The idea of an open cockpit on a Fournier glider isn't original to me. A fellow from Germany had previously made one for an RF5B. A number of open cockpit RF4D's are flying. But, it got me to thinking....could I get the feeling of that open cockpit biplane in the glider? Technically, no. Not the right engine, history, beauty of the design, etc. Yet...the increased economy and safety of the glider meant that if I could even come close, it would be a win.
Any design would have to attach to the glider. The German version seemed to use a custom-designed frame that was not easily removeable. My criteria emphasized safety and convenience. Any frame should be as good as what would come from the factory. I also wanted to be able to switch between open cockpit and enclosed canopies in a few minutes. The original canopy frames have quick release levers. Any open cockpit frame should do the same.
My first choice would be to use factory frames as a base. As luck would have it, after searching, I found a set of canopy frames in California. Purchased them and had them shipped to Seattle. They were straightened with the help of my nephews, John & Nate.
I removed the overhead supports for the canopies...but then ran into the basic issue that I'm not a welder. After a long delay, Bob Dempster directed me to Gary Charmichael & friend. Based on my design, they bent and welded 4130 tubing (stronger than the original mild steel) to create a basic framework for the new covering. Bob further helped by selling me some 4130 tubing.
Open cockpit frames in 4130 tubing
The tube frame would have to be skinned. Composite (fiberglass) would be smooth, but would involve a substantial process of making molds. Instead, if I covered the frames with cloth, that would yield a "faceted" look as the tubing would show through the fabric. For a smooth surface, a thin plywood could be attached to the frame - with or without a cloth overlay. The fuselage is thin plywood, covered with cloth. Covering the framework with clear polycarbonate was also an option. A solid plywood would blend with the fuselage and probably look better from the outside; clear polycarbonate would be more engaging for passengers. Either way, I'd need templates.
I started with paper templates. Then transferred them to 1/8" thick, bendable LitePly. I had hoped that LitePly might be the final covering, but it crushed and split too easily. Good for patterns, not good as a final covering. It could be covered with layers of fiberglass, but then that would be hard to remove from the frames if I wanted to make changes. So, I used the LitePly as templates and cut the final covering from 1/8" general purpose polycarbonate.
The plywood templates were used to cut the polycarbonate, which still required significant trimming, drilling, etc.
About 60 hours later, there weren't any more pieces to cut. After checking and rechecking the installation, there was nothing left but to roll out the glider and try a test flight. I had worked hard in the design to minimize drag and turbulence. The overall shape matched the glider. The windscreens were the same angle as the canopies. It used the same or stronger material as that from the factory. There were no changes to lifting surfaces. There were no permanent changes to the glider. It was about as streamlined and minimally invasive as I could make it. In effect, these were modified canopies.
Leather flying jackets are more than a fashion statement. It was just a bit warm on the ground, so I wore a light jacket. Not so good. Even if it's warm on the ground, a leather (at least, wind proof) jacket is the proper choice. Gloves, goggles, leather flying helmet (cloth for really warm weather) -- are all good choices.
There was a learning curve. I had not flown in a month, so the test flight was like flying a new airplane. The sight picture was different from the pilot's seat. Wind rushing was different. I got buffeting on takeoff from the propeller, but once I picked up speed, I got rushing air, but no significant buffeting. Same for the rear passenger.
I did slow flight, power on/off stalls, steep turns, etc. and found no significant difference in performance. At a bit over 100mph (into the yellow speed range), there was no deforming or any evidence that the polycarbonate wasn't up to the task of covering the cockpit. Cruise flight seemed to be about the same speed at a given rpm as the canopies. The engine ran hotter until I introduced a bit more nose down trim, then the temps returned to normal. In landing, I felt I had to give it a slightly more aft stick in the flare than with canopies. So, there might be a slight turbulence/blanketing of the rear control surfaces.
I felt as if I had plenty of stick control, just the range moved slightly more aft. From two initial flights, any handling differences are subtle to non-existent. Post-flight showed no cracks, loose screws, etc.
The exercise was a success. I could switch from canopies to open cockpit in about 3 minutes at a leisurely pace. It met the criteria of safety and convenience. The mod was particularly fun on warm, clear, high pressure days that have minimal lift for soaring. So, if you're going to go out and motor around, put the top down and enjoy it.
The project remained a success.