Piano hinge install?

[WurlyBird]

I am back to working on my engine swap and oddly enough the first thing I need to get sorted out is the cowling. I have decided to use piano hinges to secure the top and bottom but I have no info on how this is best accomplished.

What should the spacing be between rivets?

Should any adhesive be used in addition to the rivets?

[Esser]

Any reason why you want to go with the piano hinge over camlocs? The only reason I ask is I have seen some guys not having much fun with piano hinges. Just another excuse not to take the cowl off pretrip

[WurlyBird]

The Avid Mk IV cowl that I am putting on appears to have been pulled from two separate molds, top and bottom. They do not line up properly and so the top bulges out significantly between camlocks. I set it up for camlocks and that is how I found this out. There is probably an 1/8" of outward bulge between fasteners which are about 6" apart. I have been told by several people that piano hinge is very trouble free although I have no personal experience.

[Av8r3400]

I have several friends who use the hinge method for attaching upper and lower cowls on experimental planes. Properly set up, the pins go in and come out fairly easily and are not problematic to operate.

In fact the pins are so easy to operate there may be a screw needed to secure the pins.

[War Eagle]

I used piano hinges on my build because I wanted the smooth cowl look ( was after some sexy aesthetics) and I my thoughts were that a smooth cowl would eliminate some level of drag that might be caused by any other connector sticking above the cowl surface.

At first, I started out planning not only the hinge on the mating line of the cowls but also on the mating line of the boot cowl also.

As I worked on the boot cowl connection I really ran into challenges that the surfaces I was trying to mate together had very large gap distances between where each side of the hinge had to be mounted and you have to make all that gap up by putting in some shim type material in order to get each half of the hinge mounted in the same plane relative to one another. Then the curvature around the boot cowl is really very shape that I had to make the hinge in small segments. See the first picture below. The second picture shows an attempt to make up the gap between the various mating parts so the hinge halves were on the same relative plane.

I eventually abandoned the hinge assembly on the boot cowl connection because I couldn't make the places where you would intall and remove the hinge pin for these connections to look aesthetically acceptable to me.

I then opted for a very recessed countersunk screw into a rivetnut connector for every anchor point on my boot cowl. You'll see this in a later picture.

I did go forward with the hinge connection where the top and bottom cowl join together. You still have the issue where you have to install some kind of gap make up material to get the two halves of the mating hinge in the same relative plane so the hinge pin can be installed. I used hysol in all mating surfaces as well as rivets to secure the hinge halves to each cowl.
The rivets were placed anywhere from about 1 - 2inches apart. You can see this in the pictures.

Of course this took a lot of clecos and dozens of adjustments until the fit was where I wanted it. Lots of holes drilled and redrilled than filling of the ones I didn't use.

Then there was building up the surfaces and sanding of the build up in order to get everything to look and fit perfect.

The pins are installed and locked into place in each cooling air inlet on each side of the prop.

I also installed the oil acess door with a piano hinge as well. Again, I wanted nothing protruding above the surface of the cowling.

Many of the RV building guys attach their cowling with hidden hinges and they were my model for what I did.

I am including several pictures to show the progress and end results.

Fit and finish were big priorities to me and if you go this route be prepared to invest a lot of time to get it right.

[Av8r3400]

A work of art, WarEagle.

[HighWing]

I used piano hinge primarily for the reason you mentioned - top and bottom fit. I have seen tons of Kitfoxes with the bulge between camlocs. I spent a lot of time on my first Model IV eliminating that and since hinge is what we did on the Lancair, that is what I did this time.

A note: The hinge pins can be a struggle to get in and out at first, but with some run time there is the inevetible wear that loosens things up a bit. It still requires a bit more effort installing the cowl, but I think it is all worth it

Consider this is a Model IV and some things I did might not work on later models. I spaced the rivets about every three inches. I used flat head solid rivets set with a rivet squeezer. you could also use flat head pull rivets. The rivet heads are, of course, covered with Micro. I used Hysol under the hinge - Lancair. I also made a small blade and pocket at the front edge to help align the cowl halves so there is no misallignment beyond the sharp curve that will not accommodate the hinge. To store the hinge pin when removed, I put a length of carbon fiber arrow blank running aft at each side above the seat back secured with cable ties to fuselage tubing. The right angle bend on the pin is in the cockpit and can be pushed in a bit if necessary so there is no need to secure them. There is about a sixteenth inch or less between top and bottom halves. Since on the Model IV, the cowl tended to bow out at the joint line, I made a U shaped piece of 4130 drilled it hinge pin size at the right spot and Hysoled it to the door post. The hinge pin is then passed through that and through the hinge halves and it holds the aft edge of the cowl snugly to the door post top to bottom.

To make the forward blade, I trimmed off the lower cowl joggle where the hinge would be and then trimmed the forward end to the shape shown with a slight taper to the top edge. I covered it with the transparent plastic packing tape as a separator, trimmed the tape to the shape and layed up a couple of layers of glass on the inner surface of the bottom cowl. After curing separate the parts and sand a bit to make them slip in and out easily. Don't forget clecos for trial fit, etc. Nice thing about fiberglass is it is easy to fill a hole drilled in the wrong place.

Another trick I learned on the Lancair to clean up the edges between top and bottom cowl, you can lay in a layer of Micro (Microballoons and epoxy) or Feather Fill and then take something like a dinner knife or thin piece of metal and run it along the hinge line with the knife tip touching the hinge and held to either top or bottom cowl. Sand after curing, remove the pin and pop the top and bottom cowls apart. I found I had to do that step a couple of times.
Lowell

[WurlyBird]

HighWing and WarEagle, thanks for the very informative posts. You both have done some tremendous work on your planes. You both mentioned filler between the cowl and hinge to fill a gap and this reminds me of another concern. Does the hinge need to be kept flat along one axis or the other? Obviously if the two sides of the hinge are flat then the hinge will rotate smoothly. When both sides are lined up, the hinge is completely open or at 180 degrees, it seems that you can put a pretty good bend in it like WarEagle did on the boot cowl in one of the pictures. The way my cowl pieces are I envision that the hinge will be "closed" to about 160-170 degrees and it will then go around a slight curve, probably 20 degrees over a 2.5-3' distance. Is this going to work or will a wedge need to be made so that the hinge sides are aligned with one another?

[HighWing]

The hinge is just a means of connecting upper and lower edges with continuous attachment . It doesn't really flex like a typical hinge joint. When I remove my cowl, I will remove the hinge pin and then lift that side to separate the hinge sections. I suspect the ability to lift the free side a couple of inches has more to do with the flex of the top cowl fiberglass than an actual rotating movement of the opposite hinge. Like War Eagle, if you were to make an oil door that would open fully, the hinge would have to be perfectly straight. And as War Eagle suggests, there is a minimum curve that a hinged joint will accommodate - my blade protrusion accommodated that area on my cowl.
Lowell

[dholly]

Wow, I love that straight, clean look! You've convinced me to try piano hinge on my Avid+ project as it seems particularly suited to a 2-pc upper / lower cowl. Appreciate your pics and the respective workmanship, thank you gents for sharing your ideas.