Synergy 626 UAV Helicopter

My Synergy E5S 626 long range electric UAV helicopter has completed it’s initital flight testing. It is flying custom firmware in a Pixhawk. Instead of using the common power modules, which don’t work all that well in high-voltage helicopters, I am using Castle Link Live telemetry to the RC radio for power system monitoring and logging. I had thought about incorporating the Castle Link Live into the ground station. However, at this time I decided to use it only with the RC radio as I sometimes fly autonomous flights without the ground station present. I may eventually incorporate the power system telemetry data into the ground station but that will require some significant modifications since I am using watts and watt-hours to measure power consumption and fuel remaining, instead of amps and amp-hours. Amps and amp-hours is not linear as battery voltage sags during discharge. Watts and watt-hours is, and provides a better measurement of power and energy for long flight time UAV’s.

On the endurance test this helicopter flew for 28.5 minutes with 5% reserve carrying 1kg payload with two 3300 55C 12S batteries. On this test flight I had my channel 6 option set up to change the WPNAV speed to determine best efficiency cruise speed, which turned out to be be 15.5 m/s.

On this morning’s test flight with two 20C 5000 12S batteries it flew for 41 minutes with 1kg payload @ 15m/s on a longer 1.6km test flight course. The 1.6km test course has level cruise flight sections, flying crosswind, upwind and downwind, with addtional sections that vary altitude to measure performance in cruise climb and descent.

Measured power consumption in level cruise with this helicopter is 620 watts @ 15m/s and 6.6kg takeoff weight @ 1,650 rpm headspeed. It had a maximum tested range of 37km with 1kg payload in that configuration, flying a combination of level cruise and climb/descent typical with following terrain elevation on autonomous flights.

I have to modify the ESC cooling system in the heli. My ESC temp got to 72C on this morning’s test flight. I am going to design a cooling duct that pulls outside air from the nose and routes it over the HV120 ESC to reduce its operating temperature in cruise flight. The high operating temp of the ESC is not due to load, but duty cycle and the fact that it is in still air under the canopy. I could install an electric cooling fan on it, which would provide for cooler operation in hover as well as cruise. However I will experiment with ducted air-cooling first, since we have plenty of airflow over the helicopter in cruise flight to work with for cooling, and hovering is not what this helicopter is intended for.

The powertrain configuration is a Scorpion HKIII-4035-530Kv 3.4kW (4.4kW 5 sec), Castle Edge HV120 speed controller with governor, logging and telemetry, 9.971:1 main gear ratio, tail ratio 4.5:1, Rail 626 mains, Rail 96 tail blades.

Battery configurations, depending on flight time needs, can be:

  1. Single 3300 12S (147Wh) providing 14 minutes cruise flight time with 3 minutes reserve
  2. Single 5000 12S (222Wh) providing 21.5 minutes cruise flight time with 3 minutes reserve
  3. Twin 3300 12S (294Wh) providing 25.5 minutes cruise flight time with 3 min reserve
  4. Twin 5000 12S (444Wh) providing 40 minutes cruise flight time with 3 min reserve

CG is adjustable for all battery configurations with 1kg payload. Note the helicopter has higher cruise flight power consumption with the twin battery configurations. Lighter payload will increase flight time accordingly. A typical MAPPIR/GoPro camera and mount is about .25kg. Power consumption goes up or down in cruise flight at about 90watts/kg of payload weight

This is the small version of a high-performance electric dedicated UAV helicopter platform I am developing for Synergy Helicopters with the help of Matt (and Amy) Botos, the owners of the company. The larger version is a Synergy 766 UAV (coming soon). These helicopters have grease ports to grease the bearings, tail transmission that is independent of the tail boom and provides for 2 custom gearing options on the tail drive, selections of three different blade sizes for each model, 8 main transmission gearing options to accomodate different drives, headspeeds, operating voltage and blade configurations, and several other maintenance features that allow them to fly 50hr inspection and 300 hour overhaul and bearing replacement interval.

They are also very smooth-running and quiet machines. The Pixhawk on this one is mounted to the side of the frame with just two 3M round tissue tapes with no other vibration isolation whatsoever. Below is a video of the maiden auto test flight, and below the video is a plot of Attitude Roll desired vs actual to show well it tracks attitude in cruise flight once the dynamic flight rate integrator leak is turned off. There is no downstream FBL unit used, or needed.


For FAA cert for the 626 on my Section 333 I have to develop a flight manual with tested performance data and operating limitations, pre-flight procedure, maintenance intervals, H/V (Height/Velocity) diagram for autorotation procedure (otherwise known as the “dead man’s curve” among helicopter pilots), loading and CG chart, etc…

None of this applies to Part 107 operation. Section 333 pilots must be licensed (full-sized) pilots and we have less operating restrictions than Part 107. But more stringent requirements for the aircraft. The Synergy 626 has completed this phase of the performance testing and development of the performance and maintenace data. The documents will be submitted to the FAA for approval next week and this helicopter is ready to go into commerical service.

In addition to ducted air for ESC cooling I also added a ESC cooling fan. The addition of outside ducted air plus the cooling fan keeps the ESC at an operating temperature of around 50-60C in sustained high-speed cruise flight. The motor in this helicopter is quite a bit more powerful than the piston engines in my piston heli’s and the ESC cooling system was really the only mod needed.

New photo by Chris & Kristin Olson

At 367 feet/sec blade tip speed it is capable of 80 mph airspeed. And it has the raw shaft horsepower vs disc loading to do it. With the series/parallel 50V battery configuration it has the battery capacity to deliver the goods to make it happen. I chose this size vs a bigger one as the ideal compromise between power, efficiency, disc area vs takeoff weight (disc loading), and payload/fuel capacity. It can lift 18lbs (8kg) on a dead lift out of ground effect and still hover stable with it. You can lift more payload with a bigger helicopter, but IMO really should be looking at piston or turbine power if you go beyond this size. It takes insane amounts of batteries in a larger electric size to match the performance of this 626. The heavier this thing is loaded the more stable and efficient it gets. 5.8-6.0kg takeoff weight is the “sweet spot” that really makes the motor and drive happy.

With a 700 or larger you can theoretically get more flight time by running low disc loading and low blade tip speed. But that does not fit well with stability and cruise flight performance. I wanted a compromise between supreme stability and performance approaching that of a full-sized light utility helicopter in cruise speed and payload capacity vs minimum takeoff weight. It was built and designed with hours and hours of experience learning what works and what doesn’t, and this one outperforms my piston 700 hands-down (which has now been retired from commercial use, and sold).

I’m not necessarily flying it at low headspeed. It will fly at 1,100 rpm just fine. But it’s a pig at that low of a headspeed. 1,530 rpm for the single battery configurations, 1,580 rpm for the twin 3300, and 1,650 rpm for the twin 5000 has proven to be the best compromise between performance, power and flight time. Since the governor is programmed for three different headspeeds, depending on the configuration flown and takeoff weight, that has become part of the pre-flight checks on this helicopter - verify the governor setting is appropriate for the takeoff weight.

It has accumulated 12.9 hours of flight time in testing to-date. Minimum 10 hours is required.


Hah, I had a bunch of questions lined up for you with regards to your heli and you answered them all, I think.!
I was looking at the 766 as my next project, so I’ll be watching this for sure! (Although with the performance of this one, the E5S might be an option too).

My 766 build is not for this type of work. It is for heavier payload (spray pack, namely), using the helicopter to spray specialty crops. Matt has some customers already using the 766 for that in Geogia. I’m going to set one up and tweak it for the ArduPilot system so it can do that job in full autonomous flight.

It will not match the 626 for flight time and cruise performance on the same amount of power. Not without hanging insane amounts of batteries on it to provide the power. The bigger the rotor, the more power it takes just to beat air into submission.