Why did inrunner motors go out of favor?
I remember 20 years ago when inrunner motors ruled the RC airplane world. AstroFlight and their cobalt geared motors dominated the electric flying field.
What happened? What was so bad about these motors that seeing one in an airplane today is a rare sight indeed?
Classic Inrunner Motors
Inrunner motors spin their central part that is wrapped with copper windings. The magnets are on the outer casing that is stationary. Since the electrical windings spin, conductive electrical brushes must be used to make the electrical connection. The brushes caused drag that lowered the efficiency of the motors. They were also a constant source of wear and tear.
The bigger problem with inrunners is that they have a relatively high motor constant (Kv) value. What this means is that they want to spin at high RPM with little torque. Before they can be used in the typical model airplane, you must convert some of those RPMs into torque that can turn a reasonably sized propeller.
The best way to do it is by attaching a gearbox to the front of the motor. The best gearboxes are the so-called planetary gearboxes. They are expensive, easily costing more than the motor itself. They are also heavy, usually adding about 50% of the motor’s weight to the airplane.
Even worse, if you have a bad landing, it is the gearbox that is going to get it. Again, the nice gearboxes are not cheap.
Modern Outrunner Motors
Outrunners still have the copper windings on the inside, but now the magnets on the outside casing spin around. This arrangement completely avoids the need for brushes, a major source of headaches in inrunners.
Since the larger diameter outside can is what spins around, outrunners naturally develop more torque than inrunners. For a given size and weight, outrunners have much higher torque. Put another way, their Kv values are much lower. They are not as good as an inrunner with a gearbox, but it is good enough.
Outrunners have another advantage. They are designed for their shafts to be easily replaceable. The overall quality of components has gone up over the years anyway. It has been a long time since I have bent the shaft of one of my motors. But it is nice to know that for a couple of bucks I can get a replacement.
Ducted fan airplanes are still a great choice to use an inrunner motor. You need high RPMs and there is no danger of bending the shaft.
An inrunner would still use brushes, which you might want to avoid. Different windings on outrunners can be used to increase their Kv values. I think an inrunner still has the edge in this application, but in the end it is up to you.
While I’m on the subject of outrunners, let me say a word or two about their connectors.
Outrunner motors, at least the ones that I buy, almost never come with connectors soldered on. They usually come with the bullet connectors in a plastic bag for me to put on. For some reason 4 mm bullet connectors are very popular. A lot of motors come with them so you can solder them on yourself. But not always. Sometimes they come with an oddball size. I always keep a ten pack of 4 mm bullet connectors handy just in case. I recommend you do the same.