An Almost-Ready-to-Fly Basic (ARF) Trainer differs from the same airplane as a Ready-To-Fly (RTF) version in several important ways. First, ARF Trainers take longer to build, about 15 hours, than the usual RTF trainer’s 45 minutes assembly time. A very modest amount of building skill and tools are also required. Most RTF trainers can be assembled using just a screwdriver and wrench. An ARF Trainer usually requires adhesives, and proper airframe alignments. The builder must also buy and install the engine, fuel and radio systems.

Since almost any ARF Trainer is also available in an RTF version, why would a new pilot choose an ARF over an RTF airplane? An even better question is why do so when the RTF version will also probably cost much less than the ARF? Fortunately, there are several good reasons to do so:

Ø       If the worst has happened to your RTF trainer but the radio system and engine survived the uncontrolled air-ground interface, you can just purchase the ARF airframe for a fraction of the cost of another RTF. For example, the Tower Trainer ARF used in this article costs just $80 vs. about $270 for the RTF version.

Ø       Most RTF trainers arrive with very basic radio and engine systems. Almost all RTF trainers include a basic 4-channel computer radio system. The only exceptions are the Hangar 9 Progressive Trainer Systems (PTS) aircraft and the NexSTAR EP which include 5 or 6-channel systems. This limits the pilot’s future growth in the sport and usually requires purchasing a more capable radio system in the very near future.

Ø       Like the basic radio systems, RTF glow-powered trainers are often equipped with basic 40-size engines (there are exceptions including the NexSTAR and the PTS trainers which are powered by two great 46-sized engines) which limit the airplane’s performance and would probably not be best suited outside the trainer role. That means your second airplane needs a more capable engine.

Ø       Constructing an ARF airplane allows the builder to modify some airframe components to increase durability and to improve flight performance.

Ø       Since the pilot is free to choose the engine and radio systems, it is possible to buy just one advanced radio that will grow with the pilot’s increasing skills. This applies to the engine and propeller choices as well.

Photo 1           Photo 2

For this project, we choose the O.S. Max 46 AX engine. Unless you have run this engine in your airplane, it is difficult to understand just what a magnificent engine the AX is for the beginner to operate. Its power is incredible, it is easy to operate and far more reliable than such a powerful engine has any right to be.

The idle is steady at around 2,100 rpm on an APC 11 x 6 in. propeller. That is about 300 rpm less than most 46’s can manage. A lower idle speed means easier to manage landing approaches and better touchdowns.

The radio system selected was the Spektrum DX-7 2.4 Ghz with the DS821 Digital Sport servos. The DX-7 is a 7-channel advanced computer control system that will handle the pilot’s next 20 airplanes. It even has dual rates on the rudder and 5 programmable mix settings which new pilots don’t need to know about right now but will surely need in their first sport aerobatic airplane. For more information on what a 2.4 Ghz radio system is about, read the Sport Aviator articles “2.4 Ghz For The Common Pilot” and “Spread Spektrum - Are You Ready For Full Range?”

Photo 2A

Why Digital servos? Digital servos provide better centering (they return to the exact same neutral point every time) and more precise positioning when deployed than the typical analog servos installed in most RTF trainers. The extra precision increases the pilot’s “feel” for the airplane and for what it is doing. Better “centering” means that the airplane always returns to its neutral trim settings every time; making it much easier to fly.

Finally, these Digital servos are stronger; producing 72 oz/in of torque vs. the usual RTF analog servo’s ~45 oz/in. The DS 821 is also faster than analog servos at 0.19 seconds for a 60-degree movement vs. the analog’s 0.22-0.24 seconds. While all trainers will respond noticeably to the Digital’s better centering and strength, the faster servo speed will probably not show up until your first aerobatic airplane. Trainers are just not designed to move quickly enough for the faster speed to communicate itself to the pilot.

Hopefully you are planning to build an ARF trainer because of its advantages and not because you need to replace an RTF trainer that has flown its last. In either case, there are some skills and tools needed and some ways to improve the airplane.

While reviewing this article, keep in mind that the building process is covered in extra detail with some building hints and tips for a better airplane. Some of the modifications are my own ideas but have been proven over decades.

Also, don’t let the seeming complexity of this two-part article fool you into thinking that building an ARF trainer is a long, tedious job requiring Master Builder skills. It is not. The whole process takes only 12-15 hours and requires simple modeling tools. It might take you longer to study this article than it will to build the airplane.

The best way to illustrate is to build one, so here goes.

Building The Wing:

Photo 3           Photo 4

While it is bigger than the fuselage, the wing is less complicated and therefore easier to build and align. It is a good place to start. The basic task is to take all the parts in photo 3 and transform them into photo 4.

All RTF wings use an interior metal spar plus an alignment pin to assemble the wings. (For complete information on how an RTF trainer is built, read the Sport Aviator article “How to Assemble Your First RTF Trainer”.) The wing halves are held together with a small strap similar to the nylon strap used to hold the main landing gear in place. Eventually, this type of assembly begins to loosen and the airplane loses its easy trim points under stress as the wing flexes a little.

ARF Trainers use a laminated wooden spar that is epoxied in place and the wing halves are also epoxied together. This type of assembly never loosens and the wing always remains steady. Durability point one for the ARF over the RTF!

Photo 5

Photo 5 shows almost everything needed to assemble the wing. Note that the wooden spar is in two pieces. Once laminated together with epoxy, the spar will be much stronger than a single plywood piece of the same thickness. Tip: The more time an epoxy requires to set up and cure, here 12 minutes for setup and 4 hours to cure, the stronger will be the bond on porous surfaces such as wood. Since the slower “drying” epoxy remains liquid for a longer time, more of the adhesive sinks into the porous wood making an “in depth” bond rather than just a surface joint.

Photo 6           Photo 7

Actually three epoxy brushes will be required to complete the wing, not just the one shown. Two squares and lots of clamps also come in handy. Stresses sometimes build up during manufacturing of the spar pieces resulting in slight warping. Tip: When laminating the two pieces together, make sure the gap is towards the lamination’s interior as shown in photo 7. A better, straighter bond usually results this way.

Photo 8           Photo 9

Mark the two pieces as shown in photo 8 to insure a correct lamination. Brush on the 12-minute epoxy onto one piece and join the lamination. Square up the spar as shown. Do this over wax paper to protect the table from the epoxy.

Photo 10           Photo 11

After ensuring that the lamination is perfectly square, clamp it all together. After about 30 minutes, remove the clamps. In many cases, there is a slight, very slight, overlap on one side, maybe two sides? Use a 90-degree sanding block as shown in photo 11 to make everything perfectly even.

Do not remove much material. The idea is to make sure that all sides are flat so that they mate squarely against the wing’s spar sockets’ top, bottom, edges and sides for maximum strength. A solid mechanical joint is best.

Photo 12           Photo 13

Draw a center line on the laminated spar as shown in photo 12. This will be used to trial fit the wing halves together. Sometimes, the spar pocket is blocked by a little piece of wood (photo 13). Carefully use a razor chisel blade, available at all hobby shops, to remove the extra material. Do not enlarge the socket; just remove the “flash” as shown.

Photo 14          Photo 15

Trial fit the wing spar into each wing half. Make sure that the spar can be inserted into each half so that the center line is slightly inside the root wing rib (photo 14). If the spar/socket fit is too tight, remove a little material from the spar as shown in photo 15. Place the spar on a flat surface and use a sanding block. Do not hand sand without a block as that causes cavitations in the surface that will weaken the glue joint. Test the left wing half first and mark the spar “left” after it fits well into the left side.