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Technical Challenges

Solar Impulse 2 was built to take up the challenge of achieving the first round-the-world solar flight. This revolutionary airplane will have to do what no one has ever done before:  fly through 5 consecutive days and nights without using any fuel, so as to cross oceans from one continent to the next.

 

This requires the optimization of new kinds of technology and a drastic reduction in energy consumption. Solar Impulse’s 80 engineers and technicians, under André Borschberg’s leadership, have had to apply highly innovative solutions.

 

Whilst Solar Impulse is not the first solar aircraft project, it’s certainly the most ambitious. It is a real airborne technology lab with virtually endless endurance, capable of crossing oceans and continents by remaining in the air for several days and nights in a row.

DATA: Building A Solar Airplane

The Solar Impulse project in numbers:

  • 12 years of feasibility study, concept, design and construction
  • 50 engineers and technicians
  • 80 technological partners
  • more than 100 advisers and suppliers
  • 1 prototype (Solar Impulse 1, registered as HB-SIA)
  • 1 final airplane
    (Solar Impulse 2, registered as HB-SIB)

«Imagine energy reserves increasing during flight! To make this dream a reality, we had to make maximum use of every single watt supplied by the sun, and store it in our batteries. We tracked down every possible source of energy efficiency. Today, Solar Impulse is the first solar airplane flying through night and day, the first aircraft to come close to perpetual flight» 
André Borschberg

CHALLENGE #1: ENERGY TO CROSS OCEANS & CONTINENTS

SOLAR CELLS

More than 17’000 solar cells, collecting up to 340kWh of solar energy per day and representing 269.5 m2!

More precisely 17'248 monocrystalline silicon cells each 135 microns thick mounted on the wings, fuselage and horizontal tailplane, providing the best compromise between lightness, flexibility and efficiency (23%).

In order to maximize the aerodynamical performance, the plane is built with a wingspan of 72m: wider than that of a Boeing 747 Jumbo Jet!

BATTERIES

The energy collected by the solar cells is stored in lithium polymer batteries, whose energy density is optimized to 260 Wh / kg. The batteries are insulated by high-density foam and mounted in the four engine nacelles, with a system to control charging thresholds and temperature. Their total mass amounts to 633 kg, or just over a quarter of the aircraft’s all-up weight.

In order to save energy, the aircraft climbs to 8’500 m during the day and descents to 1,500 m at night.

  • Zero-Fuel
  • Batteries
  • Aerodynamism
  • Solar Cells
  • Across Oceans
  • Energy Storage
  • Fly Non-Stop
  • Lightweight Structure

CHALLENGE #2: FLYING OVER 35,000 km

MOTORS

Average power over 24-hour of a small motorbike (15 hp) with a maximum power of 70 hp (four 17.5 hp engines).

Four brushless, sensorless motors, each generating 17.4 hp (13.5 k), mounted below the wings, and fitted with a reduction gear limiting the rotation speed of a 4 m diameter, two-bladed propeller to 525 rev / min. The entire system is 94% efficient, setting a record for energy efficiency.

SPEED

Solar Impulse can fly at the same speed than a car, between 36 km/h (20 Kts) and 140 km/h (77 Kts).

At sea level: minimum speed of 45 km/h (20 Kts) and maximum speed of 90 km/h (49 Kts).
At maximum altitude: from 57 km/h (31,5 Kts) to 140 km/h (77 Kts).

  • Power
  • Motors
  • Propeller
  • Energy Efficiency
  • Speed
  • Altitude

CHALLENGE #3: BEING LIGHT AS A FEATHER… OR A CAR

LIGHTNESS

Prowess of the engineers led by André Borschberg who managed to build the entire structure proportionately 10 times lighter than that of the best glider. Every gram added had to be deducted somewhere else, to make room for enough batteries on board, and provide a cockpit in which a pilot can live for a week. In the end, it is of the weight of a small van: 2’300kg!

Stimulating innovation in the field of sheets of carbon, which now weigh only a third as much as sheets of printer paper (25 g/m2)

ROBUSTNESS

The airframe is made of composite materials: carbon fibre and honeycomb sandwich.

The upper wing surface is covered by a skin consisting of encapsulated solar cells, and the lower surface by a high-strength, flexible skin. 140 carbon-fiber ribs spaced at 50 cm intervals give the wing its aerodynamic cross-section, and also maintain its rigidity.

  • Carbon
  • Light Structure
  • Optimization
  • Minimum Weight
  • Resistance
  • Honeycomb

Solar Impulse 2:
Construction Steps

1. Concept, design, production and assembly of separate parts

2. Testing of assembled parts

3. Assembly by the Workshop Team

  • Cockpit and fuselage assembly: october-february 2013.
  • Wing assembly: november-february 2013 - Ribs, Flaps and ailerons, solar panels, fabric covering of the underside, post-cure of the wing.
  • Full aircraft assembly: March 2013 - Transportation of parts from Dubendorf to Payerne and final assembly. Vibration test of the fully assembled plane. Cleared for inauguration and flight tests!

To learn more, read our series "MAKING OF A SOLAR AIRPLANE"

JOIN THE DISCUSSION

Solar (Lego) Land

As Solar Impulse engineers slowly conclude most of the testing, many parts are laying scattered around the immense Dübendorf hangar. This is when, in the Making of process, the assembly of the solar airplane begins. Just like with Legos, the Production ...

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As Solar Impulse engineers slowly conclude most of the testing, many parts are laying scattered around the immense Dübendorf hangar. This is when, in the Making of process, the assembly of the solar airplane begins. Just like with Legos, the Production and Workshop team, responsible for building the solar airplane, take the finished pieces and put them all together. Of course, there are some slight differences between a construction game and Solar Impulse. Our solar airplane is much larger, the parts don’t connect so easily and the people assembling the plane have long passed the days of fiddling around with brightly colored plastic cubes (well, I guess that’s just an assumption…)

Led by Martin Meyer, Production and Workshop is a team of seven. With additional support from partnering companies Décision (6 people), Ruppert Composite GmbH (2 people) and Sportec AG (1 person), they follow the blueprints and technical drawings of the design engineers and proceed to bring the two-dimensional sketches to life. They’re an extremely heterogeneous team consisting of carpenters, composite specialists, mold makers, mechanics and even a gardener not to mention a design engineer, Martin. This diversity is crucial in such a versatile team.

They’re not only responsible for assembling the solar airplane, they’re also responsible for building prototypes, like the human size wooden cockpit; or smaller parts that are too complicated to outsource, such as the iPad holder that will be integrated into HB-SIB’s cockpit or the solar panels. These “little” projects can actually amount to 80 orders a month!  But the most important requirement to integrate Martin’s team is to be an ingenious and diligent handyman (and prove you were a Lego fan in your childhood).

Larger parts are outsourced to our suppliers as we don’t have the infrastructure or human resources to follow. If the assembly requires gluing, like the ribs to the wing spar, the part might be retested afterwards to ensure the bonding process was successful.

Martin has become leader of this team less than a year ago as he was previously part of the design team. “As a design engineer, I had to be more creative in terms of finding a good solution to a technical issue. Now it’s more about organizing the work and dispatching it to people depending on their capabilities. But having design experience helps me detect where the problem is when it arises.”

I guess that a multi-colored, Lego-like solar plane could be an amusing sight, but would be too attractive for birds and bees – but why not a toy model? I’m sure if Lego made a miniature HB-SIB, their target consumers wouldn’t only be kids…   

 

Photo (left to right): Jürg Birkenstock (Ruppert Composite GmbH), Peter Schindler, Simon Wyss, Martin Meyer, Stefan Stadelmann, Rolf Meier (Sportec AG), Jakob Reck (Trainee), Daniel Kober, David Fankhauser (Ruppert Composite GmbH)

Keeping the airplane balanced

Have you ever started feeling tense when, during a flight, your drink starts shaking so much that it’s about to spill and, in a moment of panic, you glance out of the window to check the state of the wings? All sorts of scenarios rush through your mind and you pray the aeronautical engineers weren’t distracted when ...

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Have you ever started feeling tense when, during a flight, your drink starts shaking so much that it’s about to spill and, in a moment of panic, you glance out of the window to check the state of the wings? All sorts of scenarios rush through your mind and you pray the aeronautical engineers weren’t distracted when building the plane.

Solar Impulse might not carry 300+ passengers, but the forces the solar plane has to withstand are proportionally the same as a commercial one: it’s physics. At Solar Impulse, the man that ensures the wings don’t collapse when strained in turbulence is our Loads Engineer, Richard Leblois. He calculates the main load pattern (the force the wings have to withstand during flight) for every single aircraft component.

Richard is present throughout the making of process. During the design phase he virtually attributes the loads to every part, via special software. Once the engineers agree on a part’s final blueprint, it’s sent into production. Let’s take the backbone of HB-SIB’s wing as an example, the wing spar: an ultra-light yet very large (over 70 meters long!) carbon fiber box. Once the wing spar is delivered, Richard works with the analysis team to simplify the tests while ensuring that the wing spar goes through all the necessary steps to be declared flight-ready. This means that real-world situations must be simulated with an intricate game of weights, test jigs and forces. Richard calculates those variables and subsequently helps develop the jigs.

Unlike other engineers on the team, Richard only gets the confirmation that his calculations were correct once he sees the plane in flight. Think of your shaking drink… Just kidding! That’s what test flights are for.

Richard has a critical role second role as the weight and balance calculator where his responsibility is to track the aircraft’s mass and center of gravity to guarantee aircraft stability. That’s when Richard laughs and says: “I basically create my own destiny: if I define the loads too high at the beginning of the process, the part will be too heavy and could become an issue for me later during the Weight and Balance assessment.”

But rest assured, when you see how far up the wings are bent during the structural tests, you would never again panic. You would simply proceed to sip you drink, relaxed, and try to enjoy the turbulent flight.

Testing of the new wing spar

Pushing the limits is a tough job, but it’s a necessity when innovation is the ultimate goal. The problem is that it’s hard to know ahead of time where the limit is. Solar Impulse crossed that thin line last year, on July 5th, 2012, during the structural test of HB-SIB’s wing spar – the central part ...

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Pushing the limits is a tough job, but it’s a necessity when innovation is the ultimate goal. The problem is that it’s hard to know ahead of time where the limit is. Solar Impulse crossed that thin line last year, on July 5th, 2012, during the structural test of HB-SIB’s wing spar – the central part of the second solar airplane’s wings. We got too close to the edge and fell overboard: i.e. the wing spar broke. Only by persevering and learning from one’s mistakes can innovation happen. In fact, this is the attitude that made it possible to build the first solar airplane able to fly day and night with the wingspan of a Jumbo Jet, the weight of a small car and an almost limitless endurance.

The new wing spar was delivered July 19th, 2013 in Dübendorf. The engineers had no time to lose and immediately began preparations for the upcoming tests of this very large and bulky part (over 70 meters in length). The first one, a torsion test and also the most difficult one, took place on Monday August 5th. Solar Impulse engineers decided to start with the one that had caused the first spar to break in order to instantly know whether the new structure would be able to handle the load. Nothing in the configuration of the test was changed. In fact, the analysis done after last year’s incident proved that the load cases, test jigs and overall setup were indeed correct.

The maximum load for the torsion test was of 4.9 tons of lead. However, the weight is applied in a way that half the load goes upward and the other half, downward consequently twisting the part. The tension in the hangar was high as the engineers looked on, in silence, secretly hoping no strange sounds would be heard.

 

The test was a complete success, to everybody’s relief. The structure of the wing spar hasn’t changed to the naked eye, but the pieces that made the cracking noise during last year’s incident, the bulk heads – small, square carbon fiber plates placed inside the spar to keep the structure from deforming – are now more numerous, adding an additional 2 kilos to the overall structure.

On August 8th, the second test – bending – took place. The wing spar is placed upside down and the weight is either focused on the inner section, the outer section (both along the x-axis, i.e. in the direction of flight) or on the z-axis (i.e. vertically). These are three separate tests to verify different flight and wind scenarios. For the bending test, a maximum of 3.5 tons are placed on the spar, basically bending the wings upwards.

The weight used for the tests is much less than what was used for HB-SIA, the first prototype solar aircraft. This is because, firstly, Solar Impulse engineers know more about load cases and what to expect from these lightweight structures and, secondly and most importantly, because the HB-SIB motor gondolas will be placed in a different position than the first plane, modifying the load requirements.

This series of tests will be completed by the end of August from which point the “dressing” of the wing will start. Solar panels, ribs, fabric and motor gondolas will be added to the structure bringing the solar wings slowly to life…

HB-SIB is ready to land

While the logistics of the final leg of the Across America mission –Washington D.C. to New York City - are being finalized, Solar Impulse engineers are advancing the construction of the second solar plane, HB-SIB.

On Thursday June 27th, ...

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While the logistics of the final leg of the Across America mission –Washington D.C. to New York City - are being finalized, Solar Impulse engineers are advancing the construction of the second solar plane, HB-SIB.

On Thursday June 27th, they did one of the landing gear tests to check the structure’s solidity at the time of landing. This first test was a dynamic one, meant to verify the reactivity of the landing gear during a hard landing.

Inside the landing gear structure there is a thing called a “crash element” which is made out of aluminum honeycomb: very light but also extremely resistant. This part is supposed to handle an impact of up to 4.3 tons and, during a tough landing, it is designed to collapse in a structured way at a defined force to avoid damaging other parts of the plane.

Solar Impulse engineers set up a pretty ingenious jig to simulate this scenario. The landing gear was placed on the ground horizontally while a crate filled with 2 tons of lead was hung on a cable next to it. During the test, the crate is pulled back and released causing an impact of the lead with the landing gear. The crash element takes the shock while also releasing a certain amount of force to avoid collapse.

The crash element is already delivered with the proper force it needs to be collapsed. The test is therefore mostly designed to check the landing gear structure and how it reacts to dynamic loads. The engineers simply verify the resulting g-loading (stress) caused by the adjusted crash load to understand how much friction occurs inside the landing gear (friction inside the crash element’s cylinder can cause higher g-loadings before it collapse).

A similar test will be done again (ultimate load), but the force will be applied statically and this time up to 6 tons. 

Slowly coming to life

HB-SIB: Solar Impulse’s second generation aircraft currently under construction in Dübendorf (Switzerland). Designed to fly around the world in 2015, the testing phase is slowly coming to an end while completion of the airplane is scheduled for the end of 2013. Flight tests will start ...

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HB-SIB: Solar Impulse’s second generation aircraft currently under construction in Dübendorf (Switzerland). Designed to fly around the world in 2015, the testing phase is slowly coming to an end while completion of the airplane is scheduled for the end of 2013. Flight tests will start early 2014.

HB-SIA: Solar Impulse’s first generation aircraft. Initially designed to prove that a solar airplane can fly day and night, it has exceeded expectations by flying across countries, connecting continents and now flying coast-to-coast across the United States from San Francisco to Washington D.C. and New York City.

This week Solar Impulse engineers are busy testing the cockpit fairing at the RUAG Large Subsonic Wind Tunnel in Emmen (Switzerland). It’s one of the largest structures in Europe and is used for a variety of tests including aircraft development, automotive research, rain tests and boat aerodynamics. The maximum blowing speed is of 68 m/s. The wooden mock cockpit will be used, enclosed in a polyurethane foam shell with covering material on the external surface. These tests are meant to verify the airworthiness of the fairing, the cabin door, and simulate the behavior of the materials during flight.

The key of this test is to see the ability to jettison the cockpit’s door during an emergency situation but also to see the overall behavior of the structure when winds are blowing from different angles. During a similar test on HB-SIA, the cockpit was mounted on the roof of a car and driven around the runway. This time Solar Impulse has decided to use a wind tunnel because all the variables that need to be tested are easier to control. In fact, the wind tunnel has a turn table module which allows tested objects to be rotated around their axis. This function allows the engineers to control the wind direction to the nearest degree and consequently study the behavior of the cockpit and materials in different situations such as during certain manoeuvers, cruising speed as well as emergency situations.

Still, this is not the only test that’s taking place at Solar Impulse. The Iron Bird, testing of HB-SIB’s electric system, is ongoing with electric engineers and electricians hard at work to ensure that all of the cockpit’s control systems are working properly when connected to each other. And finally, preparations are being made to setup the landing gear test which should take place in the coming weeks.

While HB-SIA continues its journey Across America, and certainly its last adventure, HB-SIB is slowly coming to life impatient to become the next solar airplane and discover the world. 

Photo: Courtesy of RUAG


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