AstroFlight 020 Brushless Motor

Model Airplane News, Jul 1998 by Shaw, Keith

WHAT'S THIS? A review of a small motor written by the guy who's famous for trying to deplete the world's Ni-Cd supply? Well, if truth be told, I really like small airplanes. I have many fond memories of the dozens of l/A planes I designed and flew in the '60s and '70s. They had great performance, were inexpensive to build and easy to transport and store. Overall, they were the perfect solution for a college student. However, there are also less than pleasant memories of the gooey mess, shrieking noise and continuous mechanical failures of the powerplant.

There is currently (ouch!) a renewed worldwide interest in small aircraft, powered by little, inexpensive ferrite motors of the Speed 400-type. Except for some tiny, exotic pylon racers and a few multi-motor designs, though, the performances I've witnessed have been quite mediocre. Any attempt to extract higher power from these small ferrite motors dramatically shortens their life, sometimes down to less than a dozen flights! I find that the joy of flying electric aircraft is dampened if I need to fiddle constantly with a motor. Bob Boucher of AstroFlight* is a very clever man who has always understood the attraction of small planes. Along with his line of larger motors, AstroFlight's brushed 05 and 035 cobalt motors have provided reliable, efficient power for several decades now, enabling thousands of people to enjoy trouble-free, small electric planes. About the only drawback to these motors was that they were a little heavier than appropriate for the really small planes in the lM2A class. THE NEW KID ON THE BLOCK AstroFlight's newest motor, the brushless 020, is already revolutionizing the smallplane field. Even though it is smaller and lighter than the ubiquitous Speed 400, it is virtually maintenance free, much more efficient and more powerful by far. The motor is only 0.95 inch in diameter and 1.7 inches long and weighs a mere 2.3 ounces. It features dual ball bearings and a full IA-inch shaft, which is noticeably tougher than the 3/32 shaft common with other small motors. Since its design is brushless, a specially designed matching AstroFlight controller is required. It is about the size of a postage stamp, features two microprocessors and a BEC and is hard-wired to the motor to eliminate the weight and space of the connectors that would be required for the necessary eight connections (three power leads and five sensor leads). Two different windings are available: a 7 turn that's mostly for direct-drive applications and a hotter, 6-turn variant for pylon, ducted fan and gear reduction use. You can also choose from two controller options: the standard one is for use up to 15 or 16 amps and, for an additional $25, the enhanced version is for currents up to 25 amps. In addition to the direct-drive model, there are two geared versions available with either a classic offset spur design (3.3:1 ratio) or a more expensive coaxial planetary design (4.4:1 ratio).

Brushless motor designs have many inherent advantages. Since there are no brushes to replace, there is no maintenance to perform. A nice bonus is the lack of brush dust that eventually contaminates an otherwise clean airplane. Brushless motors are much more efficient due to the lack of mechanical brush drag and electrical losses at the brush/commutator interface. Efficiency is also helped by better cooling, as the windings are in direct thermal contact with the outside of the motor case. Remember, heat is the enemy of any motor. A minor but important point is the almost total lack of generated RF noise that can play havoc with the radio link.

For those people who enjoy "virtual modeling" by playing with motor performance simulation programs, all the motor constants are available at the AstroFlight website at http://www.astroflight.com, along with a vast amount of other good info and links. I must stress that these programs are based on an extremely simple motor model. They do not include factors like copper heating, brush drag, interface losses, hysteresis, or circulating currents. My tests regularly show the brushless 020 performing much better than the programs predict, while the small brushed ferrites come out worse than expected.

During extensive testing on both prototype and production units, I was impressed with the power of this little motor. It can easily handle 100 watts with the standard controller and nearly 200 watts with the beefed-up 12-FET controller, while demonstrating efficiency of 80 percent or better. On seven cells, the 7-turn standard motor will spin a Cox 6x3 or a Master Airscrew S.5x4.5 at better than 16,000rpm. The 7-turn geared (3.31:1) motor can spin even larger 8x7 and 9x7 props (see chart). The little BearKitty I demonstrated at the 1997 KRC has a 6-turn motor with the 3.3:1 Astro gearbox, a standard controller and seven SR Magnum 1250 cells. It turns a 9x7 prop at about 7,500rpm, which gives the 24-ounce, 200square-inch fighter almost unlimited vertical performance while regularly doing rigorous aerobatics for seven to eight minutes. The pylon setup (6T and enhanced controller) winds the CAM 4.7x4.7 prop at over 20,000rpm and turns the typical 65 to 70mph Speed 400 racer into a 120mph rocket. I have also witnessed stellar climb rates on converted hand-launch gliders equipped with the planetary geared 020 and a CAM lx8 folding prop. WHAT ABOUT EFFICIENCY? Judging the merits of a power system requires some careful thought. The first major point is efficiency, expressed as a percentage to quantify the amount of power really turning the prop compared to the electrical power being drawn from the battery pack. The input power is expressed in watts-the product of the battery voltage times the current drawn. So if a motor is 80 percent efficient, 80 watts of every 100 watts drawn from the pack turns the prop, while the remaining 20 watts ends up as generated heat. A higher-efficiency motor can turn a prop faster (or a bigger prop at the same rpm) than a lower-efficiency motor at the same input power. An example might be helpful. Most Speed 400-style motors are only about 50 percent efficient (although with careful break-in, timing advancement and prop selection, they may achieve 60 percent for a limited number of runs). For best longevity, the Speed 400 should stay below a maximum of 10 amps. The input power from a 7-cell pack is about 70 watts (assuming 1 volt/cell under load), but only 35 watts make it to the prop. An 80 percent motor such as the Astro brushless 020 would deliver 56 watts to the prop for the same 70W input. This is a 60-percent increase in real power for better aerobatics or higher climbs. Alternatively, one could reduce the throttle and get a 60 percent longer motor run at the same performance level as the 50percent-efficient motor.