Fantrainer (oz1339)

About this Plan

Fantrainer. Scale radio control model of the RFB Fantrainer 600 aircraft, for Cox .049 power.

Note there is an RCGroups thread https://www.rcgroups.com/forums/showthread.php?65703-RFB-Fantrainer-600-electric-conversion by Paul Willenborg himself (dated 2002) where he converts this design to brushless electric power with a Mega 16/15/4 motor.

Update 03/05/2021: Added article, thanks to JimPurcha.

Quote: "ARE YOU FASCINATED by ducted fans, but unwilling to sink megabucks into a model airplane? Do you want to fly a model jet, but have a small, grass runway? Do you like scale models, but aren't sure you can handle a plane that turns into an air-to-ground missile if the motor quits? If you answered yes to any of these questions, the RFB Fantrainer is the airplane for you.

The Fantrainer is small and powered by an inexpensive Cox TD .049. Construction is very low-tech and, although it's somewhat unconventional, it can be built easily by anyone who has built a few kits. Small fields are no problem, since the Fantrainer is designed for hand launching. Flight characteristics are excellent: It's very fast and acrobatic under power, it's very gentle and has a good glide when the motor stops. Best of all, it's cheap! Most modelers have a TD .049 stuck away somewhere, and a lot of wood can come from the scrap box. If you had to buy everything that goes into this model, it would cost approximately $100, but my prototype cost about $50.

Building the Fantrainer isn't difficult, but it's certainly very different. Four building features are worth noting:

1. A lathe will make the duct construction much easier.
2. You'll need an 05-size electric motor to make a starter (you can probably borrow an old one from someone who runs R/C cars.)
3. Part of the plane must be finished with dope and tissue, but don't panic! The parts that require tissue are flat, sheeted surfaces; it's really quite easy.
4. Unless otherwise specified, all balsa must be 4- to 6-pound light stock (eg Sig contest stock or similar material). All 1/8-inch plywood is lite-ply, and the sheet shouldn't be horribly warped.

The Fantrainer is built much like a full-size airplane. Various sub-assemblies are com-pleted and then brought together for final assembly. I'll start with the duct, since it's the most difficult part.

Duct Construction: The duct is made of 1/64-inch ply and 1/16-inch balsa, which are wrapped around a 4-3/8-inch-diameter cylindrical form. I searched for a bottle this size, but never found one, so a friend turned a wooden form on his lathe for me. It's 1-3/4 inches thick.

Cut a strip of 1/64-inch ply to 2-1/8 x 16 inches with the grain running the short way. Unless you have large ply sheet, you'll have to glue several pieces together, but this isn't difficult if you work on wax paper and use CA with kicker. Be sure to keep it straight! Block-sand one end of the strip to a feather edge. Roll the strip around the form, starting with the sanded edge. Glue the overlap with 5-minute epoxy and bind it with rubber bands. Don't get glue on the form!

While the ply is drying, cut three strips of 1/16-inch balsa to 2-3/8 x 15 inches, again with the grain following the short side. Butt one end of a balsa strip against the end of the 1/64-inch ply. Roll it around the form, mark the overlap, and cut off the excess. Apply a thin coat of epoxy to the balsa, and wrap it tightly around the form with rubber bands or masking tape.

When the epoxy dries, sand the joint to remove the small step caused by the joint in the ply. Glue on two more layers of 1/16-inch balsa, being sure to stagger the joints. Bind each layer tightly and allow each to dry before adding the next layer.

Remove the duct from the form and sand it to the cross-section shown on plans. Note that it tapers to 1/16 inch at the trailing edge. If you used a turned form, it's easiest to do this is on a lathe. It can be done completely by hand, but it's tedious work, and you must be very careful. The detailed sketch on the plans shows the leading edge of the duet, My research on ducted propellers convinces me that a light bell-mouth with a smooth, rounded edge is very important for efficiency, especially at low speeds.

Apply several coats of dope to the inside of the duct. It's a breeze from here!

Forward Fuselage: Cut out the formers, the fuselage sides and the nose block. Draw center lines on the formers, the nose block and the building board, and former lines on the fuselage sides. Also mark the motor-mount location on F6.

Glue 1/8 x1/4-inch strips to the top edge of the fuselage sheets. Glue 1/2-inch triangle stock to the bottom edge. Severe curves will probably require kerf cuts in triangle stock. Because the fuselage is flat on top, it's easiest to build it upside-down. Glue F3 to the sides, then add the other formers and the nose block; be sure to keep it all straight. (Again, kerf cuts in triangle stock make it easier to bend.) Note that F5A, F5B, F6A and F6B are installed later, during final assembly.

Add the 1/8-inch bottom sheet (one piece, lengthwise grain front nose to F4, and cross-grain from F4 to F6) and the 1/2-inch top nose block (sand the curve at the rear of the block before attaching). Carve and sand the forward section of the nose block to shape; cut a slot in the bottom sheeting for the lower keel. Cut openings in the sides for the wing spar and the rear fuselage attachment. Drill F6 for motor-mount screws.

Canopy: This is another tedious part. Start with a 3 x 3 x 15-3/4-inch balsa block. Sand one edge to match the curve of the top nose block. Hold the block tightly in place and trace the outline of the fuselage onto the bottom surface of the block; then draw the canopy on the side of the block. Run the block through a hand saw in one direction, tape the pieces together, and cut the other side.

Carve, plane and sand to rough-shape. The front quarter of the block is a simple cylindrical curve; the rest is compound. Carefully check the fit at the nose block..."

Update 24/11/2023: Added further article from RCSA Nov 1991, as referenced by Pete Rose, thanks to RFJ.

Supplementary file notes

Article.
Article (RCSA, 1991).

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