Introduction

Driving on wind power is nothing new. In the Netherlands, Maurits van Nassau, later Prince of Orange, introduced the sand yacht in 1862, designed by inventor Simon Stevin. They drove with an unprecedentedly high average speed of 40 km/h for that time from Scheveningen to Petten.

Figure 1 First Dutch sand yacht. Foto: Wikipedia: Zeilwagen .

A wing more effectively converts the airspeed into a force. Especially in the case of low vehicle friction, high speeds can be obtained. For example, the Greenbird. Since 2009, Richard Jenkins has set the world record for land sailing with this vehicle with a top speed of +/- 200 km/h (3 to 5 times the wind speed).

Figure 2. The Greenbird. Photo: Peter Lyons .

Figure 3. Airflow aircraft wing. Gerben49, CC BY-SA 3.0,

Lift force

A wing converts the airspeed into a force perpendicular to the direction of the airflow. At an aircraft, the air flows opposite to the flying direction, resulting in a force vertically upwards. That is why one usually speaks of lift force.

For a vehicle with a vertical upright wing, the lift force is horizontally directed. The lift force can be divided into a forwarding force and a force in lateral direction (figure 4).

A balanced wing sail

Figure 4. Balanced wing sail.

Wings usually consist of the main wing and a flap. The Angle of Attack and the flap deflection angle determine the generated lift force. Both angles are adjustable, meaning the lift force is adjustable between 0 and a maximum. Due to better efficiency, the maximum lift force of a wing sail is about twice a comparable traditional sail. The tail wing ensures the balanced wing sail always has an optimal Angle of Attack. The tail wing balances the torque caused by the main wing sail. The tail wing works the same way as the tail of an airplane. In Figure 4, the lift force generated by the wing causes a counterclockwise torque. At the same time, the tail wing generates clockwise torque. In case the wing sail is in balance, both generated torques counterbalance each other.

True wind, speed and apparent wind

The airflow around the wing of an aircraft is completely determined by the speed of the aircraft. For traditional sailing ships, true wind mainly determines airflow. For fast sail crafts, like the Greenbird, the apparent wind angle and speed change according to the combined speed and direction of each the craft and the true wind. It’s called apparent wind. The apparent wind is the vector sum of the velocity of the headwind (which is the velocity a moving object would experience in still air) plus the velocity of the true. The apparent wind speed is usually higher than the actual wind speed unless the wind is coming from behind. A wind-powered vehicle is, therefore, partly self wind generating.

Wind-powered vehicle

Figure 5 Animation wind-powered vehicle with speed limit.

On a wind-powered vehicle, we must therefore calculate with apparent wind. Apparent wind generates forwarding force and lateral force. The forwarding force determines the vehicle's speed. The lateral force can cause the vehicle to tip over. As shown in the animation in figure 5, increasing vehicle speed is rapidly increasing the lateral force, resulting eventually in tipping over. For the Greenbird, this was solved by building an extremely wide vehicle. However, this solves the tipping over but makes a road-legal vehicle impossible. For the WindWheeler, we limited the lateral force by maximizing the vehicle speed. Regenerative braking is forcing the speed limit and, the surplus of power is generating electricity.