IN THE FIRST article of this portion of MA's "From the Ground Up" series I reviewed RTF and ARF aircraft that are suitable for new RC pilots. The second article covered building a generic RTF trainer, using available glow- and electric-powered versions as examples. You can assemble RTF trainers in 20 minutes to an hour, using common hand tools. No adhesives and few building skills are required.

    RTF trainers represent a leap forward for beginning modelers. It is possible to get into the air with a quality aircraft in less time than was once required to unpack a wood kit. Still, constructing a wood kit has several advantages that RTF pilots miss, one of which is that it allows the pilot to select an upgraded radio system and engine.

    Building a wood kit allows the pilot to modify the airframe to increase performance, appearance, and durability. Wood kits allow the modeler to "swap out" heavy parts by making identical parts from lighter wood. Built-up wooden kits are usually 5%-10% lighter than the identical ARF. Less weight makes any sport aircraft fly better.

    Constructing a wood kit is also fun and educational. This series will eventually cover how to build a wood-kit aircraft "from the ground up." Every model pilot needs to gain this experience, but constructing a wood kit has two drawbacks: it takes one to two months to complete and requires advanced building skills.

    Surely there must be a compromise between RTFs and wood-kit models. There is, and it is an "ARF."

    ARF aircraft require some building skills, but nowhere near those required to complete a wood kit. ARFs come in enough pieces that the builder can make almost as many modifications as he or she can when building a wood kit.

    The pilot supplies the radio and engine systems, allowing upgrades. So much work is factory-completed that the average ARF trainer requires just 20-25 hours to go from box to air. Almost the only advantages left for a wood kit are its lighter weight and unique appearance.

    There are far more ARFs available than the relatively few RTFs. It is impossible to include every ARF in this installment, as I did in the June issue with the RTFs. Instead, in the next few "From The Ground Up" articles I will cover building and modifying one ARF trainer: Hobbico's Hobbistar 60 Mk III.

    This 60-size advanced trainer features a semisymmetrical-airfoil wing, allowing inverted maneuvers. However, the generous wing area and dihedral give the Hobbistar basic-trainer abilities as well.
 
    If you want to learn about many other fine ARF trainers available, you can read about them in MA's Sport Aviator online magazine at www.masportaviator.com.

Getting Tacky About Adhesives: ARF construction requires adhesives. These aircraft do not bolt together as RTFs do. A review of adhesives is a good idea before I start gluing tab A into slot 12 and get my six left fingers stuck together.

    ARF construction typically employs three types of adhesive.

    Two-part epoxy is used where strength is important. Five-minute epoxy is great for parts that require strength but little alignment time, such as aileron-servo mounts. A 12-minute epoxy is best if some alignment time and extra strength are required. This adhesive is used for stabilizer and vertical-fin attachment. Use 30-minute epoxy if maximum strength is required, as when joining wing halves.

    There is a difference in the final bond strength between the different epoxies when they are used on porous surfaces such as wood. The five-, 12-, and 30-minute epoxies seem to have the same final film strength, but a longer "dry" time (epoxies bond chemically—not through evaporation) results in better adhesive penetration into porous materials. The deeper the penetration, the stronger is the total final bond strength.

    For some reason—possibly lower solids content—the typical two-part epoxies in hardware stores do not always have the strength of epoxies that are meant for model construction. Past experience has shown me that the epoxies sold in various hobby outlets are usually best for ARF construction.

    The various cyanoacrylate-type adhesives are important in ARF assembly. The thin variety is used for hinge installation, and medium consistencies are used for most other construction steps. A small bottle of cyanoacrylate accelerator is handy for bridging gaps or building reinforcing fillets.

    Cyanoacrylate bonds are surface only, making them weaker than epoxy joints. Unlike epoxy, cyanoacrylate does not have much antirotational strength.

    Having some old-fashioned yellow aliphatic-resin glue—ol' reliable carpenter's glue—helps construction. Carpenter's glue penetrates like epoxy, forming a strong bond in wood only. It's not as strong as epoxy, but this one-part adhesive is easier to use; no mixing is required. In areas that require strength and alignment time but with room for tack-gluing the pieces in place with thin cyanoacrylate, carpenter's glue is a good alternative.  

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