Motor

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

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 commutator, more power, and an overall reduction of electromagnetic interference.

Basic data of our BLDC motor:

BLDC motors need electronic speed controllers for their operation. Controller is mounted on top of main whell box, and is cooled by three 12V ventilators.

FCU

Special 57mm size, FES Control Unit instrument was designed for really simple FES operation. It is equiped with sunlight readable high resolution color display. The screen is really bright, in fact brighter than most displays out there, and is readable in the strongest of sunshine! It indicates important parameters like, RPM, Power, Voltage, Current, Batterie 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 toggle switch to turn on instrument and a button with rotary encoder which acts as throttle selector and propeller brake. Press on rotary button, have some additional functions like alarm confirmation, etc

On right side of cockpit or other suitable location is located master switch under red safety cover, which powers up the motor controller and main contactor which than connect batteries to controller.

FCU was developed in cooperation with LXNAV, LZ design and Pipistrel.

Propeller

We developed a special very light carbon fibre foldable propeller. The propeller opens quickly using centrifugal force when rotation starts. It is 1.0m in diameter and each blade weighs only 230 grams. The blades are slightly bent to take the shape of the front surface of the fuselage. During propeller folding, pitch of blades is automaticly reduced when they rotate closer to the fuselage. Additional drag of the folded prop is then quite minimal. Probbably less than for instance if you have wing wheels instead of small wing skids, or tail wheel instead of tail skid. Maybe like bug wipers!

The propeller can be stopped in any position but horizontally might be preferred so that small disturbance of air is on the same line like gap of canopy. The carbon fibre spinner has recesses for the middle section of propeller and a central hole for cockpit ventilation and for cooling the electric motor. The vent can be closed during flight but must be open for motor operation.

Early tests shows that even at very strong sideslip air holds propeller blades close to fuselage surface, so additional spring was not neccesery.

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

You can continue to use the original nose cone in your trailer!

Batteries

The drive battery of the FES consists of 24 (or 28) lithium ion polymer cells pruduced by Dow Kokam. We use high power cells with capacity of 40 Ah. We pack cells into special boxes byself. Cells are wired in serial and operates at a voltage range of 75 to 100 volts (or 90-118V). There is a total of 3,6kWh (or 4,2kWh) of energy available in the two battery boxes, each of which weight 13,5kg (or 15,5kg). They are located in reinforced fuselage area behind the wings. The batteries fully compensate the weight of motor and propeller in front.

These batteries are also used to supply power for all instrumentation on board through a DC/DC converter. For safety reasons we still use one small batterie (could be tail batterie) which acts as a buffer.

Because the power consumption of the instruments is only one-one thousandths of the motor consumption at full power you will now finally have enough power for your transponder, radio, flight computer and PDA. Actually for the whole season, if you don't use engine!

If batteries are discharged during flight using the motor, they are easily accessible from the top of the fuselage so that you can take them out for charging.

Charging of batteries

An external 1000W programable Li-Po charger is used for charging, together with BMS- balancer unit.  Balancer monitors the voltage of each cell in the pack and balance them above preset Voltage. Cells are ballance to precision of 5mV. Balancer can be used even when charger is not connected.

Charging time of one pack of batteries is about 3 hours if they are fully discharged. If you used only 50% than charging time is also 50% shorter. LiPo batteries are without memory efect, so you can charge them also if they are not fully discharged!

Optionaly is available also 500W charger (additional 2kg), located in sailplane, for overnight recharges (goal flights, flyings from airfield to airfield etc).

Now we are testing new fast 3000kW charger which can charge both packs simultaneously in only 2h! 

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.