| Meanwhile, many modelers have had fine success converting small, rubber-band-powered models to micro RC. A good selection of plans is available for scratch-builders, ranging from simple stick fuselage models to more elaborate scale projects. A good start for plan projects are the Model Airplane News downloads, and many others are available from rcstore.com. Meanwhile, a number of modelers have successfully resized plans from older magazines at photocopy stores that have this capability. |
Assuming you have selected a small flying model, let's go over some of the equipment options available and look at a few features and compatibility.
Receivers. Several types of receiver are currently being produced for the micro market. In general, all of them incorporate an electronic speed control (ESC) for small brushed motors. The ratings on these ESC circuits vary, so you need to make sure that the ESC can handle the motor selected. All of the current receivers are designed to operate from a single Li-poly cell (or equivalent voltage, if you're using Ni-Cd or NiMH batteries). At least one of the receivers, the Plantraco HXF900, operates on the 90OMHz frequency band, which eliminates all conflicts with equipment on lower frequencies (of course, you must use Plantraco's matching HFX900 transmitter). Several of the receivers are relatively wide band, and to function without interference, they require that adjacent channels not be in use. Others are narrowband and can operate safely in a crowded environment.
Most of the available receivers can drive actuators directly, while several have outputs that can drive servos. A few are capable of handling brushless motors' speed controls along with other features. The small flying model market is changing rapidly, so it pays to compare features and prices.
Actuators. In general, all of the actuators being produced for the micro market are designed along the same lines and operate in the same way. A coil is wired to a special driver in the receiver, in which a proportional voltage is generated depending on the transmitter-stick position. A magnet pivots within this coil and moves in response to the changing voltage. With the proper centering setup, the movement of the magnet is truly proportional to the transmitter stick. The primary difference in these actuators is their size. The smallest weigh only a few tenths of a gram and produce very light forces. Larger units can weigh several grams and operate the controls on larger or faster models.
Servos. Several microservos are currently available, with more likely to appear. Those in the current crop are very light (1.3 to 1.7 grams) and generate substantially more force than a magnetic actuator. Of course, they must be mated to a receiver with outputs intended for servos. As a result of the effort to keep the weight of your small flying model to a minimum, the gears are exposed, and this requires some special consideration in the installation and operation. The gears must be kept very clean, and no wires can be allowed to foul them. A small particle of balsa dust between the motor pinion and the spur gear can jam the servo completely; of course, a loose wire would have the same effect. If they're operating in a clean environment, these devices can be expected to provide many hours of trouble-free service.
Weight and Balance Key to Getting Off The Ground
Motors and gearboxes. Originally, the choice of drives was quite limited. The main commercial unit was the KP-00, which featured an M20LV motor geared about 2.7:1. This unit worked best with a small U-80 propeller and could provide useful thrust for models that weighed between 0.75 and 1 ounce. In a quest to increase the thrust, a number of us used the same motor but increased the gearing to as much as 6:1. This allowed modelers to use a larger propeller and provided a substantial increase in thrust with no additional drain on the battery. Meanwhile, smaller and lighter models began to appear, and now there are a variety of drives based on pager motors that do quite well. Because of our strong interest in aerobatic flying-especially 3D-micro brushless motors have also started to proliferate. Weighing only 5 grams, one of these motors can produce 60 grams of thrust on a single Li-poly cell! Of course, a special controller and receiver that can accommodate it are needed. Other lighter and less powerful micro brushless motors are also available.
Propellers. GWS is the main supplier of micro propellers, and they offer an extensive range of sizes. Meanwhile, there are the smaller U-80; several blades meant for rubber-band-powered models; a selection of carbon-fiber props; and for us "hardcore" modelers, carved wooden props. Follow the drive manufacturer's recommendation for the proper match (or the prop listed on the plan), and be prepared to experiment a bit for the best match in flight.
Batteries and chargers. This is a simple list. Li-poly cells are pretty much the basic source of power for all micro models. It becomes a matter of choosing the right capacity for your application, and the vendor or plan will help with that. Most dealers carry a single-cell Li-poly charger that is small and portable with adjustable settings. Never use a charger that's intended to charge a different type of cell! I taped up a pack of flashlight cells to power my charger, and this has worked very well, especially when traveling.
The Right Stuff
The primary material for homemade, scratch-built small flying models is balsa wood. The rub is the widely varying weight and quality of this precious material. Almost universally, micro models need a high-quality, light grade of balsa for success. Though it may be possible to find wood like this in a hobby shop, it is pretty unusual. I have resorted to shopping over the Internet with good success. By way of example, a sheet of [fraction one-thirty-second]-inch-thick, 3x36-inch balsa should weigh about 6 grams, while a similar sheet of [fraction one-sixteenth]-inch-thick balsa should weigh about 12 grams. This is in the range of 6 to 7 pounds per cubic foot.
Several sizes of music wire are very useful, as is an assortment of small carbon-fiber rods. If you plan to cover your small flying model with tissue paper, the light-grade Esaki brand works best. If plastic film is desired, RA Microlite is by far the most suitable. Delta Satin Spray Magic (found at craft stores) is an excellent wood primer, while Krylon Crystal Clear spray fills tissue nicely. Automobile spray touch-up paints are very useful for a color finish. I generally dilute these 50:50 with slow-drying auto paint thinner and apply them with a simple airbrush. CA adhesive is excellent for structure, especially the medium-grade applied sparingly with a fine tip. Carpenters' Wood Glue also works well on structures. Elmer's White Glue is most useful for attaching tissue to a frame. In all cases, use just enough adhesive to make the joint without excess buildup. Weight is an enemy of small flying models, and it takes a careful approach to learn to build light but strong.
My toolbox is fairly simple; it contains a craft knife with a really sharp blade, single-edge razor blades, scissors and several sanding blocks with 150- and 220-grade paper. I frequently use several pairs of small pliers and wire cutters as well as my Moto-Tool equipped with a cutoff wheel or sanding burrs. A set of miniature drills, nos. 61 to 80 (and a pin vise to turn them), will also be very useful. A soldering iron with a small tip often comes in mighty handy. I purchased a triple-beam scale from a discount house, and it has been quite helpful for keeping track of weights. As you progress, no doubt you will run across special-use tools that fit your building efforts nicely. I enjoy good tools, and a complete list of my toolbox's contents would be beyond the scope of this article. Just get what you need, and enjoy using it.