SURVEYOR'S NOTEBOOK

How to Measure Solar Impulse Success?

By Rik van Hemmen

In March of 2015 Solar Impulse started its around the world adventure and today it brought the adventure to completion; an around the world flight entirely on solar power. As I noted in an earlier blog this is a first order achievement that has only occurred a few times in human history.

Still it is difficult to make real sense of this achievement at this stage.

As a matter of fact, I have noticed that the Solar Impulse people and its website have struggled with it too, they note that many records have been broken, but these records provide little context. I am not sure why this is, but possibly it is related to the complexity of the achievement. This was a flight around the world, but there have been many flights around the world and to really figure out why this flight was special is complicated.

While we have been provided with lots of data, the Solar Impulse website provides little insight into the meaning of the achievement. As a matter of fact, I found it easier to follow the BBC coverage of the event rather than to access the Solar Impulse website, which had lots of bells and whistles, but little to hang my hat (head?) on.

I asked our intern, Sara Zaky, to make a little spreadsheet on some information that was provided by BBC’s excellent coverage of Solar Impulse.

I was particularly curious about times and speeds and that data had not been clearly provided.

As the spreadsheet shows, the actual flying time was 23.39 days with an average speed of 76 kph. Interestingly, this is very close to the speed provided in the Solar Impulse graphic for the craft (70 kph). I am not sure whether that speed is actual still air level flight speed or not, and when I look at that speed many details of my university aircraft performance classes come back, and it makes me wonder about speed while climbing, and gliding speeds, all of which, in some form or another, are needed to analyze the performance of this curious craft. Still, it would indicate that there was not a lot of tailwind, which makes it even more impressive.

While the average speed in flight was 76 kph, the actual average speed of the whole adventure (about 16 months) was more like 4 kph, which is just about walking speed. To me it also has never been clear if Solar Impulse charged its batteries while on the ground before takeoff or took off with empty batteries and on solar power alone. These details matter, because it affects the actual average speed of the voyage. If one day of solar charging is needed before a take off, the average speed of the voyage is actually less.

I looked at these number to figure out if Solar Impulse beat sailing around the world, and at this stage I would say that the sailing around the world (45 days, but a more constrained route) is faster than flying, and it may take a while before there is a solar powered aircraft that is sufficiently rugged to beat sailboats.

However, when one considers the engineering achievement of Solar Impulse, we are talking about something else all together. Solar impulse went from first solar powered flight to around the world in just 7 years. (Note that I consider Solar Impulse to be the first solar powered flight, since any other efforts that I am aware of could not sustain solar powered flight with its onboard solar cells)

This data should show how special the Solar Impulse achievement was:

  • Powered Flight:
  • First powered flight: Wright Brothers, 1903
  • First powered English Channel Crossing flight: Bleriot, 1910
  • First powered around the world flight: Douglas, 1924
  • First non-stop powered around the world flight: Rutan, 1986
  • Human Powered Flight:
  • First human powered flight: Gossamer Condor, 1977
  • First human powered English Channel Crossing flight: Gossamer Albatross, 1979
  • First human powered around the world flight: Still waiting
  • Solar Powered Flight:
  • First sustained solar powered flight: Solar Impulse 1, 2009
  • First solar powered English Channel Crossing flight: Effectively done the day Solar Impulse 1 flew across the US in 2011.
  • First solar powered around the world flight: Solar Impulse 2, 2016
  • First non-stop solar powered around the world flight: Not yet, but if you can keep a human alive aboard Solar Impulse 2 for 23 days, in theory that can be done soon.

In effect, one team went from demonstrating sustained manned solar powered flight to flying around the world in 7 years. It took the world (I mean everybody in the world, plus the technology drive of a world war) 21 years to do this in powered flight. Very impressive!

Congrats Solar Impulse team; you are an inspiration to the world in engineering can do!