Sliding Canopy

WARNING - This is just an account of my experiences - do not use this as an authoritative guide - contact Van's or your TC if you are not sure

The canopy has the reputation of being one of the most difficult areas of construction of an RV aircraft. <y experience confirms this. I have decided on the sliding canopy version of the RV-7, because I have flown similar setups (Grumman trainer, Grumman traveller, Grumman tiger) before. I like the idea of being able to taxi out to the run-up bay with your elbow hanging out over the wing. It's cool!

My experience with the Vans RV-7 sliding canopy has confirmed the often-stated feeling that Van has not quite covered this area in his instructions and drawings. Let me explain from first principles:

1. The perspex bubble is a different shape from the sliding canopy frame and windscreen. Generally, the bubble has not enough curvature to match the curvature of the frame and roll bar bows. This has some important consequences, chief among which is the FACT that, when you attach the perspex bubble to the sliding canopy frame, the bubble pulls the frame out of shape, making it less curvaed and sagitally wider than it was in free space. So why does Van instruct you to carefully bend the shape of the canopy frame so that it aligns perfectly with the fuselage sides and rear fuse skin - when attaching the bubble will simply pull it out if shape? I dunno, beats me!

2. Because of the afre-mentioned difference in curvature, the free windscreen does not match the shape of the roll bar and front fuse skin. It has to be pulled into shape by clecos or screws into the roll bar, and little aluminium "clips" that drag the leading edge of teh windscreen down onto the fuse surface. One consequesnce of this is that the rear edge of the windscreen will almost always need trimming. It souldn't, because when you make the "BIG CUT", the rear edge of the 'screen and the leading edge of teh sliding canopy should meet perfectly without trimming. Be prepared to trim to get it o work.

1. Preliminary

Both the slidiing canopy and tip-up versions of the RV-7 come with a large, floppy perspex moulded bubble, which contains the basic perspex structure of the windscreen and canopy. The major difference is how this bubble is divided into two separate components by a large coronal (that is an anatomical term!) cut which divides the bubble into two separate sections, front and back. In the tip-up version, the cut is made more to the rear, yielding a small rear window and a large front canopy. This canopy tips up and away from the fuselage to allow cabin access. In the sliding canopy version (my version) the same bubble is cut more to the front, yielding a smaller front section (the windscreen, which is fixed to the fuselage). amd a larger rear section which is mounted on a steel frame which can roll from back to front, much like a spitfire or hurricane canopy (Biggles rides again!) - this is the "sliding canopy" which I am trying to build here.

The sliding mechanism relies on three load-bearing points: at the front there are wheel assemblies (Image 1) which run in extruded aluminium section (image 2) and carry most of the sliding canopy weight. At the rear there is a single long aluminium "T" section on which runs a teflon block which can slide to and fro as the canopy is opened and closed. (image 3). There is a sturdy welded steel frame to which the perspex sliding canopy is attached.. Image 4 shows one of the two rear lugs which will eventually slide into a teflon block to hold the closed canopy in position. Image 5 shows one of the front wheel assemblys. Images 5-8 show various stages in the construction of the rear runner over which the rear teflon block slides as the canopy is opened and closed.

wheel assy

wheel track

top rear slider

rear lugs to lock posn

wheel posn adjusts

track for rear slider

measure and..

.. drill

The first image below shows the detail of how this runner is fastened to the top fuse by -6 size countersunk screws . I stuffed up and drilled the runner with larger holes than indicated, so I have had to replace the -6 gauge screws with larger -8 gauge screws and nuts - no big deal. One modification I did employ was to set the position of this runner in what seemed the best place to give the best location of the rear of the sliding canopy, and install ONE ONLY screw at the front, and a strip of duct tape at the rear to hold it in lateral position. You need to do this while shaping the canopy, because duct tape will just not hold the runner in place while it is experiencing loads from the sliding canopy. The idea was that if this position was not optimum, I was free to alter the locations of the other screws appropriately, amd extend the hole of the installed screw to allow it to move to the new optimujm location. The second, third and fourth images are (1.) a drawing and (2+3)pictures showiing the teflon bearer which slides along this runner, and the final shape of the runner as it is riveted together. Image 5 shows the runner installed with black duct tape on the rear top fuse skin. Image 7 shows the canopy frame in place, with the runner supporting the rear canopy. Image 8 shows the vertical spacing of the front bow of the canopy frame relative to the roll bar. The gap is a bit big, and later I had to fix this by sawing off some of the front tube which located the front wheel assemblies.

rear runner track screws

runner track detail

bending and..

drilling and..

rivetting the runner track

runner track in posn

canopy frame rough posn

fronts not aligned

More of the critical stuff is shown below. Image 1 is a rear view of the canopy frame and roll bar (the thick one at the front). Image 2 shows the detail drawing of the left roller bearing. It is formed by a teflon tube mounted into a powder-coated angle weldment which has a steel tube which snugly fits into the ends of the front bow of the canopy frame. Image 3 shows the locating rear pins which are welded to the rear frame. When the canopy is closed, these should slide neatly into two holes, one of ech side, which are in two rectangular plastic teflon blocks. Image 4 shows the details of these blocks and their mounts. Obviously the final positions of the runners and these pins and blocks are critical, and should only be finalised when the canopy frame and perspex are complete and attached to each other. Image 5 shows how a small G-clamp can hold the canopy frame and roller together. I found it more useful to use adhesive tape as a shim to give a tight, but adjustable, fit.The rough vertical position of the canopy frame should be about 3/8" above the roll bar (image 8). The true position is not available until you make the perspex canopy and windscreen and attach them both to the canopy frame and roll bar respectively - so Iwaited until this stage before finalising this arrangement (which is done by drilling holes through canopy fram and roller mount tube, front to back, and inserting AN3 bolts and nuts.Image 8 shows that things are not quite right, because the horizontal square bar of the canopy frame is not yet parallel to the fuselage deck - it has to be parallel. Clearly, you cannot go much further with this without getting stuck into the perspex bubble, to make the sliding canopy and windscreen. Image 8 shows that there is about 1/16" "inset" between the fuselage side and the square section horizontal member of the canopy frame, as per spec. However - when you add the bubble to the frame, the bubble distorts the frame by about 1/2", so you really should bend the canpy frame so that it is about 1/2" less wide than the fuselage, when not attached to the rollers in their guides. Van's instructions are difficult to understand here. They tell you about this expected spreading of the canopy frame when the perspex bubble is attached, but they ask you to bend the fram so that its final shape (no bubble) lines up with the fuselage sides. Remember, the roller guides will CONSTRAIN the rollers to their correct location, however if you follow Vans instructions, you surely will end up with these rollers exerting an outward force on the guides because of the spreading action of the perspex canopy on the frame. This is exactly what happened to me.

rear view

roller mount detail

rear locating pin

rear block and mount

clamping roller to frame

This height is important

side view - not parallel

inset is about 1/16", but..

The next series of pictures were taken about a year after those above. You can see that the canopy side "inset" was still about 1/16" as per Van's instructions. Image 2 shows how to track the effect of bending the frame. You need a decent ruler (image 2) to measure the width of the frame at critical positions (e.g. at the front near the rollers (image 3), and at the rear, near the rear locating pins (image 4). I bent the frame to get these dimensions exactly right to give 1/16" inset all around the canopy frame sides. It was clear that the front bow of the canopy frame was much too high to match the apex of the roll bar, so I had to hacksaw off about 1/4" - 3/8" of roll bar to allow it to sit lower (image 5). At this stage it was clear that I needed to attack the dreaded perspex bubble.

Canopy frame side inset about 1/16"

You need a good ruler..

to check dimenstions front..

and rear.

front bow ends needed a trim

The first 5 images below show some of the idiosycrasies of the canopy frame. Images 1 and 2 show how the square section side bar really does not line up with the front bow - there is a gap! Image 3 shows the rear canopy runner/support - it has a countersunk hole waiting for a countersunk screw to attach it to the upper fuselage. Images 4 and 5 show that, with the canopy frame in its forward resting position, there is a 1/16" inset between the rear fuse skin and the rear bow of the canopy, as per spec. However, I am not going to finally l;ocate this baby until I have the perspex canopy attached to teh frame, and am trying to line up the rear sheet skirts with the fuse. To my mind, the fore/aft position of this component is probably the one thing that will best determine whether we have a good fit at the rear of the canopy, between the canopy shet and the fuse skin. I don't want any cold breezes entering the cockpit through any gaps!

2. Finding the best position for the frame in the bubble.

Many previous builders report that the sliding canopy instructions are none too clear, and I also found this to be true. The canopy consists of a clear perspex bubble which is riveted to a powder-coated steel frame. Ideally the frame shape should fit the bubble shape, but this only happens when the frame is located exactly in one front-back position. You cannot easily find this position, because the frame has a tube (about 1/2" diameter, 1/2" length) sticking up into where the canopy perspex will be. This tube takes the shaft of the rotating latch mechanism, and the first thing you need to do is find the "best " position of the frame to fit the shape of the bubble, note the position of the tube, and use a unibit to drill an appropriate hole so the tube can pass through the perspex. I decided to tape a couple of thicknesses of lengths of corrugated packing box material to the top of teh centre member of the frame. These lay snugly againts the bar, exactly matching its shape, and aligning well with the edge of the 1/2" tube (images 6 and 7). This meant that I could find the exact fore/aft postion where the shape of this bar best matched the shape of the bubble (image 7). The last image shows the final vertical position of the front canopy bow relative to the roll bar, after cutting about 3/16" off the front tube setions that fit over the runner wheels. I am sure every canopy frame is different here, so you need to cut as required (which may not be 3/16").

square side frame bar

square side frame bar

rear runner csk hole

1/16" inset rear canopy

1/16" inset rear canopy

fitting frame to bubble

use cardboard filler

vert alignment good

Putting the canopy back on the fuse I noyed that the rear locating lugs seemed to be in their correct positions (images 1 (from side) amd 2 (from above). Also the rear fuse inset seemed good (image 3), ans did the vertical insets (images 4 and 5) and the side bars seemed to be parallel to the fuse skirts (images 6 and 7). Also the tempertature in teh shed was a warm 27 deg C (82 deg F), and so I nervously decided it was a good time to start cutting perspex.

A centre line was marked (image 1), and the excess perspex trimmed from the bubble. I tried using a die grinder, but my little compressor was not up to this task. The best system for me was to mount a cutting wheel in my portable Panasonic battery drill. It was a little slow and heavy, but it did the job well. The edges were smoothed with wet/dry sandpaper of various grades (image 2). It was tedious work, but not that difficult. Next the final position of the sliding canopy frame (with cardboard filler bits) was re-checked and I just dived in and drilled a hole for the latch shaft (image 3). Next Ireplaced the frame in the bubble, measure the specified edge clearances (read the plans in detail for these, e.g. 1/2" at rear), marked the bubble cut lines and used duct tape to seal the edges of teh plastic covering sheets on both sides (images 4-6). Image 7 shows the bubble, supported by a roll of foam, after these edges were trimmed and smoothed. Image 8 shows the frame back in position with all edge clearences as measured, and the "BIG CUT" line marked on the bubble.

centre line

edges smoothed

hole for canopy latch

mark cut lines and seal..

before trimming edges

all around bubble

edges cut all around

final frame posn matches

The Big Cut

Double checking that the latch shaft was positioned in its hole, and that the centre member of the frame was over the centre line of the bubble, I clamped the frame in position and marked a cut line across the bubble as specified (image 8 above) and sealed the plastic either side of the cut line. I then used my Panasonic drill and a cutting wheel to slowly and carefully cut the canopy in two. It took about 20 minutes and there were no issues. I used tape to support the cut as it progressed, so there was NO possibility of gravity pulling on the cut. Once it was complete, i removed all the supporting tape and found I had a sliding canopy and a windscreen! (image 3). Image 4 shows the rear section of bubble sitting over the rear caonpy frame in position - notice how the bubble's radius of curvature is much larger than the frame's. The top latch shaft hole has weld material forming a gusset which interferes with the perspex adjacent to the hole in the perspex. I removed this material with a fine file, so there was no pressure on teh perspex from the gusset, and sealed the streel with Van's powder coat repair paint (Image 5). The next job is to clamp the bubble to the rear frame and drill 3/32" holes every 2" through the perspex and the top centre fram member (images 6-8). these holes will later be expanded to 1/8" (frame) and 5/32" (perspex), to take retaining rivets.

sealing the big cut

making the cut

and now there are two..

initial fit

trim gusset and paint

centre holes

clamped in posn

all is lined up