Don't be too quick to write off the use of piano hinge for the cowl. I had the fancy quarter turn fasteners on my Midget Mustang. They were heavy and expensive, but they worked VERY well.
My RV-3 has hinge pins for the cowl at the split, top-to-bottom, and at the firewall on both the top and bottom cowl and I've come to LOVE them. The original builder did a fantastic job of making sure everything fits and the hinge pins aren't being used to tweak anything in place or suck anything in. He also included some bent needle nose pliers that have a hole drilled in the jaws that fits nicely over the hinge pins to aid in removal and installation. Next time you're up flying, plan to come visit me at CO12 and we'll remove and reinstall the cowl. I've become quite proficient with the recent engine rebuild. I'd fly it over to Boulder, but I'm out of service right now because my prop needs to be serviced.
FWIW, the Glasair 1RG I recently purchased also uses hinges for the cowling and I'm very happy with the arrangement.
If done right the hinge pins on the cowl work fine. My RV-3 had a little piece of hinge to retain the bent pin handle at the front of the cowl. My RV-8 has pins that are inserted from the cockpit side so the exterior hing line is smooth and pin is hidden. That requires a longer pin with nylon tube guide to keep the pin aligned when inserting. Nice and clean but more work.
If you go with Skybolt / Camloc type fasteners you might consider leaving them loose without retainer washers. While you will have to put them in a cat food can when taking off the cowl they won't fight you like retained ones can do while you try to pry off the cowling and all the little lock pins are catching in the holes.
As far as canopy hold-open devices go, I like one that doesn't impose a bending load in the canopy when closing it like an air strut can. With the mechanical folding link there is zero resistance when lowering and no twisting load on the frame in either direction. Not saying an gas spring would be a no-go, especially if a lower design pressure than normally used in automotive and commercial applications.
Although the canopy still isnít on yet, it was time to get an idea of what sort of geometry was available for the canopy frame cross-stiffeners (the ones front and back that go side to side). These are going to be foam core, fiberglass and carbon. Hereís me with a 1.5Ē cushion under me and a 1Ē cushion plus a 2Ē foam spacer for the seat back. With my age and the pandemic, I let few visitors in to the house but with the garage door open and makes all around, they can come into the garage. One took this photo.
The c-clamp was holding a dummy cross-stiffener in place at the panel, something to give me an idea. But the main thing was all the space I have between me and the seat bulkhead. Some of that can go to the cross-stiffener and some to the hold-open device. And that design is being narrowed down. A possible idea that I hinted about earlier wonít work but another might.
The latch assembly has the usual forward and aft latching bars. Iíve added an aft-canopy latch point, remember, so I have two of them. In an effort to minimize the build-up of friction, these bars attach to rod-end bearings at the latch itself, and those will be secured with jam nuts. There are other ways to do that, such as small holes in the bars and safety wire around the rod-end bearing pins, but Iím comfortable with this.
The next issue was how to hold the rods while gently tightening the jam nuts. The approach I chose was to get some aluminum hex bar and drill it out and glue them on. VAF member Rick Brennan kindly did the drilling on his mill. Thanks Rick! Here are the rods and the bit of hex, which is only to hold the rods while snugging the jam nuts.
Hereís a quick look into the canopy-less cockpit as I had it at one point. Before too long, I removed the 2Ē foam spacer under the seat cushion but who knows - once the canopy is on I can play with that. Also the cushions are not RV-3 cushions. They were made for a different application all together, and Iím merely re-purposing them, to use a semi-dumb word. After this photo, I added another, final, coat of the dark gray interior paint.
Some cockpit work needed before flight.
One thing that I needed to do was finalize the conceptual design for the hold-open link. Right now that’s planned to be a bar, pivoted on the aft fairing frame cross-member 5” radius from the canopy hinge pin. In the picture below, those are the white lines. The link is the blue lines, and close to where they cross will be the sliding connection to the aft seat bulkhead. Virtually all other geometries caused the link to interfere with either the harness or the seat back.
The tentative idea is to have the top end of the link have a notch that drops over part of the sliding connection fitting. A spring will add assurance that it will stay there, and it ought to open and come to a stop and stay there. To close the canopy, lift the link past its catch and then close the canopy.
Yes, this will protrude above the turtledeck and be visible, regrettably. But it’ll be under the canopy rather than in the outside air.
Possible link shapes: either a flat aluminum bar or a tube similar to the canopy latch but with different end terminations.
The wooden blocks are to provide spacing for this. They gradually go to 1/8” from the current 3/4”, so that the aft canopy cross-member will be near the seat bulkhead for the aft latch bar. My head and shoulders clear that nicely.
The hold-open link attaches to the seat bulkhead and to the aft canopy cross-member, which I hadn’t built yet. The forward cross member will go under the shade that’s integral to the canopy fairing and can be 3/4” deep. The aft one goes between the pilot and the seat back frame so it has to be thinner. I made a foam blank from 1/2” PVC foam. Since it appeared hard to fiberglass, I also made several practice pieces.
The top two pieces used the foam I was planning on using for the front cross member, 2 pcf construction foam. The bottom piece is a sample of the 5 pcf PVC foam aft cross-member foam that I’d tried to heat-form and got too hot, while the piece above it was a trial piece of the same foam that I’d wrapped with 45 degree BID, with the glass wrapped along the bottom and along the top. The top two pieces and the middle piece were spiral-wrapped with 90 degree BID. Unfortunately the wrap wasn’t at the 45 degree angle and I couldn’t make it so.
The middle piece wasn’t intended to be practice. It was supposed to be the beginning of the aft cross member. But the lay up was also sufficiently uneven and in places not attached to the foam core so I scrapped it.
This attempt did use heat-formed PVC foam. I formed it to be parallel to the seat back on the right but 3/4” away for that hold-down bracket, and within 1/8” on the left for the latch bar.
The one I was working on today merely goes from one side to the other with those spacings, no bend needed.
Once I got it covered with 45 degree BID, I applied some carbon. Here’s the first pass on that going on. So far, this will fly. I’ll let this cure and then do the edge that’s on the bottom in the photo, to build up bending stiffness.
Got the aft cross-brace glued in but not yet tabbed with glass to the fairing. As-is, it significantly stiffens the entire fairing/frame assembly.
The front cross-brace has the carbon on the outer surface only so far, and that surfaceís +/- 45 degree plies. Itís lagging the aft one but coming along.
This period Iíve been taking some time off work because Iíve been going down to the airport and flying. I have a Cessna and finally the wind is less and the forest fire smoke has gone, at least locally, and itís pretty nice. Itís a curious position - whichever plane Iím spending the day with, the other is getting shorted.
The forward cross-bar has all its carbon now and is done. What's interesting to me is that the two cross-bars have carbon caps top and bottom and that's the only carbon I've needed for stiffness so far. I bought a lot more than I ultimately needed.
This has been slow going. I'd add some glass or carbon, let it cure, and it takes the better part of a day before I can work on it some more. And there's been a lot of thinking in between actually doing things.
One thing I've learned is that you you want a project, then the approach I've been taking is excellent. If you want a flying airplane, either buy one or build a mature kit exactly per the manual. Make no changes. Many years ago, a friend told me something important. He'd built a Volmer amphibian so he knew what he was talking about. He said about his recently-purchased Cessna 170, "I didn't have to built anything. All I did was sign my name a few times." The lesson stuck and after a few years I've had a flying airplane at hand ever since. A good thing too, since I flew my Cessna 180 earlier this week and this RV-3B won't fly for a while longer.
Okay, philosophy lesson over, let's get back to it.
I've been working, even more slowly than normally, on the hold-open latch. The basic concept is simple enough. In this photo, the retainer plate on the front of the stop is missing but you can see the two white bushings and their screws that it attaches to.
To unlock it, merely lift the bar and close the canopy. That part works just fine.
There's a bit of some eccentricity at the outer connection to the bar since it not only pivots around it's attachment screw, it has a small rotation about a yaw axis as well. That is accommodated by a slightly oversize hole in the end of the bar. That part also works just fine.
So what doesn't work so well? The bar itself rotates to a more vertical position as the canopy closes and the bar's inboard tip moves aft. It contacts the roll bar. I've been tweaking things to prevent that and perhaps the next posting will show more. At the moment, having dropped and lost one of the only two white bushings that I have, it's disassembled.
Dave, did you perhaps consider a small gas strut to hold open the canopy? It looks like some hours have already gone into creating the latch. Just a thought, when I saw it. And I guess whatever the solution it will need to allow the canopy to zip off per Vans design if it ever accidentally opens in flight.
Paul, that was one of my first ideas. The other was a tape-measure tape used as a folding strut.
I decided against the gas strut partly because at that time I couldn't figure out how to connect it to the fuselage and partly because I was concerned that it wouldn't break free if it needed to. Now that the canopy frame is further along, I can see ways to attach it; the breaking free issue is probably minor but remains.
I bought a tape measure and tested the tape. It would not support the load intended for it. Also, repeated folding of the tape let it take a permanent set, making it even more buckle-prone. A further problem was that the mounting points would have needed to be carefully oriented so that the folded tape didn't interfere with either the canopy frame or me. While the idea has merit, the execution would have taken more time than I was willing to spend with it.
The current concept has a couple of break-away possibilities. The first is the guide bushings and screws at the top of that small bracket. They are #8 and cantilevered from the bracket. The second is the bracket itself at its attachments to the bulkhead.
Have a look at the coil spring devices used to hold hatches on boats open.
Good one, thanks.
Who knows, maybe if I need an alternate..... These come in a variety of sizes from numerous vendors. But I don't know if they'd carry the anticipated load of up to 100 pounds in the open position, in tension. Also, I expect that when they straighten out they'd snap straight and that could be a problem. In many ways, this is a heavier version of the tape measure tape I was considering. While this has better ends, for my purpose anyway, otherwise it's just bulkier and heavier. But off the shelf, an important consideration.
So far I'm content to work with the concept I've pictured in my photo above. This is probably a good time to present the "Five Rules of Good Engineering." I didn't write these but I learned from them.
1. Part of the art of being a good engineer is knowing where
to steal good ideas.
2. Get it good enough and get on with it.
3. The best way to solve an engineering problem is to guess the
4. Any **** fool can figure out another way to do it; i.e., the
first solution to a problem is often entirely adequate.
5. If you can't fix it, feature it. There's always a way to market
the problem as a benefit.
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