It is very difficult and time consuming to describe how the wind (or water ) interact with a drag type turbine. In some phase of rotation sail moves at the same direction with the wind, while in the other it moves against it. The sail also has two faces. While the wind acts on one face at one half of the rotation, it acts on the other face in the other direction. To simplify description of the wind and turbine interaction we come up with the following conventions in this site. Please note that, older pages are not written with this convention in mind and they will be updated when ever time permits.
A positive motion of the sail is used to indicate the sail is moving in the same direction with the direction of the wind. A negative motion of the sail means that the sail is moving against the wind direction.
By positive face, it is meant that the wind hits the sail surface where the sail is in positive motion. For the Flap-Turbine the flaps are in a CLOSED position in that face. On the other hand, by negative face it is meant that when the wind hits the sail the sail is in negative motion. For the Flap-Turbine the flaps are in an OPEN position in the negative face. You should notice that, by this convention negative face of a sail is always the opposite of the positive face.
Both positive and negative faces of a drag type turbine generates drag against the wind during its operation. By convention, a positive drag will be described as drag generated on the positive face of the sail when the sail undergoes a positive motion, while a negative drag will be generated by the negative face of the sail undergoing a negative motion. The challenge for the drag type wind turbine designer is to make the positive drag as large as possible and keep the negative drag as small as possible. The useful power generated by the drag type turbine is proportional to the magnitude of the difference between the positive and the negative drags.