Patent Description:
From <CIT>, a method of fabricating a thermoplastic torque box assembly is known. The method includes providing a plurality of braided thermoplastic tubular spar caps, connecting one or more connector elements to one or more of the braided thermoplastic tubular spar caps, and laying up an inner thermoplastic facesheet in a continuous manner around the one or more braided thermoplastic tubular spar caps. The method further includes attaching a plurality of skin panel stabilization elements to the inner thermoplastic facesheet to define four torque box side portions, laying up and attaching an outer thermoplastic facesheet in a continuous manner around the plurality of skin panel stabilization elements, and heating at an effective temperature and an effective pressure the thermoplastic torque box assembly. Another example can be found in document <CIT>.

Embodiments of the invention are defined by the dependent claims.

A system embodiment includes: a system comprising:a wing panel; a spar of circular or oval cross section having a cylindrical shape, disposed in the wing panel, wherein the spar comprises: an inner face sheet of the cylindrical shape; an outer face sheet of the cylindrical shape, an upper cap ;a lower cap; and a honeycomb core connected between at least a portion of the upper cap and the lower cap; wherein at least a portion of the honeycomb core is disposed between the inner face sheet and the outer face sheet.

In additional system embodiments, the wing panel comprises a semi-rigid shell. Additional system may further include an unmanned aerial vehicle (UAV), where the wing panel may be attached to the UAV. In additional system embodiments, the honeycomb core and outer face sheet may go around an entire circumference of the spar. In additional system embodiments, at least a portion of the honeycomb core and the outer face sheet extend past at least one edge of the upper cap and overlap onto the upper cap. In additional system embodiments, at least a portion of the honeycomb core and the outer face sheet may extend past at least one edge of the lower cap and overlap onto the lower cap.

In additional system embodiments, the spar may have a varying thickness. In additional system embodiments, the spar may be thicker at the upper cap than at the lower cap. In additional system embodiments, the honeycomb core may provide stabilization to the spar between the upper cap and the lower cap. In additional system embodiments, the honeycomb core stabilizes a cross section of the spar against flexing of a spar walls, such that the spar withstands positive and negative out-of-plane bending loads about a chordwise principal axis of the spar.

In additional system embodiments, the upper cap may be distal from the lower cap. In additional system embodiments, the upper cap and the lower cap are made of carbon fiber. In additional system embodiments, the honeycomb core may be made of aramid fibers.

The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principals of the invention. Like reference numerals designate corresponding parts throughout the different views. Embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which:.

With respect to <FIG>, a wing panel <NUM> of an aerial vehicle, such as an unmanned aerial vehicle (UAV), with a spar <NUM> having a honeycomb core <NUM> is depicted. UAVs are aircraft with no onboard pilot and may fly autonomously or remotely. In one embodiment, the UAV is a high altitude long endurance aircraft. In one embodiment, the UAV may have one or more motors, for example, between one and forty (<NUM>) motors, and a wingspan between one hundred (<NUM>) feet and four hundred (<NUM>) feet. In one embodiment, the UAV has a wingspan of approximately two hundred sixty (<NUM>) feet and may be propelled by a plurality of propellers coupled to a plurality of motors, for example, ten (<NUM>) electric motors, powered by a solar array covering the surface of the wing, resulting in zero emissions. Flying at an altitude of approximately sixty five thousand (<NUM>,<NUM>) feet above sea level and above the clouds, the UAV is designed for continuous, extended missions of up to months without landing.

The UAV functions optimally at high altitude and is capable of considerable periods of sustained flight without recourse to land. In one embodiment, the UAV may weigh approximately three thousand (<NUM>,<NUM>) lbs.

The wing panel <NUM> may include a light-weight, semi-rigid shell <NUM> that serves as an aerodynamic fairing. The spar <NUM> may include an upper cap <NUM>, a lower cap <NUM>, and side walls comprised of an inner face sheet <NUM>, the honeycomb core <NUM>, and an outer face sheet <NUM>. In one embodiment, the honeycomb core <NUM> and outer face sheet <NUM> go around the entire circumference of the spar <NUM>. In one embodiment, the honeycomb core <NUM> and outer face sheet <NUM> extend slightly past the edges of the upper caps <NUM> and lower caps <NUM> and overlap onto the upper caps <NUM> and lower caps <NUM>. The spar <NUM> may be of varying thickness, such that the spar <NUM> may be thickest at the upper cap <NUM>, thinner on the lower cap <NUM>, and some other thickness on the sides with the honeycomb core <NUM>. The upper caps <NUM> and lower caps <NUM> may be sized according to load, and the thickness of the upper caps <NUM> and lower caps <NUM> may vary along the span of the spar <NUM>. In one embodiment, the upper cap <NUM> thickness varies from <NUM> (<NUM> inches) to <NUM> (<NUM> inches) and the lower cap <NUM> thickness varies from <NUM> (<NUM> inches) to <NUM> (<NUM> inches). In one embodiment, the honeycomb core <NUM> thickness is <NUM> (<NUM> inches) thick inboard and <NUM> (<NUM> inches) thick outboard.

The honeycomb core <NUM> allows for stabilizing the otherwise thin side areas of the spar <NUM>. The sandwich formed by the core <NUM> and the inner and outer face sheets <NUM>, <NUM>, respectively of the spar <NUM> provides higher moment of inertia, hence more side wall stability that results in more overall strength of the spar <NUM>. More specifically, the honeycomb core <NUM> stabilizes the cross section of the spar <NUM> against flexing of the spar <NUM> walls, such that the spar <NUM> may withstand positive and negative out-of-plane bending loads about a chordwise principal axis <NUM> placed on the spar <NUM>. The upper cap <NUM> is generally of greater thickness than the lower cap <NUM> because the positive out-of-plane bending moments typically are of a larger magnitude than the negative out-of-plane bending moments. Furthermore, the upper cap <NUM> is also generally of greater thickness than the lower cap <NUM> because the material of the upper cap <NUM> and the lower cap <NUM> is better at taking tensile stress than compressive stress.

With respect to <FIG>, the spar <NUM> with the honeycomb core <NUM> is shown in cross-section. In one embodiment, the spar <NUM> is generally cylindrical as a result of being laid up on a cylindrical male mandrel. Generally, internal stresses caused by non-uniform thickness around the circumference of the spar <NUM> will result in a slightly oval cross-section of the spar after removal from the mandrel. In one embodiment, the spar <NUM> is made of carbon fiber. In one embodiment, the honeycomb core <NUM> is made of lightweight, low-density, corrugated aramid fibers, such as Kevlar (or Nomex). In another embodiment, the core is made of foam.

The spar <NUM> may be manufactured with a cylindrical mandrel to shape the spar <NUM>. Sheets (e.g., plies) of carbon fiber may be laid down to form the inner face sheet <NUM> and the upper and lower caps <NUM>, <NUM>, respectively. In one embodiment, the inner face sheet <NUM> and caps <NUM>,<NUM> are cured before applying the honeycomb core <NUM>. The honeycomb core <NUM> may then be wrapped around the thin areas on the side of the spar <NUM> and the outer face sheet <NUM> applied such that the honeycomb core <NUM> is sandwiched between an inner face sheet <NUM> and an outer face sheet <NUM>. Generally speaking, a plurality of carbon fiber face sheet plies <NUM> (see <FIG>) are wrapped around the spar <NUM> and the cap plies are stretched along its length until the desired thickness is achieved; therefore, the upper portion of the spar <NUM> may get more plies than the bottom and side portions, due to the greater desired thickness in order to withstand compression loads associated with positive out-of-plane bending. In one embodiment, the inboard ends of the caps (including the upper cap <NUM> and lower cap <NUM>) of the spar <NUM> may be approximately twice as thick as the outboard ends of the caps (depending on load).

<FIG> shows the thickness of the honeycomb core <NUM> beveled or "panned down" at the edges; such that the outer face sheet <NUM> ramps down from the top of the honeycomb core <NUM> to the level of a base <NUM>. At this junction, the outer face sheet <NUM> and the inner face sheet <NUM> come together and the honeycomb core <NUM> is no longer needed since there is sufficient stability at the caps <NUM>, <NUM>. Additional stability is provided at the other "buildups" in places along the length of the spar <NUM>. Said buildups may include reinforcements associated with, for example, wing panel spar joints, payload mounts, motor pylon mounts, and landing gear pod mounts.

With reference to <FIG>, a wing panel spar joint <NUM> is illustrated. More specifically, the wing panel spar joint <NUM> may be located between two adjacent spar sections 134a,b of spar <NUM>. In one embodiment, the two adjacent spar sections 134a,b have different diameters such that the end of one spar section can fit into the end of the other spar section with some clearance. For example, the diameter of section 134a may pan down to fit into section 134b. The distance that one spar section (e.g., section 134a) might protrude into the end of the adjacent spar section (e.g., section 134b) may be approximately <NUM> times the diameter of the smaller spar section. Bending moments may be transferred from one spar section to the other through a pair of annular spacers 138a,b, one at each end of the overlapping sections 134a,b, such the two spacers 138a,b are separated by a distance of approximately <NUM> times the diameter of the smaller spar section 134b. Each spacer 138a,b may include an outside diameter that just fits within the inside of the larger spar section 134a and a hole that fits securely around the outside of the smaller spar section 134b. In one embodiment, both spacers 138a,b are bonded to the outside of the smaller spar section 134b. In one embodiment, eccentric spacers may be used where the hole for the smaller spar section 134b is not in the center of the spacer 138b, such as when a dihedral break angle is desired at the wing panel spar joint <NUM>.

With respect to <FIG>, each spar section 134a,b may be constructed of lightweight, sandwich side walls and the honeycomb core <NUM> described above. For example, the spar <NUM> includes the upper cap <NUM>, lower cap <NUM>, and side walls comprised of the inner face sheet <NUM>, the honeycomb core <NUM>, and the outer face sheet <NUM>. The upper cap <NUM> and lower cap <NUM> of each spar section 134a,b may be constructed of relatively thick layups of unidirectional carbon fiber and epoxy where the fibers are oriented along the length of the spar section 134a,b. Each cap <NUM>,<NUM> may occupy up to <NUM>° of the circumference of the respective spar sections 134a,b. Each cap <NUM>,<NUM> may be sized to carry the required spar bending moment within predetermined cap stress thresholds. The caps of the smaller spar section 134b may be thicker than the caps of the larger spar section 134a, because both spars sections 134a,b carry approximately the same bending moment, yet the smaller spar section 134b must carrying the bending moment with less cap separation (and potentially less cap width). The thicknesses of each spar <NUM>,<NUM> may have a range from <NUM> (<NUM> inches) to <NUM> (<NUM> inches). In one embodiment, the different cap <NUM>,<NUM> and side wall thicknesses produce outsides of the spar sections 134a,b that may me slightly eccentric and not cylindrical.

Along most of the length of the spar <NUM>, the shear is modest and may be easily carried by the lightweight sandwich side walls, which may be sized for cap stability; however, the shear is high where the spar sections 134a,b overlap. This is because the force in a cap must ramp from full force where a spar section starts to overlap the other spar section to zero at the end of the spar section. The force may be transferred to the spar side walls within the overlap section as shear. The side walls in the overlap section must, therefore, be heavily reinforced. In one embodiment, the side walls are reinforced with a plurality of plies <NUM> having mostly ±<NUM>° orientation. The plies <NUM> may be wrapped around the full circumference of the spar <NUM> at the spar sections 134a,b to properly tie into the caps <NUM>,<NUM>. The shear walls in the smaller section 134a may be thick enough that no honeycomb core <NUM> is required. In one embodiment, as load is transferred from the caps to the joiner shear wall, the cap cross-sectional area may ramp down to save weight.

At the two spacer locations 138a,b, the shear may be gathered up and transferred to the spacers 138a,b. The shear transfer may be facilitated by hoop plies 136a (shown in yellow), which are unidirectional plies having <NUM>° orientation with respect to the spar <NUM> axis. In order to properly tie into joiner shear plies 136b (shown in green), the hoop plies 136a may be wrapped around the full circumference of the spar <NUM>. The hoop plies 136a may be slightly wider than the spacers 138a,b, thus resulting in additional thickening of the spar tube wall. In one embodiment, of the smaller spar, the spacers 138a,b are bonded on top of (e.g., around) said thickened areas of the spar section 134b.

Generally speaking, the edges of the plies <NUM> are stepped, such that abrupt transitions may be avoided. Additionally, the different types of plies <NUM> are interleaved (e.g., mixed). In one embodiment, the interleaving may include a plurality of cap plies 136c (shown in blue), then a joiner shear ply 136b, then a hoop ply 136a, followed by additional cap plies 136c. In one embodiment, the cap plies 136c run relatively parallel to the spar <NUM>. In one embodiment, wherever cap plies 136c are laid on top of the ramping edges of other plies (e.g., hoop plies 136a, joiner shear plies 136b), a small "joggle" is produced. The joggle may be tolerated in locations where the joggle is small and the overall layup of the plies <NUM> is thick. The hoop plies 136a may be biased toward the outside of the layup of the plies <NUM> to minimize such joggles that the hoop plies 136a may produce under the caps <NUM>,<NUM>.

Claim 1:
<NUM>. A system comprising:
a wing panel (<NUM>);
a spar (<NUM>) of circular or oval cross section,
having a cylindrical shape, disposed in the wing panel (<NUM>), wherein the spar (<NUM>) comprises:
an inner face sheet (<NUM>) of the cylindrical shape;
an outer face sheet (<NUM>) of the cylindrical shape
an upper cap (<NUM>);
a lower cap (<NUM>); and
a honeycomb core (<NUM>) connected between at least a portion of the upper cap (<NUM>) and the lower cap (<NUM>);
wherein at least a portion of the honeycomb core (<NUM>) is disposed between the inner face sheet (<NUM>) and the outer face sheet (<NUM>).