  In order to further expand the flight envelope of quadrotors under actuator
failures, we design a nonlinear sensor-based fault-tolerant controller to
stabilize a quadrotor with failure of two opposing rotors in the high-speed
flight condition (> 8m/s). The incremental nonlinear dynamic inversion (INDI)
approach which excels in handling model uncertainties is adopted to compensate
for the significant unknown aerodynamic effects. The internal dynamics of such
an underactuated system have been analyzed, and subsequently stabilized by
re-defining the control output. The proposed method can be generalized to
control a quadrotor under single-rotor-failure and nominal conditions. For
validation, flight tests have been carried out in a large-scale open jet wind
tunnel. The position of a damaged quadrotor can be controlled in the presence
of significant wind disturbances. A linear quadratic regulator (LQR) approach
from the literature has been compared to demonstrate the advantages of the
proposed nonlinear method in the windy and high-speed flight condition.
