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Building The U. Maine Trainer

- Constructing The Main Wing -

(last update:   27 Jun 2012)

The main wing of the U. Maine trainer is, like much of the rest of the aircraft, constructed of ordinary 2 inch building insulation foam.

As described earlier the wing block is cut to provide enough stock for both halves of the main wing.

Fuselage and wing blocks from 2 inch building foam

In the picture above the wing block is the lower block.

Plan the wing cuts carefully...

In the picture below the wing block is on its edge with the two wing section templates placed for clearance.

Lining up the templates on the wing stock

To get clean edges, there needs to be enough space between the templates for the hot wire to pass. There should also be enough clearance from the outer edges of the stock so as to avoid any dents and dings on the surface.

The one problem with using this tight spacing is that it is difficult to use a stop block during the cut. The problem manifests itself in the photo below.

Stop block interference during wing cut

The height of the stop block is higher than the tail of the wing section template. The black line in the photo suggests how high the stop block may be. This means that the hot wire will get stopped by the block before it has fully exited the foam.

On the one hand this isn't good, on the other hand it won't really matter in this case because much of the foam on the template tail will be discarded anyway. As long as there is enough extra tail length to accommodate any distortion, caused by the stop block, it may be left in place.

The stop block is good to have so that there is something to pull against, as the hot wire is drawn through the foam. Without the stop block, a fair amount of downward pressure must be applied to keep the foam from getting dragged across the bench, during the cut.

Note that the stop block may be useful on this first wing only. This cut will have the full block face to distribute the block load, and the stock's rectangular shape makes things square. Once the first section is cut out the remaining stock piece will no longer be square, and will not lie flat. This can make use of the stop block tricky.

However, if there is enough overhang on the tail, leader, and offset from the stock faces, it may be possible to use the stop block for both. Note that the height of the stop block may need to be increased. If this all seems a bit confusing, try a cut or two, and you will get the idea... foam is cheap. If you mess up a wing section or two, it's no big deal.



Cutting speed is important. If the hot wire is pulled too hard it will bow too much, and then the airfoil profile will be distorted.

Below is an example of how the hot wire bows during a cut. This is the block of wing stock after the first cut. Note the bowed discoloration in the middle of the block.

Pull arc in stock after first wing cut

Here is another view with the discoloration highlighted.

Highlighted pull arc in stock after first wing cut

The other half of the wing needs to be cut from the stock. The picture below shows one of the templates fixed to the end of the wing stock. The other template has yet to be fixed to the other end, as a mirror image.

... also seen in the photo are the bow marks left behind on the surface of the first hot wire cut...

Prepping for the cut of the second half of the wing

Here's a picture of what happens when the cut isn't pulled from the leading edge to the trailing edge of the wing section.

Poor results when pulling in the wrong direction

The lower surface of this wing was pulled in the wrong direction, trailing edge to leading edge. It was also probably pulled too fast. The bow/arc of the hot wire has dramatically softened the corner at the leading edge of the wing. This effect is most notable in the center of the wing, where the corner is almost flat.

Whenever the shape is complex (tight radius on leading edges... etc.), it is best to slow down, and/or cut that section first (leading edge to trailing edge). The arc of the hot wire, caused by the drag on wire as it cuts through the foam, will have less of an effect on gentle tapers, like the trailing edge of a wing.

Be cautious about too hot a wire!

A hotter wire will reduce the bowing or arc of the wire as it cuts the foam. But it may not be worth it.

Don't be too quick to turn up the heat on the wire. This can cause more problems than it solves. Among other things is can melt the foam too much. This can form a thick dense layer at the surface of the cut, which makes final shaping more difficult. Too much heat can also cause an uneven cut, which will again make more work.

It is usually better to increase the tension on the wire to reduce the arc during the cut. Of coarse, too much tension can cause the wire to break.

There is a balance between tension, heat, and wire arc during a cut. You'll just have to find it...

... ailerons from the wing blanks...

There should now be two wing blanks ready for shaping, and aileron cutting.

The shaping is pretty simple in that the leading edge, and trailing edge template extensions need to be cut away. They don't need to be cut close. Whatever remains should be sanded away with a sanding block or bar. The leading edge needs to have a smooth radius. The trailing edge needs to be narrow but not sharp. It can also be slightly rounded. Try for a radius of less than a millimeter

Cutting the trailing edge to create an aileron is straight forward. In the photo below a sanding bar is used as a straight edge and a measuring tool. The sanding bar is about 53 mm (2 inches) wide, 56 cm (22 inches) long, and makes for a reasonably sized aileron on this size wing. The cut is made along the full length of the wing.

Cutting the ailerons from the wings

In the above picture a small knife is used to mark the cut. The cut should then be made perpendicular to the top surface of the wing but this is not critical.

A large skill knife can then be used to cut through the foam. One could also do this with the band saw. If a band saw is used, the work table will need to be tilted to allow for a perpendicular cut, with respect to the upper surface of the wing.

Making this angled cut is relatively straightforward because the bottom surface of this wing section is flat. Other wing sections do not have flat bottom surfaces, and making specific angled cuts can get tricky.

Here's what all the foam looks like once it has been cut.

All the major pieces cut out

The two wings, with their respective ailerons are in the center. The forward part of the fuselage is in the upper right corner. The rudder, and horizontal stabilizer are center right.

There are no pictures of cutting out the rudder and horizontal stabilizer. The two pieces were cut from 1/4 inch building foam, using the paper templates previously mentioned.

All these parts now need to be put together to form a flying aircraft. Before that can happen they need some reinforcement.

Reinforcing foam parts to form a functional aircraft.