A robust state estimator is presented by fusing the aerodynamic model of multi-rotor UAVs with measurements from optical flow and other low-cost sensors such as IMU, magnetometer, and ultrasonic sensor. Due to the particular aerodynamics of multi-rotor UAVs, the body velocity in the rotor plane is able to be measured by the accelerometer. We therefore propose a novel state estimator by fully exploring the characteristic of aerodynamics of multi-rotor UAV. Our state estimator is fast and easy to be implemented. We have tested our estimator with different platforms in different scenes. Experimental results show that our estimator performs robustly in low light conditions where existing methods usually fail.