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Home > Library > Smart Spars: Intrinsically-Smart Composite Structures Smart Spars: Intrinsically-Smart Composite Structures Christopher Pastore and Moishe Garfinkle Novacomp 2006
An intrinsically-smart box-beam fabricated by continuous-winding without discontinuities is described and aeronautical applications relating to wing and rotor stabilization suggested. Wings with heavy tip loads such as fuel, weapons or lift engines are subject to adverse pitch-twist coupling under harsh weather conditions or during severe maneuveres, as are high aspect-ratio wings of aircraft serving HALE missions. Wing spars with intrinsic beneficial twist-bend coupling can ameliorate these adverse effects. The intrinsically-smart load-bearing box-beam architecture described herein can be fabricated by continuous-winding using conventional winding equipment and exhibits reversible twist-bend coupling. The proprietary continuous-winding architecture of the coupled beam was developed by M. Garfinkle, evaluated by C. Pastore and S. Greenhalgh, and analysed by C. Pastore, A. Bogdanovich and A. Birger. A smart textile composite is a structure tailored to exhibit a desirable
elastic deformation behavior not necessarily proportional to the imposed
load. An example of such a structure would be a box-beam so tailored that
an imposed cantilever load results in twisting as well as bending, although
no torsional load was imposed. Reversible behavior would be exhibited
if in addition an imposed torsional load results in bending as well as
twisting, although no cantilever load was imposed. Such a structure is
said to exhibit twist-bend coupling. The composite fabrication technique
required to produce such a complex response can be either intrinsically-smart
(passive) or extrinsically-smart (active): The symmetry and balance of the composite filament plies controls the elastic deformation response to loading of the composite structure. Extrinsically-Smart (Active) The sequence of actuation of piezoelectric or magnetostrictive actuators embedded between the composite plies controls the elastic deformation response to loading of the composite structure. Nomenclature Figure 1A illustrates a conventionally-wound box-beam, which is both symmetric and balanced. Because the windings have opposite angles on opposite sides of the beam, the preferential bending axis is normal to the longitudinal axis of the beam.
Figure 1. Balanced and Unbalanced Winding Architecture
Figure 2. Elastic behavior of Uncoupled (U) and Coupled (C) Box-Beams
Figure 4. Fabrication of Coupled Box-Beam by Patch Layup
In response to the crucial concerns associated with manual-layup of critical load-bearing box-beam members an alternative continuous-winding technique has been developed denote the non-symmetric quasi-unbalanced architecture. This construction has been described in detail by Garfinkle; Greenhalgh, Pastore & Garfinkle; and analysed by Bogdanovich, Pastore, Greenhalgh & Birger. Box-beams fabricated using this proprietary technique exhibit pronounced twist-bend coupling.
Figure 5. Fabrication of Quasi-Unbalanced Box Beam Two balanced symmetrical box-beams as shown in Figure 1A are wound in opposite directions and stacked as shown in Figure 5A. The combination is obviously non-symmetrical as the upper and lower winding angles are identical as seen in Figure 5B, yet the beam is ostensibly balanced. Performance of Quasi-Unbalanced Box-Beams Five prototype box-beams were fabricated from Uniweave fabric comprising 12K AS-4 carbon yarn in the warp direction and 200 denier E-Glass yarn in the weft direction. The warp yarn comprised 98% of the fabric weight. For each demonstration beam the uniweave was wound around two foam cores without laps, producing two oppositely wound helices. The two cores were then stacked and saturated with Shell 8132 resin with U40 hardener. Consolidation occurred in a closed mold at ambient temperature under a pressure of 100 kPa. The beams were then post-cured at 120 C for at least four hours. The measured winding angles of the five beams tested were 17, 29, 43 and 65 degrees as described in detail by Greenhalgh, Pastore & Garfinkle. The cross-sectional dimensions of the beams were nominally 50 mm x 50 mm.
Figure 6. Elastic Behavior of a Quasi-Unbalanced Box-Beam It is evident from Figure 6 that the observed twist-bend coupling Z is highly dependent on the fiber-placement angle. The coupling exhibited at the optimum placement angle was far greater than would be expected, with Z exceeding unity. Accordingly the twist angle exhibited significantly exceeded the bending angle directly imposed by the bending load. The curve shown was derived by Bogdanovich, Pastore, Greenhalgh & Birger using meso-volume representations of the box-beam, with the numerical data converging with finer meso-volume meshes. Application Considerations For load-bearing purposes of course longitudinal fibers would be required in the fabrication of the quasi-unbalanced wing spar as in usual practice as shown in Figure 7. The spar would be covered with a balanced wound or braided overwrap.
Figure 7. Load-Supporting Smart Box-Beam Bending of a box-beam is accompanied by warping of the sidewalls, as shown in Figures 8A and 8B. If the loading is excessive the sidewalls will fail by buckling.
Figure 8. Warping of Box-Beam Sidewalls Under Load As shown in Figure 8C however, the internal web of the quasi-unbalanced box-beam constrains the sidewalls, both increasing the resistance of the box-beam to bending deformation and to buckling failure. Discussion The first full-scale application of bend-twist coupling in wing construction involved the X-29 experimental aircraft with swept-forward wings. Although such wings delay compressibility effects and exhibits relatively benigh stalling characteristics they do exhibit divergent twist-bend coupling (Z>0) wherein upward wing bending results in upward wing pitching, destabilizing the aircraft in roll. For the X-29 the solution was to fabricate the entire wing skin using the patch-layup technique shown in Figure 4, with the fabric patches overlaping around the leading and trailing edges of the wing as described by Allburn, Retelle, Krone & Lamar. The resulting wing, shown in Figure 9A, exhibited convergent twist-bend coupling (Z<0). .
Figure 9. X-29 Wing with Unbalanced Fiber Layup The manual fabric layup of the X-29 wing, with its accompanying fabrication
inconsistancies, mitigates against the practical application of this labor-intensive
tailoring technique. As might be expected evidence of delamination of
the wing layups was found as a result of aerodynamic loading under flight
conditions. In contrast, utilization of a quasi-unbalanced box-beam shown
in Figure 7 as the wing spar, had the spar existed at that time, would
have resulted in the wing shown in Figure 9B: exhibiting convergent twist-bend
coupling (Z<0) but amenable to conventional wing construction.
Figure 10. Behavior of Coupled Airfoil The conventional wing structure (Z=0) shown in Figure 10A, when subject
to abnormal aerodynamic loading must rely on the elastic constraint of
the wing structure alone to effect damping. Such limited damping can prove
inadequate under severe loading associated with abrupt maneuvers or can
lead to oscillations when wings with substantial tip loads are involved,
as described by Goorjian, Tu & Guruswamy. When sufficiently severe
such adverse loading can lead to structural failure. Accordingly conventional
wing structures must be constructed significantly more rigidly than abnormal
aerodynamic loading would dictate, and consequently heavier. Summary
Figure 11. Potential Applications of Quasi-Unbalanced Wing Spar The convergently coupled wing structure with the quasi-unbalanced architecture is particularly advantageous for aircraft with very high aspect-ratio wings designed for loitering flights, highly maneuverable aircraft with tip stores such as ordinance, and VTOL aircraft with tip-mounted lift engines, as shown in Figure 11. Aeroelastic tailoring will permit such aircraft to be significantly more effective in conducting their missions.
References M. Garfinkle - United States Patent 5,269,657 Aerodynamically Stable Airfoil Spar; Dec 1993 - All rights Reserved E. S. Greenhalgh, C. Pastore and M. Garfinkle, A Continuous-Fiber Composite Wing Box-Beam Exhibiting Twist-Beam Coupling; Composite Engineering; 3(1993)691 E. S. Greenhalgh, C. Pastore and M. Garfinkle, Aeroelastic Smart Spar; Composite Manufacturing; 4(1993)195 A. Bogdanovich, C. Pastore, E. Greenhalgh & Birger; Analytical and Experimental Results for a Quasi-Unbalanced Composite Double Box Beam Spar; Proc. Am. Soc. for Composites, 8th Tech. Conf. Cleveland, ON Oct 95 M. Shirk, T. Hertz & T. Weisshaar; Aeroelastic Tailoring - Theory, Practice and Promise; J. Aircraft; 23(1984)6 J. Allburn, J. Retelle, J. Krone & W. Lamar; X-29 Revives the Experimental Aircraft; AIA Aerospace America, Feb. 1986; p 30 M. Goorjian, E. Tu & G. Guruswamy; Aeroelastic Computations for Wings with Preloaded Tips; Ames Research Center; NASA Tech Brief ARC-11753 H. Atanasoff & A. Vizzini - A Mfg. Process for Open-Mold Mechanically Coupled Composite Box Beams with Foam Tooling; AIAA/ASME/AHS/ASC, 28th Structures, Structural Dynamics & Materials Conf.1989 L. Corso, D. Popelka & M. Nixon; Design, Analysis and Test of a Composite Tailored Tiltrotor Wing; Am. Helicopter Soc., 53rd Annual Forum; Virginia Beach, VA; 29 Apr 97
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