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 27kg of batteries could compensate a nose-heavy moment. But its construction needed to be strong enough.

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. We also developed our own speed controller which is light, small and, most importantly, reliable. Controller is mounted on top of main whell box.

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 lithium ion polymer cells pruduced by Kokam. We use high power cells with capacity of 45 Ah. Cells are wired in serial and operates at a voltage range of 75 to 100 volts. There is a total of 3,6kWh of energy available in the two battery boxes, each of which weight 13,5kg. 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. 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. Unlike lead acid batteries, Li-Po batteries can be stored for 3 or 5 months without significantly losing charge.

A special Li-Po charger with balanced charging must be used. This simply means that the charger monitors the voltage of each cell in the pack and varies the charge on a per-cell basis so that all cells are brought to the same voltage.

Charging time of batteries is about 4 hours using the cell balancers.

Life expectancy of batteries is expected to be 10 years or more than 1000 charge-discharge cycles. At this point the battery will still have 80% of the original capacity.