Analysis of Stability, Response and LQR Controller Design of a Small   Scale Helicopter Dynamics
  This paper presents how to use feedback controller with helicopter dynamics state space model. A simplified analysis is presented for controller design using LQR of small scale helicopters for axial and forward flights. Our approach is simple and gives the basic understanding about how to develop controller for solving the stability of linear helicopter flight dynamics. 
Hardian Reza Dharmayanda, Taesam Kang, Young Jae Lee, and Sangkyung   Sung