Day 2 Velis ground runs: What is the Remaining Flight Time, RFT, in an eplane?

The e-plane team of three UW Aviation students (Gabriel, Kyra and Tyler), WISA researcher Paul Parker, and WWFC staff continued measuring the Pipistrel Velis Electro’s performance in a series of ground runs prior to flying the plane in the air.

One key difference between the e-plane and fossil fueled training aircraft is that you have a shorter flying time due the battery holding less energy than a tank of avgas. The conventional plane has a fuel gauge to tell you how much fuel is left. The e-plane has a screen showing the SOC, or State of Charge remaining, as a percent of the total battery capacity. It is functionally similar to the fuel gauge. The SOC goes steadily down, the same as the fuel gauge goes down over time. The rate of the decline depends on your power setting. This was measured and shown in our Day 1 ground runs.

The computer screen shows the SOC at the top centre for easy reading: 80% for both front and rear batteries, in the photo. The time of 47 minutes shown below the SOC is the Remaining Flight Time or RFT.

The RFT value is calculated to help pilots plan their flying activities and landing options. This is a useful number, but pilots need to be aware that it does not follow a simple declining pattern like the SOC. The RFT depends on both the amount of energy in the batteries, SOC, and the power setting or rate of discharge. In other words, changing the power setting to spin the propellor faster or slower will change the remaining flight time. Changes in other electrical loads will also have an influence, but these are typically minor. The power setting changes are illustrated with a simplified power lever sequence to represent changes in flight.

The team conducted a second set of trials that modified the steady discharge model of the first set of trials. The steady discharge was initiated from 90% to 70% SOC and then power was increased to 48kW (the maximum continuous power setting, for 90 seconds). This represents a decision, for example to climb to a higher altitude. Then the power setting was returned to its initial setting until the 50% SOC point was reached. At that point a lower power setting (15kW for 180 seconds) was made, for example, representing a decision to slowly descend on your way toward the airport. Finally, the power setting was returned to its initial level. The results from adding the increase and decrease to the power setting were only small changes in total mission time for each power setting.

However, the more interesting observations were the changes in Remaining Flight Time. Changes in power settings clearly changed the RFT. For example, in the 25kW discharge trial when the power was increased at 70% SOC the RFT dropped from 30 to 15 minutes because the battery was being depleted almost twice as fast. Conversely, when the power setting was lowered to 15 kW at 50% SOC, the remaining flight time increased from 22 minutes to 28 minutes. The lower power setting also meant a slower decline in RFT. Pilots need to remember that this is not a fuel gauge. The time displayed as Remaining Flight Time is a function of both the amount of energy in the battery and the rate of discharge set by the power controller lever (throttle equivalent). It is an inverse relationship: increasing power decreases RFT and decreasing power increases RFT.

The RFT is a new number added to the computer screen in the cockpit of e-planes for pilots to use. Understand how it changes and use it wisely.