Safety Latch, Part 3 – 1 hr

Tuesday Oct 9, 2012

To complete the safety latch installation I made two strips of 1/8 inch thick UHMW plastic, 3/4 wide. One is 6 inches long and the other is 2-5/8 long. I laid out four holes on the longer strip and drilled #40 pilot holes. Then I aligned that strip to the center of the roll bar with the front edge flush to the face of the roll bar and match drilled holes in the roll bar. In the photo below that part is clecoed in place.

I enlarged the holes in the roll bar to #29 which is the tap drill size for a #8-32 thread. I tapped the holes in the roll bar next and enlarged the holes in the UHMW to #19 which is the clearance hole size for the screws. I also match drilled two holes through the short UHMW srtip using the longer strip as a guide. With my countersink cutter I countersank two holes on the long strip (right side) and the two holes in the short strip for flush flat head screws.

Here is how they are installed (below). I used four #8 screws from the finishing kit which I’ll have to replace. The short strip overlaps the long strip providing a bumper stop for the latch handle.

Here is how it works. With the canopy closed and the latch handle unlocked the canopy is free to open. The spring in the handle provides a little friction between the handle shaft and the UHMW block that helps resist any tendency for the handle to rotate by itself. That is a nice added benefit of this design.

If the handle were to rotate by itself with the canopy closed (such as while moving the airplane with a tow bar) the edge of the handle will bump against the edge of the longer UHMW strip preventing it from going under the roll bar. This avoids a very awkward problem of a locked canopy with no one inside. The photo below shows the latch handle in that position. I verified that the canopy opens and the handle doesn’t catch on the roll bar because the front edge of the UHMW is flush with the roll bar.

To latch the handle for real you simple pull down slightly (.16 inch) and rotate it to the fully locked position. The handle slides under the long UHMW strip and bumps up against the edge of the short strip which lets you know without looking that it is all the way home. Ready to fly! Plus the UHMW strips prevent the paint from getting scatched up on the bottom of the roll bar.

As far as I am concerned this solution looks nice, feels solid, and fixes a nasty flaw in the stock design. I credit Dave B. on Vans Air Force for the basic idea which he posted in 2007. I modified the approach with a stiffer spring partially counterbored into the UHMW block and I added the bumper stop. If you like it, of course you should feel free to copy the design.

Safety Latch, Part 2 – 1.5 hrs

Monday Oct 8, 2012

Tonight I aligned and match drilled the safety latch UMHW block to the canopy bow frame. Nothing unusual here except to note that I aligned the block such that the handle touches the underside of the roll bar without the spring compressed. Since the spring will compress about .16 inch I should have room to install a .125 inch thick UMHW strip on the bottom of the roll ball and still have about .035 inches of clearance with the spring compressed. The UMHW strip will also serve as a bumper that will prevent the handle from rotating under the roll bar unless the handle is pulled down compressing the spring.

 

Here is a shot with the handle rotated into the latched position. It moves smoothly and the handle does just barely touch the roll bar. Looking good so far. Next I have to make the UMHW strip and install it.

Safety Latch, Part 1 – 2.5 hrs

Sunday Oct 7, 2012

The next task is to install the safety latch handle. This handle provides a backup to the main latch to prevent the canopy from opening in flight. It is essentially a catch that rotates under the roll bar to prevent it from lifting. The stock design is weak because there is nothing to prevent the handle from rotating under vibration and it has been known to rotate to the latched position on the ground with no one inside the cockpit which makes it difficult to get back into the canopy, to say the least. I have seen several modifications to prevent this. Jason Beaver has a nice solution using ball detents for the open and latched positions but you need an end mill to cut notches in the side of the latch handle tube and I don’t have an end mill so I chose to go with a spring preload approach. I will have to pull the handle down slightly to move it into the latched position so it won’ t be possible for vibration alone to rotate to the handle.

I found a suitable compression spring at True Value Hardware. It is 5/8 OD and just large enough to fit over the latch tube. I needed to enlarge the hole in the UHMW block on one end to a depth of .81 inch for the spring. I happened to have a 7/16 diameter end mill bit which I chucked up in the drill press and put a band of tape on it .81 inches from the end. I marked a circle 5/8 diameter around the 1/2 inch hole at one end of the UHMW block and raised the table until the bit was plunged .81 deep in the hole. Then I just turned on the drill motor and guided the block around by hand to enlarge the 1/2 inch hole to 5/8 using the mark as a guide. I just worked slowly keeping the block flush on the bottom. It was more like filing the hole interior than cutting. You could call this a make-do mill job with a drill press. Try it at your own risk.

 

I walked the cutter around the hole and gradually it got larger until I could slip the spring in. It’s not perfectly round but its close enough for this job. Since the spring is longer than .81 long there is still about 3/4 inch that sticks out of the block allowing for the compression needed to provide a preload on the handle. In this picture you can see the spring inserted and uncompressed.

 

I drilled a #40 hole near the end of the tube for a cotter pin and installed it with the kit flat washer which preloads the spring. At this stage it seems to operate nicely requiring a few of pounds of force to pull the washer down flush to the UHMW block with a travel of about 0.16 inch. It snaps back when released. I plan to use a 1/8 inch UHMW strip on the bottom of the roll bar as a bumper and surface for the handle to ride on. I’ll show that next time. But I still need to align and install this portion on the canopy bow frame.

 

It was time to check the dessicant containers on the engine. They were purple going pink so I knew it was time to bake them out. Here is one before bake out.

 

Here it is after removing the pellets and baking them at 250F in the oven for about 15 minutes. What a difference. These are now back in the  engine and good for another month or so.