Dübendorf, 8 May 2012 – If one was to look at this quiet industrial suburb of Zürich, one would never think that something revolutionary was going on. In fact, not even within the military compound would someone imagine that spectacularly innovative activities were taking place. 

Yet, behind the silence of the large military hangars, innovative minds are at work developing what will be Solar Impulse’s second generation aircraft: the HB-SIB.

Squeezing through the sliding doors of the hangar, one can see individual aircraft parts lying here and there, scattered work stations in various corners of the loft-like space and silently efficient engineers circulating around the premises, each absorbed in their own individual tasks. On Tuesday 8th of May, however, the four teams of Solar Impulse’s Dübendorf site all came together to witness the first structural integrity test for what will become HB-SIA’s big brother.

The first of a series of tests was done on the gondola - the part which carries the motor, the batteries and supports the airplane’s propeller. In fact, every piece that is designed and built needs to be analyzed and tested for two things:

• Limit load: the force that a given part is expected to bear during flights. Each flying part of the aircraft’s structure has to be tested to its limit load.
• Ultimate load: meant to push the limits of a given part’s resistance. During the ultimate load test, 50% more weight is added to verify a given part’s reaction to extreme conditions. Such conditions, however, are never expected to occur but strictly serve as a safety margin. Ultimate load tested parts are not to be used on the aircraft, strictly acting as a test part. The airplane itself, and therefore each new part, must be certified by the Swiss Federal Office of Civil Aviation (FOCA). Ultimate load tests are the key to achieving certification for production and the eventual certification for the aircraft to fly.

To undertake the test, the gondola was hung on a metal frame and, little by little, Solar Impulse’s structural testing team added weight. The suspense in the room was great and, with each additional weight, the tension in the hangar grew exponentially. As the last weight was placed, silence reigned. Should the gondola not support the final objective of 1.3 tons, it would result in months of added work.

Seconds later, the relief and satisfaction of the on-looking engineers could be felt in their collective sigh of relief: the test was a success!

Compared to the first gondola of the HB-SIA prototype, whose main structure consists of carbon fiber tubes and connectors covered by a foamy eggshell, the new casing is an integral part of the gondola, making it more resistant to light rain and humid climates; a requisite for the tour around the world.