Selecting Your First ARF Trainer
By Frank Granelli
It has been a long, but hopefully educational and not too difficult, road to this point. In the preceding "From the Ground Up" installments, Bob Aberle covered what an airborne radio-control system is, how to choose one, how to install one, and how to keep one working well. He also covered electric motors, batteries, and the basics of electric-powered RC flight. The articles before those detailed the mysteries of the modern glow engine in all its incarnations.
If, at times, it seems that there is more information than you really want, it is because the "From the Ground Up" concept is to present everything that a new RC pilot will need to know in the first few years. It is hoped that learning the technical aspects of model aviation from these articles or from fellow pilots might be more fun — and less expensive — than being educated by "Fractured Balsa University." With this in mind, let's move on.
At this point we have an engine and a radio system. Nice, but we need something to house these items. It would be convenient if this housing for the engine and radio were also able to lift the assembly off of the ground. There is such a structure, and we call it the airframe, the model, the airplane, or other less-polite names when all is not working well. Whatever it is called, the airframe is the next topic. Specifically, I will be discussing ARF and RTF aircraft.
Types of Model Airframes
Yes, there are many other types of model airplanes. Some are built from wood kits — assembled one stick at a time — and then covered. Some consist of fiberglass fuselages and Styrofoam (foam) wing cores with balsa sheeting. Some are built straight from plans. In that process, the builder cuts the wood into all the pieces of a kit and then starts the assembly. Last, there are modelers who design their own aircraft, draw the plans, create the kit, and then build it. Future "From the Ground Up" installments will detail all of these airplanes.
The following articles will cover the types of airframes available and which are best suited to learning the basics of RC flight. Along the way I will include some thoughts about how to modify RTF and ARF airframes to improve performance and durability. Proper balancing is important, as is building the airframe straight and true. RTF and ARF appearances are usually predetermined, but there are some ways in which even beginning pilots can make minor changes to the looks to set their aircraft apart from all the others. Radio setup can be optimized on many models — even on some RTF airframes.
The sheer number of ARF and RTF aircraft is astounding. There are:
- Scale models that replicate full-scale airplanes.
- Competition models meant for aerobatic-performance contests.
- Basic trainers, biplanes, second aircraft, sport models, 3-D performers, and some that defy any logical description.
All of those accomplish their design tasks far better than their compatriots of just 20 years ago. But new RC pilots will usually find it best to begin with a "basic trainer"-type aircraft.
Why a Basic Trainer First?
Why begin with a good basic trainer design? It is possible to learn RC piloting on any type of model, but sport or scale aircraft react quickly to control inputs, leaving less time for the new pilot to plan the next control inputs. Also, takeoff, in-flight, and landing speeds can be higher with the more reactive models. Sport and scale airplanes demand more from the pilot than most basic trainers. In addition, their higher airspeeds and quicker control responses mean that this "more from the pilot" must happen more quickly. This requirement that the pilot be "further ahead of the airplane" makes learning to fly RC with a sport airplane more difficult and time consuming.
There are so many excellent sport ARF designs that make great advanced trainers or second models that many new pilots want to use them as their basic trainers. This is possible, but sport aircraft have certain drawbacks in that capacity. Although high-wing advanced trainers such as the Midwest Aerobat may resemble basic trainers and even have similar wing loadings (the weight each square foot of wing area must support), their design criteria are different, as are their wings' airfoils.
The Aerobat and its cousins, which include Hobbico's Avistar, Hobby Lobby's Bonnie 20, and Hangar 9's Arrow, feature symmetrical (or semisymmetrical) wing airfoils that have less lift than equivalent flat-bottom trainer airfoils.
Symmetrical vs. Flat-Bottom Airfoils
A symmetrical wing is curved equally on the top and bottom, and a semisymmetrical wing is curved on the bottom but more so on the top. The symmetrical wing allows for easy inverted flight, outside loops (loops with the airplane upside-down, performed using down-elevator), faster aileron response, and good snap roll/spin performance. But landing speeds are higher and the symmetrical-airfoil wing is more responsive, making these second aircraft sensitive to control inputs.
The symmetrical airfoil actually has slightly less aerodynamic drag than an equivalent flat-bottom wing. This means that the aircraft gains more airspeed than a basic trainer if the student pilot lets the nose drop in a turn. Aerobatic trainers will not "balloon" as much (raise the nose and climb) in this situation as basic trainers would. That is a plus for them.
However, the increasingly rapid descent in the turn usually causes the new pilot to input substantial up-elevator, causing the aircraft to rocket upward or tighten the radius of the turn. Learning to make level turns is the first step in becoming an RC pilot, and aerobatic trainers make that more difficult.
These second airplanes are designed to take a newly soloed pilot beyond basic flight and into the exciting world of aerobatic flying — not to be the best basic trainer.
Some scale aircraft would seem to be ideal basic trainers and do use flat-bottom airfoils, but they are better suited as second models. The Piper Cub, for instance, is extremely short-coupled; the tail is close to the wing, given its wing's large span and chord. Without perfectly set aileron differential, one aileron moves upward much farther than the other moves downward, and the Cub needs coordinated rudder/aileron input to make proper turns. Coordinated turns and aileron differential can be daunting for most new RC pilots and is seldom necessary in a basic trainer.
The Cub's short fuselage and narrow landing gear make ground handling troublesome for new pilots. Its light wing loading and large wingspan compound landing difficulties because the aircraft tends to land on the main gear and keep the tail up. This usually means that the wing drops to one side, pulling the aircraft off in that direction, or the model ends up on its back.
Yes, a new pilot can begin on a Cub or similar scale model, but learning is easier on a basic trainer that is designed expressly for teaching RC piloting.
Design Features of a Good Basic Trainer
A good basic trainer has several design features to ease the learning process. Most important is that the wing usually has a flat bottom and a curved, or airfoil-shaped, top section.
As far as I know, only one basic trainer — Hobbico's Hobbistar 60 — uses a semisymmetrical airfoil, and its wing is made overly large to compensate. Without delving into total eye-numbing detail about why an aircraft flies, this type of wing produces more lift than a similar symmetrical wing with equal airfoil shapes on both sides.
The air flowing over the curved top section of a flat-bottom wing must move faster to cover the longer, curved distance than the air flowing over the straight bottom. According to Daniel Bernoulli's theorem of gaseous density, the faster a given amount of gas moves, the less dense it must become if all other conditions remain the same.
If the air above the wing has reduced density, the pressure it exerts on the wing's top is lower. This means that there is a "low-pressure" area above the wing. The wing tends to move upward into the low-pressure area, taking the rest of the aircraft with it.
But then how does an aircraft with a fully symmetrical wing fly? Bernoulli is less relevant for that wing, but, fortunately for all sport and aerobatics pilots, Sir Isaac Newton remains in the house.
If the wing is pointed upward to the airflow even a few degrees — called a positive angle of attack — much of the air striking the wing's leading edge is redirected downward. By Newton's third law, for every action there is an equal and opposite reaction. Therefore, the wing is "pushed" upward, creating lift.
This is a gross oversimplification of why an aircraft flies. There is still much debate about this subject even after more than a century. However, this basic explanation serves to illustrate why a flat-bottom wing has more lift per square foot than a symmetrical airfoil.
Of course, Newton's law also affects the flat-bottom airfoil — maybe even to a greater degree than it does a fully symmetrical wing. But Bernoulli and Newton work together on a flat-bottom wing, generating that extra lift.
Extra lift means slower takeoff and landing speeds, lower stall speeds (the speed at which the wing stops producing lift and begins resembling an anvil), fewer bad habits such as snap stalls in tight turns, and generally an all-round more rewarding learning experience.
High Wing, Dihedral, and Stability
It helps if the wing is placed on top of the fuselage (a high-wing configuration) and has some positive dihedral; the wing is bent in the middle so that the wingtips are higher than the center section. The fuselage below the high wing imparts a slight pendulum effect — but every bit counts in a good trainer. The dihedral reinforces this pendulum effect.
Combining these two features helps to create an aircraft that will tend to stay in wings-level flight during straight flight and gentle maneuvers. They also help the pilot recover the aircraft from most turns. Fewer pilot corrections are therefore required in all of these flight regimes.
Dihedral has other effects, but some are not so beneficial. It allows the rudder to be used to bank and turn the aircraft better, but too much dihedral results in a model that requires continuous aileron input to remain in a banked attitude. This is bad because it teaches a new RC pilot bad habits. On the other hand, a reasonable amount of dihedral, such as the three inches on Hangar 9's Alpha 60 RTF, adds a great deal to a trainer's teaching abilities.
Power-Plant Selection
Power-plant selection is also important when studying a basic trainer. As far as I know, all RTF basic trainers that are powered by engines are two-stroke designs; these include Hobbico's NexSTAR. If the pilot prefers to use a four-stroke engine, he or she must select an ARF.
There are also a few electric-powered basic trainers. Some, such as Hobbico's SuperStar EP Select, are complete, four-channel RTFs. Others, such as Horizon Hobby's HobbyZone Firebird Commander, are basic, two-channel RTFs.
Engine-powered ARF basic trainers such as Midwest's Aero-Star 40 and Lanier's Explorer 40 offer the new RC pilot choices of radio system and engines that are unavailable in RTF models. However, this luxury comes at the cost of additional assembly work that could require basic model-building skills. RTFs require only that the new pilot be vaguely familiar with which end of the screwdriver points toward the work.
The amount and type of assembly work involved in ARFs and RTFs (it's hard to call preparing those models for flight "building") is all that separates the two. As you can see in photos, they look almost identical. And the two types do fly the same since there are few airframe-performance differences. There are performance differences, however, if the pilot equips an ARF with more powerful engines/motors and more capable radio systems than are usually found in an RTF.
With the exceptions of the Hobbico NexSTAR powered by the O.S. Max .46 FXG engine, the HobbiStar 60 Mk III using the O.S. Max .60 LA engine, and the Hangar 9 Alpha 60 equipped with the Evolution .61 engine, all RTF glow-powered aircraft currently use .40 cu. in. power plants. No matter how good a .40 might be, and all of today's engines are good, a hot .46 offers more performance.
Except for the Hangar 9 Extra Easy 2 with the five-channel JR XF-421 computer radio system, all RTF basic trainers use analog four-channel radio systems for control. The NexSTAR does have an installed flight-stabilization system, which is similar to an autopilot but without direction control, but its transmitter remains a good four-channel analog system.
Whether a pilot chooses an RTF or an ARF airframe as a basic trainer is his or her choice. Both offer excellent aircraft and performance. But as good as these aircraft are, there is always room for improvement and for pilot individuality.
Next month I'll build an RTF trainer and make a few easy improvements. Following that, I'll look at the ARF world and show you how to make those aircraft look different and perform better.
Until next time, you can review many of these aircraft on Sport Aviator, MA's online magazine, at www.masportaviator.com.
— Frank Granelli 24 Old Middletown Rd Rockaway, NJ 07866
Flat-Bottom Basic Trainer Wing Section
Air flows faster over the curved top section than it does along the flat bottom section.
Symmetrical Sport Wing Section
Air flows at equal speeds over both top and bottom sections.
Since air must travel a longer distance over the curved top of a flat-bottom wing in the same amount of time, it must move faster than the air below the wing. The faster the air moves, the lower the density.
Top and bottom, air moves the same speed over a symmetrical airfoil. Therefore, the wing must be pointed up to have any lift. Total lift is less than with a flat-bottom wing, but the symmetrical wing can fly equally well upright or inverted.
Transcribed from original scans by AI. Minor OCR errors may remain.
