Published: 14.05.2020 by FVA
To prepare the engine runs for the EMRAX 228 HV used, we followed the certification regulations of JAR 22 (Section H Subsection Test Bench Runs) and then made a proposal to the LBA (German FAA) for our approach to the engine runs. This proposal was approved by the LBA for the battery electric prototype.
In the first stage, the FVA-30 is to fly battery-electric only, and then a range extender (REX) is to be retrofitted in the second stage. In the second stage, the additional power available from the REX and the long duration of cruise flight will place a higher and longer load on the motors than on the battery. We will therefore perform part of the endurance test on an engine test bench, as our battery is the limiting factor for the partial runs due to the energy content and cooling. For this purpose, we will simulate the propeller with a load machine and the battery with a battery simulator or appropriate power supply on a motor test bench and thus test only the motor and the inverter (see tables below for motor runs I and II). In addition, we will only test one motor and inverter each, since the two drive trains on the left and right are identical.
Later, ground runs of the battery-electric prototype from Phase 1 with battery and propeller installed will be conducted with two performance profiles based on real-world use of the FVA-30 and similar to the battery module tests (see tables below for engine runs III and IV).
Analysis of the approval guidelines and test design
JAR 22.1843 – Vibration test: As an external rotor and axial flux motor, the electric motor transmits the torsion directly via the rotor to the propeller hub. Since the length of the electric motor shaft is short while the diameter is large, the motor is not expected to cause critical vibration. Additional information from the manufacturer: “EMRAX motor features: […] – No vibrations”.
JAR 22.1845 Argumentation Power measurement: The motor is measured together with the inverter on a test bench. On the one hand, voltages and currents on the battery side and, on the other hand, the voltages, currents and speeds output by the inverter, including torque and the speed at the drive shaft, are to be measured and compared. The battery is replaced by a battery simulator. The power measurement only takes place in the expected speed and power range of the propeller and the expected voltage range of the battery.
JAR 22.1849 – Endurance test: We will perform a part of the endurance test on a motor test bench, since the battery is the limiting factor for the partial runs due to its energy content and cooling. For this purpose, we will simulate the propeller with a load machine and the battery with a battery simulator or appropriate power supply on a motor test bench and test only one motor and the inverter each (see partial runs 1 and 2, tables below). Ground runs are then performed with the battery and propeller installed, with two performance profiles based on real-world use of the FVA 30 and similar to the battery submodule tests (see sub-runs 3 and 4).
JAR 22.1849 (c) – Load profiles of the partial runs: The starting power will be slightly above the maximum permanently permissible motor current of the EMRAX 228 HV, but below the maximum permissible current, which is limited to 2 minutes according to the manufacturer. We therefore do not want to exceed these 2 minutes in the test. In addition, the full takeoff power of the FVA-30 is required for short runways, as noted in JAR 22.51-takeoff, for about 35 seconds until the obstacle height is passed. Thus, the pilot has approximately 85 seconds to reduce power to the highest continuous power. Therefore, we want to adjust the partial run according to JAR22.1843 (c) to the engine run according to Table I (below). In addition, with highest continuous power available, the 4 minute climb to 360 m required by JAR22.65 is expected to be cut in half. Thus, from a flight performance standpoint, it should not be critical to limit the maximum takeoff power to, say, one minute and test the engine for a duration of two minutes. This results in a safety factor of 2 for the duration of the starting power. In addition, the temperature sensor on the motor coil monitors the temperature, so that the pilot can be warned in time if the starting power is called up for too long and the cooling is not sufficient for this. We will additionally perform a second partial run (Table II, see below) on the test bench, in which the highest continuous power is called up for 15 minutes directly after the 2 minutes of starting power (without cooling run). This represents the most critical and real load case for the engine cooling and the engine.
JAR 22.1851 – Operational behavior testing: Operational behavior is also tested on the test bench, ground tests and subsequent flight testing. In addition, we will test engine behavior on the engine test bench, such as the transition behavior between operating stages, the acceleration behavior of the load assumption and the behavior at overspeed.
JAR 22.1853 – Testing of engine components:
(a) Additional partial runs with performance profiles based on the real use of the FVA 30 and similar to the battery submodule tests.
(b) Temperature limits are set for the motor winding and magnets and ball bearings ranging from -40°C to 120°C. This is in accordance with the manufacturer’s data sheet.
(c) The EMRAX motor has a built-in temperature sensor to measure and monitor the winding temperature.
Detailed design of the engine test runs
TEST BENCH – MOTOR RUN I | Order | Duration [min] | Operating State | |—|—|—| | 1 | 1 | Start-up (idle) | | 2 | 2 | Starting power | | 3 | 1 | Cooling run (idle) | | 4 | 2 | Starting power | | 5 | 1 | Cooling run (idle) | | 6 | 2 | Starting power | | 7 | 1 | Cooling run (idle) | | 8 | 5 | 75 % max. continuous power | | 9 | 2 | Cooling run (idle) | | 10 | 15 | Max. continuous power | | 11 | 2 | Cooling run and shutdown | | Overall | 34 | |
TEST BENCH – MOTOR RUN II | Order | Duration [min] | Operating State | |—|—|—| | 1 | 1 | Start-up (idle) | | 2 | 2 | Starting power | | 3 | 15 | Max. continuous power | | 4 | 5 | Cruise flight power | | 5 | 2 | Cooling run and shutdown | | Overall | 25 | |
TEST BENCH – MOTOR RUN III | Order | Duration [min] | Operating State | |—|—|—| | 1 | 1 | Start-up (idle) | | 2 | 2 | Starting power | | 3 | 5 | Max. continuous power | | 4 | 10 | Cruise flight power | | 5 | 2 | Cooling run and shutdown | | Overall | 20 | |
TEST BENCH – MOTOR RUN IV | Order | Duration [min] | Operating State | |—|—|—| | 1 | 1 | Start-up (idle) | | 2 | 2 | Starting power | | 3 | 10 | Reduced continuous power | | 4 | 3 | Cruise flight power | | 5 | 2 | Cooling run and shutdown | | Overall | 18 | |