Design constraints propel electric aircraft design innovation

Article By : Bill Schweber

An electric motor-powered aircraft offers insight into design constraints but also invites new options.

“Why didn’t they just add this function?”

This type of casual, offhand remarks causes frustration on my part, especially if it comes from someone with little or no understanding of the reality of engineering tradeoffs in design.

Design is all about making decisions within a square defined by parameters of power, performance, practicality, and cost at its corners. Some designs will do almost anything to be at one of those corners and are only lightly constrained by the other ones, but most designs strive to find an acceptable “sweet spot” somewhere within that box.

So it is with the all-electric airplane. Regardless of your views on the wisdom, desirability, and near-term practicality of such a craft, it’s a fascinating design exercise. This was made clear in the recent article “How I Designed a Practical Electric Plane for NASA” in IEEE Spectrum, where Tom Neuman (at the time, a student at Georgia Tech) explained the many considerations and iterations he went through in his award-winning design for a NASA competition to design a four-seat electric aircraft. This contest wasn’t about an artist's conception with vague objectives and pretty drawings; it required serious analysis and simulation.

__Figure 1:__ *This four-seat electric airplane design uses ideas and components from the Toyota Mirai automobile, which is powered by fuel cells. (Image source: IEEE Spectrum)*

His contest-winning craft uses fuel cells as the primary power source, and Neuman clarifies how use of electric motors for propulsion changes some of the usual fundamental assumptions about aircraft design and component placement. Equally important, he is candid about the approaches he investigated that turned out to be impractical, and why. In short: it’s mostly about power/weight ratio, and managing power and energy resources.

That’s an aspect of engineering design that is often ignored in so-called “success” or “how we did it” stories, which tend to portray the design process as a linear, deterministic journey with little backtracking or venturing into dead ends where decisions have to be re-evaluated and even abandoned. It’s especially aggravating when politicians pass laws defining specific engineering accomplishments and time frames, as if these can be called up by mandate.

I suspect that it’s part of human nature to recall the better decisions and suppress the abandoned ones. However, part of the benefit to the audience is to explain decisions that were not in the final design. These could be the result of outright mistakes (“Oops, we misread the data sheet,” or “What do you mean they changed the requirements?”) or, more likely, the realisation that a design decision had undesired or unacceptable consequences.

I’ve read many articles and books about leading-edge designs that have resulted in successful or even unsuccessful outcomes. The best ones give insight into the realities of engineering design and processes, and show how it is both an interactive process and often a frustrating one, including known and unknown problems. (The few movies about engineering projects tend to focus on breakthrough and success, rather than abandoned paths.)

Among the earliest is The Soul of a New Machine by Tracy Kidder, a classic about minicomputer design at Data General (What’s a minicomputer? And who was Data General?). I recently added two more to my favorites list (see “My Summer (Re)Reading List”):

• Pinpoint: How GPS is Changing Technology, Culture, and Our Minds
  by Greg Milner, about the history and impact of GPS

• The Right Kind of Crazy by Adam Steltzner (with co-writer William Patrick), about the amazingly successful Curiosity project which landed that 2000-pound rover on Mars in August 2012.

Are there any memorable books or articles about the engineering-design process (and not only the glory of success) that you’ve read? Are there any movies that realistically capture it?