These requirements narrow the trade space and allow for the selection of a motor and corresponding power electronics topology, which will feed off the direct current bus from the power system.
For the initial search, the three engines referenced in Table 1 above were scrutinized to identify appropriate torque and power requirements. Initially, only direct-drive options were considered, as this enables the lowest-risk installation (although with increased weight and/or loss of efficiency). Additionally, the initial motors considered for the feasibility assessment were COTS options – which were generally not purpose-built for aviation (e.g. heavy).
Certainly, one of the major issues associated with direct drive motors is that the high torque required to drive the propeller results in a heavy motor that may not be operating at its most efficient point. For example, Figure 2 shows the power output of two candidate motors (YASA 400  and YASA 750 ) overlaid with the estimated power output of the Rotax 912S gasoline engine vs. propeller speed. (For reference, the Rotax 912S is unusual in light aviation engines, as it employs a 2.43:1 gearbox between the engine and the propeller. The O-360 and IO-550 motors referenced in this study are both direct drive.) Although this figure shows that the YASA 400 should meet the power output vs. propeller speed as a replacement for the Rotax 912S, the torque vs. propeller speed curves shown in Figure 3 tell a different story. Here, it becomes apparent that the heavier YASA 750 may be necessary unless a lower-torque, higher RPM propeller is used, or a gearbox is added to the motor.
Not surprisingly, the torque requirements imposed by a direct-drive propeller ended up driving the COTS motor selection. Thus, the power capabilities of many of the motors selected as part of the initial scaling studies were significantly higher than the power output necessary. A snapshot of the candidate motors and their associated parameters are given in Table 2. It should be noted that one major difference between ICEs and electric motors is that electric motors can have contingency power and torque ratings over 50% in excess of their maximum continuous values; it is usually the motor cooling in this case that limits maximum continuous power and torque. Hence, while some motors may seemingly have lower maximum continuous torque values, some consideration was made for the amount of time the motor would need to operate at higher torque (for example, takeoff).
Table 2: Initial electric motor parameters for three representative aviation engine replacements.
Replacement Class Electric Motor Mass Max. Cont. Power
Max. Cont. Torque
Efficiency at Cruise
Rotax 912S YASA 750  33 kg 50-90kW 400 N-m 92% Lycoming O-360-A4M UQM PowerPhase HD 250  83 kg 150kW 360 N-m 92% Continental IO-550N Siemens 261kW [30,31] 50 kg 261kW 1000 N-m 95%
Figure 2: Shaft power vs. propeller speed for two candidate replacement electric motors as compared to Rotax 912S.
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American Institute of Aeronautics and Astronautics