1. Introduction
The high aspect ratio flexible flying wing (HARFFW) is a quite favorable configuration in unmanned aerial vehicle (UAV) development because of its excellent aerodynamic performance. Usually, the rigid body pitch mode frequency of the flying wing will increase due to the tailless design, and the elastic modal frequency decreases due to the aspect ratio and flexibility of the wing increases. The coupling of the short period pitch mode with the wing elastic mode will result in a special type of aeroelastic instability problem, which is called body freedom flutter (BFF). There are two folds of hazards coming from the BFF: (1) When dynamic pressure is lower than the flutter boundary, the handling quality of the aircraft will be adversely affected by the elastic mode of the wing. (2) Once the dynamic pressure is reaching the flutter boundary, the aircraft will undergo severe pitching oscillations and diverging wing bending motion, leading to a catastrophic aircraft failure.
Theoretical analysis methods for the BFF problem include classical frequency domain method [
1], state space method [
2], robust modeling and analysis framework research method [
3], fully coupled linearization method under aeroelastic trimming conditions [
4]. The theoretical analysis of BFF shows that for the HARFFW, the BFF speed is significantly lower than that results calculated based on the cantilever wing model; the elastic degrees of freedom of the aircraft, the structural characteristics of the fuselage and the wing structure characteristics will have significant impact on the BFF characteristics.
Since the BFF problem may affect the flight dynamics of flexible configuration with large aspect ratio or large slender ratio, several modeling schemes for this special configuration are developed. Comprehensive theoretical models [
5] are established regarding flight dynamics and aeroelasticity coupled with large body movement and small elastic deformation. Moreover, improved aeroelastic analysis method are developed considering additional terms, i.e., the influence of unsteady aerodynamic force and gravity, combing with the control system [
6]. High fidelity aeroelastic simulation analysis is also studied by implementing the computational fluid dynamics (CFD) method [
7]. More recently, flight dynamics model considering elastic effect are developed based on mean axis motion equation [
8]. The research on the multi-disciplinary optimization framework for the solar powered high altitude long endurance (HALE) UAV shows that the design of HALE aircraft is mainly subject to the stiffness constraints, which is different from the strength constraints of the traditional aircraft.
Beside the theoretical analysis, the BFF test, including wind tunnel and flight flutter test, are also quite complex elements of the testing campaign. Comprehensive experimental validation studies of BFF usually include the design and processing of the flexible flying wing aircraft and the formulation of the test scheme [
9,
10], updating of theoretical analysis model by ground vibration test [
11], and verification of theoretical analysis by wind tunnel test [
12]. When the theoretical model is updated according to the ground vibration test data, the accuracy of model modification can be improved by optimization algorithm according to the complexity of the model. One should pay close attention to the influence of model support system on the test results in wind tunnel test.
In recent years, the X-56A multi-utility technology test bed (MUTT) [
6], which is used to study the BFF and active flutter suppression of BFF vehicle, has completed the flight dynamics simulation and aeroelastic simulation, the aeroelastic modeling method is verified through the flight test data [
7]. Based on the scaled model of X-56A MUTT, BFF vehicle conceptual design [
8,
9], theoretical modeling and analysis [
13,
14] and active flutter suppression controller design [
15,
16,
17,
18] studies have been completed and verified by closed-loop flight tests have been carried out [
19,
20].
The traditional wind tunnel test include hard supporting fixtures [
21,
22], including abdominal support, tail support, back support [
23], etc. With the development of mechanism technology, parallel support system and rope traction support system have emerged [
24,
25,
26]. However, for the BFF wind tunnel test, it is necessary to release the rigid body freedom of the model to simulate the “free flight” state, especially the pitch and plunge degrees of freedom. In the BFF wind tunnel test for flying wing half model, there are side wall track support system [
27], a lateral support system installed at the bottom of the wind tunnel [
28]; in the wind tunnel test using full-span model, there are beam support system [
29] and flexible support system [
30].
To the best of the authors’ knowledge, there is no public report available on the body freedom flutter wind tunnel test of the full-span flying wing model which is capable of free flying. In this work, a BFF flying wing UAV model is designed and a novel quasi-free-flying support system is introduced into the full-span BFF wind tunnel test.
The rest of the paper is organized as follows. In
Section 2, the basic theory of BFF analysis is introduced. In
Section 3, the structural finite element method (FEM) modeling and aeroelastic modeling of the UAV along with the BFF wind tunnel test are introduced, and the preliminary theoretical analysis is conducted. In
Section 4, the design and processing of UAV are completed, and the theoretical analysis model is modified by the ground vibration test. In
Section 5, the quasi-free-flying suspension system design and the comparison between the test and theoretical results are supplied; in the final Section, the overall work is summarized and pertinent conclusions are drawn.