Ironed Foamboard Hinges

I have been designing, building, and flying with dollar tree foam for almost 3 years. One of the aspects that I have had the hardest time with (because I seldom crash) are the hinges. The 50% score cut and bevel, even with the hot glue reinforcement, delaminates and sometimes is just too flexible for very precise control. On my last few planes, I have experimented with an alternative hinge method. This can only be done if the paper on both sides of the foam at the hinge has not been cut already (no 50% score cut). That means this technique is for scratch-builders. This method shortens the total wing chord across the hinge by 1/8 inch. Plan accordingly or just don't finish cut the wing/tail/etc. until after the hinge has been made.

You will need: ruler, stiff sharp-cornered straight edge, pencil, iron, and foam wing.

Step 1 - Layout the hinge-line

This involves measuring, drawing, and double checking the hinge reference line on both sides of the foam.

This must be accurate or else the flaps will be skewed.

Step 2 - Cut control surfaces

Cut the control surface free so that the creasing and ironing processes do not bend the rest of the wing. (The above picture is wing with 3 one inch hinges. That made the ailerons easier for the 5gm servo to move.)

Step 3 - Crease the hinge

I use a bar of 1/8" thick steel that has a very square edge (or a hardwood block with a sharp straight edge) for this. I line the edge up with the drawn reference lines and press the edge into the foam. I press one corner into the paper right on the line. I alternate pressing the paper on both sides of the foam sheet closer together. Do this on a solid, flat work surface.

Step 4 - Fold and work control surface

Bend the control surface back and forth at the hinge until it easily deflects about 45 degrees.

Step 5 - Iron and fold the hinge line

This is the trickiest part. Use extra hands if you can find them. (Just don't burn them accidentally). Lay the wing on a table with the control surface off the edge. (A more solid table with a nice sharp edge is best. I don't recommend an ironing board. They are way too flimsy.) Bend the control surface down about 45 degrees and hold it there. Then use an iron (set to high) to melt the foam through the paper evenly on both sides of the hinge line. Work in quick even passes. Don't try to do too much at once. Try to melt the foam evenly only out to about 3/16" on each side of the hinge line. 

Then put the iron down and bend the control surface up and down while the foam cools. You should be able to bend it a little farther than 45 degrees the other way. (note that the paper is beginning to form a groove at the hinge line.)

Step 6 - Flip wing and repeat Step 5

Lay wing down on the other side with control surface off the edge. Bend control surface a little more than 45 degrees down and iron.

Step 7 - Keep repeating steps 5 and 6

The objective is to melt the foam under the paper and work the hinge until the paper on both sides of the foam meets at the hinge line. 

The flap should be able to bend nearly 90 degrees with minimal resistance. 

Pointers

This can be somewhat tricky to get right.

• So, make absolutely sure your measurements and creases are accurate and that you iron and bend evenly along the hinge line. Otherwise...
• I recommend practicing on scraps of foam before trying it on a wing.
• There is a chance that the control surface may not turn out exactly the way you want and might require remaking the wing. Make the control surfaces before folding the rest of the wing so that if it doesn't turn out the first time, you haven't already spent a lot of time making a wing. 
• It is better to iron in several short sessions (letting the wing and hinge cool in between) than trying to do it all at once. (Holding the iron close to the wing for very long can cause the wing to warp.)
• Longer hinge lines are easier to warp and can be stiff if they aren't perfect. Cutting the hinge line at strategic locations makes the hinge easier to make and much more flexible.

Pros

The melted foam sticks to the paper better and hardens making a much more precise hinge line.

The hardened foam also doesn't get squished when the control horn is pulled.

The hinge looks cleaner and won't delaminate.

Cons

You might have to remake a wing.

It takes more time

Someday, I might make a hinge press that does all of this in one step, but until that happens, this will just have to do.

Good Luck FT Bloody Wonder with ironed hinges.