Getting Started in Radio Control Electric Aircraft
One of the fastest growing areas of radio control in recent years is electric aircraft. Many factors have contributed to this growth including quickly advancing battery technology and construction techniques, coupled with a shortage of good flying sites for larger, noisier aircraft, such as those that are glow or gas powered. Electrics are quiet. They produce no exhaust fumes, and can be quite small and light—some small enough, in fact, that they can be flown indoors in a large gymnasium or hangar.
An improvement in motor technology has also provided greater interest in the field. Motors have gotten smaller, more powerful, with better magnets, and now there are brushless motors with less friction, less wear, and less maintenance.
There are several components to electric flight and we will touch on each briefly.
Although just about any model aircraft can be electric powered, performance will be far superior if the model is designed specifically for the purpose. The key factor for a good performing electric model is lightness—the airframe must be strong, but more importantly light. Electric models can be of almost any size, but the most popular are in the small to medium size range. The smaller models may be only a couple of feet in wingspan, weigh merely ounces, and have only a stick for a fuselage.
For a model to be classified as an “Indoor Flyer”, it should be extremely light (ounces, in the single digits!), fairly small wingspan, be easily controlled, and fly very slowly. There are some exceptions where the model may fly faster, but it must be extremely agile and quick on the controls to contain its movement in the confined space—not for the faint of heart! Indoor models are meant to be flown indoors in such facilities as a large gymnasium, an arena, or a hangar. They may also be flown outside, however, they should be restricted to almost calm conditions. Their slowness in flight also means their wind penetration is almost nil.
The next step up is the “Park Flyer”. These models are heavier and slightly faster than their indoor counterparts, penetrating wind slightly better (not very strong winds, mind you), but are still able to fly in a reasonably confined area such as a park or large yard. This is a very popular category of electric model and currently accounts for almost half of the selection available.
There is another term used to describe light electric models and that is the “Slow Flyer”. Different people use this term in different ways, however, it usually refers to both Indoor and Park Flyers. Some consider it a category between indoor and park, where it is slightly larger and heavier than a standard Indoor model, but can still be flown in a reasonably large venue indoors or outdoors.
There are three basic types of power systems used in electric flight. These are: direct drive, where the propeller is fastened directly to the motor drive shaft; gear drive, where the prop is connected to the motor through a series of gears to reduce the speed of the prop compared to the motor speed; and ducted fan, where a multi-blade fan unit is mounted directly to the motor and operates inside a shroud or duct. Ducted fan systems, although less efficient than prop systems, are ideal for jet-style projects.
Direct drive systems use smaller propellers rotating at the motor speed and are suitable for models that fly faster. Gear drive systems turn the prop slower, allowing for larger props and suitable for slower flying models. The motors that drive these systems vary widely in size and power and newer technology is offering some interesting design changes.
The standard electric motor is that developed by Mabuchi, one of the worlds largest electric motor manufacturers. These motors carry designations that are based on the physical size of the motor—the length of the motor can in millimeters, times ten. Two of the most popular sizes are 38mm and 54mm which are designated 380 and 540, respectively. Often, these get rounded up and get prefaced with “SPEED”, a name coined by Graupner, one of Europe’s most prominent model manufacturers. The “380” motor would then be referred to as a “SPEED 400”.
Astro Flight makes cobalt motors especially for models using rare earth magnets. These motors have a different designation which is based on the size of a glow engine of equivalent power. Their Cobalt 05 would be, in theory, equivalent to a .05 glow engine, or a typical ½A. This is only an approximation and should be taken with a “grain of salt.” Other variations on electric motors include “brushless motors” and “External Rotary Brushless Motors”.
Brushless motors have windings that remain fixed (so the wires that send power to them can be soldered directly) and the magnets rotate with the shaft. The disadvantage of these motors is that a special controller is required to supplied pulsed current to the windings to provide the changing induction and thus motion. Early versions also required “sensors” to sense the position of the armature to aid in supplying the proper pulse. Newer versions are able to accomplish this without the additional sensor and are called “sensorless” brushless motors.
External Rotary Brushless Motors are unique in that the entire outer can rotates with the shaft. The windings are mounted on a sleeve to a backplate, which is essentially a motor mount. The drive shaft fits into the sleeve, supported by bearings and is fixed, along with the magnets, to the outer can.
Although brushless motors require a more complex controller, they have the great advantage of having no brushes to wear out, meaning less maintenance, and a lower friction. Dirty brushes and poor contact is never an issue.
Batteries have come along way in the past number of years and continue to evolve, almost monthly. Capacity per size/weight unit is always increasing, meaning you can get either greater performance or longer run times for the same size pack as you may have used in the past.
Your battery pack supplies the power that runs your motor. Battery packs are made up of cells of batteries and come in a wide range of sizes and capacities. The voltage of a battery pack depends on the number of cells and the type of cell. You now commonly find battery packs with Nickel Cadmium (NiCd) cells, Nickel Metal Hydride (NiMH) cells, or Lithium Polymer (Li-Po). All of these cells are rechargeable and have a reasonably long lifetime of many charge/discharge cycles.
NiCd batteries were developed by Sanyo in 1963, have been in use with model aircraft the longest and have evolved greatly since first introduced. Capacities and charge rates have improved dramatically and the cells are available in a wide variety of sizes. Each NiCd cell has a nominal voltage of 1.2 volts and is generally considered discharged when reduced to 1.1 volts. Some variations of NiCds have the ability to deliver quite high current rates and, can be recharged at a high rate. Generally, NiCd batteries can be charged and discharged over 500 times.
NiMH batteries were developed in 1990 and are very similar to the more common NiCd. A Nickel Metal Hydride cell has a voltage of 1.2V and cell shapes and sizes are similar to NiCds. One of the major differences between NiCds and NiMH cells is that for a comparable size cell, the NiMH will have a higher capacity (double in some cases) and be a bit lighter, but cannot be charged or discharged at as high a rate due to a higher internal resistance. This is of concern in high-demand current applications. NiMH packs cannot deliver as much current and would be better suited to models where duration and light weight are more of a factor than power and speed (the trade off between performance and duration will be discussed later). NiMH, like NiCd cells, can be charged and discharged over 500 times.
Li-Po batteries are the most recent addition to electric flight and are quite different from the previously mentioned types. Their claim to fame is high capacity, light weight. Li-Po have 3.6 volts per cell with a high energy density, minimizing battery size and weight. They are also not packaged similarly to the NiCd or NiMH, but are typically square and flat. The newer versions of these polymer based cells are also capable of over 500 charge/discharge cycles.
In most cases, the battery pack that powers your motor, also powers the radio components mounted in the aircraft. This saves the weight of a separate pack. The on-board electronic components have a function called BEC (battery eliminator circuit) that ensures the proper voltage to the receiver and servos from the battery pack.
Obviously, you would not want the battery pack to become so fully drained from powering the motor that the receiver and servos would no longer be able to function. Protection to prevent this from happening is provided through an “auto cut-off” feature in the speed control. Once the voltage level of the battery pack is reduced to a certain level, this function will turn the motor off leaving enough battery power to keep on-board electronics functioning until the plane can be glided in for a landing.
Performance vs Duration
This would be a good time to discuss the trade-off between performance and duration. Any given battery contains a certain amount of electrical energy or power. You can either take a lot of power out of a battery for a short period of time or you can take a small amount of power out for a longer period of time. This is the trade-off between performance and duration. If you want to go fast and have all kinds of climbing power you are going to do it for a short period of time. If you want to have long flights, you are going to have to preserve power. You cannot have your cake and eat it too!
Of course you can also compromise where you can get some moderately hot performances for a medium length of time. In any case, the goal is to have the most efficient setup possible with the smallest amount of wasted energy so that almost everything goes towards your flight performance.
Given that all batteries used to power electric aircraft are rechargable, a charger will be necessary for continued enjoyment of the hobby. If life was simple, there would be one charger that will fast charge any type of pack quickly and without supervision, have all the features you need including reporting each pack’s capacity, and sell for less than $20 . . . . alas, life is not simple. There are a myriad of chargers available, each with their own set of features in ranges from simple to complex, from cheap to expensive. The charger you need will depend on many things—the type of cells you are charging, the number of cells in the pack, the capacity of the pack, whether you want a discharge or cycling function, whether you are charging in the field or from an AC source, and not least of which, your budget.
Some chargers can only be used where there is 110VAC power available. These are called AC chargers. Others are powered by only a 12VDC source and these are called DC chargers. Some have the capability to operate from both power sources and are called AC/DC chargers.
DC chargers are ideal for keeping you flying at the field but will be of little help at home to maintain your batteries if you don’t have a 12VDC source there. Likewise, AC chargers are great for home use but will do you little good while out at the field. AC/DC units are good choices if you only want to buy one charger! Some chargers can only charge up to a certain number of cells while using 12VDC as the power source.
There are three types of chargers you can buy and many variations on each. These are a fixed rate charger, a timed charger, and a peak charger. The fixed rate is usually AC powered, you plug it in the wall, plug your battery into it, and it charges at a fixed current rate. They have their place when charging at low rates, however, caution should be taken as these fixed rate chargers can overcharge your battery pack. Most have no provision for reducing the charging current to trickle once fully charged and your pack can be damaged.
Timed chargers are usually basic high-rate chargers. They are cheap and not very sophisticated in design. They may be powered by either AC or DC or both. You simply connect the charger to the power source, attach your battery, and set the timer (usually 15 to 20 minutes). The charge current is supplied to the battery until the timer runs out at which time it simply shuts off. These chargers are not the best solution in most situations. The main thing they have going for them is that they are generally inexpensive. If your battery is not fully discharged before charging with these timed chargers, it can be overcharged, damaging the battery. Also, you are never sure that your battery gets a full charge.
Peak chargers are the ideal solution, but again there are many variations. Usually peak chargers are designed for a specific type of battery, i.e.. NiCd, NiMH, or Li-Po as the operation has to be different for each. Some chargers are designed to handle more than one type. Taking NiCd cells as an example, the charger monitors the voltage of the pack while the charge is taking place. When a NiCd is fully charged it will stop increasing in voltage and will actually drop back in voltage slightly. The charger detects this drop in voltage and automatically shuts off or reduces to a trickle charge rate. This is called a “Peak Detection Charger”.
Peak chargers have many benefits: They are simple to use—usually just connect the power source and battery and push a button. They will not overcharge your battery pack. They always give your battery the maximum charge possible. Peak chargers cost a bit more but they are very much worth it! They are simpler, safer, and will generally save you the difference in cost in ruined battery packs from overcharging with the other types of high-rate chargers.
When choosing a charger there are many things you must consider. Some of these include: What power source do you have—AC or DC? What types of cells do you have? How many cells are in the packs you are charging? How fast do you need these packs charged?
When using anything other than an overnight or trickle charger, ensure that the charger is being attended should something go wrong. Highrate chargers always have the potential for overheating a pack, or worse, and someone should always be around to intervene if a problem arises.
Some electric aircraft can use the same radio equipment normally used for combustion powered aircraft while others require equipment with smaller components. Receivers and servos are now available in incredibly small sizes and this has contributed to the popularity of indoor electric flying where size and weight are critical.
The main airborne components of a standard radio system are the receiver, a servo for each control function, a battery pack, and a switch harness to control the power going to the components. For electric models, an electronic speed control is necessary and replaces one servo to control the throttle of the motor. As mentioned earlier, a Battery Eliminator Circuit (BEC) is usually used so that power can be obtained from the battery powering the motor, eliminating the need for a separate pack to power the radio.
The size of the aircraft and the power required to execute a specific function will determine the size of the servos required—larger servos have more torque. The same holds true to the speed control—the bigger the motor and the more current it will require, the larger the speed control will have to be both in size and in its current carrying capability.
Receivers will vary in size in relation to the number of channels (functions) they can handle—7-channel receivers are generally larger than 4-channel units. Also, some receivers have been made smaller by having limited range. Indoor models do not require long range capability as the model will rarely get more than a couple hundred feet from the person flying it. Receivers used strictly for this purpose can be smaller, lighter and still operate perfectly at that distance. It should be noted, however, that these receivers should not be used outdoors where the model flies at any distance, and they are definitely not to be used with combustion powered aircraft as range can quickly be lost endangering both the aircraft and people and property in the vicinity.
Electronic speed controls also come in a variety of sizes and power capabilities. Some are so tiny that they appear to be merely an “interruption” in the wire going between the receiver and the motor! These would generally be suitable for the smallest and lightest of models, requiring minimal power. The greater the power handling capability of the motor controller, usually the larger it will be physically.
Speed controls may have many added features including: built-in BEC (as described earlier); auto-cutoff (also described earlier); safe power-on arming that ensures motor will not accidently turn on; softstart; auto-shut-down on loss of signal; and others. Brushless motors require different speed controls than brushed motors.