With small e-planes aiming for the pilot training market, a key question is how many times can you take-off, fly a standard circuit, and land before you have to recharge? Of course, batteries will improve in the future, but what is practical with today’s technology?
Pilot training typically includes at least five hours of circuit training where the pilot practices taking off, flying a standard pattern around the airport, and landing again. The exercise is often called a ‘touch-and-go’ because as soon as you land safely and start to slow down, you accelerate again and take-off. Student pilots need to repeat the exercise to become proficient at both taking-off and landing, two of the essential pilot skills.
Practicing circuits with their repeated take-off and landing sequence seems to be an ideal initial application for e-planes as no time and energy is spent flying to a distant practice area. Instead, you focus on learning at the local airport. Therefore, it was the first flying exercise that we (WISA and WWFC) tried with GAUW.
The Pipistrel Velis Electro’s Pilot Operating Handbook (POH) suggests that each circuit will use approximately 10% of the batteries’ state of charge (SOC). A variety of factors influence the amount of energy needed for a circuit, however, if you make your final landing with the recommended reserve of 30% SOC and start at 100%, then simple math would suggest that up to seven circuits could be completed on a single charge and leave ample reserve. If you fly to less than 30% SOC recommended reserve, 15% SOC is set as the limit where you should not make another circuit because the battery is getting close to depleted and the safety margin is too thin.
Graph 1 shows the discharge sequence for the first flight that three pilots had in GAUW. In each case, the pilot was accompanied with an instructor. The first pilot (P1) made six circuits, the second pilot (P2) made five circuits and the third pilot (P3) made seven circuits before stopping to recharge. Each flight ended with 30-34% SOC remaining. As the graph shows, the SOC declines rapidly during the initial full power take-off stage of flight, then has a slower decline while maintaining altitude during the downwind leg of the circuit and then remains flat while the power is off (or minimal) during the descent to land stage. As the P3 line shows, this flat section of descent to land occurred at approximately each 10% level of the SOC, demonstrating that the aircraft performed as expected with seven circuits using 70% SOC. Including the preflight check and taxi, each flight lasted approximately 45 minutes.
As mentioned, various factors influence how many circuits can be completed in a flight. One important factor is other traffic. In the case of the P2 flight, a commercial jet took off and this required an extended circuit where the pilot flew farther (and changed runway) to accommodate the departure of the jet. This is shown on the graph as a long steady decline in SOC between about 26 and 32 minutes as the aircraft maintained altitude.
Another important factor is weight. Carrying one or two people makes a big difference. P1 flew circuits with an instructor and then flew circuits solo. The dual flight had six circuits and 33% SOC remaining for an average of 11% SOC per circuit. During the solo flight, P1 flew three circuits and returned with 74% SOC averaging just over 8% SOC per circuit. The difference in energy used is illustrated during the downwind stage of the circuit where a power setting of 20 kW was used to maintain altitude during the P1 flight with two people and 18 kW was used for the solo flight. Given that the empty Velis weighs just 421 kg, the addition of one or two 80 kg people represents a significant increase in the total weight. (Note: The maximum take-off weight allowed is 600 kg.)
Overall, the number of take-offs and circuits completed before recharging the batteries in the e-plane depends on several factors, including: the reserve SOC left in the battery, traffic conditions and weight. When flying with pilot and instructor, five to seven circuits were completed with a 30% SOC reserve remaining. When flying solo, an extra circuit can be expected because of the reduced weight.
Graph 1: First Circuit Flights of the Velis Electro