Body Freedom Flutter Research Articles

Effect of Inertial and Constitutive Properties on Body-Freedom Flutter for Flying Wings

The class of high-altitude long-endurance aircraft often has very slender wings with minimal structure. This leads to large trim deflections and coupling between the vehicle flight dynamics and wing vibrations. When this coupled behavior becomes unstable, it is referred to as body-freedom flutter. Body-freedom flutter behavior is dependent on the inertial and constitutive properties of the wing, as well as the fuselage. This relationship is explored for an aircraft representative of the Horten IV flying wing, using an efficient yet rigorous analysis that relies on geometrically exact, fully intrinsic beam equations and a finite-state induced flow model, which is implemented in the computer code NATASHA (which stands for nonlinear aeroelastic trim and stability of high-altitude long-endurance aircraft). The wing inertial and stiffness properties were calculated using a realistic representative section using the section analysis tool called variational asymptotic beam section analysis. Trade studies on the body-freedom flutter behavior were performed by varying the fuselage properties and the internal wing structure, as well as examining the effects on the flutter speed, flutter frequency, and flutter mode shape.

PrandtlPlane Joined Wing: Body freedom flutter, limit cycle oscillation and freeplay studies

Dynamic aeroelastic behavior of a joined-wing PrandtlPlane configuration is investigated herein. The baseline model is obtained from a configuration previously designed by partner universities through several multidisciplinary optimizations and ad hoc analyses, including detailed studies on the layout of control architecture. An equivalent structural model has then been adopted to qualitatively retain similar aeroelastic properties.Flutter and post-flutter regimes, including limit cycle oscillations (LCOs), are studied. A detailed analysis of the energy transfer between fluid and structure is carried out; the areas in which energy is extracted from the fluid are identified to gain insights on the mechanism leading to the aeroelastic instability. Starting from an existing design of control surfaces on the baseline configuration, freeplay is also considered and its effects on the aeroelastic stability properties of the joined-wing system are investigated for the first time.Both cantilever and free flying configurations are analyzed. Fuselage inertial effects are modeled and the aeroelastic properties are studied considering plunging and pitching rigid body modes. For this configuration a positive interaction between elastic and rigid body modes yields a flutter-free design (within the range of considered airspeeds).To understand the sensitivity of the system and gain insight, fuselage mass and moment of inertia are selectively varied. For a fixed pitching moment of inertia, larger fuselage mass favors body freedom flutter. When the moment of inertia is varied, a change of critical properties is observed. For smaller values the pitching mode becomes unstable, and coalescence is observed between pitching and the first elastic mode. Increasing pitching inertia, the above criticality is postponed; meanwhile, the second elastic mode becomes unstable at progressively lower speeds. For larger inertial values “cantilever” flutter properties, having coalescence of first and second elastic modes, are recovered.

Body Freedom Flutter of a Blended Wing Body Model Coupled with Flight Control System

Aeroservoelastic model is developed for a blended wing body aircraft coupled with flight control system. Body freedom flutter of such aeroservoelastic system is analyzed and trend studies considering varied loop gain value, center of gravity of the balanced weight and wing stiffness are delivered to determine their influence on body freedom flutter characteristics of this aeroservoelastic system. It is found that the closed loop flutter speed decreases dramatically compared to that of the open loop case. The loop gain value and the wing stiffness have strong influence on the closed loop flutter characteristics. It is noted that the static unstable unaugmented blended wing body aircraft can be stabilized by proper designed flight control system, however, the closed loop body freedom flutter speed is still quite low which needs further consideration in flight control system synthesis.

Body Freedom Flutter Study and Passive Flutter Suppression for a High Aspect Ratio Flying Wing Model

Normal mode and flutter analysis are conducted for a high aspect ratio aft swept flying wing model, and body freedom flutter is found to be the most critical aeroelastic instability for this air vehicle model. To determine the influence of various kinds of design parameters on BFF characteristics, eight factors are considered in the parametric study, i.e. wing vertical bending stiffness, weight and center of gravity of the wing root payload and the wing tip mass balance, wing half span, aft swept angle and the station of wing body blended line. After the parametric analysis, the mass and center of gravity of the wing root payload are selected as design variables, and the baseline model is utilized in the design optimization study subject to critical flutter speed constraint. Finally, the optimal mass balance design is suggested to suppress the body freedom flutter phenomenon passively and maximize the payload.

Model Reduction for Aeroservoelastic Systems

A model-reduction method for linear, parameter-varying systems based on parameter-varying balanced realizations is proposed for a body freedom flutter vehicle. A high-order linear, parameter-varying model with hundreds of states describes the coupling between the short period and first bending mode with additional structural bending and torsion modes that couple with the rigid body dynamics. However, these high-order state–space models result in a challenging control design, and hence a reduced-order linear, parameter-varying model is desired. The objective is to reduce the model state order across the flight envelope while retaining a common set of states in the linear, parameter-varying model. A reduced-order linear, parameter-varying model with tens of states is obtained by combining classical model reduction and parameter-varying balanced realizations reduction techniques. The resulting reduced-order model captures the unstable dynamics of the vehicle and is well suited for the synthesis of active flutter suppression controllers.

Effect of multiple engine placement on aeroelastic trim and stability of flying wing aircraft

Effects of multiple engine placement on flutter characteristics of a backswept flying wing resembling the HORTEN IV are investigated using the code NATASHA (Nonlinear Aeroelastic Trim And Stability of HALE Aircraft). Four identical engines with defined mass, inertia, and angular momentum are placed in different locations along the span with different offsets from the elastic axis while fixing the location of the aircraft c.g. The aircraft experiences body freedom flutter along with non-oscillatory instabilities that originate from flight dynamics. Multiple engine placement increases flutter speed particularly when the engines are placed in the outboard portion of the wing (60–70% span), forward of the elastic axis, while the lift to drag ratio is affected negligibly. The behavior of the sub- and supercritical eigenvalues is studied for two cases of engine placement. NATASHA captures a hump body-freedom flutter with low frequency for the clean wing case, which disappears as the engines are placed on the wings. In neither case is there any apparent coalescence between the unstable modes. NATASHA captures other non-oscillatory unstable roots with very small amplitude, apparently originating with flight dynamics. For the clean-wing case, in the absence of aerodynamic and gravitational forces, the regions of minimum kinetic energy density for the first and third bending modes are located around 60% span. For the second mode, this kinetic energy density has local minima around the 20% and 80% span. The regions of minimum kinetic energy of these modes are in agreement with calculations that show a noticeable increase in flutter speed if engines are placed forward of the elastic axis at these regions.

Engine Placement Effect on Nonlinear Trim and Stability of Flying Wing Aircraft

The effects of engine placement and sweep on the flutter characteristics of an aft-swept flying wing resembling the Horten IV are investigated using the Nonlinear Aeroelastic Trim and Stability of HALE Aircraft. The analysis was validated against the published results for divergence and flutter of swept wings and found to be in excellent agreement. Moreover, the modal frequencies and damping obtained for the Goland wing were found in excellent agreement with published results based on a new continuum-based unsteady aerodynamic formulation. This aircraft exhibits a nonoscillatory yawing instability, expected in aircraft with neither a vertical tail nor yaw control. More important, however, is the presence of a low-frequency “body-freedom flutter” mode. The aircraft center of gravity was held fixed during the study, which allowed the aircraft controls to trim similarly for each engine location and minimized flutter speed variations along the inboard span. The maximum flutter speed occurred for engine placement just outboard of 60% span with engine center of gravity forward of the elastic axis. The body-freedom flutter mode was largely unaffected by the engine placement except for cases in which the engine is placed at the wing tip and near the elastic axis. In the absence of engines, aerodynamics, and gravity, a region of minimum kinetic energy density for the first symmetric free-free bending mode is also near the 60% span. A possible relationship between the favorable flutter characteristics obtained by placing the engines at that point, and the region of minimum kinetic energy is briefly explored.

Nonlinear Aeroelasticity of a Very Flexible Blended-Wing-Body Aircraft

Blended-wing-body (BWB) aircraft with high-aspect-ratio wings is an important configuration for high-altitude long-endurance unmanned aerial vehicles (HALE UAV). Recently, Northrop Grumann created a wind tunnel model under the Air Force’s High Lift over Drag Active (HiLDA) Wing program to study the aeroelastic characteristics of blended-wing-body for a potential Sensorcraft concept. This paper presents a study on the coupled aeroelastic / flight dynamics stability and response of a BWB aircraft that is modified from the HiLDA experimental model. An effective method is used to model very flexible BWB vehicles based on a low-order aeroelastic formulation that is capable of capturing the important structural nonlinear effects and couplings with the flight dynamics degrees of freedom. A nonlinear strain-based beam finite element formulation is used. Finite-state unsteady subsonic aerodynamic loads are incorporated to be coupled with all lifting surfaces, including the flexible body. Based on the proposed model, body-freedom flutter is studied, and is compared with the flutter results with all or partial rigid-body degrees of freedom constrained. The applicability of wind tunnel aeroelastic results (where the rigid-body motion is limited) is discussed in view of the free flight conditions (with all 6 rigid-body degrees of freedom). Furthermore, effects of structural and aerodynamic nonlinearities as well as wing bending/torsion rigidity coupling on the stability and gust response are also studied in this paper.

Parametric aeroelastic stability analysis of a generic X-wing aircraft

This paper discusses the dynamic aeroelastic stability of a generic X-wing aircraft configuration. The analysis model includes rigid-body degrees of freedom and unsteady aerodynamic forces generated using the doublet lattice method. Design parameters are varied to evaluate the trends in dynamic aeroelastic stability. X-wing rotor blade sweep angle, ratio of blade mass to total vehicle mass, blade structural stiffness cross coupling, and vehicle center of gravity location were parameters considered. The typical instability encountered is body-freedom flutter involving a low-frequency interaction of the first elastic mode and the aircraft short period mode. In most parametric cases, those cases having the least static longitudinal stability demonstrated the highest flutter dynamic pressures. As mass ratio was increased, the flutter boundary decreased. The decrease was emphasized as center of gravity location was moved forward. As sweep angle varied, it was observed that the resulting increase in forward-swept blade bending amplitude relative to aft blade bending amplitude in the first elastic mode had a stabilizing effect on the flutter. b [B,] [C' [C] El [F] GJ [7] ^ [K] [Kf] L Lb [M'] mb nif mr [M] q

Comment on 'General formulation of the aeroelastic divergence of composite swept-forward wing structures'

O can only wonder at the reasons for the incessant refinements to an academic formulation of an aeroelastic problem that does not exist in flight. Reference 1 and many of its references have addressed the problem of divergence of a cantilevered composite forward-swept wing (FSW), whereas a FSW airplane will flutter in flight rather than undergo a quasistatic divergence. A number of papers' have shown that the flutter speed of a FSW aircraft is lower than the divergence speed of its cantilevered wing. A limited study' of a simple two-dimensional wing/fuselage with only a rigid-body degree of freedom in plunge also showed that the instability is always oscillatory for a large range of fuselage to wing mass ratio. (An earlier attempt to study the divergence characteristics of an unrestrained FSW configuration by a quasistatic stability analysis was unsuccessful, but indicated the necessity of a dynamic stability analysis.) A number of other papers show that an oblique-wing aircraft does not diverge either, but has a flutter speed higher than the divergence speed of its cantilevered FSW component. Weisshaar has compared the oblique wing and FSW situations: An important conclusion of these (oblique wing) studies was that the inclusion of the freedom of the oblique aircraft to roll as a rigid body during aeroelastic oscillation enhances the stability of the system. In particular, the aeroelastic divergence mode of instability associated with cantilevered forward-swept wings disappears and is replaced by a low-frequency, oscillatory instability that occurs at a higher flight speed than does divergence. (This is unlike the freely flying FSW, which experiences body freedom flutter at an airspeed lower than its divergence speed.) The inclusion of pitch and plunge rigid body freedoms further modifies the low-frequency instability speed, but not to the extent seen with the inclusion of rigid body roll freedom. The main purpose for this Comment, however, is to criticize the use of aerodynamic strip theory. The assumption of strip theory is, of course, a sine qua non for a closed-form solution. The assumption has no compensating errors in a static aeroelastic problem, in contrast to the flutter problem where it sometimes enjoys some compensation (An even number of mistakes), viz., a conservative aerodynamic stiffness may offset an unconservative aerodynamic damping, depending on

Experimental aeroelastic behavior of forward-swept graphite/epoxy wings with rigid-body freedom

An analytical and experimental investigation was made of the aeroelastic flutter and divergence behavior of graphite/epoxy forward-swept wings with rigid-body pitch and plunge freedoms present. A complete, two-sided, 30 deg forward-swept wing aircraft model was constructed and mounted with low-friction bearings in a lowspeed wind tunnel. Four different ply layup wings could be interchanged on the model, namely, [02/90]s, [152/0]s, [302/0]s, and [ -152/0]s. Wind-tunnel tests on the free*'-flying models revealed body freedom flutter, bending-torsion flutter, and a support dynamic instability that could be eliminated by proper adjustment of the support stiffness. Good agreement with linear theory was found for all the observed instabilities. Additional tests on the models with rigid-body pitch only gave lower critical speeds, while tests on the cantilever wings gave higher speeds. The [152/0] s wing gave the best tailored aeroelastic behavior.

The fundamentals of body-freedom flutter

SummaryThe object of this paper is to uncover the nature of the destabilising coupling that is the major cause of body-freedom flutter and to see whether a simple cure for the flutter can be found. To do this frequency-coalesence theory is applied to a simple aircraft. It is shown that the aircraft is liable to flutter if it has a sweptforward wing and a positive ‘tail-off’ cg margin or a sweptback wing and a negative cg margin but a simple cure for the flutter does not appear to exist.

Control of a forward-swept-wing configuration dominated by flight dynamic/aeroelastic interactions

An active control system concept for an aeroelastic wind tunnel model of a statically unstable forward-sweptwing configuration with wing-mounted stores is developed to provide acceptable longitudinal flying qualities while maintaining adequate flutter speed margin. On forward-swept-wing configurations, the inherent aeroelastic wing divergence tendency causes strong flight dynamic/aeroelastic interactions that, in certain cases, can produce a dynamic instability known as body-freedom flutter. The carriage of wing-mounted stores is shown to severely aggravate this problem. The control system developed combines a canard-based stability augmentation system with an active divergence/flutter suppression system that relies on wing-mounted sensors and a trailing-edge device (flaperon). Synergism between these two systems is exploited to obtain the flying qualities and flutter speed objectives.

Dynamic stability of flexible forward swept wing aircraft

This paper describes potential vehicle instability modes for forward swept wing aircraft and other divergence prone aircraft. Examples show that either body-freedom flutter or aircraft aeroelastic divergence may occur depending upon the airplane planform geometry and mass distribution. These vehicle instabilities may occur at speeds very different than the clamped wing aeroelastic divergence speed.