Limit cycle oscillation for an airfoil in transonic air flow considering aerodynamic nonlinearity is studied. Transonic nonlinear aerodynamic forces are calculated by solving Euler equations or Navier–Stokes equations with respect to harmonic airfoil plunging/pitching motion, and then the aerodynamic describing functions and the superposition principle are used to build an equivalent linearized aerodynamic model in the frequency domain; hence, the transonic limit cycle oscillation solutions can be obtained by the frequency domain flutter analysis method. The procedures for building the aerodynamic describing function and obtaining the limit cycle oscillation solution are described in detail. Four examples are adopted to verify the accuracy of the present method. The Isogai wing model with NACA 64A010 airfoil is used as the first example to verify the accuracy of the transonic linear flutter solution of the present method. Another NACA 64A010 airfoil model with different structural parameters is then adopted to study its transonic limit cycle oscillation characteristics considering only aerodynamic nonlinearity, and the NACA 0012 airfoil model with both aerodynamic and structural nonlinearities is investigated for its transonic limit cycle oscillation properties. The NLR 7301 airfoil is studied for its limit cycle oscillation characteristics in inviscid and viscous transonic flows, respectively. The results obtained by the present method are in good agreement with those obtained by the existing research with the harmonic balance method and time-marching method. The limitations of the present method are also discussed.

Aiming at the experimental test of the body freedom flutter for modern high aspect ratio flexible flying wing, this paper conducts a body freedom flutter wind tunnel test on a full-span flying wing flutter model. The research content is summarized as follows: (1) The full-span finite element model and aeroelastic model of an unmanned aerial vehicle for body freedom flutter wind tunnel test are established, and the structural dynamics and flutter characteristics of this vehicle are obtained through theoretical analysis. (2) Based on the preliminary theoretical analysis results, the design and manufacturing of this vehicle are completed, and the structural dynamic characteristics of the vehicle are identified through ground vibration test. Finally, the theoretical analysis model is updated and the corresponding flutter characteristics are obtained. (3) A novel quasi-free flying suspension system capable of releasing pitch, plunge and yaw degrees of freedom is designed and implemented in the wind tunnel flutter test. The influence of the nose mass balance on the flutter results is explored. The study shows that: (1) The test vehicle can exhibit body freedom flutter at low airspeeds, and the obtained flutter speed and damping characteristics are favorable for conducting the body freedom flutter wind tunnel test. (2) The designed suspension system can effectively release the degrees of freedom of pitch, plunge, and yaw. The flutter speed measured in the wind tunnel test is 9.72 m/s, and the flutter frequency is 2.18 Hz, which agree well with the theoretical results (with flutter speed of 9.49 m/s and flutter frequency of 2.03 Hz). (3) With the increasing of the mass balance at the nose, critical speed of body freedom flutter rises up and the flutter frequency gradually decreases, which also agree well with corresponding theoretical results.