction for which acrylics are well known.
A 1/8" fastener should fit loosely in its hole before the soft pop rivet is set. I used AACQ 4-4 rivets.
Similarly, a hole for an 832 screw fastener should be drilled oversized with a 3/16" drill. Be careful not to tighten these loose fitting screw fasteners too much…snug is good enough. Over-tightening will surely cause cracks around the drilled holes.
Cutting A Large Opening
A safe way to do this is by first drilling a 1/4" pilot hole in each corner for the proposed cut-out. I made sure the sharp cutting edge of the drill bit was honed flat before drilling the initial 4 holes to avoid the risk of the bit digging in.
Next, I used a 3/4" hole saw to cut the large radius corner holes for the window vent opening. These large holes were cut only part way through from the outside, and then completed from the opposite side.
The straight lines of the cut were made using a small (3" x 1/32") abrasive disc chucked in a drill. I had to be extremely careful not to let the disc cut too far or to accidentally jump out of the line of cut. That risk is great! Using a handheld hacksaw blade is slower but safer.
Next, the opening’s edges in the canopy were beveled and sanded to provide a snug smooth fit for the similarly beveled edges of the soon to be installed PLEXIGLAS window.
After a good fit was achieved, I attached the bottom half of the piano hinge to the canopy using soft rivets (AACQ 4-4).
Incidentally, my canopy has a slight curvature in the area where the window vent is located. A flat piece of PLEXIGLAS, therefore, did not fit perfectly although it would have been acceptable.
To obtain a nicer fit and contour, I heated a flat piece of PLEXIGLAS in the oven (350° F. for about 5 minutes) until it was limp and flexible.
I then removed the piece from the oven using a couple of pot holders and pressed the ‘glass down over a slightly curved piece of metal for a few minutes while it cooled.
Do not lay the PLEXIGLAS directly on the oven rack while it is heating as it may pick up some of the oven rack’s features. Instead, lay it on a plywood board or a small sheet of metal while it is getting hot.
Next, the top half of the piano hinge was fitted and riveted to the window vent to complete the installation. A few useful installation details may be seen in the accompanying drawing.
Notice that two plastic twist latches are installed to secure the vent window in its normally closed position. These will later be replaced by two smaller clear plastic swivel tabs.
If you have a less than perfect fit of your window, I suggest you try laying a strip of electrical tape along the beveled edges and trimming the excess away with a sharp razor blade. Perhaps a light bead of bath tub caulk might serve as well or better to completely seal out any wind noise.
A thin film of caulk should not immobilize the vent window in the event you want to open it…but you better check that out beforehand for yourself.
I no longer have a cracked canopy. Not only that, my RV-6A now has a Bonanza-class window vent that works quite well, thank you.