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Record-breaking wind-powered car gives a glimpse of the future

21st February 2013


Just as the Formula 1 technologies of today filter down into the production cars of tomorrow, so Richard Jenkins, the designer of the wind-powered Greenbird, believes the technologies behind his record-breaking vehicle could herald a new era of environmentally-friendly transportation. Jon Severn reports.

On the morning of 26th March 2009 at Ivanpah Dry Lake on the California/Nevada border, USA, the Ecotricity Greenbird broke the world land speed record for wind-powered vehicles. Greenbird recorded a top speed of 126.4mph (203.4km/h), and sustained a speed of 126.2mph (203.1km/h) for the required time of three seconds, beating the previous, American held, record of 116mph (186.7km/h), set by Bob Schumacher in the Iron Duck in March 1999 at the same location (Fig.1).

The Greenbird project is a partnership between Ecotricity (which describes itself as the UK's first and largest independent 'green' electricity company) and engineer Richard Jenkins. It builds on Jenkins work as founder of the Windjet project, which he started in 1999 while studying mechanical engineering at Imperial College, London. All-British, the Greenbird team is independent, passionate and committed to a vision of a zero-carbon transportation future.

From the early Windjet designs to the latest Greenbird, Jenkins has carried a vision and passion for wind-powered travel. He brings extensive design, engineering, construction and piloting experience to the project. Ecotricity, founded and run by Dale Vince, brings to the partnership a vision for, commitment to and experience of renewable energy, wind technology and engineering.

Both Jenkins and Vince have successful histories of entrepreneurship, independence and world-class achievement against tremendous obstacles. Together, this team is shaping a glimpse of the future through cutting-edge research and design, and then the practical application of that work in producing new technologies for wind engineering and transport.

Vehicle design

As well as being elegant, sophisticated and futuristic, Greenbird is nevertheless based on familiar principles borrowed from aeronautical, sailing and wind technologies. In conventional vehicles, more power almost always equates to more speed. With land sailing, however, more wind does not necessarily mean more speed. Instead a design is required in which aerofoil lift (translated to forward thrust in a land-based craft) is maximised but, more importantly, drag is minimised. Greenbird tackles this problem by combining racing yacht, aeroplane and Formula 1 racing car concepts and technologies (Fig.2).

The main structure of the vehicle is made from carbon-fibre composite materials that have the high stiffness and strength required to transfer the wind loads on the Greenbird's 'sail' (actually a rigid vertical wing) into traction forces on the ground. And the structure really does have to be extremely strong, with the vehicle taking huge forces when travelling at high speeds: Greenbird only weighs 600kg but at high speeds it is transferring up to one tonne of side force into the ground.

Crucial to the success of the design is harnessing and optimising airflow in an efficient way. Greenbird's vertical wing generates thrust in the same manner that an aeroplane wing generates lift. The pressure differential over the wing creates forces that are translated into forward thrust. To prevent the craft from tipping over, a horizontal wing uses the effects of aerodynamics and weight to keep the Greenbird firmly on the ground (Fig.3).

The design of the vertical wing, combined with the low drag and rolling friction, delivers exceptional efficiency, enabling the Greenbird to travel at between three and five times the real wind speed, depending on the surface traction.

Importantly for the record attempt, Greenbird is able to travel faster than the prevailing wind speed thanks to the phenomenon known as apparent wind. Apparent wind is the wind flow across a sail that is created by the combination (the vector sum) of true (or prevailing) wind and the relative wind velocity of the forward movement of the craft. When all conditions are optimised, the apparent wind is greater than the true wind. Sailing perpendicular to the true wind allows the fastest speeds to be achieved.

Starting the vehicle is a simple matter of removing the chocks from in front of the wheels, at which point it accelerates slowly up to about 50mph (80km/h), but then gathers momentum and takes just eight seconds more to reach 100mph (160km/h). With no brakes, stopping the vehicle involves reversing the wing to create aerodynamic drag, which gradually slows it down.

Greenbird has a pencil-shaped fuselage with a continuous structural roof that protects the two occupants. The front cockpit is somewhat cramped, containing the steering wheel and 'throttle,' while all of the electronics and timing gear are contained in, and monitored from, the rear cockpit.

Evolution

The Greenbird is an evolution of Jenkins' WindjetMkIV, which recorded speeds of 90mph (145km/h) in winds of 20-25mph (32-40km/h) in 2007. These test runs suggested key modifications that have been incorporated into the Greenbird design. For example, the main tail was redesigned to control power and acceleration more accurately, the rear of the vehicle has been reworked to put more load onto the rear tyre, and the steering position has been moved from the rear cockpit position to the front cockpit position to give better visibility.

Finally, Greenbird uses a shorter, stiffer outrigger, so that it relies more on weight to provide heeling moment rather than aerodynamic downforce.

A major design challenge is striking the right balance between speed and controllability. Going fast means transferring large forces to the ground, for which the vehicle needs to be strong and relatively stiff. But this presents problems for a structure that also needs to absorb bumps over a range of speeds, so sophisticated suspension is required. Greenbird builds on the Windjet design, using a pneumatic front suspension to deal with the bumpy desert surface; the suspension provides a degree of compliance between the vehicle and the surface, giving a smooth ride at high speed.

A hydraulic mechanism is used to steer the craft, giving it greater reliability than an earlier design that used a mechanical steering link. The pneumatic front suspension is very intensive on the components involved, and the mechanical steering link broke at the front wheel. Installing rugged hydraulic steering has removed this weak link.

While an earlier prototype had brakes, these became clogged with dirt during a test run, creating drag; they were therefore removed, with deceleration now relying on aerodynamic drag when the sail is reversed.

Future developments

With the land-speed record broken, the team now has its sights on new challenges. Plans are in place for an assault on the ice speed record in the winter with a Greenbird ice craft equipped with blades instead of wheels. There is still some debate within the team over whether ice or land will be faster.

Longer term, there is potential to reap benefits from the technologies incorporated within Greenbird in terms of both renewable energy supply and wind-powered vehicles suitable for everyday use. This comes at an opportune time, as Vince and Jenkins see the world at a watershed: the age of fossil fuels is passing and there is a need to progress (or revert) to renewable energy sources. Greenbird does more than symbolise this; it offers the potential for technologies that could make a real difference.

In the words its founder, Ecotricity is all about changing things - the journey to low-carbon living and the next Industrial Revolution. It empowers people to bring about this change through their electricity bills; by choosing to purchase electricity from this company, consumers are boosting directly the demand for new wind turbines. The technology used in the development of the Greenbird, and lessons learned from the record attempt, will ultimately be incorporated into designs for the next generation of super-efficient micro turbines, a technology that Ecotricity is developing to bring wind power to the masses.

For his part, record attempts aside, Jenkins is trying to raise a serious question: with fossil fuels running out, how are we all going to get around in a world without oil? He believes the answer may be wind-powered cars (Fig.4). The Greenbird, of course, is a single-purpose, thoroughbred machine, optimised for breaking records - but it is not a vehicle in which you could go to the shops. However, Jenkins says that a second-generation wind car is not too far away, and this could be one that you really could use daily.







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