Correction: A photograph of a rotor used last month to illustrate a feature article on blade manufacturer Aerpac was incorrectly captioned. The photograph was of blades for the experimental Flexhat rotor tested by the Dutch national laboratory, ECN, at Petten, and not of blades for an Enercon E40, as stated. The Flexhat rotor has partial passive pitch control. We apologise for the error. Ed.