There were problems with the stall induced vibrations that caused cracks. The non-rotating edgewise natural frequency of the blade is measured and amounts to 2.3 Hz. This is too close to the higher harmony 6p. That is why Ngup enhanced the edgewise natural frequency to 2.6 Hz. The reinforcements can be done at reasonable cost. After the tests in our workshop the two prototype sets were tested in strong wind conditions.