Starting Configurations

Connecting the EM and ICE with a clutch bearing so the EM overruns the ICE complicates the midair restart of the engine. With the help of Capt Cary Wilson at AFRL, the Condor team obtained a small starter motor that mounts to the pull start hub at the rear of the GX25. Figure 31 shows the starter motor assembly and gearing as well as the assembly mounted to the engine through the simulated bulkhead. The starter motor is made by FEMA, which is a small company located in Germany. The starter motor takes 12 V power and has a one-way bearing so that it can turn the engine, but the ICE will not drive the starter motor once started. There are limited options for COTS ICE starters in the appropriate size, and all of them, like the FEMA motor, require custom mounting hardware specific to end user's ICE. Therefore, when issues arose with the FEMA motor, other commercial choices were virtually non-existent.

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Figure 31: Starter motor assembly for Honda GX25 (left), starter motor mounted on ICE (right) From a proof of concept standpoint, the starter motor is capable of starting the GX25 engine, which was demonstrated successfully on the bench. However, the setup has a number of issues identified in testing. First and most notably, the start/restart capability is unreliable. The starter motor only started the engine about 50% of the time during a 30 s attempt. To get even 50% reliability, the starter motor could not simply be supplied with a 12 V power source like that available from the step down converters intended for the avionics system. The starter motor consistently required power from a deep cycle lead acid battery with a full charge, 13.8 V, for an effective start. The starter motor assembly has a mass of 0.5 kg, making it a substantial weight addition to an approximately 13.6 kg (30 lb) aircraft. The starter motor would occupy the area directly behind the engine bulkhead, space dedicated to fuel and batteries. Although slim, the starter motor is directly in the middle of the space, making it difficult to fit fuel or batteries around it.

The Honda GX25 is not well equipped for a starter motor. The pull start shaft on the back of the GX25 is not concentric, even visually. This is of little consequence for a pull start which rotates the shaft only 2-3 times during the starting pull. It is a large issue for a starter motor which performs dozens of rotations. Over time, the alignment, or lack thereof, caused significant wear on the teeth of the large starter motor gear, wear that can be seen in a close inspection of Figure 31. Based on the manual starter used when the starter motor was not

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connected, it is the author's opinion that a faster motor, different gearing, or both would be required to reliably start the engine. After proving the starter motor concept on the dynamometer, the starter assembly was replaced with the cone and a manual starting motor like that used on hobbyist aircraft. As the cone was secured to the misaligned pull start shaft, vibration in the cone made manual starting challenging as the handheld motor would drift off of cone.

Due to the reliability issues, the Condor team decided to push mid-air restart to a future version of the aircraft and pursue a more reliable ground start only solution. This choice only eliminates testing one portion of the concept: mid air restart. To test EM-only power, the ICE can run in idle and the EM can propel the airframe. For an acoustic or loiter test, the engine could be killed and the aircraft could land under electric power. Obviously, to be fully operational, the ability to restart the engine to fly home would be required. Still, a starter motor is conceptually possible and the remainder of the concept could be tested more reliably without it.

Eliminating the starter motor is not as straightforward as just using a manual starter and an external battery. When the system is mounted to the aircraft, the operator is unable to access the rear of the engine to use a hand held starter. The one-way bearing eliminates the possibility of starting the engine from the front using a manual starter and propeller cone; spinning the propeller in the direction of starting the engine will overrun the engine due to the one-way bearing. Therefore, the author reverted to the manufacturer's recommended solution: use the pull start that comes with the engine. The CESI support contractors created a mount to fit the pull starter between the engine and aircraft bulkhead, allowing for pull start of the engine on the ground. The pull starter only moved the propulsion system 1.3 cm (0.5 in) forward since the mounting plate for the starter motor was eliminated in the process. The pull starter is more reliable and safer than even a traditional manual starting system since the operator can stay completely behind the plane of the propeller. The pull start allows the testing of all concept objectives except for mid-air restart itself, which is discussed as an objective for future work in

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Chapter V. This concludes the discussion of ICE integration. The chapter continues with a discussion of the EM integration.

5. Electric Motor and Batteries