It's weird enough that we drive around in machines powered by dead plants and animals from millions of years ago, but flying around in machines powered by sunbeams?! The latter part of this peculiar reality is the result of more than 10 years work, which is currently seeing the Solar Impulse aircraft circumnavigate the globe fuelled by nothing more than sunshine.
While it is easy to label this project as a flight of fancy for engineers – a technical exercise to prove the possibilities without practical purpose – Solar Impulse has become all about proving people wrong. While practical outcomes are abundant (more on that later), the project has broken frontiers and is capturing the imagination of people young and old in much the same way as the early years of flight.
Engineering is often seen more widely as safe, calculated and sanitised, and it is all too easy to forget about those inspiring engineering success stories from yesteryear. Solar Impulse along with Bloodhound SSC, but very few others, have taken the torch and are doing amazing things no one thought possible. And that is making everyone sit up and listen.
"We see ourselves as pioneers but we also want to inspire," said Marc Baumgarten, a lead engineer on the project. "We want to do something that people say 'wow' and capture that spirit of adventure by technical knowhow like the space race did in the 1960s."
Solar Impulse 2 is currently on a round the world flight, which is the culmination of decades of iterations and development. But perhaps the biggest leap forward in solar flight has come in the last five years, as the team stepped up the capability of the originator, Solar Impulse 1.
"There is a long history of flight, but some of the biggest developments have been in the last five years and are now flying in Solar Impulse 2," added Baumgarten.
The most striking change has been to the overall wingspan of Solar Impulse 2. Now, at over 70m, it is just 8m shy of the world's largest airliner, the Airbus A380, and is actually greater than a Boeing 747-800. This shows the size of the project, and the aircraft itself. This is no flimsy small glider with batteries, it is a proper aeroplane.
The top of the wing, fuselage and tail plane are covered in photovoltaic cells that trickle charge four large lithium-ion batteries (weighing 633kg), which power four large motors that turn propellers. It all provides enough propulsion to cruise at a normal 27,000ft (8,500m) and 87mph (140kph).
But, at over 2,300kg, it is a wonder that Solar Impulse ever gets off the ground. And this was one of the biggest challenges for engineers: weight.
"The plane needs a lot of batteries, and batteries are heavy," explained Geri Piller, head of structural analysis at Solar Impulse. "Yet the aeroplane gets only a small amount of energy from the solar cells, so it has to be really light overall."
The batteries are essential in providing enough power. Unable to be reduced, these are a deadweight that engineers have had to design around.
Making everything lighter
Solar Impulse 1 already used most of the obvious tricks of the trade in terms of reducing weight, with all the easy pickings exhausted. It meant that any further weight reduction was going to be a daunting challenge.
"The challenge was huge as we had to make a large more powerful aircraft, but also make everything lighter and sturdier," said Piller.
The challenge was exemplified in the wingspan that needed to be increased nearly 10m. The increase, and corresponding greater surface area, was needed to go from 11,628 photovoltaic cells rating the aircraft at 45kW peak, to 17,248 photovoltaic cells and 66kW peak.
In addition, the motors increased from 7.5kW to 13kW, and propellers from 3.5m to 4m. But perhaps the biggest weight gain came from the batteries, almost doubling output from 4 x 21kWh to 4 x 41kWh, adding an additional 183kg. It was clear that more than just good design was required, thorough analysis and simulation was needed.
"We had to simulate to get the strongest lightest structure possible," said Piller. "If we were not able to simulate and try all those different iterations and optimise the structure in such as a way that the whole structure become highly optimised, I don't think we would have got off the ground."
The structural analysis team used Femap with NX Nastran software, from product lifecycle management (PLM) specialist Siemens PLM Software. While some simulation had been used on Solar Impulse 1, the challenge facing the team would be pushed to breaking point.
Despite hours of in-depth simulation effort, in July 2012 during the final structural test of the wing spar, the central part succumbed to the load and failed. It was a harsh reminder of the close margins that the aircraft design and construction has had to work within.
Many types of analysis
Swiss engineering company AeroFEM was contracted to perform special analysis like aero-elasticity and rotor dynamics. The analysts initially used the CAD geometry of the wing's outer surfaces to create a simple model to look at load paths. Later, using Femap, they added 3D solid elements representing the Kevlar-aramid paper honeycomb core for more detailed analyses such as local and global buckling.
Disproportional size v weight increase
The single-seat cockpit of the plane that will fly around the world is tiny, just 3.8m3. But it's actually three times larger than the cockpit of the Solar Impulse 1. However, despite being three times larger, it weighs less than twice as much as the original, 60kg from the original 42kg.
The wing, also, consists of a Kevlar honeycomb core covered with advanced carbon fibre composite. As well as additional weight from the increased wing area, Solar Impulse 2 also flies that bit faster, meaning its wings have to withstand greater loads.
As Femap has its own modelling functionality, composite materials that make up a large portion of the aeroplane could be improved. Carbon fibre plies were highly optimised to meet the new loading conditions with the least amount of added weight. This also allowed the team to go from a carbon fibre weave weighing 100g/m2 to 25g/m2.
Similarly, while the motor gondola has to carry a heavier load but the weight increase was kept to a minimum. This was done by, in part, by changing from a framework structure with a fairing to a sandwich structure. In addition, FEA was used to optimise components such as facings and spar caps.
The future of solar flight?
The deliverable in this pretty amazing engineering effort is that solar flight offers the potential to replace satellites. Satellites are hugely expensive to build and launch, and once in space they are virtually untouchable. Maintenance is not an option.
Solar flight, offers the possibility for drones to fly high in the atmosphere out of the way of normal flight paths, they can land and take off for maintenance, change hardware, update, or whatever. They can be quickly repurposed to a disaster area, improve mobile phone coverage during large events, or sent to a crash site. All could be able to be done more quickly than waiting for a satellite to get in position.
So while some might label this as a modern day flight of fancy, actually it has real potential to change the world.
