1. Nomenclature
2.
Fabrication of Coupled Box-Beams
3.
Performance of Quasi-Unbalanced Box-Beams
4.
Configurational Considerations
5.
Application Considerations
5.
Specific Applications
7.
Combined Techniques
8. Summary
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 effect of combining these diverse techniques is discussed.
1. Nomenclature
The winding sequence of continuous-wound intrinsically-smart box-beams is denoted by balance and symmetry. A winding is denoted symmetric when the fiber orientation does not change with a 180° rotation of the beam and denoted balanced when the sum of the plies oriented at any angle from the longitudinal axis of the beam is equal to the number of plies oriented at an equal angle but in the opposite direction, taking into account the windings on both the top and bottom surfaces. Balance is required for fabrication by continuous-filament winding.
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.
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Figure 1B illustrates a non-symmetrical,
unbalanced box-beam. Because the winding angles are the same on
opposite sides of the beam, the preferential bending axis is essentially
the winding angle. Hence on bending this beam will exhibit twist-bend
coupling. Non-symmetry is required for coupling. The flexural
behavior of the balanced (uncoupled) and unbalanced (coupled)
architecture are shown in Figures 2U and 2C, respectively.
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Coupling is measured analytically as the incremental change in twist angle with bend angle, as shown in Figure 3. The beam is shown twisting upwards with upwards bending, indicating divergent coupling.
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2. Fabrication of Coupled Box-Beams
Unfortunately, an unbalanced box-beam cannot be fabricated by continuous winding as is evident from Figure 1B. Rather it must be fabricated from L-shaped or U-shaped patches of unidirectional fabric, the former shown in Figure 4A. These patches are then positioned on a mandrel, as shown in Figure 4B.
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This construction was utilized by Atonasoff & Vizzini to study the coupling behavior of the non-symmetrical unbalanced architecture. This fabricating technique is also being studied by Corso, Popelka & Nixon for engineering application purposes. Because individual patches must be manually laid-up process uniformity is difficult to maintain and mechanical properties can be inconsistant. The greatest concern involving the application of such discontinuous fabric construction in life-critical applications however is the presence of longitudinal seams all along the corners of the resultant box-beam. These are zones of weakness and can constitute potential delamination-initiation regions.
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.
3. 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.
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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.
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).
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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.
Accordingly, the conventional wing construction, which does not induce convergent twist-bend coupling as shown in Figure 10U, can be retained, with the quasi-unbalanced smart wing spar shown in Figure 10C inducing the desired convergent twist-bend coupling (Z<0).
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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.
In contrast, the convergently coupled wing structure (Z<0) shown in Figure 6B, when subject to abnormal aerodynamic loading, can rely on significant aeroelastic restoring forces to effect damping of the wing structure, as described by Shirk, Hertz & Weissharr. Accordingly cost-effective convergently-coupled (Z<0) wing structures can be constructed less rigidly than required by conventional wings, and consequently can be both lighter and cheaper.
6. Specific
Application
a.
Resistance of wing spars to torque
loading is all as important as the resistance to deflection loading.
In fact tip oscillations induced by adverse air forces on wings
with heavy tip loads can be strong enough at resonance frequencies
to cause wing failure. The BA 609 tilt-rotor aircraft designed
for general-aviation applications is a prime example of an aircraft
with heavily tip-loaded wings.
The components of a short section of the complex built-up wing-support structure of the BA 609 is illustrated by Clements in Figure 11A. Fabrication of the components requires extensive hand-layup, as does fabrication of the finished wing shown in Figure 11B.
Consider now the configuration of the BA 609 wing supported by a proposed bend-twist coupled spar. The principal load-supporting structure comprises wound box-beams as shown in Figure 12A, essentially eliminating manual fabrication of not only the beams but the overwrapping. Moreover box-beam construction will permit a lighter structure than the present built-up structure because load-bearing seams are eliminated along the wing shown in Figure 12B.
The coupled-spar wing will not only resist torque loads imposed on the wing but will actively compensate for wing twisting moments arising from lift-engine twisting about its support structure.
b. Larger aircraft, either military or commercial, require fuel cells in the wing structure. These can take many forms, but to be compatible with the coupled spar probably tubular cells, such as illustrated in Figure 13 would be the most practical.
The coupled spars themselves would serve as fuel cells without diminishing their effectiveness.
7. Combined Techniques
Perhaps the greatest benefit of smart structures will ultimately be attained by combining the architecture of the intrinsically-smart passive structure with the control of the extrinsically-smart active structure. Coupling would be attained using fiber architecture while fine control would be achieved using actuators. In this manner large elastic deformations with fine control could be realized without causing any structural delaminations.
8. Summary
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 14. Aeroelastic tailoring will permit such aircraft
to be significantly more effective in conducting their missions.
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