Weight Power and Dimensions
The most important thing to keep in mind before choosing a motor is its weight and dimensions. We would all agree that extra weight added to a model to achieve the correct centre of gravity is undesirable. I personally would prefer to have a larger, heavier and more powerful motor than a smaller less powerful motor and a lump of lead in my models. Sometimes there is no choice but to use lead, but just don’t forget about the relationship between the weight of your motor and the centre of gravity of your model. The dimensions of a motor are obviously important. Don’t buy it if it won’t fit in your model.
You will want a level of performance suitable for the type of model you are powering. A 3D model will need thrust greater than 1:1, and a scale WW1 biplane will need considerably less. Here is a table giving performance in Watts per pound. Remember that if you are running your motor above its maximum efficiency the Watts per pound rule won’t be accurate, as a higher percentage of the Watts going into the motor will be producing heat instead of power.
70-90 watts/lb. Trainer and slow flying aerobatic models.
90-110 watts/lb. Sport aerobatic and fast flying scale models.
110-130 watts/lb. Advanced aerobatic and high speed models
130-150 watts/lb. Lightly loaded 3D models and ducted fans.
150-200+ watts/lb. Unlimited performance 3D models.
Motors: Should you get an Inrunner or an outrunner?
Now you have an idea of the weight and power you will need for your model, what sort of motor is best, an inrunner or outrunner? Both have their pros and cons.
Inrunners are constructed with the magnets attached directly to the shaft, which is surrounded by the copper windings. Because the magnets are close to the shaft it spins very quickly. This means they produce high rpm but low torque. This high rpm can be converted into torque by using a gearbox (see the section below on gearboxes).
Inrunners are more efficient and powerful, but need a gearbox to drive large propellers. They produce high revs per volt (Kv) compared to outrunners. For models requiring a small prop running at high speed like a Zagi (wing), pylon racers and ducted fans, inrunners without gearboxes are popular. Once a gearbox is used there are even more pros and cons. Gearboxes are an extra expense, require maintenance and can be noisy, but you will still get the best efficiency and power with a geared inrunner spinning a large prop. This is the reason why all competitive F5b models still use geared inrunners.
Outrunners are constructed with the copper windings on the inside. The shaft is attached to a “bell”, or casing that contains the magnets, which spin around the copper windings. Because the extra weight of the bell and magnets are further out from the shaft it acts like a flywheel. Generally outrunners produce lower RPM at higher torque than inrunners due to the way they are made. This enables an outrunner to spin a larger prop without a gearbox.
This means no maintenance, quieter operation and cheaper purchase price (no gearbox). These factors outweigh the higher efficiency and power of the inrunner for most sport flyer's.
kv is simply the revolutions per minute (rpm) an electric motor will spin at per per volt when under no load. You could think of high and low kv like the difference between a high performance 2 stroke racing motorcycle engine compared to a 4 stroke Harley Davidson motorcycle engine. Just say they put out approximately the same horsepower, but the 2 stroke does it at 11,000rpm and the 4 stroke does it at 3,000 rpm. The same can usually be said for high and low kv electric motors. Assuming the same voltage, a high kv inrunner with a small diameter propeller would be perfect for a high speed model like a pylon racer, and a low kv motor with a large diameter prop will be better for power i.e. getting a sailplane to altitude, or 3D manoeuvres like prop hanging. Kv is determined by the number of winds or turns. This is the amount of times the copper wire has been wound around each stator pole. More winds = low kv, less winds = high kv.
kv has two main implications.
A high kv motor will spin faster than a low kv motor at the same voltage. This means you may choose to use a high kv motor if you are limited to a lower voltage battery pack. An example of this would be in 7 cell glider competition (7 NiMh or NiCd cells at 1.2 volts a cell = 8.4 volts). A lower kv motor could not produce enough rpm at 8.4 volts to be competitive, so a higher kv motor is used.
If you are not limited to a particular voltage, a lower kv motor can be used at higher rpm by using a higher voltage. Large outrunners with a kv of 200 to 300 are a good example of low kv high voltage motors. Make sure you consider the voltage limit for any motor you are considering.
Choosing an inrunner and a gearbox isn't as complicated as it sounds, it is basically the same as choosing an outrunner but you add the downshift of the gearbox to the calculation.
When choosing an inrunner you usually have two sizes to choose from. I'll use Feigao for this example, Feigao have their motors listed on their site (http://www.feigao.com/sdp/85838/4/main-102731.html) with a complete set of numbers on every motor and they even have suggestions for different set-ups. Feigao call their smaller diameter motors (27.6mm) "380" and their larger ones (36mm) "540". Both come in three different sizes, Small, Large and XLarge. These are copies of Hacker inrunners, "380" for the B40 size and "540" for the B50 size which makes it very easy to find an successful set-up to copy if you search the net. The ratios are the same for both Hacker and Feigao gearboxes 4.1:1 for "380" size and 6.7:1 for "540" size.
But what does that 6.7:1 mean ? To get the kv of the prop shaft with a gearbox, simply divide the kv of the motor by the ratio of the gearbox. If we take the FG540-07S as an example, it's a 5070kv motor but using a gearbox with a ratio of 6.7:1 you get a kv of 5070/6.7 = 757. This would fit a 3S setup for a hotliner perfectly. If you want to use it to it's max amp rating (93A) check that your Lipos are up to it (Check C Rating in this Guide). Personally I go a little easier on the Feigao motor/gearbox combinations when copying a Hacker set-up, as Hacker are are a higher quality product.