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Figure 3: Shaft torque vs. propeller speed for two candidate replacement electric motors as compared to Rotax 912S.

In all cases, the motors in Table 2 are liquid cooled, and the mass associated with the motors does not include the mass of the cooling system or the motor controller (inverter). The addition of this equipment will increase the total powertrain weight by 10 kg or more, depending on the architecture. Also of note, the motors selected here are quite heavy, with the exception of the Siemens 261 kW motor, which achieves a specific power output of over 5 kW/kg. However, this motor is not yet for sale on the open market, and has only been announced in press releases [30]. Unfortunately, other direct-drive motors in this power and torque class were so heavy that they were not included in the comparison table. The fact that a purpose-built aviation electric motor could achieve such high power-to-weight ratios is encouraging, and other concepts will certainly be considered once the conceptual design of the demonstrator to support a funded flight program begins in earnest. In the meantime, the heavier masses were used as a hedge to provide additional mass margin.

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Figure 3: Shaft torque vs. propeller speed for two candidate replacement electric motors as compared to Rotax 912S.
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In addition to the motors listed in Table 2, Siemens has been developing a geared 65kW motor [31] suitable for aviation applications that, until recently, was used in the Pipistrel Alpha Electro. This motor, at 13kg (including gearbox and inverter), again achieved near 5kW/kg, showing the possibilities associated with purpose-built aviation electric motors. Another potential low-cost concept, albeit at higher mass, would be to marry the gearbox from a Rotax 912S with a YASA 400 motor, which would yield acceptable torque at higher efficiency, though perhaps at a similar mass vs. the YASA 750. These, and other options, will persist as the demonstrator program moves forward.

D. Demonstrator Candidates More than a dozen candidate aircraft were considered as retrofit candidates, ranging from very light single

engine aircraft to twin-engine designs. All used powerplants within the range of the replacement classes mentioned above, and were scrutinized considering the power architectures defined from the Boeing scaling study. Of these aircraft, six specific configurations were evaluated through more thorough analysis to identify any salient trends. To conduct this evaluation, the following ground rules were used:

 Revised maximum gross weight (after retrofit) cannot exceed nominal gross weight – Though it is certainly possible to operate certified aircraft at higher gross weights than shown on the type certificate (particularly since NASA is its own airworthiness authority within its test ranges), the gross weight limits of the candidate aircraft were held fixed to the certified aircraft to level the playing field. If reasonable performance could not be possible at the posted gross weight, the aircraft would not be considered suitable.

 Revised maximum zero fuel weight cannot exceed nominal maximum zero fuel weight – Some aircraft have a maximum fuel weight that is less than the gross weight of the aircraft, and this was also respected to reduce/eliminate need for more extensive structural analyses.

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