Electric Conversion of Large RC Models

I have been in and out of the RC hobby several times in the past thirty years, most of my previous encounters being with glow-powered model airplanes in the .40 to .60 size range. My current return to the hobby began a couple of years ago, and differed from previous experiences in that I decided to concentrate exclusively on electric aircraft. I started with small foamies, but soon missed the more significant feel of larger aircraft and began to focus on larger models. Most of the airplanes designed for electric flight are still relatively small, that is one to two pounds gross weight. But conversion of large models intended for glow power is now not only technically feasible, but almost a trivial effort. In fact, given modern battery and motor technology, the viable size of the model is limited only by the number of batteries you can afford. My current collection of electric conversions includes several .20 size airplanes (e.g., a Model Tech Cub), a couple of .40 size models (a Hangar 9 T-34 and a Hangar 9 Twist), and my most recent addition of a .60-size Hangar 9 P-51, which I expect to fly this spring.

In this article, I will discuss a few key issues in the selection and conversion of a glow model to electric. There are a number of articles in the current model literature that offer more detail. This discussion is limited to the bare essentials necessary to convert a model and, as a member of the club, I offer more focused assistance to anyone who wants to tackle a conversion.

Although it is obvious, when you think about it, the conversion only involves the power system. The rest of an electric aircraft is virtually identical to a glow-powered model. (Granted, electric motors don’t vibrate and the airframes can thus be built significantly lighter, but that’s generally irrelevant when converting an ARF.) So we need to discuss four things: the battery, the motor, the speed control, and the propeller. Everything else is the same. Assembly of an electric system is perhaps a bit trickier than selection of a glow system because the battery, the motor, the propeller influence each other significantly and must be selected as a compatible set. But let’s start with batteries and we’ll worry about the matching a bit later.

First, let me acknowledge my distinct preference for lithium polymer (LiPo) batteries. While I have heard the horror stories of “venting with flames,” I have been using them for a year now and have never had a problem. Using the AstroFlight 109 LiPo charger, I have never even had a battery get warm during the charging operation. (Of course, I still charge them in a fireproof container and watch them pretty closely.) While I acknowledge the potential danger of this technology, LiPos in my basement seem less a risk than residual gasoline in a fuel tank. And the fact that they pack nearly three times the energy per unit weight as the closest alternative battery makes the choice of LiPos in large conversion projects a no-brainer for me.

The troublesome issue in electric conversion has always been weight. Although LiPo technology significantly diminishes this concern, it remains a significant design consideration. Most large electric motors have a weight comparable to their glow counterparts. The Model Motors AXI 2820 series, comparable to .20 size engines, weighs about 6 ounces. The AXI 4130 series, roughly comparable to something in the .60 to .90 (two-cycle) class, weighs about 14 ounces. The batteries are somewhat heavier than fuel, however. I’ll use a typical Kokam 2000 MaH cell for the following illustrations. Assembled into a pack, this cell weighs about 2 ounces. I generally like to power my aircraft to give me a comfortable 10-minute flight (of typical flying with 3 minutes reserve). This requires about three cells for each .10 cubic inches of equivalent glow displacement. Thus, a .20-sized aircraft would need six cells, a .40 size would need twelve cells, and a .60 size would need 18 cells. This is, of course, a starting point; my 3D-capable Hangar 9 Twist only has eight cells, but then I only fly it seven or eight minutes. But, as a rule of thumb, this suggests that the batteries for a .60-sized airplane will weigh about 36 ounces, or about 24 ounces more than the fuel for the comparable glow engine.

A few things are worth noting about LiPos. First, a cell is a single 3.7-volt LiPo cell. Rarely do I work with cells, but rather packs of two or three cells that I connect together to form an even larger battery. Connecting them in series (+ to -) gives more voltage; connecting in parallel (+ to +) gives more capacity. The cells must be connected in the proper series-parallel configuration to match the requirements of the motor and propeller. Two sets of three-serial packs connected in parallel would be denoted 3S2P. Individual cells are rated in terms of the milliamp hours (just like NiCad flight packs). They are also rated in terms of their discharge rate, expressed as a C-multiple. C for a 2,000 MaH pack is 2 (I’ll use the ampere scale for discharge in this article). A cell with a discharge rating of 10C is capable of providing 20 amperes. One with 20C is capable of providing 40 amperes. Wiring two cells, each capable of 40 amps, in parallel results in a battery capable of 80 amps. Cells with higher discharge ratings tend to be heavier, because of their construction. I generally design my systems to discharge at 6C or less. A 6C discharge will result in a 10-minute flight (with no reserve). Higher discharge rates are useful for burst applications, such as motor gliders. The cells in my collection have discharge ratings from 8C to 15C. (Note that the cells in a battery pack must all have the same capacity and discharge rating.)

A little multiplication reveals that an electric model will weigh about 20% more than a comparable glow model. It is generally wise to keep the wing loading of models of this size under 30 ounces per square foot of wing area, with the upper limit being about 32-34. This suggests that the glow version of the model you are planning to convert should probably have a wing loading under about 26 ounces per square foot.

While glow power is usually described in terms of engine displacement, electric power is described in terms of watts. Wattage has a direct correspondence to horsepower, one horsepower being equivalent to 746 watts. While it is the wattage output to the propeller that is important, inputs watts are much easier to measure. It depends on the efficiency of the power system, but output is generally about 75% of input. Input is calculated as the battery voltage times the applied current in amps. The general rule of thumb for motor power is that input wattage should be between 50 and 100 watts per pound. A power system drawing 50 watts per pound will result in a rather docile aircraft with a long, scale-like takeoff. My Cub and T-34 conversions are in this class. My experience with aircraft in the 100-watt/pound range is that they tend to jump off the ground after a takeoff run of between two to five feet. The Twist certainly does this; I'm not quite sure what the P-51 will do.

The majority of my electric motors are Model Motors AXI outrunner motors. These are neither the best nor the cheapest electric motors available, but I have found them a good value and have generally had good experience with them. Outrunner motors are a bit unusual in that, while the face of the motor (which is fastened to the mount) remains stationary, the rest of the motor rotates. While I initially found this a bit disconcerting, I got used to it and find I really like the silent power these gearless, high-torque motors provide, spinning their relatively large propellers. Selecting an electric motor is a bit more intimidating than selecting a glow engine, though. The “problem” comes from the fact that electric motors are very versatile in the amounts of power they can supply. While more fuel and a bigger propeller don’t necessarily increase the output of a glow engine, more batteries and a bigger propeller almost always result in more power from an electric motor. Of course, too many batteries and too large a propeller result in less efficiency, which generates more heat, and may melt the motor. Fortunately, two useful computer programs, MotoCalc and ElectriCalc, help to avoid this situation. These programs take as input data the size and shape of the airplane and its desired use and duration to suggest motors and batteries. While I rarely take its recommended systems, I almost always use MotoCalc to evaluate several of my own alternatives. It can tell me, for a selected motor and propeller size, how many cells I will need for my desired performance and duration. As a bonus, it also makes predictions regarding how the airplane will fly.

Between the battery and motor, you need an electronic speed control (ESC). The ESC switches the power on and off very quickly and gives the effect of unlimited variations in speed between a few RPMs and the maximum RPM the system is capable of. (While a variable resistor would adjust speed by wasting unwanted voltage as heat, the modern ESC offers a much more efficient switching technology and produces very little heat.) The ESC plugs into the throttle slot of the receiver and, from that point, behaves much like a conventional throttle. Except for a couple of other features.

The AXI motors I mentioned are brushless motors. What this means is that they have no commutator and thus no brushes to make sparks, carbon dust, or electrical noise. They are, in fact, little three-phase motors, technically very similar to the three-phase motors used in much larger industrial applications. The ESCs for brushless motors convert the direct current from the batteries to three-phase electric current of the frequency necessary to achieve the desired speed of the motor. This is actually quite a miraculous conversion, but easily available to us for the price of an ESC.

The second feature often found on ESCs is a battery eliminator circuit (BEC). The BEC converts the higher voltage of the motor battery to 5 volts for use by the receiver. This offers several advantages including no separate receiver pack to carry, no separate pack to charge (or forget to charge), and a virtual guarantee that the motor will quit before the receiver does. There is a potential trouble spot in the current BEC technology, however, that problem being the fact that the conversion is typically passive. This means that the difference in voltage is dropped through resistance, which turns electrical energy into heat. If the difference is too great, too much heat will be generated and the BEC (and the ESC) will burn. ESCs can often be used with more voltage than can their associated BECs. Be very careful to observe the limitations so as not to destroy the BEC and the associated equipment (the airplane, for example).

There is a way around the BEC voltage limitation and this is by the use of a Universal BEC (UBEC). The UBEC is a switching power supply that can convert higher voltages to 5 volts efficiently with little generation of heat. I use UBECs in all of my models of .40 size and greater.

That pretty well covers the electrical aspects of conversion. The remaining mechanical issues are balance and prop clearance. Electric motors usually turn slower than comparable glow motors and thus use longer propellers. In selecting a propeller, it is important to maintain adequate ground clearance. I generally prefer at least 2”, but have occasionally settled for 1-1/2”. Figuring the ground clearance prior to purchasing the airplane can be troublesome, since the distance from the propeller shaft to the ground can usually be determined only by putting the parts together and measuring it. I generally find it useful to buy the airplane first and design the power system after I have weighed and measured the parts in the kit. If ground clearance is an issue, methods of dealing with it start with the selection of a power system that turns a shorter propeller faster, maybe considering a three-bladed propeller, and possibly even changing the tires or modifying the landing gear.

Regarding balance, my only real advice is to plan to achieve balance through the placement of the batteries. Think of the batteries as a two-pound balancing resource. Generally, having to add weight beyond the batteries to an electric airplane indicates a lack of planning. But also keep access in mind when placing the batteries. I generally remove my batteries through the hole left when I remove the wing. Some modelers cut hatches, but I usually find the wing hole much more convenient and less damaging to the integrity of the airframe. Keep in mind that you will remove the batteries after every flight. We are admonished not to charge LiPos in the airplane, citing potential fire danger. My more practical motivation is that I want to get the dead pack out so I can put a live one in and resume flying.

I think that covers the essentials of electric conversion. But I’m sure I’ve left out plenty of details. If you need more information, you can generally find me at the club meetings and I love to answer questions.

Electric Conversion of Large RC Models

C. David Vale Princeton, New Jersey

C. David Vale
Princeton, New Jersey