## 1 Introduction  
Conventional helicopters use the rotor as the main lift component. In high-speed forward flight, the main rotor is operated in a complex mode. It provides forward thrust while providing the upward lift. This working mode suffers from disadvantages. Helicopters have notably good low-speed characteristics and can vertically take off and land (VTOL), which minimizes the requirements for a landing field. The helicopter can complete many rescue missions and heavy transport missions. In contrast, the fixed-wing aircraft offers excellent performance in the high-speed range. In a high-speed flight condition, the fixed-wing aircraft, which has a pure lift generator (wing) and a pure thrust generator (propeller), is much more efficient than a helicopter. Aviation pioneers have attempted to combine fixed wing components with the helicopter to design a better configuration since the mid-20 th century. For attack demands, many compound configurations were created to achieve better speed performance, enhanced cruise efficiency and higher altitude characteristics for aircraft such as the AH-56A Cheyenne, the Piasecki X-49A Speedhawk of 2007, the Sikorsky X2 demonstrator of 2008 and the Eurocopter X3 of 2010. The cruise speed of the compound helicopter exceeded 250 knots [1], which is a truly impressive result. The strategies of these existing products vary but the common point is that they all use propellers to create extra thrust and a wing is required to unload the rotor, except for the X2, which makes use of coaxial rotors. A fundamental compound helicopter configuration [2] uses a wing to unload the rotor and a propeller to create forward thrust in forward flight. There is some research that focuses on the application of full-scale compound helicopters. In References [3] and [4], the authors investigated the trim results and the dynamic behaviors of the compound configurations similar to X2 and X3. It was discovered that the compound configuration demonstrates higher maneuverability in certain special maneuvers. In References [5; 6; 7], the NASA Heavy Lift Rotorcraft System Investigation program gave a comprehensive analysis on multiple compound helicopter configurations. CAMRAD II program was adopted to conduct overall design and optimization. NDARC (NASA Design and Analysis of Rotorcraft) software was utilized to design a slow-rotor, which can reduce aerodynamic drag in high speed. In Reference [8], a software was designed to optimize and visualize a multi-dimensional compound helicopter design problem. In Reference [9], a compound helicopter modified based on the GENHEL UH-60A model was considered for control optimization design. However, the algorithm was time-consuming for recent practical applications. Since the volume of research into compound helicopters is small and the existing compound products are military demonstrator types, the available flight data is quite limited and additional work is required in the practical domain. Meanwhile, with the rapid development of the unmanned helicopter nowadays, the great potential of compound helicopter miniaturization cannot be denied [10]. More practical research into compound helicopters can be conducted in the small-scale domain. In the small-scale domain, some experience has been accumulated upon modeling methods. [11; 12] gave linear dynamics models of CMU (Carmegie Mellon University) R-50 helicopter under hover and 15 m/s forward flight condition. The models were obtained by using system identification method in CIFAR (developed by NASA Ames Research Center, CA, USA) software [13]. Similarly, in Reference [14], the hover and 9 m/s models of 5 kg Raptor 50 helicopter were identified from CIFAR. [15] established the nonlinear model of a 10 kg helicopter based on first physical principles. The whole model proved to have good accuracy up to 15 m/s. Unfortunately, it is difficult to find more research that focuses on small-scale helicopters above 15 m/s forward speed.