Multibody Aircraft Research Articles

Trim Analysis and Structured H-Infinity Control of Wing-Docked Multibody Aircraft

A wing-docked multibody aircraft is a novel configuration being explored as a balanced solution between efficiency and structural resilience for very long-endurance aviation. The presence of wingtip hinges complicates the dynamics and control of this class of aircraft but has thus far only been analyzed in specific cases. In this work, we make a comprehensive exploration of parameter space of the multibody trim problem and stability and demonstrate the treatment of the control problem using the robust H∞ synthesis technique. We propose a simplified simulation approach to the static trim analysis and demonstrate key observations relating to the configurations and open-loop dynamic stability of multibody aircraft in general. An interpolated parameter-scheduling model using the hinge angle as a scheduling parameter is established, based on which the structured multimodel H∞ synthesis method is employed to synthesize the robust flight controllers. Simulation results show that the designed controllers are capable of configuration control within the range of possible trim solutions and have also achieved good control effectiveness in path following and disturbance rejection. The proposed trim analysis and control techniques serve to provide a systematic benchmark study on wing-docked multibody aircraft dynamics, which is valuable to the development of this research area.

Dynamics modeling and flight characteristics of folded multi-body aircraft

This paper studies the dynamic modeling method of fold-able multi-body aircraft based on the dynamics theory of multi-body system. Considering the multi-body aircraft as a complete multi-rigid body system, the dynamic model for the free unfolding process of the multi-body aircraft after horizontal launch is established. On the basis of the multi-body dynamics model, the coil spring moment model and the damping moment model for the flexible connection between the flight units are added. Taking a certain type of fold-able three-section wing aircraft as an example, the numerical simulation calculation is carried out. The influence of the flexible connection stiffness coefficient between the flight unit on the dynamic characteristics of the fold-able multi-body aircraft in the free deployment process and the relationship between the stiffness of the flexible connection and the maximum initial folding angle are studied. Finally, the stability envelope between the stiffness coefficient of the flexible connection and the maximum initial folding angle is determined.

Modal Analysis and Flight Validation of Compound Multi-Body Aircraft

The flight dynamics of a compound aircraft are highly coupled and significantly different from those of a single aircraft. In this paper, the characteristic flight dynamics of a three-unit connected aircraft are analyzed in detail. By distinguishing the aircraft’s modes into a symmetric mode group and an asymmetric mode group, the mechanisms by which the relative roll modes of the compound aircraft significantly impact the rigid-body modes of the vehicle become clear. The trim configuration of a compound aircraft has a profound influence on its stability and flight modes. In addition, the relative roll modes can be separated from the full-state model by appropriate simplifications, and the modal coupling mechanism and evolution law can be analytically derived. The key parameters which have a significant influence on the trim configuration and flight modes of the aircraft are pointed out in this paper. Finally, the common failure modes of compound aircraft are identified. A flight experiment with test data verifies the correctness of the above theoretical analysis.

Experimental Investigation of the Coupling Process of Laminar Wings at Their Tips

In this paper, we address the influence of geometric parameters relevant to the lateral in-flight coupling of two laminar wings at a Reynolds number of . The aerodynamic loads are determined by means of detailed force and pressure measurements for different wing spacings and angle-of-attack combinations. When the spanwise spacing between the wings is reduced, increased lift and decreased drag coefficients are noted, which are mainly explained by the alteration of wing-tip vortices. We also show that the gap flow developing between both wings at small distances induces a suction effect, that is, strong lateral forces of opposite signs acting on the two wings. When the angles of attack of both wings are varied for a fixed-wing spacing, a strong interaction between the two wings is found to affect the lift and drag coefficients. The results presented in this paper help understand the changes in aerodynamic loads that occur during the wing–tip-coupling process, and thus may promote the design and operation of such procedures in for future multibody aircraft.

Conceptual design and flight test of two wingtip-docked multi-body aircraft

To overcome the drawbacks such as large wing deformations, poor performance encountering gusts, limits in taking off and landing, inconvenience of transportation of High-Altitude Long-Endurance (HALE) Unmanned Aerial Vehicles (UAVs), a new conceptual aircraft called wingtip-docked Multi-Body Aircraft (MBA) has attracted lots of attentions. Aiming to investigate the feasibility of this concept, two UAV models were designed, manufactured and connected by a wingtip-docking mechanism, which only allows the relative roll motion between the two aircraft. The trim solution of the two connected aircraft is firstly obtained by solving the developed nonlinear flight dynamic equations, followed by the stability analysis based on the linearized model. The results show that the connected aircraft is inherently unstable and cannot fly without a reasonable flight control system. A set of Proportional-Integral-Derivative (PID) control laws was then developed and implemented in the two experimental aircraft. The success of the flight tests show that the flight control can effectively eliminate the unstable motion and the wingtip-docked MBA is controllable and feasible.

Modeling and Control of an Articulated Multibody Aircraft

Insects use dynamic articulation and actuation of their abdomen and other appendages to augment aerodynamic flight control. These dynamic phenomena in flight serve many purposes, including maintaining balance, enhancing stability, and extending maneuverability. The behaviors have been observed and measured by biologists but have not been well modeled in a flight dynamics framework. Biological appendages are generally comparatively large, actuated in rotation, and serve multiple biological functions. Technological moving masses for flight control have tended to be compact, translational, internally mounted and dedicated to the task. Many flight characteristics of biological flyers far exceed any technological flyers on the same scale. Mathematical tools that support modern control techniques to explore and manage these actuator functions may unlock new opportunities to achieve agility. The compact tensor model of multibody aircraft flight dynamics developed here allows unified dynamic and aerodynamic simulation and control of bioinspired aircraft with wings and any number of idealized appendage masses. The demonstrated aircraft model was a dragonfly-like fixed-wing aircraft. The control effect of the moving abdomen was comparable to the control surfaces, with lateral abdominal motion substituting for an aerodynamic rudder to achieve coordinated turns. Vertical fuselage motion achieved the same effect as an elevator, and included potentially useful transient torque reactions both up and down. The best performance was achieved when both moving masses and control surfaces were employed in the control solution. An aircraft with fuselage actuation combined with conventional control surfaces could be managed with a modern optimal controller designed using the multibody flight dynamics model presented here.

Applying Eigenstructure Assignment to Inner-Loop Flight Control Laws for a Multibody Aircraft

Unmanned aircraft used as high-altitude platform system has been studied in research and industry as alternative technologies to satellites. Regarding actual operation and flight performance of such systems, multibody aircraft seems to be a promising aircraft configuration. In terms of flight dynamics, this aircraft strongly differs from classical rigid-body and flexible aircraft, because a strong interference between flight mechanic and formation modes occurs. For unmanned operation in the stratosphere, flight control laws are required. While control theory generally provides a number of approaches, the specific flight physics characteristics can be only partially considered. This paper addresses a flight control law approach based on a physically exact target model of the multibody aircraft dynamics rather than conventionally considering the system dynamics only. In the target model, hypothetical spring and damping elements at the joints are included into the equations of motion to transfer the configuration of a highly flexible multibody aircraft into one similar to a classical rigid-body aircraft. The differences between both types of aircraft are reflected in the eigenvalues and eigenvectors. Using the eigenstructure assignment, the desired damping and stiffness are established by the inner-loop flight control law. In contrast to other methods, this procedure allows a straightforward control law design for a multibody aircraft based on a physical reference model.

FLIGHT DYNAMICS AND STABLE CONTROL ANALYSES OF MULTI-BODY AIRCRAFT

Multi-body aircraft is a new concept aircraft consisting of multiple small unmanned aerial vehicles hinged-connected by wing tips. This concept aircraft can integrate the performance and advantages of two types of flying platforms: small unmanned aerial vehicles and high-altitude long endurance unmanned aerial vehicles. It has great development potential. Based on the Newton-Euler equation and on the lifting line method, it establishes a multi-body aircraft flight dynamics model, and analyzes the trim and stability characteristics of the multi-body aircraft system that allowing the relative degree of freedom of rolling motion with the dual-aircraft combination. The results show that, different from the traditional configuration aircraft, the multi-body aircraft system has an unstable relative motion flight mode, which is dominated by relative rolling motion. This configuration aircraft cannot fly stably in an uncontrolled situation. Based on the completion of the dynamics modeling, the Proportion-Integration-Differentiation control method is used for stable control. The stable control loop is designed separately for each single aircraft to achieve the purpose of stable flight. The simulation results show that the control method is effective and can quickly stabilize the divergent flight dynamics system.

Scalar Reference Governor for Constrained Maneuver and Shape Control of Nonlinear Multibody Aircraft

A control system for constrained control of a nonlinear multibody aircraft is described which exploits a combination of a scalar reference governor and a Linear Quadratic Integral (LQ-I) controller for maneuver load alleviation, trajectory control and shape control. A nonlinear multibody aircraft model for longitudinal flight is used to develop and validate the control system performance. The scalar reference governor is designed based on the linearized model and ensures that the imposed state and control constraints are satisfied. The mismatch between the linearized model used by the reference governor for prediction and the nonlinear dynamics of the multibody aircraft is accounted for in the reference governor design. The paper demonstrates the feasibility of the adopted control approach, including constraint enforcement, based on the nonlinear model simulations of pitching maneuvers.

Exploring multi-stage shape optimization strategy of multi-body geometries using Kriging-based model and adjoint method

This paper deals with an efficient and high-fidelity design strategy for wing-body configuration. According to the nature of the design space and the number of design variables, aerodynamic shape optimization is carried out at each design stage by using a selective optimization strategy. In the first stage, global optimization techniques are applied to wing planform design with a few geometric design variables. In the second stage, local optimization techniques are employed to wing surface design with many design variables, which can maintain a sufficient design space with a high Degree of Freedom (DOF) geometric change. For global optimization, the Kriging method in conjunction with a Genetic Algorithm (GA) is used. A searching algorithm exploiting Expected Improvement (EI) design points is introduced to efficiently enhance the quality of the initial Kriging model for the wing planform design. For local optimization, a discrete adjoint method is adopted to obtain sensitivity information by fully hand-differentiating the three-dimensional Euler and N–S equations on an overset mesh topology. By the successive use of the global and local optimization methods, the drag of a multi-body aircraft configuration can be minimized for inviscid and viscous flow conditions while the baseline lift and wing weight are maintained. Throughout the design process, the performances of the test models are improved, compared to those with the single stage design approach. The performance of the proposed multi-stage design framework is evaluated by the drag decomposition method.

DESIGN AND SIMULATION OF SEMI-ACTIVE LANDING GEARS FOR TRANSPORT AIRCRAFT1*

ABSTRACT This paper describes an integrated design process for control laws for semi-active aircraft landing gears, addresses interfaces required for the implementation of different computer aided engineering (CAE) tools used in the process, and presents a multibody aircraft simulation model. Three control laws, a skyhook-type controller, a fuzzy-logic controller, and a state feedback controller, are designed, and the control parameters of all three controllers optimized using a multi-objective parameter optimization. Finally, simulation results for passive, fully active, and semi-active systems are compared. *Communicated by D. Bestle.

Vibration Control of Flexible Multibody Aircraft During Touchdown Impacts

A method is presented for the dynamic analysis and vibration control of large scale flexible multibody aircraft during the touchdown impacts and rollover motion. The rollover motion is simulated by modeling the uneven runway profile as a stationary zero mean space dependent random process defined by its spectral shape. The prescribed motions at the two landing gears will then represent a different time dependent random process with spectral shapes changing with the change in velocity of the aircraft. Composite materials which provide higher stiffness-to-weight ratios are used as a passive control system to reduce the aircraft vibration. The results of the numerical example presented showed that the use of composites can have a significant effect on attenuating the vibration of the aircraft during the touchdown impact and rollover motion. The numerical results are obtained using the general purpose computer program DAMS (Dynamic Analysis of Multibody System).

Timoshenko beams and flexible multibody system dynamics

A method for the dynamic analysis of flexible multibody systems that accounts for rotary inertia and shear deformation effects is presented. Flexible components in the system are discretized by using the finite element method. Because of the large rotations of the system components, a set of reference co-ordinates are employed to describe the motion of a selected body reference. Kinetic and strain energies are derived for each element, thus identifying the element mass and stiffness matrices which account for the rotary inertia and shear deformation effects. A new set of time invariant element matrices that describe the coupling between the reference motion and elastic deformations is developed. These matrices need be evaluated only once in advance for the dynamic analysis. It has been, shown that the form of these matrices as well as the mass and stiffness matrices are significantly affected by the inclusion of rotary inertia and shear deformation. The formulation presented in this paper is exemplified by a two-dimensional flexible multibody aircraft. Numerical results indicate that rotary inertia and shear have a significant effect on the dynamic response of large scale flexible multibody systems with large angular rotations and in which the reference motion and elastic deformation are coupled.

Effect of geometric elastic non-linearities on the impact response of flexible multi-body systems

The intermittent motion behavior of large scale geometrically non-linear flexible multi-body systems due to impact loading is investigated. Impacts and the associated impulsive forces are incorporated into the dynamic formulation by using a generalized momentum balance. The solution of the momentum equation provides the jump discontinuities in the system velocities and reaction forces. Flexible components in the system are discretized by using the finite element method. Because of the large rotations of the system components, a set of reference co-ordinates are employed to describe the motion of a selected body reference. The rigid body modes of the finite element shape functions are eliminated by using a set of reference conditions and accordingly a unique displacement field is defined. In order to account for the inertia and elastic non-linearities which are, respectively, the results of the large rotations and finite deformations, the system inertia and stiffness characteristics have to be iteratively updated. Two numerical examples of different nature are presented. The first example is a high speed slider crank mechanism with a flexible connecting rod. In the second example, however, the dynamic response of a flexible multi-body aircraft during the touch down impact is predicted.

Transient analysis of flexible multi-body systems. Part II: Application to aircraft landing

This investigation presents a method for transient analysis of a large-scale multi-body aircraft consisting of interconnected rigid and flexible bodies that undergo large angular rotations. Elastic components of the aircraft are discretized using the finite element method. The system equations of motion and nonlinear algebraic constraint equations describing joints between different components are written in the Lagrangian formulation using a finite set of coupled reference and modal coordinates. The system differential equations of motion and algebraic constraint equations are computer-generated and integrated forward in time using an explicit-implicit direct numerical integration algorithm coupled with a Newton-Raphson type iteration in order to check on constraint violations. Impact and intermittent motion events are accounted for by using a generalized momentum balance that predict jump discontinuities in the generalized velocities as well as jump discontinuities in the system reaction forces. The formulation presented and the computer program developed are used to simulate the impact between the landing gear and the runway. The method is also used to predict the dynamic behavior of the aircraft during the traverse of an abrupt elevation change in the runway.

Pulse control of flexible multibody systems

An active pulse control method is developed to reduce the vibrations of multibody systems resulting from impact loadings. The pulse, which is a function of system generalized coordinates and velocities, is determined analytically using energy and momentum balance equations of the impacting bodies. Elastic components in the multibody system are discretized using the finite element method. The system equations of motions and nonlinear algebraic constraint equations describing mechanical joints between different components are written in the Lagrangian formulation using a finite set of coupled reference position and local elastic generalized coordinates. A set of independent differential equations are identified by the generalized coordinate partitioning of the constraint Jacobian matrix. These equations are written in the state space formulation and integrated forward in time using a direct numerical integration method. Dependent coordinates are then determined using the constraint kinematic relations. Points in time at which impact occurs are monitored by an impact predictor function, which controls the integration algorithms and forces for the solution of the momentum relation, to define the jump discontinuities in the composite velocity vector as well as the system reaction forces. The effectiveness of the active pulse control in reducing the vibration of flexible multibody aircraft during the touchdown impact is investigated and numerical results are presented.