Radio-controlled helicopters can be incredibly powerful today. 
I interviewed a proud Finnish hobbyist.

I remember when I was a kid and wanted a radio-controlled (RC) model plane. They were quite expensive back then, but nothing like RC-helicopters were. Those started at 2000 euros or so, and were pretty much exclusively powered by gasoline engines (or methanol mixed with nitromethane if I remember right).

Fast-forward a…couple of years, and you can buy a small electric-powered indoor helicopter from any supermarket for 20 euros. I even have one. Big surprise. It’s awesome.

Times they are a-changing. Thousands of moneys no longer needed.
The times they are a-changing. Thousands of moneyz no longer needed.

But, the actual RC-hobby actually goes light-years behind those small toys. I was pretty much unaware, until our newest summer employee mentioned a small custom electric motor being sold online.

The catch?

The motor had a peak power of 21 kilowatts, at a weight of roughly 700 grams.

Let me stress that.

That’s huge.


That’s three times better than the power-densest electric motor listed on Wikipedia.

That’s almost half-way to being a space shuttle turbo-pump.

Absolutely amazing. Wordless.

Hence, I was pretty much forced to make a story for my research group’s Facebook page.

The very motor that triggered all this was unfortunely German-customized. Luckily, its owner directed my to the proud owner of an identical powermill, hand-wound and modified here in Finland. He kindly agreed to an interview.

You can find the original piece here. But, I also wanted to save it on a less-fickle website that Facebook. And what could fulfil that purpose better than my blog?

So, please enjoy a brief glimpse to the wonderful world of RC-helicopters:


Recently, we spotted something extremely interesting online: a small customized electric motor for sale, on a forum for radio-controlled helicopters What made it special, though, was its 21 kW peak power with an absolutely-incredible power density: slightly less than 30 kW per kg. For those unaware, that’s 7.7 times higher than that of the current Formula 1 engines, and almost 3 times better than the freaking jet engine of a Boeing 777.

Think about that for a second.


Obviously, this kind of awesomeness shall not go unrewarded. Below you can find the first-ever AaltoElectromechanics Interview of Tero, the proud owner of an identical motor (the one for sale had been customized in Germany, but Tero’s is fully Finnish-wound). It’s the longest piece we’ve ever published, but we still strongly suggest you check it out. It’s worth it, promise.

Off we go.

Q: So, please tell us something about yourself. How did you end up with RC-helicopters as a hobby? Have any formal education on the field?

A: I’m Tero, a 46-year-old male from the city of Tampere, currently working in healthcare communications. I started on helis after moving to a new city: I didn’t know anybody, and wanted spend my time on something “reasonable”. My education as a car and electronics technician made the beginning quite a bit easier, although of course you have to study a bit and make a lot of questions. Mostly the “why” kind rather than “how”.

Helis briefly

Q: Could you tell something about this hobby for us complete outsiders? There’s apparently quite a bit of selection available today?

A: There’s indeed a huge range in RC helicopters; from the 20€ ready-to-fly models (pictured on the header) you can find in any supermarket, to the huge scale-models powered by jet turbines and costing more than ten grand each.

However, the most popular one among hobbyists is a so-called 3D-helicopter powered by an electric motor. 3D in this context means that the heli can perform acrobatic movements in the air; such as flying upside-down or sideways strafing in any position. To anybody seeing it, it looks as if it’s defying the laws of physics 🙂 There are even World Championship competitions, in several separate classes. Indeed, Finland has a couple of competitors often ending up on the podium, at the World and European Championship levels.

The helicopters can be divided into two, or actually three classes:

  • The traditional “bod & boom” helis, actually looking like what you usually associate to a helicopter. They come in all sizes, from the small 200 mm rotor diameter to the 1.8 meter 3D helis. Scale helis are typically large, with blade spans even from 3 meters upwards.
  • Multirotors for aerial photography. On the lower-end, you can simply go and buy e.g. the DJI Phantom. But, the more serious filming drones are pretty much always self-manufactured without exception, or at least heavily modified from factory models. As such, they typically carry a larger film- or a still-cam.
  • FPV (first-person view) helis: the controller wears virtual reality glasses which replay the view transmitted by an on-board camera. Ergo, they’re “riding” the helicopter. These are typically multirotors with a 250 mm blade span and four motors. They even have a special sport called FPV-racing, which has become hugely popular in the recent years, and is indeed the fastest-growing sub-sport at the moment.

Electric Propulsion

Q: How common are electric motors in the RC-field today?

A: Electricity is indeed the most common form of propulsion at the moment. This is due to e.g. the high power density offered by electrical motors. However, nitromethane-burning internal combustion engines are currently making a strong comeback, and are expected to become significantly more popular in a few years. This is mainly due to some new models introduced by one manufacturer. Lower skill requirements also factor in, i.e. a lower-powered heli is easier to control.


Q: Nice! Let’s focus on electric-powered RC-technology from now on. If I wanted to start this hobby now from scratch and build a helicopter myself, what components would I need besides a motor and a battery?

A: Let’s use the most typical 3D heli as an example. Starting from zero, the heli itself is obviously needed, most commonly as an assembly kit. Add to this four servos: three for controlling the main rotor blades, and one for the tail. Additionally, a speed controller is needed for supplying the electric motor. Obviously, a radio transmitter is needed for the user, and a receiver for the heli end. Nowadays, a gyro is also required for mechanical stabilization of the device. Finally, a charger is needed for the Lithium-Polymer batteries. As a rule of thumb, something like 1000 euros is needed for everything to get started, unless you choose to opt for the highest-end equipment.


Q: Tell me more about the speed controller. It’s some kind of a power electronics component supplying the motor from the battery, right?

A: Correct. There’s a huge selection of these to choose from, with the price generally increasing with the required maximum current. For the smallest helis, the controllers typically cost for example 40 to 50 euros, and are capable of supplying 50 Amperes continuously, and 80 A momentarily. On the upper end of the price range, the numbers might be something like 200 and 400 Amperes with a price of 800 euros.

The motors used

Q: Let’s move on to the actual motors. What is the most common kind? What kind of power-to-weight ratios do the un-customized models offer?

A: A permanent magnet brushless DC-motor (BLDC) is the most common type by far. The individual motor models basically only differ by their size and the so-called kV number, i.e. the number of revolutions a minute per Volt of supply voltage. The motor that actually sparked this discussion has a rated power output of 4.8 kW and peak power of 6.7 kW, although both of these numbers are a slightly pessimistic estimate by the manufacturer. With the factory mass of 740 grams, this translates to a nice 6.5 continuous and 9.1 peak kilowatts per kilogram.


Q: That’s already quite nice. How commonly do hobbyists then customize their own motors?

A: That’s quite rare in fact, probably because of the current availability of quite powerful off-the-shelf motors. But, sometimes you do still face a sort of “only too much is enough” scenario, and you have to re-wind a motor to get the required power-to-weight ratio.

The 21 kW machine

Q: Alright, let’s finally get to the hot topic – the custom motor in question. How did you get such an incredible power-to-weight ratio out of the motor? How did you change the winding?

A: The biggest change is indeed the winding. The original motor had a so-called multistrand winding, with each turn wound from 22 strands of 0.33 mm wire. This was changed into a single-wire winding with 1.7 mm copper wire. The basic reason for this choice was the improved cooling due to better ventilation, and the sheer increase in copper area. The latter translates to more torque and power with the same kV number.


Q: That kind of power density suggests a high-speed machine. Am I correct?

A: Helis are an example of the so-called constant-speed machines, meaning that the rotational speed of the main rotor is kept approximately constant with the speed controller. The better the motor and the controller are able to do this under varying loads, the more consistently and “evenly” the heli will behave. This particular motor has a kV number of 410 (Editor’s note: for the for-sale motor, the number was 402. Minor difference.) With a 2×7-cell LiPo battery in a 14-cell configuration, this translates to approximately 1800 rpm for the main rotor, and 16 700 rpm for the motor itself.

Power requirements

Q: So that’s not an ultra-high speed machine after all, making the huge power output even more impressive! But finally, what everybody probably wants to know: is this kind of power density actually needed somewhere, or is more like a “because I can” situation? 😀

A: Both-and 🙂 For me, this all was triggered by the fact that I own one of the biggest 3D helis you can buy (see the picture below for scale), with a flight weight of roughly 7 kilograms. The heli has rotor blades with a length of 800 mm, corresponding to a 1800 mm blade span. This alone needs some power. This need is exaggerated by my “big air” flight style: meaning wide-line maneuvers with maximum blade pitches, requiring the highest amount of torque and power. The default motor simply couldn’t keep up with this, partially because my flying skills are not the smoothest possible for the equipment. It’s like a traditional “power replaces skill” situation 🙂

The helis also come in sizes bigger than your typical supermarket toy.

As example maneuvers, I could name e.g. a sudden 180-degree direction change chomping over 400 A peak currents, or a long “pedal to the metal” banked turn with a continuous requirement of 250 A.

Editor’s note: I took the liberty of linking a video here, just so you get some idea of what these RC-helis are capable of:

Q: On behalf of the entire research group, thank you so much for the interview!

For more information on the Finnish heli community, please check the dedicated forum 🙂

Phew, that was easily to longest piece I’ve ever written for this site.

Next time, speed optimization in Matlab. Until then!


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Incredible Power: Interview with an RC-Hobbyist

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