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Making a Rudder Fairing with Acrylic
Click on picture above for larger view
Even though it is not mandatory, I wanted to have a beacon atop my CH601 HD rudder. I also wanted it to look nice and case little additional drag (the original CH601 design is “draggy” enough, thank you very much).
This is how the idea for making a transparent fairing for the beacon was born.
I did a little research on the Internet, and found useful references on handling and shaping acrylic.
For instance:
There is also some information at the FAA site, Flight Standards Service, Continuous Airworthiness Maintenance Division, AFS-300, AC 43.13-1B.
(As the search can be time consuming, I have a couple of documents I can share with anyone interested – email me at carlosfsa@yahoo.com.)
The process is quite simple, after you figure things out.
First, I needed a mould over which to form the part. I had the shape of the topmost rib on the ruder, so I just reduced that by the thickness of the acrylic sheet and used it as a starting point.
I cut two pieces of MDF, half an inch thick, and glued them together.
Then, on with some
sculpting work. I used a rasp file, utility knife and sandpaper to get the shape
I wanted. By the way, working with MDF creates a fine dust that gets everywhere. Very messy.
The mould looked something like this, when done:
I needed a frame with a whole that would accommodate the mould plus the thickness of the acrylic. In other words, the hole should be able to accommodate the rudder tip rib. That was made from MDF 0.5" thick as well.
The mould is then placed atop a base of the same size as the frame. A piece of wood, with a footprint slightly smaller than the mould, rises it above the base by about 2 cm.
The frame and the base are attached to each other with carriage bolts and nuts with washers.
Spacers located at the four corners prevent the acrylic from being stretched beyond the bottom of the mould. These spacers should be as high as the bottom of the mould minus the thickness of the plywood frame.
What’s missing is the
acrylic, and a way of attaching it to this contraption. I made an additional
frame out of 0.25” plywood, shaped like the first MDF one. The acrylic piece is
then “sandwiched” between these two frames, and kept in place with a number of
screws.
Of course, the acrylic piece should be drilled at the points where the screws will go through, or it risks cracking.
Now, to actually make the acrylic fairing, I followed these steps:
1. Pre-warm the kitchen oven to about 300 F (if the temperature is set too high, the material will create bubbles).
2. Cut a piece of acrylic large enough to cover the hole in the frame, and to allow for proper fastening with the screws.
3. Assemble the “sandwich”: MDF frame, acrylic, and plywood frame.
4. Place the sandwich over the mould and fasten it lightly in place with the nuts and washers.
5. Cover the corners of the wooden structure with aluminium foil, to avoid damage from heat.
6. Place the gadget in the middle of the oven and wait 10 – 15 minutes.
7. Take the “cake” out (don’t forget the oven mitts) and quickly tighten the nuts, thereby forcing the acrylic to take the shape of the mould, a few millimetres at a time.
8. Repeat steps 6 and 7 until the frame hits the spacers.
I achieved acceptable results in 1 – 2 h.
After the assembly cools off, the acrylic piece is removed and trimmed to size.
The only bad thing about this process is that the inside of the fairing takes the texture of the MDF mould, and needs to be polished. I used a Dremmel tool and a polishing paste (“Mother’s”, available at Canadian Tire), with good results. Note: it is necessary to use the smallest diameter buffing attachment and the slowest speed, or heat will create additional marks on the acrylic.
It might be possible to coat the mould with something that will yield a formed part that needs not polishing, but I don’t know what would stand the 300 F heat.
The final product can be seen in the photographs. The fairing itself is perfect, but the beacon itself isn’t satisfactory: I used an automotive lamp and red lens, but the results weren’t that good: the lamp bulb got too hot and started deforming the red lens.
I have put this part of the project in the back burner, as I have a ton of other things to do (like building the rest of the airframe), but expect a simple solution will be available when I get around to it.
Actually, LEDs have just made an appearance on the aviation scene. A bit expensive, but prices are coming down.
For example, position lights made with LEDs are now available for experimental aircraft (http://www.killacycle.com/Lights.htm), and Whelen has announced its own (TSOed) LED based position lights and beacons (http://www.whelen.com/pb/11015.pdf).