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. :D

[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.

[Esser]

I guess a little time and detail go along way. I have seen RV guys mess with their hinges for 20 minutes trying to get their cowl back on. If it can be made to be fairly easy to put back on I would think about doing it as it looks so nice,

[HighWing]

James, Thinking about your question further, I don't think I answered it. As the hinge is bent, it will remain flat so yes some accommodation will be needed to correct the bedding of the hinge. Lancair folks use clecos a lot when positioning panels that are being glued together. It is messy and requires cleaning the clecos from resin residue from time to time.

The procedure is to fit the hinge in position with clecos. It will result in gaps in areas between the hinge and the cowl skins. This is the way I took care of that. A half inch drum sander in a dremel is great for trimming spots to get the hinge positioned so the outside surfaces are close to being flush with each other - don't sand the glass too thin. - After everything fits closely mark the hinge halves as to which goes where on the cowl halves. Pull the hinge pin so the positioning of the top and bottom parts are done separately. Cover the mating surface of the hinge half with clear packing tape wrapping the tape around the back side of the hinge covering both sides and trimmed to clear the hinge cuts. Mix a batch of thick Micro or feather fill, spread it in line on the cowl half where the hinge will be located. Reposition the hinge and cleco the units together. Carefully remove the excess micro to prevent it from getting in the hinge cut area. Put top and bottom cowls together and replace the pin. You may want to remove each cleco one at a time to wipe them with an acetone wipe to remove excess micro. Allow to cure.

After the clecos are removed, the parts can be separated to remove the tape separator for contour and scuff sanding. After scuff sanding the mating surfaces of the hinge and the cowl, they can be hysoled together. Use the clecos to position and follow with the rivets. I also dipped the rivets in the hysol before squeezing.

Final surface contour is accomplished by using the micro/Feather Fill and spread it on the hinge area to level both sides. This is when the knife run along the hinge will open the hinge joint enough for separation and clean up.

I hope I thought of everything this time - someone let me know if not.
Lowell

[WurlyBird]

Again, thanks HighWing. You certainly got the important points for me. I had "played" with the hinge a bit and it is possible to have a bend in the hinge while it goes around a slight curve but I did not know how difficult it would be to secure properly. Filling with micro to allow the hinge to be flat while it curves makes a lot more sense. I was out in the garage between posts and there is really not that much fill that will be needed.

[HighWing]

I guess a little time and detail go along way. I have seen RV guys mess with their hinges for 20 minutes trying to get their cowl back on. If it can be made to be fairly easy to put back on I would think about doing it as it looks so nice,

Maybe the blades on the front of my top cowl helps. The only problem I had was early when everything was very tight. It was hard to get the hinge pin started. Now it probably takes less time than it took to fit and close all the camlocs. I also don't need a screwdriver for the main cowl - but still do for the ring cowl up front.
Lowell

[WurlyBird]

So I have since installed the piano hinge on the cowl and on a whim I conducted an experiment with great results. The piano hinge will accept a compound curve.
I started at the nose on the upper cowl, which has more curvature to it, drilled and clecoed every six inches or so, moving aft and keeping the hing at the proper height from the cowl. I removed the clecoes, installed the hinge wire and re-clecoed the hinge. I again worked forward back in six inch increments securing as I went. The hardest part was working inside the cowl while trying to hold it in the right position. After the initial install I attached the hinge every 1.5 to 2". The parts are now epoxied and riveted and function perfectly.

[Esser]

Take any pictures of the install?

[WurlyBird]

I will try to get some pictures in the near future but I am sort of living in the woods on a government funded camping trip at the moment. I will forwarn you that it is not pretty though. I used blind rivets as I have not been able to find good instruction on solid rivets and have only dabbled with them. Once envy thing is done the inside of the cowl will probably be painted over to cover it up a bit.

I will try to get those up soon (thee weeks ish).