Darren's Blog

Motors 101 Part 1

Motors 101

Questions to tackle for this series:

How do I choose a motor for my airplane?
How do I ensure I will not burn my motor up?
Are Brushless motors worth the extra cost?
What is the difference between Brushed and Brushless motors?
Can I just buy ONE motor and use it for all my airplanes?
Which motor is Best?

First, a few definitions common to all motors:

Brush: An electrical contacting point between the rotor and stator.
Rotor: Rotating magnet or coil portion of the motor.
Stator: Stationary portion of the motor.
Kv: Revolutions per Volt

How do I choose a motor for my Airplane?

With so many motors available, choosing the right motor can be daunting. In fact, several set-ups can work for the same airplane. For example, I have flown a GWS Formosa with brushed 300 and 400 motors and Brushless Mega 16/15/5, Himax 2025-4200, and Himax 2015-5400 motors. The key is to match power output with the weight and desired performance of the model. More power is not always better. The Mega was easily the most powerful, but the extra weight and vibration was very hard on the airframe. The Himax was enough for unlimited vertical, after that, more power became less useful.

So the first step is to answer a series of questions about your model:

1) How heavy is it and how do I want it to fly?

To fly as a trainer or basic sport plane with good climb performance and duration, 100W per pound is a popular rule of thumb. For a high performance (EDF, 3-D, Pylon racer, Fast unlimited vertical aerobatics) 150-200W per pound is a better goal. These Watts per pound are Ready to Fly. Be sure to include your battery and estimated motor weight in the calculation.

If you are not certain how much the motor will weigh, another rule of thumb (for 2005) is to add 2oz per 100W for a brushed motor or 1oz per 100W for a brushless motor. Then add 5oz per 100W for the battery. (Note on Li-Poly vs NiMH/NiCaD: as of today, power density is roughly equal so Li-Poly essentially buys more duration for a given weight, not more power. This may change as the weight of 20C Li-Poly cells decreases).

Once the target power is calculated, chose a motor that will comfortably supply the desired power. I chose motors rated 150-200% more than desired output. Larger motors will add too much weight, smaller ones may be damaged in hot weather, long runs, etc.

Example: I want to convert a Lanier Stinger .10 model to electric power. Because of the size, (36” wingspan) it should fly at approximately 2lbs. Since I want it to perform above average aerobatics, I’m going to shoot for 150W per pound, or 300W output.

For 300W, a few options for brushed motors would be:
Guapner Speed 500, Kyosho Lemans APL

Some Brushless options:
Mega 16/25/x, Aveox 27/13/x, Astro 020, Hacker B40, AXI 2814

Note the brushed motors weigh ~6oz and the BL motors come in between 3-4oz (the AXI outrunner being the heaviest at 4.6, but will not need a gearbox).

2) Outrunner, Direct Drive or Gearbox?

I will cover the specific advantages and disadvantages of gearboxes, DD and outrunners later, but nothing outweighs personal preference here. For brushed motors, most applications will benefit from a gearbox, though some speed applications can use Direct Drive and small propellers.

3) Which battery will I use?

So here we are to the Chicken or the Egg question. Do I choose a battery to match a motor or a motor to match a battery? For reasons I will discuss in Battery 101 section, for today 2005 technology, I’ll assume most want to use the popular 3 cell Lithium or 8-10 cell NiMH/NiCad sizes. If you chose higher voltages (4s+ or 12+ cells), you’ll want to go with lower Kv (higher turns) motors and smaller cells. If you chose the 6-7 cell or 2s Li-poly route, bigger cells and higher Kv (lower turns) motors.

4) Speed, Thrust, or balance?

Motor rotation can be converted into two types of power: Speed and Thrust. Pylon racers, flying wings, and Jets crave speed. 3D planes require maximum Thrust. Most planes benefit from a mix. It is a common myth that Direct Drive is for speed and gearboxes are for thrust. Not entirely true. A low Kv motor will spin a large, thrusty prop Direct Drive. A high Kv motor will turn a small prop very fast for high speed.

Going back to my Lanier example, suppose I have chosen the Mega 16/25/x family and 3s Lipos for power. I need 300W so I know I need ~30A draw on 3s lipos. For a racing set-up, I could go for a Mega 16/25/3 and a 5x5 prop. For a more balanced aerobat, I’d use a Mega 16/25/4 and a 9x6 prop; and for a 3D conversion, a 16/25/6 and a 12x6 – all direct drive.

5) Gearboxes

Gearboxes allow a greater flexibility using a single motor. If gearbox use is intended, it is usually best to pick the highest Kv motor available for safe use (most inrunners are safe to 50,000RPM+) because most applications will benefit from lower electrical resistance of lower Turn motors. Gears can then lower amp draw to allow larger prop use. For 3s Li-poly or 8-10 cell use, even 5000-5500 Kv motors can be properly geared to spin relatively large hover props.

With gearboxes, there are now too many variables for simple Rule of Thumb selections. Generally, to spin a big prop you will want a high gear ratio (6:1 or more) to lower the load on the motor and limit amp draw. For a speed, a lower (2:1) ratio will be more likely. But depending on the specific motor Kv and battery chosen, this may not be true.

Most manufacturers provide a chart showing several gear ratios, batteries, and props with a corresponding amp draw and power out. But some do not and many are incomplete. Here, a simulation program such as Motocalc or posting questions on RC Groups will help finalize the right ratio and prop if you are unsure.

It is always good to measure the amp draw of your set up, particularly when using a gearbox. Even if someone is using your exact set up, any binding in a gearbox or wobble in a shaft can dramatically increase amp draw endangering your motor, ESC, and battery.

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That covers motor selection. Next we will examine all the specifications used to keep your motor operating safe, risks of pushing specs, failure modes, and basic repair of common motors.

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