July brought record high temperatures to many parts of Canada. Even the normally cool Vancouver Island had six days in a row above 30°C at the Campbell River airport (CYBL), including a new record of 33.4°C on July 8th. Did these hot temperatures affect the performance of the e-plane based there? Did battery temperatures rise unexpectedly?
Let’s look at the data. Sealand Flight operates a Pipistrel Velis Electro at Campbell River, they flew most of these hot July days and have kindly allowed us to access the data recorded by their aircraft.
Graph 1 shows the State of Charge (SOC) of the aircraft’s batteries versus time. The SOC decreases from 100% at take-off to around 30%, the minimum reserve that is recommended when landing. The legend notes the outside air temperature (OAT) at take-off for each flight. The observed slope for each flight is similar. A cooler weather flight (13°C) is added for comparison. It fits right in the middle of the hot weather flights (26°C+), indicating no difference in performance.
Graph 1 Battery Discharge Profile During Flight, %SOC vs Time
A simple measure of the rate of energy consumption of the e-plane is %SOC/min. We have used this measure in previous blogs and will use it here to compare performance of the flights at the various temperatures observed this summer.
Graph 2 compares the energy consumption rate on multiple days and multiple flights. The high initial consumption of 3-4 %SOC/min identifies the take-off and climb portions of the flights. Another clear pattern is the flat lines at 1.5-2 %SOC/min when the plane is cruising. Finally, the low points at 0.5 %SOC/min represent descent portions of the flight. The rate of energy consumption is shown as consistent for the different phases (climb, cruise, descent) of flight across multiple flights. The outside air temperature (OAT) is noted in the legend for each flight.
Graph 2 Battery Depletion Rate During Flight, %SOC/min vs Time
The consistency observed in hot weather flights leads to the next stage of our analysis where we compare these flights to those at other temperatures. To simplify the presentation, we combine flights at a similar temperature into a single line to compare the groups.
Graph 3 compares the rate of change of SOC (how fast the battery is discharging) with the average power over that interval (1% SOC). The flights are grouped by OAT at take-off. A line of best fit is calculated for each temperature grouping and all are very similar. The slopes of the lines indicate that for each 1 kW increase in power, the rate of energy consumption will increase by roughly 0.07 %SOC/min. In other words, increasing power by 10 kW will increase the rate of energy consumption by 0.7 %SOC/min, or an extra 7% SOC every 10 minutes. The slope of the line, or the increase in energy consumption, is smallest (0.067) for flights with an OAT in the 21-25°C range. Higher temperature flights (26-30°C) had a slightly steeper slope of 0.069 while lower temperature flights had a steeper slope of 0.074 or 0.075. However, the differences in the slope of the lines for each group of flights were small in comparison to the spread in the values of the dots. The lack of differences is especially strong when looking at the 15-25 kW power setting that is used most for cruise flight. Overall, the differences between groups of flights at different temperatures are not statistically different for the small number of flights observed. In other words, when flying in a temperature of 30 °C the e-plane delivers the same power as when flying in the 20s.
A second important question is whether flying in hot weather will see the battery temperature rise. Again, we looked at flights with the outside air temperature over 25°C (Graph 4). The battery typically starts at a temperature a couple degrees above outside air temperature. This is caused by charging the battery just before the flight. The charging process heats the battery and causes this higher starting point. In all cases, the maximum increase in temperature was only 2-3°C and the battery often then cooled a degree or two later in the flight. Clearly, the cooling system has ample capacity to keep battery temperatures under control during high-power phases such as take-off and climbing.
An important observation is that the battery temperature remained well within the normal range of 11-50°C. It never approached the caution temperature range of 51-57°C, or to the maximum temperature, red line, of 58°C. Each flight demonstrated a stable battery temperature with a fluctuation range of just 2-3 degrees. The temperature range during all eleven flights was 26-34°C. The flights included different lesson patterns such as circuit training and cruises. The repeated take-offs of circuit training would be expected to generate the most heat as the power setting is set high for the climb. However, the cooling system worked as designed and battery temperature remained stable. (Not surprisingly, the temperature also remained well above the minimum temperature of 5°C.)
Overall, the e-plane was designed to handle hot days with battery temperatures remaining well within the normal range. The discharge rates or energy used for various flight activities at high temperatures remained similar to those at lower temperatures.
Technically, the flights were perfectly normal. However, from a human factors point of view, it is probably fair to comment that the pilots would be feeling the heat more than the plane. Remember to use sunscreen and to keep hydrated on hot summer days at the airport.