Reproduced with kind permission from
Charles Varvaro of www.lsss.homestead.com There are several types of motors to consider when powering our models. HIGH QUALITY BRUSH MOTORS have ball bearings and use cobalt, neodym, or other rare earth magnets that maintain their efficiency even when subject to high heat. They have adjustable timing, large brushes and commutators, and high quality springs. The purpose of these is to provide good contact for the brushes on the commutator at high rpm. These motors have good "efficiency" meaning that they can turn much of the electricity going into them into power to the propeller. BRUSHLESS MOTORS have all the feature of the high quality brush motors without the brushes. You can not use brush motor controllers on brushless motors. Brushless motors dissipate heat very efficiently through their entire cases. These are the most efficient motors. Some of the newer sensorless motors cost the same or less than the high quality brush motors. See Aveox for detailed information on how brushless motors work. MEDIUM QUALITY BRUSH MOTORS such as those used to power model race cars usually have ferrite magnets, replaceable brushes, adjustable timing, and ball bearings. Most operate at such high rpm that they can only be used with reduction gearing in model airplanes. "CAN MOTORS" are the least expensive and least efficient motors. Nevertheless they are more than adequate to power our models up to about 3 lbs. in weight. They usually have ferrite magnets the can lose efficiency with heat and time. These motors are sealed in a cylinder or "can" in which you can not change the timing or replace brushes. They generally do not have ball bearings. Note that I have heard great things about the Graupner 480 Speed motor sold by Hobby-Lobby and others. Some of our club members are using the 480L reverse direction motor as a pusher motor on their small wings. They report speeds in excess of 70 MPH. Hobby Lobby says these motors can propel model planes with up to 150 sq. in. wing areas at 84 MPH. This is when they are powered on seven cells using a 4.7 x 2.4 propeller.  | "can" motors and "can" motor with gearbox. |
ELECTRIC MOTOR TERMINOLOGY TIMING is the relationship between the position of the armature and motor magnets and the time that the electromagnets change their polarity inside the motor. It is generally changed by rotating the end bell of the motor which changes the position of the brushes relative to the commutator. By turning the end bell against the direction of the motor rotation the rpm will increase, but so will the current draw and heat the motor produces. In brushless, sensorless motors the electronic speed controller adjusts the timing automatically. Advancing the timing too much will make a motor less efficient. The trick is to find the right timing for maximum power and efficiency. Some motor manufacturers like Aveox and Astro Flght enclose timing directions with their motors. TURNS are the number of times wire has been wrapped around the motor armature. WINDS are the number of strands in the armature wire. More turns and winds mean a more efficient motor. The larger mass of winds and turns around the armature also means heavier weight, more mass to accelerate and lower rpm. Model racing car motors that need high rpm and fast acceleration use low turn and low wind motors. This is not a concern with most airplane models. MOTOR SIZES This can get confusing. Astro Flight, the pioneer in electric motors for model airplanes, started using terms like 05, 20, etc. to describe their motors. This was a way of comparing the power output of their motors with that of wet fueled model airplane engines. Graupner and others use terms like speed 500, 600, 700 etc. to describe their motors. The smaller the number, the smaller the motor. Many brushless motors have designations that use a combination of the number of motor turns and the dimension of the motor. The best thing to do is to look at the motor specifications. The HobbyLobby web site and catalog does a great job in describing motors and their potential uses. Selecting an efficient propeller is very important in getting the most out of an electric motor. Propellers lose efficiency when turning very high rpm. Reducing the speed of the prop by increasing pitch and diameter increases efficiency. The ratio of propeller pitch to diameter, P/D, needs to be above 0.8 for best efficiency. GEARING Using seven cells, speed 600 size motors typically turn 18,000 or more rpm without a load while drawing 3 or 4 Amps. Put an 8 x 4 prop on these motors and the rpm goes down to around 12,000 while Amps go up to 25 or more. These motors operate most efficiently at high rpm and do not have much torque to turn large, high pitch, more efficient propellers. The way to accomplish this is through gearing.  | gear boxes and pinior gear (notice large offset from center of motor to propeller shaft) |
Gearing allows our motors to turn high rpm at low amps while swinging efficient propellers. For example my Aveox 1406/3 speed 600 size motor on 7 cells will turn an 8 x 4 prop at 13,000 rpm for a prop speed of 50 MPH while drawing 28 Amps. If I use a 3:1 ratio gear box with an 11 x 11 prop the rpm goes down to 5,100 while the prop speed goes up to 53 MPH and the current goes way down to only 15 Amps. The end result is a cooler running, more efficient motor and almost double the flight times. If my model can take the extra weight I add extra battery cells to further increase motor performance. See the review of the Midway Models EMBAT for how this combination works.  | Planetary in-line gear boxes |
The drawbacks to gearing are that the gear box can block cooling air to the motor, add to the weight of the plane, and add to the length and sometimes circumference of the motor. The motor also has to be retimed to run in the opposite direction from the prop. If the plane has a narrow nose it will probably need a planetary in-line gear box. These gear boxes have little or no prop shaft offset, allow the motor to turn in the same direction as the propeller and are usually more expensive than standard gear boxes. Belt drives allow the motor and propeller to turn in the same direction, but can be large and heavy.  | Belt Drive gear box |
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