a combined fixed and rotary wing aircraft may operate in vertical takeoff mode relying on the rotary wing , exclusively , and may completely change over to flight support by the fixed wing at higher advance ratios . advance ratios may exceed not only the typical advance ratios of less than 0 . 5 but may exceed 1 , and may even exceed 2 . at the higher advance ratios , the rotary wing may be completely unloaded , the aircraft relying on the fixed wing for vertical support and airfoil lift . to maintain stability in the rotary wing , configuration , autorotation must continue . to power autorotation , without presenting a large area and drag , an anemometer - type flap system may selectively open and close to increase drag on the retreating blades , and provide minimum drag on the advancing blades .