Quadrotor is a type of unmanned aerial vehicle that has been widely used in many applications, such as, policing, surveillance, aerial photography and agriculture. Conventionally, the control of quadrotor flight direction is accomplished by varying speeds of motors or manipulating torques. In this paper, a novel mechanism is proposed. The mechanism uses stepper motors to control the arm length for changing flight directions, while maintaining motors' speed at constant. A mathematical model has been created. The analysis results have shown that varying arm length can effectively control the moment of bending of quadrotors. Increasing the length of arms can result in the increase of the moment of bending without changing speed of motors, thus saving energies. Experimental results have shown that the new mechanism is able to carry more payloads which the motor speed can be utilized fully at 100% while the flight direction is been controlled by changing of the arm length compared to conventional flight control mechanisms.
