Solar airplanes

The Solar Impulse stopped in our neighborhood this week and is off to continue its trip around the world.  It was stuck here for five days because it was too windy for this absurd, delicate, folly to fly safely, but things have calmed down and it’s ready to resume its 30 mph odyssey. I think the whole thing is silly, though as long as people are willing to fund it as a lark or can make money from sponsorships, it’s harmless. Nothing about solar power (or aviation) is being learned from the project that could not be inferred from engineering and economic calculations. The critical constraint on such a thing, of course, is the refusal of the sun to shed more than about the power consumption of a toaster on a square yard of any surface, so to carry a pilot and his lunch, and the batteries that keep it under way (and therefore aloft) at night, the aircraft has to be (i) enormous and (ii) exquisitely delicate, pushing the limits of material science.

It is also impractically at risk from  turbulence, eddies even in steady flows that can apply forces in opposite directions to different parts of the plane.  This turbulence is probably the reason very large, necessarily delicate, lighter-than-air rigid craft like the Shenandoah, Macon and Akron failed. I predict that if the Solar Impulse 2 voyage fails, it will be loss of the aircraft in unexpected weather, possibly clear air turbulence and not even a violent storm.


Author: Michael O'Hare

Professor of Public Policy at the Goldman School of Public Policy, University of California, Berkeley, Michael O'Hare was raised in New York City and trained at Harvard as an architect and structural engineer. Diverted from an honest career designing buildings by the offer of a job in which he could think about anything he wanted to and spend his time with very smart and curious young people, he fell among economists and such like, and continues to benefit from their generosity with on-the-job social science training. He has followed the process and principles of design into "nonphysical environments" such as production processes in organizations, regulation, and information management and published a variety of research in environmental policy, government policy towards the arts, and management, with special interests in energy, facility siting, information and perceptions in public choice and work environments, and policy design. His current research is focused on transportation biofuels and their effects on global land use, food security, and international trade; regulatory policy in the face of scientific uncertainty; and, after a three-decade hiatus, on NIMBY conflicts afflicting high speed rail right-of-way and nuclear waste disposal sites. He is also a regular writer on pedagogy, especially teaching in professional education, and co-edited the "Curriculum and Case Notes" section of the Journal of Policy Analysis and Management. Between faculty appointments at the MIT Department of Urban Studies and Planning and the John F. Kennedy School of Government at Harvard, he was director of policy analysis at the Massachusetts Executive Office of Environmental Affairs. He has had visiting appointments at Università Bocconi in Milan and the National University of Singapore and teaches regularly in the Goldman School's executive (mid-career) programs. At GSPP, O'Hare has taught a studio course in Program and Policy Design, Arts and Cultural Policy, Public Management, the pedagogy course for graduate student instructors, Quantitative Methods, Environmental Policy, and the introduction to public policy for its undergraduate minor, which he supervises. Generally, he considers himself the school's resident expert in any subject in which there is no such thing as real expertise (a recent project concerned the governance and design of California county fairs), but is secure in the distinction of being the only faculty member with a metal lathe in his basement and a 4×5 Ebony view camera. At the moment, he would rather be making something with his hands than writing this blurb.

6 thoughts on “Solar airplanes”

  1. Artifacts are important. Even artifacts that don't do anything directly useful. They can inspire people in ways that engineering projections can't. (I'm not being original here, rather parroting one of the conclusions of Carver Mead and Lynn Conway, who are credited with jumpstarting the VLSI revolution that gave us all those cute devices. The chips they taught their students to design didn't typically do useful things by themselves, but they had an enormous psychological and marketing impact on the field, in ways that just looking at Moore's Law didn't.)

  2. "Wilbur, Orville, nothing about gasoline power (or aviation) is being learned from this hop-down the beach project that could not be inferred from engineering and economic calculations." "Well, actually, we did those calculations, and we think there is a real upside." "The critical constraint on such a thing, of course, is the refusal of the engine to shed more than about the power to clear a small sand dune." "We think the technology has more potential and is worth pursuing. A correspondent in the Channel Islands has done some interesting . . ." "What-evurr, enjoy. Bye."

    1. There is no analogy whatever between the energy density of current sunlight and the potential output from an engine burning fossil fuel that stored up that energy over millions of years of photosynthesis. I was not talking about the energy collectible from sunshine with current technology, which will surely be pushed closer to the 1kw/m2 limit…but not above it. Other non-negotiable, non-engineerable constraints on aircraft design are the refusal of gravity to accelerate things down at less than 32ft/sec2, and the refusal of A to be larger than F/M.

  3. Airbus have flown, and propose to sell, a real electric plane running on batteries. The first E-Fan has a flight duration of one hour, the production version two, which is (they claim) enough for basic pilot training. Expected improvements in battery power density could make electric planes commercial reality. Airbus don't bother with solar panels, yet. In theory they could be useful range extenders for long-haul electric planes, as the intensity of sunlight is higher at altitude.

  4. Actually, you've understated how bad solar powered winged craft have it, compared to dirigibles.

    Lift for a dirigible scales with volume, while lift for a winged craft scales with area. So, as you make a dirigible larger, it can keep the same shape, while as the winged craft gets larger, it has to get more spindly.

    Drag for both scales as area, of course, so for the dirigible, as it gets larger, the available interior volume and mass per unit drag goes up, while for a winged craft it stays roughly constant. So the solar craft gains no power or cargo advantage as it gets bigger, unlike the dirigible.

    If a winged craft relies on an interior power supply, it's available power scales with volume, too, so it has the power to overcome drag more effectively at large sizes, traveling faster, which increases available lift. But the solar winged craft has power that scales with area, so it gains no power advantage, either, in getting bigger.

    Solar actually works well with a dirigible, though, because the power and the drag are both scaling with area. At least the power to drag ratio doesn't suffer as it gets bigger.

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