FES Motor

After evaluating available electric motors on the market, it was clear that there was no suitable electric motor which would fulfil our requirements. The weight of motor could be only about 7kg so that 30kg of batteries could compensate a nose-heavy moment. But its construction needed to be strong enough and suitable for installation.

We decided to design our own electric motor. After two years of work, many tests and a few development versions, we have created a state of the art brushless DC electric motor. Our BLDC motor offers several advantages over brushed DC motors including higher efficiency and reliability, reduced noise, longer lifetime (no brush erosion), elimination of ionizing sparks from the commutation, more power, and an overall reduction of electromagnetic interference.

Basic data of our standard FES electric motor:

  • Out runner BLDC brushless synchronous permanent magnet motor with electronically controlled commutation system 3 phase
  • 20kW continuous power at 118V
  • Up to 23kW for shorter time
  • Rotor rink diameter 180mm
  • Length 100mm
  • Weight of motor is only 7,3 kg
  • Efficiency up to 95%.
  • Rotor position by hall sensors

BLDC motors need special Controllers for their operation. Controller is mounted on a cooling rib on  top of main wheel box, and is cooled by three ventilators.

FES FCU Instrument

FES Control Unit instrument (FCU) was designed for simple FES system operation. It fits into standard hole of 57mm diameter. It is equipped with sunlight readable high resolution colour display. The screen is really bright, in fact brighter than most displays, and is readable in the strongest of sunshine! It indicates important parameters like, RPM, Power, Voltage, Current, Battery capacity, Remaining time and Temperatures. In case that one of the parameters is out of limit, alarm is activated together with sound signal. FCU also count total engine running time, and some other parameters.

There is only one switch to turn ON instrument and a knob with rotary encoder which acts as throttle and propeller brake. Press on throttle knob, have also some additional functions like message confirmation, etc. Having external potentiometer as a throttle is also possible!

On right side of cockpit or other suitable location is located "Power switch" (as a key switch or as toggle switch under red safety cap) which powers up the motor controller electronic  and main contactor which connect Battery packs to controller.

FCU instrument was specifically developed for FES system in cooperation with leading producer of gliding instruments LXNAV, who is also producing it for LZ design.

FES foldable propeller

We developed a special very light carbon fibre foldable propeller. The propeller opens quickly using centrifugal force when rotation starts. It has 1.0m in diameter and each blade weighs only 240 grams. The blades are slightly bent to take the shape of the front surface of the fuselage. During propeller folding, pitch of blades is automatically reduced when they rotate closer to the fuselage. Additional drag of the folded propeller is really minimal as proved by accurate Idaflieg flight performance measurements.

After propeller is stopped by electronic braking, it is automatically positioned to horizontal position. The composite spinner has recesses for the middle section of propeller blades and a central hole for cockpit ventilation and for air cooling of the electric motor. The ventilation can be closed during soaring flight and must be opened when motor is running.

Pitot measurement point is on top of vertical stabilizer, and works fine also during powered flight.

Nose cone in your trailer needs to be slightly modified, so that is not pressing to the propeller blades.

FES Battery packs

The traction Battery packs of the FES consists of 28 high quality Superior Lithium Polymer cells produced by world leading LiPo battery producer Kokam. We are using special high power cells with capacity of 43 Ah each. We pack cells into special composite boxes by our self. Cells are wired in serial and operates at a voltage range of 90-118V. There is a total of 4,2kWh of energy available in the two Battery packs, and each weight 15,5kg. They are located in reinforced fuselage area behind the wings. The batteries fully compensate the weight of motor and propeller in front.

Our latest GEN2 Battery packs have integrated BMS (Battery Management System) and RADSOK high power quick connectors!

Traction batteries are also used to supply power for all instrumentation on board through a DC/DC converter.  No other battery onboard is required anymore.

Power consumption of all instrumentation is very small (5W to max about 30W) compared to the motor consumption at full power. So you will finally have enough power for your radio, flight computer, PDA and transponder.

If batteries are discharged during flight using the motor, they still have plenty of power for instrumentation. It is easy to take them out of the fuselage, for charging.

Charging of FES Battery packs


An external 1200W programmable Li-Po charger (or two 600W chargers) is used for charging. Inside of each Battery pack (GEN2) is integrated advanced Battery Management System - BMS unit. BMS monitors the voltage levels of each cells, and their condition by calculating internal resistance of each cell in the pack. At end of charge it balance cells to equal voltage in precision of 2mV, using advanced balancing algorithms. Charging current, starting and stopping of charging is controlled by BMS.

Charging time of one battery pack with 1200W charger is about 150 minutes (+balancing time which depends on differences between cell voltages) if it is fully discharged. If you have used only 50% of available energy, than charging time is also 50% shorter. LiPo cells do not have memory effect, so you can charge them also if they are not fully discharged! Charging process can be monitored on PC using dedicated FES BMS control software.

Quick onboard 3000W charger is available optionally. Both packs fully discharged can be charged simultaneously in less than 2h! At least 16A fuse on grid is required.  

Unlike lead acid batteries, Li-Po batteries can be stored for months without significantly losing charge. Self discharge rate is less then 1% per month at room temperature!

Cell manufacturer claims that at discharging with 1C rating (horizontal flight) life expectancy of batteries is around 1500 cycles. After that the battery will still have 80% of the original capacity.