ASCI 530 Blog Post 2: Weeding Out a Solution

     A hypothetical UAS has been designed for precision aerial application of fertilizer, however it is overweight. The offending subsystems are Guidance, Navigation, & Control (GNC) and the payload delivery system. In the current state, the vehicle will not be able to meet the advertised performance without using fuel reserves.

Solution

     The first step to solving the problem should be a relatively quick assessment of the GNC and payload systems to find obvious weight penalties associated with the off-the-shelf hardware. This could be as simple as a lighter enclosure for GNC avionics or removing excessive hardware or material from the spray system. Assuming there are no simple solutions, the systems engineer (SE) should take a holistic approach. While GNC and payload may have exceeded their estimated weights, the solution should not necessarily be limited to those areas as they might currently be the most economic designs. Each subsystem team should be directed to identify at least two ways to save weight, the associated cost, and any negative effects. For example, replacing the landing gear wheels with skids may save weight at a low cost, but pass on higher maintenance costs to the user if the skids wear faster than tires. The SE should evaluate the compiled list of ideas and select one or a blend of ideas that incur the least cost to the company while minimally impacting the overall vehicle characteristics.

Table 1. Example of total system cost-benefit analysis for reducing vehicle wight.

     In this example scenario the SE analyzes the benefits of the potential solutions. Removing the battery backups appears to have the best cost-benefit ratio, however a loss of control resulting from an engine-out situation may present unacceptable risk to users or fail to meet certification standards. Similarly, removing heat sinks from computer processor boards may impose constraints in hot weather climates. The SE could also select multiple solutions such as the landing gear skids and thin walled fertilizer tank to meet the design goal.

     For future iterations of this UAS the company should implement three additional practices. First, a requirements-based design process should be used, which will ensure traceability of all configuration choices back to a verified need from the customer (Loewen, 2013). This is directly tied to the second practice of gathering operational data from the current UAS and conducting user surveys. The database can be used to determine requirements for follow-on systems, complete cost-benefit analysis as a function of market demand, and make appropriate sacrifices in the event testing reveals deficiencies. Since the payload versus range problem is critical for the aerial application UAS, the company should also construct a payload-range diagram. This will assist marketing personnel and customers in understanding the capabilities of the vehicle (Ackert, 2013).

Figure 1. Example of a Range-Payload diagram. Reprinted from Aircraft Payload‐Range Analysis for Financiers, by S. Ackert, 2013. Copyright 2013.

     If a range-payload diagram had been drawn for the current UAS design, it might have shown that the performance was acceptable under certain situations (consistent with the marketing campaign) or guided better decision making. For example, if the demonstrated range was found to be between Point B and C in Figure 2, and the results of a market survey yielded few users needed ranges greater than Point B, than perhaps fewer, if any, changes could be made.

Conclusion

     This essay has briefly explored the crisis that ensues when testing reveals serious design deficiencies. A typical cost-benefit example was provided to aid configuration change decisions. Additional recommendations were also provided for future projects.

References

Ackert, S. (2013). Aircraft Payload-Range Analysis for Financiers. Retrieved October 30, 2016, from http://www.aircraftmonitor.com/uploads/1/5/9/9/15993320/aircraft_payload_range_analysis_for_financiers_v2.pdf

Loewen, H. (2013). Requirements-based UAV Design Process Explained. Retrieved October 30, 2016, from https://www.micropilot.com/pdf/requirements-based-uav.pdf