|MAKING FLEXIBLE CONNECTIONS
Foamies have led us to new ways of building and flying model airplanes. They allow us to assemble a plane in hours rather than days or weeks. Because they involve very little investment in terms of money and labor, we no longer need to fear pushing our flying skills to the limit.
The procedures followed for building with foam differ from those used when working with balsa. For example, there are various ways to hinge the control surfaces, and the method you choose will depend on the type and thickness of the foam. Let’s look at a few hinging techniques.
MAKE THE HINGE LINE
The Dremel tool is perfect for drilling holes in the control surface for Robart Hinge Points.
The first step when hinging any surface is to bevel the edges to be joined. If you use thin foam for your plane, you might not need to do this; simply hinge the surfaces.
Most foamies need beveled edges at the hinge line. Use a sharp hobby knife and a metal ruler to do the cutting. Put the ruler along the edge of the foam surface and, holding the knife at a 45-degree angle to the ruler, make one continuous cut; draw the blade from one end of the surface to the other. Have some type of cutting mat underneath the foam so that your cut is clean. A sharp knife is very important for producing a clean edge that’s ready to be hinged. A dull knife will leave a jagged edge that you’ll have to sand smooth.
You could also use a sanding block to bevel the edge. I like Great Planes’ Easy-Touch Bar sander with 150-grit sandpaper. I put the edge to be beveled at the edge of the workbench and then drag the bar sander across it at a 45-degree angle. Work across the edge until the entire surface is beveled correctly.
Charger RC’s Super Beveller (introductory price: $5.99) can handle foam of up to 5/16-inch thickness. Pull it along the edge of the surface to be beveled from one end to the other; the result will be a perfect cut every time.
Make a bevel in the control surface by using a metal ruler and a hobby knife.
Clean surfaces ensure that the hinge material will bond solidly and last for the life of the plane. Foam is sometimes slightly oily or dirty, so clean it to guarantee a secure bond. A quick wipe of rubbing alcohol cleans foam without destroying it.
Then be sure to let the foam dry completely before you hinge it-especially if you use tape as a hinge material.
Another way to ensure a good bonding surface for tape hinges is to dab thick, foam-safe CA onto the surface where the tape will go. Wrap plastic wrap around your finger and spread the CA over the surface. Then lightly spray on CA accelerator, clean off the excess accelerator with alcohol, and allow the surface to dry. It will then be ready for the tape hinge.
HINGE MATERIALS & HOW TO USE THEM
The Super Beveller is a new tool from Charger RC. It makes beveling the edge very easy to do.
Use foam-safe CA to make a thin layer of glue on the foam where the hinge tape will go. This makes for a smooth, solid surface for the tape to bond to.
Clear packing tape seems to be the most popular hinge material. It’s readily available, it sticks to surfaces very well, and it’s inexpensive. Also good is clear first-aid tape; it’s quite flexible and sticks well to control surfaces.
I put the two pieces to be hinged on a flat surface and line them up in their proper positions. I keep them in place with small weights such as beanbags. I like to keep a small gap (1/16 inch or less) between the two surfaces at the hinge line to allow complete 3D movement. I put the tape across the two surfaces with the hinge line along the tape’s center. After attaching the tape, I use a balsa stick to press out all the air bubbles and further strengthen the hinge bond. I flip the piece over, weight it down again and put tape on the other side; I make sure that the top and bottom pieces of tape stick together at the 1/16-inch hinge-line gap.
You can use CA hinges on foamies as long as you meet certain conditions. The foam should be thick enough for you to bevel a 45-degree angle on both the top and the bottom edges of the control surface and still have a 1/16- to ?-inch-thick flat edge. Cut your hinge slots with a sharp hobby knife, and insert the hinges just as you do when you hinge a balsa plane. Put five or six drops of foam-safe, thin CA on both sides of the hinge at the hinge line. Foam does not have the same moisture content as balsa, so it will take the CA a while to cure; let it do so overnight.
Robart Hinge Points are also good for thicker foam. Start by measuring the positions of and drilling the holes for the hinge points. I drill holes in foam with a Dremel tool. A Dremel spins the bit faster and is easier to control than a regular drill gun.
Epoxy works well for the hinges, and polyurethane glue is another good choice because it expands around the hinge as it dries. This glue requires moisture to cure, so wet the hinge point before you push it into the glue-lined hole. Whether you use epoxy or polyurethane glue, you should protect the hinge “knuckle” with oil or petroleum jelly. Polyurethane glue will expand out of the glue joint and onto the hinge “knuckle,” so let it dry before you attempt to remove the overflow.
Check out these hints, and you’ll have a strong, durable hinge joint. Foamies might not last forever, but there’s no reason why their hinges can’t.
Some foam models come with compressed foam hinges. I had one that fluttered on a high speed
pass, separated the elevator and crashed, destroying the plane. This technique applied before
first flight to the controls would have saved that plane.
I would love to know where I could get the beveller too.
I hound it
Hi John, I use a couple of other excellent techniques that work great. Foam-Tac hinges make a full length hinge with no gaps and are very flexible and durable. Blenderm tape also works extremely well for gap free hinges and is very tenacious. These two products/techniques work extremely well on most foams that are not too thick. You can find out about them and their use by searching on the Web.
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