The Cessna 337 is a unique twin-engine design that utilizes a tandem “push-pull” configuration; that is, both engines are along the centerline, with one forward of the crew compartment, and one aft. This eliminates thrust asymmetry due to engine failures, and could enable the replacement of a single engine with an electric motor powered by the hybrid SOFC-electric power system. The other engine would remain on the airframe to provide risk mitigation in the event of a power failure of the experimental electric powertrain or power system. A larger aircraft, the Cessna 337 has a useful load of approximately 1600 pounds, with an additional exchange weight of around 350 pounds for the single engine (given that some of the other engine-driven electrical systems will remain).
5. North American/Rockwell OV-10
The final demonstrator concept considered would not entail replacing a primary propulsion system, but rather would leverage a podded installation of a completely independent electric propulsion system. The OV-10 is a large twin turboprop aircraft (Figure 5), and NASA has two in storage. By leaving the twin turboprop engines in place, the OV-10’s large payload capacity enables freedom to select a range of options that could be sized to anything that could fit, given that the electric motor would not need to provide primary propulsion to the aircraft. The high tails and short fuselage of the OV-10 enable a podded test propulsor to be located above and aft of the crew compartment. Another interesting risk mitigation is the availability of ejection seats, given that these are surplus military aircraft.
Figure 5: One of NASA’s North American/Rockwell OV-10A Broncos.
E. Initial Concept Selection Each of the concepts discussed above represents a feasible, albeit potentially challenging, platform for the flight
demonstration of a hybrid SOFC-electric power system. To further identify concepts, the team conducted a modified Pugh assessment of the six remaining alternatives (the Tecnam was really two alternatives, given the spilt vs. centralized power system options). This Pugh analyses considered the Columbia 300 as the baseline concept, and scored each concept via the criteria from subsection A according to the following subjective criteria:
1 – Much worse than baseline, 2 – Somewhat worse than baseline, 3 – More or less equivalent to baseline, 4 – Somewhat better than baseline, and 5 – Much better than baseline.
Additionally, each metric was assigned an importance value – High, Medium, or Low. The scores for each
concept were taken as a weighted sum of importance and Pugh criteria, where metrics labeled as High importance
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American Institute of Aeronautics and Astronautics
resulted in a 3x multiplier, Medium importance a 2x multiplier, and Low importance as a 1x multiplier. The resulting Pugh evaluation and weighted scores are given in Figure 6.
Figure 6: Modified Pugh evaluation matrix for demonstrator selection.
Overall, the results showed a few trends. First, no single concept was truly awful – all seemed to be a reasonable compromise for each of the demonstration criteria. Four of the concepts – the Cessna 172 with SuperHawk STC, Tecnam P2006T with single power system, Cessna 337, and OV-10, gave similar results and stood out slightly from the others. Surprisingly, the Columbia 300, which served as the original point of departure for this exercise, came in lower. This is because potential issues with integration into the limited volume and balance envelope of the aircraft led to increased risk for integration of the propulsion and power system as compared to the other alternatives. Additionally, while it is encouraging that the Siemens motor exists at the appropriate power level at a very competitive weight, it is not yet available on the market, nor does it have a significant amount of test data that has been published. Given the lack of other suitable motors, this uncertainty is reflected in the Pugh evaluation. As more information on the motors becomes available, the Columbia 300 may become a more attractive option.
One factor that significantly boosted the attractiveness of the Cessna 172 with the SuperHawk STC was the fact that the baseline aircraft has a fixed pitch propeller. All of the other aircraft (save the podded installation on the OV- 10) would replace a hydraulically-driven constant-speed propeller. Certainly, it is possible to use a fixed pitch propeller for those applications, but it comes at a significant loss in performance that may otherwise make the system-level benefit appear less attractive.
The OV-10 podded installation, while initially attractive, does come with a fair amount of its own uncertainty and costs. The cost to bring these aircraft back to flightworthy status, including arming of the ejection seats, may make cost and schedule a more important issue. Hence, it was eliminated from consideration after many discussions with NASA’s flight test pilots and maintenance personnel that care for these aircraft.
To move forward with the initial design, the team wanted to push towards replacement of primary propulsion to identify the “pain points” associated with sizing the power system to fit within the mass and volume constraints on the retrofit aircraft. Therefore, the Cessna 337 was also dropped, though (like the OV-10), it may be considered for future programs as programmatic and technical risk tolerance (or lack thereof) is clarified during demonstrator program maturation. Thus, the Cessna 172 with SuperHawk STC emerged as the preferred candidate, with the Tecnam P2006T as a backup, should the volume and mass constraints of the Cessna prove too challenging. Due to the availability of the asset, th