Sunday, May 22, 2022
Home » Product Reviews » O.S. Brushless 700-800 Class Electric Heli Motors

O.S. Brushless 700-800 Class Electric Heli Motors

Electric Power from the Masters of Nitro!
This article was originally published in RC Heli Pilot December/January 2016 issue.
Story & Photos By Chuck Bassani038-1


Power Supply: 12S LiPo (44.4 – 50.4V) Can Diameter: 2.3 in. (58mm)
Body Length: 2.12 in. (54mm)
Poles: 10
Shaft Diameter: 0.24 in. (6mm) Shaft Length: 1.6 in. (40mm) Weight: 18.5oz (525g)
Lead Wire Length: 4.3 in. (110mm) Rated Voltage: 44.4
Rated Current: 100A
Maximum Current: 150A for 5 seconds No-load Current: 2A/10V
Phase Resistance: 15 miliohms
Phase Inductance: 26uH
Maximum Efficiency: 85%
Kv: 520
Turns: 12


Power Supply: 12S LiPo (44.4 – 50.4V) Can Diameter: 2.3 in. (58mm)
Body Length: 2.4 in. (62mm)
Poles: 10
Shaft Diameter: 0.24 in. (6mm) Shaft Length: 1.2 in. (30mm) Weight: 21.16oz (600g)
Lead Wire Length: 3.9 in. (100mm) Rated Voltage: 44.4
Rated Current: 100A
Maximum Current: 150A for 5 seconds No-load Current: 3.1A/10V
Phase Resistance: 14 miliohms
Phase Inductance: 25uH
Maximum Efficiency: 90%
Kv: 490
Turns: 12


O.S. has long been regarded as THE premiere manufacturer of nitro engines. Highly regarded for their quality and longevity, almost anyone that participates in RC has probably owned an O.S. engine at some point. However, with the popularity of electric power expanding at an exponential rate, it looks as if O.S. has figured out that in order to stay relevant, they’re going to need to expand their product offerings into electrics as well.

Not long ago, O.S. broke into the electrics when they released a line of brushless motors for airplanes. Shortly after that they expended into the helicopter market with brushless electric motors for 550 – 650 class machines. Most recently O.S. has graced us with two brushless electric motors for 700 – 800 class helicopters; the OMH-5830-490 F3C motor and the OMH-5825-520 3D motor.

TYPE: Brushless Electric Motor for 700 – 800 Class Helicopters
FOR: Beginners to Experts
PRICE: OMH-5830-490 F3C Motor: $399.99
OMH-5825-520 3D Motor: $349.99

• Motor (w/pre-tinned leads) • Instruction Sheet
• Decal Sheet

• Appropriately Sized Bullet or Solder Lug Connectors

• Two Kv options; 490Kv (F3C) and 520Kv (3D)
• 2-Year Limited Warranty

• The OMH-5830-490 F3C motor comes with a shorter 30mm shaft

To be honest, when asked to review this motor my original intention was to test it in my Goblin Urukay Competition and then replace it with a more ‘popular’ brand of motor when the flight tests were complete. In retrospect, that intent was most likely due false expectations as I thought there must be good reason why O.S. motors aren’t more main stream. I guess I wasn’t really expecting anything special from it. Well, as the old saying goes “Don’t knock it till you’ve tried it”, that most definitely applies here. My plan has been scrapped. The way it effortlessly propels the Urukay around the sky, it’s staying right where it is. I’m looking forward to putting it to some hard work this coming flying season.

The OMH-5825-520 3D motor installed in my Goblin Urukay Competition.


O.S. markets these motors as F3C and 3D specific versions. With almost identical performance ratings and only a 30 Kv difference, let’s take a closer look at what sets them apart from one another.

Foremost is the Kv rating; 490 for the F3C motor and 520 for the 3D motor. That 30 Kv difference is important, as it allows the F3C motor to develop more torque at a lower head-speed while the 3D motor can produce the torque at the higher head-speeds required for quick control inputs. Although O.S. doesn’t publish the motor power ratings, given the 12S (44.4V) and 100A current ratings, we can deduce that both of these motors can churn out approximately 4.4 Kilowatts of power with short bursts of 6.6 Kilowatts in reserve. With regards to efficiency, the F3C motor is rated at over 90% maximum efficiency whereas the 3D motor comes in at over 85% maximum efficiency. The extra efficiency of the F3C motor is much appreciated as FAI F3C routines generally require longer flight times. Lastly, with regards to performance, both motors feature a heat-reducing stator design that helps improve power output while at the same time facilitating the aforementioned efficiency ratings.

There are some notable physical differences between the two motors as well. The 490Kv F3C motor has a 30mm shaft length while the 520Kv 3D motor has a 40mm shaft length. Why this difference? I honestly haven’t got a clue, so perhaps O.S. can chime in and let us know how this came to be. I will say this, however, the 30mm shaft length on the F3C motor does render that motor’s shaft too short for a select group of helicopters utilizing a lower motor shaft / pinion support bearing. Size wise, the 490Kv F3C motor is 8mm taller and 75g heavier than the 520Kv 3D motor. Also note that the cooling vents on the top of the motors differ between the two.


The OMH-5825-520 3D motor was installed in my Goblin Urukay Competition. For those not familiar with this heli, it is a 12+ pound bird with a 3-bladed head and tail and swings 725mm main blades.

Test flights were performed with telemetry recording all the key helicopter and ESC metrics. Cruising around at 1850 RPM, the motor was drawing on average 43 amps. Startup and fast collective changes showed the motor drawing upwards of 150 amps. With full collective climb-outs (12.5 degrees of pitch), the motor showed no signs of bogging and the telemetry indicated as much … indicating a constant rock-solid RPM. In 3D flight, the motor did demand some more juice; around 75 amps average with peaks upwards of 200 amps.

Also tested were some low head-speed flights. 1350 RPM hovering and forward flight showed the motor was sipping a mere 35 – 40 amps. In all cases, the motor came down relatively cool – a definite indication that it wasn’t being overworked at all. All-in-all, I’m very pleased with this motor’s performance.


O.S. has made an enormous stride into projecting their expected performance and quality image into the electric helicopter motor genre with these two additions. I was quite surprised at how well the motor performed in a relatively heavy machine. At this point all I can add is that I hope O.S. continues to expand its electric motor line!


O.S. ENGINE, (217) 398-8970
HOBBICO, (217) 398-8970