Why did inrunner motors go out of favor?
Vintage Electrics
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 Fans
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.
Outrunner Connectors
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.
Related posts:
Figuring Out Unknown Motor Specs
Measuring Motor Constants: Introduction
Measuring Motor Constants: Rm and Kv Method 2
Measuring Motor Constants: Voltage Constant (Kv)
Simplified Motor and Propeller Selection Method
Motor Voltage Constant Kv
Measuring Motor Constants: No Load Current (I0)
Measuring Motor Constants: Power Factor
Measuring Motor Constants: Tools
Matching a Propeller to a Motor
Motor I0 and Rm Constants
Measuring Motor Constants: Winding Resistance (Rm)
RT @creyes123: Inrunner vs. Outrunner Motors: Why did inrunner motors go out of favor? #rc http://t.co/Lmaj1oCK
Geared in runner fan here! I was in the game when the geared Astro cobalt was ‘it’ – still have one! – and Tom Cimato’s MaxCim geared sensored inrunners were the new ‘kid on the block’ for bigger, high powered sports. When what brushless were around had mostly two speeds – Stop and Flat Out – MaxCims had speed control that is still better than anything around today. The other way MaxCim scored was using MEC gearboxes. Gear ratio swaps involved a pinion that cost little and a couple of hex keys – a cheap but effective way to optimise a motor to a prop and model. But that’s still far more complex than today’s game where folk buy an entire model and bolt it together in minutes…
Indeed. I still have that fancy Astro inrunner from who knows how many years ago. I have never flown it, either. I could probably buy 20 outrunners for what I paid for it.
I remember the MaxCims, but don’t know if I ever owned one.
You are mixing brushed and brushless motors here. There are brushless in-runners as well. They put the windings on the can and magnets on the rotor. They have the same properties as brushed inrunners of high Kv, but they are far more efficient and powerful than a normal brushed motor. They are a good choice for RC cars/trucks where there pretty much has to be gearing.
You could also make a brushed out-runner. In fact, some physics experiment kits that have you build a simple electric motor use that design with a coil of wire rotating around a bar magnet.
You are right. In fact, I believe the inrunner that I’m holding in the video is a brushless inrunner. A sensored brushless inrunner, at that. But it still has a high Kv, which is really the point that I was trying to make. I wanted to keep the presentation as simple as possible, which is why I did not mention it. Thanks for helping keep me honest.
I have to say, I had never heard of a brushed outrunner. I suppose it makes sense.
You’d need to do some digging outside of our hobby to learn where and why it originated, but the outrunner, wherever it came from, got to us after the inrunner in brushless circles.
Gearboxes – Tom Cimato went with the open framed MEC box to allow his two motors to use a wide variety of gear ratios. They were not as ‘streamlined’ as epicyclic ‘boxes, but were much simpler to fit, mount and swap ratios. An epicyclic box gives you one ratio, and you’re stuck with it!
It also needs far more skill, effort and, sometimes, tools to even lubricate it. That they tend to be noisier than the old MEC boxes is almost secondary.
I had two MaxCim motors and gearboxes. Their only problem was that the motors and ESCs were ‘sensored’ – you had to allow for a five wire connection between motor and ESC, albeit thin wires, plus the usual power feed cables. The plus side of that was smooth, consistent speed control across the full speed range that no modern motor can come close to. Getting that control harness into place was no real issue, as the motors were powerful enough to fly large models very ‘adequately’ and thus there was always plenty of space around the motor.
Unfortunately, they went ‘missing’ during a house move, as stuff does. They could still hold their own to this day. I also had an Aveox direct drive inrunner with sensored ESC – their first departure from motors aimed at competition electric duration flying, where the motor only really needs to be off or at full power. It too was powerful, with excellent speed control, though it could only turn a relatively small propellor.
Memories! Thank you for the walk down memory lane.
Aveox left the hobby market years ago, but they are still going strong making motors and speed controls for industrial applications.
I remember those MEC gearboxes. Ugly, even if they got the job done.
That is a really good point about the sensored motors and speed controls running better. It’s always about what is good enough and what costs less, isn’t it?
Hello Carlos, I appreciate very much your comments about all your developments and other subjects.
Here are 2 subjects I did not see yet:
1) when you compare gas /glow to electric, you did not mention that the power of a gas /glow is basically OUTPUT power, while the power of an electric is input power. Also you did not compare total weigth of a gas/glow to an equivalent brushless system.
2) I searched to find information about some technical reason for not putting 2 brushless motor in parallel. Do you have some explanation for that? The goal is to have a twin engine plane with the closest identical speed. Any idea?
Continue your presentations, it is highly appreciated!
Enjoy flying
Hi.
1) That’s a good point (regarding input vs. output power). Yes, there is a difference in the weights of the different power systems. Like I said, in practice, it is not nearly as big of a deal as it used to be.
2) Are you talking about using two brushless motors with one speed control? A brushless ESC receives feedback from the motor as it turns. It is not like a brushed motor. I have been told that you can hook up two brushless motors to one speed control, but one of them may not start automatically. I have never tried this myself.
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