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Building The U. Maine Trainer

(last update:   12 Dec 2012)

-UnderConstruction-


To provide a cost effective plane to build, and fly, the U. Maine RC flying club developed a simply constructed, high wing, park flier class, electric trainer. The plane is built using ordinary 2 inch hard building insulation foam. The kind of stuff you would find at you local building center or what's left over from your neighbor's recent remodeling job. Sometimes it's pink, sometimes it's blue. Add a little fiberglass, a flight pack, some other bits, and you should be flying.

The picture below is of one of the pink foam versions.

U. Maine trainer after a flight

The above plane has trained several RC pilots. Plenty of dents, and dings.

The design was developed by Brian Barainca, and is loosely based on a set of Goldberg Eagle II plans he has left over from a build years ago.

The plane has been shrunk considerably, and club members have put their own twists on the design as they build.

It's a nice feature of building your own plane. You can personalize the design. As long as one doesn't go too far, the plane should fly OK. But we'll go into that later.

So... How does one scratch build a reasonable trainer?


Getting started...

Normal wood working tools will be needed.

RC hobby tools are a big help but they are not required. Having such tools at hand simply makes things faster, and more precise. You can do without them though.

A normal wood working tool that makes things quick and easy is a band saw.

Typical Bandsaw

You can get away without one but it is a great help keeping things square. It doesn't necessarily need to be as big as the one in the picture above.

One item that will be required is a hot wire foam cutter. It is hard to find a less complicated, and less expensive device to cut wings.

The simple one used by the club is shown below.

Hot wire foam cutter

If you search the web for "hot wire foam cutter" you will find many commercial, and DIY hits. The RC club's cutter is built of stainless rods, Nichrome wire, a piece of left over 1 x 1, and some clip leads.

Electrically this unit is pretty scary, and I caution anyone who builds such a tool in this fashion. Especially the use of an AC power cord. Someone who didn't know better might plug the cutter directly into the wall. DO NOT DO THIS!!! Aside from the fact that someone might get electrocuted, you are almost sure to set something on fire. You might also be blinded by molten metal, as the Nichrome wire or clip leads vaporize. If you are lucky the breaker will trip before your shop burns down.

Below is the "fire starter"... I mean hot wire cutter's... power supply.

Hot wire foam cutter power supply

Many hot wire foam cutters use an automotive battery charger. This is far safer but such devices have a limited ability to change their voltage output, so the design of the hot wire cutter is a bit more specific. It is entirely possible to overheat the Nichrome wire. It is also possible to overload the battery charger.

But this is a discussion about how to build a plane, not a hot wire foam cutter.

Do some homework on the hot wire foam cutter, and buy or build something safe, and easy to use.

You will need two airfoil templates. A bunch of such things can be seen below.

Wing templates

These templates are made of aluminum. It is possible to make them out of wood. If wood is to be used as a material, it is best to use hard wood. Plastic won't do because the hot wire cutter glides along the templates when a wing is cut. A hot wire cutter will happily cut a plastic template, as well as the foam.

How these templates are used, will be covered later.

A utility knife will come in handy, as will scissors, hot glue, drill bits, a drill... all the usual stuff including normal hand tools.

One additional special tool which will likely be needed is a soldering iron. This is principally needed for connecting the various electronic components used to power the model. This is another topic, and tool that will be covered later.


Templates...

Something else that is important to locate are some templates. There is no set of plans per se. The plane is built using a set of six templates.

These templates are used to rough out the major parts of the plane. The builder is then free to further shape the plane.

A set of templates

In the above photo the horizontal stabilizer, rudder, and fuselage side profile templates are shown.

If the templates are not available, the rough drawings below should help.

Below is a rough dimensioned drawing of the horizontal stabilizer, and elevator.

Rough dimensioned drawing of horizontal stabilizer, and elevator

The fiberglass reinforcement strip is added after the stabilizer is cut from 6 mm (0.25 in) building insulation foam. The hinges can be typical RC plane hinges, or duct tape. If duct tape is used, it would be wise to have the tape span the length of the stabilizer/elevator. You should also keep an eye on the tape. If it releases in flight, you will end up with a mangled pile of foam, likely not in a place you intended.


Cutting out the fuselage, and wing stock...

A reasonable place to start is grab a piece of 2 inch thick building foam, and figure out if the fuselage will fit on it. The side template can be used as a guide

Fuselage template on block of 2 inch building foam

The idea is to cut out a block that is big enough to accommodate the fuselage but small enough to make cutting easier. The drawn rectangle on the foam will be cut out on the band saw first.

Cutting out the fuselage block from 2 inch building foam

If the stock won't fit in the band saw use a skill knife. It may take a couple passes with the knife.

Finishing the fuselage block cut from 2 inch building foam

In a similar manner the stock can be cut for the wing. In this case a known good wing is used to set the rough size of the wing stock.

The photo shows a complete wing on some 2 inch stock. The wing is constructed in two halves, so measuring for the rough size of the stock is done from one wing tip (the left side of the photo) to the center of the wing, plus a little extra.

Estimating the wing block cut from 2 inch building foam

Make some measurements, and draw some lines to make foam use efficient.

Laying out the wing block cut from 2 inch building foam

Then cut out the stock to help make working with the material easier.

Cutting out the wing block from 2 inch building foam

Once again, a skill knife can be used if the foam won't fit in the band saw.

Before you measure and make the final cut on the wing stock, be sure the faces of the block are clean.

Squaring up the wing stock on 2 inch building foam

Just trim a small amount of foam off for a clean edge. Using the band saw's miter gage keeps the cuts square.

Measure the desired length of one half the wing span. Here 20 inches is used, but there isn't any reason why they couldn't be a bit longer. Say 24 inches. The two halves of the wing span will be cut from this single block. The overall length of the wing will be about 40 inches.

Measuring the wing length block cut on 2 inch building foam

Longer wings make for more docile handling, and slower speeds especially for landing. Smaller wings make for snappier maneuvers but everything happens much faster on landing. For a trainer it is almost always better to have a bigger (longer) wing.

Now there should be two blocks of foam. One for the fuselage, and the other for the two halves of the wing. The vertical and horizontal stabilizers will be cut from other stock.

Fuselage and wing blocks from 2 inch building foam

Below is a rough idea of how the wings will be cut out of a single 2 inch foam blank.

Fitting both wings in one 2 inch building foam block

How this is done will be covered later.


Roughing out the fuselage...

Next the fuselage will be rough cut from the fuselage block. It doesn't hurt to pin the fuselage template down on the foam. In a pinch, a nail can serve as a pin. A "T" pin would be better, since it won't leave as large a hole in the material. Tape could also be used to hold down the template. The problem with tape is that it can tear the template, and in some cases, rip up some foam when it is removed.

Pinning the fuselage template to the stock

After tracing around the perimeter of the template, your fuselage block should look something like this...

Fuselage tracing on the stock

Use the bandsaw, and a hobby flat saw to rough cut the fuselage...

Rough cutting the fuselage

When you are done it should look something like this...

Finished fuselage rough cut

There is one more thing to be done to complete the rough cut of the fuselage.

The empennage taper needs to be cut.

Marking out this taper could be done with a template. However, it can also be done with some measurements...

Marking the empennage taper

Flip the fuselage block upright on its bottom plane. At the narrow end of the rough cut fuselage block, find the center, then go about an eighth of an inch either side. These two outer marks will be used to set the narrow end of the taper. The taper will run from the trailing edge of the wing back to the end of the tail.

In the photo below, the marks at the tail end are on the left. The marker tip is at the main wing trailing edge saddle position.

Overall marking points for the empennage taper

Use a straight edge to help guide the marker as the taper is drawn on each side.

Right side line for the empennage taper

When done, it should look something like this...

Empennage taper lines

Use the band saw to cut along the taper lines. Be sure to hold the bottom of the fuselage flush to the band saw table.

Cutting along the empennage taper lines

Here's a top view of the fuselage empennage taper.

Top view of fuselage empennage taper

That's all there is to the rough cut of the fuselage.

The only thing left to do is to shape and sand the rough edges, as shown below,

Shaping the fuselage after the rough cut

OK... leave the fuselage alone for a while...

Time to get started on the main wing.