A curved, inflated, tubular beam consists of braided fibers and axial fibers on an elastomeric barrier. The construction can be accomplished on a short, straight mandrel. The curvature along the beam can be varied to suit the design needs. The angle of the braid in the bias fibers determines the inflated curvature when axial fibers situated within the braid along the inside of the curvature constrain the elongation on the inside of the curvature. The curved shape can be reinforced by having tape cemented to the outside of the inflated tube. While very small and very large tubes can be perfected for a range of inflation pressures and beam strengths, the preferred embodiment is a 12 3/4-inch diameter tube, 60 feet long forming an arch for a tent 30 feet wide and 24 feet high.

BRIEF SUMMARY OF THE INVENTION 
1. Field of the Invention 
This invention applies to structural support elements which are tubes of 
flexible material inflated to develop their rigidity. They consist of 
fibers for strength and elastomeric material to contain the inflation 
medium. Such an inflatable tube is especially useful where light weight is 
an advantage and where compact storage of the uninflated item is desired. 
Inflatable, arched beams are used in rapidly deployable shelters. The 
advantages over conventional, rigid, structural elements are a reduced 
time and less labor being required to erect the shelter plus a lower total 
weight for transportation. 
Inflatable beams are also used for spars in deployable wings. Wings with an 
arch, such as parafoils and para-gliders, require curved spars. The 
advantages of using an internally-pressured structure to support the wing, 
compared to ram-air inflation of the entire wing, are a reduced line count 
and an improved structural stability on high aspect-ratio wings. The 
benefits are increased performance and improved safety. 
2. Description of the Related Art 
The current method of construction of inflated, curved, tubular beams 
involves the manufacture of beams having the same arched or curved shape 
as is desired in the final part at the time of the fabrication. The 
fabrication can also include the use of coated fabrics which are cut, 
laid-up, and fabricated to the desired shape by stitching, adhesive 
bonding or thermally welding fabric pieces together. Another method is to 
braid fibers directly onto a curved mandrel duplicating the desired shape 
of the final part. With this method the axial fibers fill the full 
circumference of the tube and fix the shape of the tube. The fibers 
braided on a bias angle are set at a very high angle to the axis. They 
contribute primarily to hoop strength. A given mandrel size and shape is 
required for a given, specific curved beam or arch. 
A curved mandrel poses special problems in the positioning and traversing 
of the mandrel with respect to the braiding machine. It can be difficult, 
if not impossible, to remove the part from the mandrel. Also, the mandrel 
sweeps through a large amount of floor space. These difficulties are 
greatest for the very large arched beams that are parts for large 
shelters. A technique is needed to reduce the size of floor area required 
for the braiding of the beam fibers and to allow changes in shape without 
involving complete substitution of large mandrels. 
Low-pressure fabric beams are known. Arches are formed in these by darts or 
seams. Such beams are typically inflated to 3 to 6 psi. They have very 
limited capability for large shelters. 
Tubular weaving is known, as is the method for pulling unequal lengths of 
warp yarn to produce curved tubes. The difficulty in maintaining 
consistent tension in the fill yarns where the yarns change directions to 
the opposite layer of the weave has prevented beams made in this way from 
achieving more than a small fraction of their design burst strength. 
Failure modes have been sudden and explosive. 
Two-dimensional and three-dimensional braiding of curved beams on curved 
mandrels is known. Arched beams of small size have been produced by this 
method. The difficulty and cost of producing and manipulating very large 
curved mandrels is a severe limitation of this method of construction. 
SUMMARY OF THE INVENTION 
The present invention is a curved, inflated, tubular beam which gets its 
curvature, variable or constant along its length, upon inflation as 
determined by the arrangement of the fibers comprising its outer surface. 
This arrangement of fibers includes bias fibers applied by a braiding 
method and axial fibers interwoven with the braid, applied over only a 
fraction of the tube's circumference. The bias fibers are applied at an 
angle that would cause the tube to elongate upon inflation, were the axial 
fibers not present. Because the axial fibers are present on only one side 
of the tube, and constrain that side of the tube to a fixed length, the 
tube curves upon inflation instead of elongating. The degree of curvature 
can be closely controlled, and optionally varied along the length of the 
tube, by controlling the angle of the bias fibers. This design of fiber 
arrangement can be manufactured economically by braiding onto a straight 
mandrel. The bias angle can be accurately controlled by controlling the 
mandrel traverse rate in proportion to the speed of the braider. 
The completed beam consists of a tube of thin elastomeric film over which 
the fibers are braided. The elastomeric tube is placed over the mandrel, 
then an adhesive is applied to the tube just prior to applying the braid. 
The elastomeric tube serves the dual purposes of stabilizing the fiber 
arrangement and of providing an inflation gas barrier. Optionally, the 
braided fibers can be impregnated with an elastomeric solution that forms 
the gas barrier after curing. Also, optionally, the fibers can be 
impregnated with a solution that forms a tough outer surface after curing. 
The strength and number of fibers in the braid are engineered to be 
adequate for the intended pressure in the inflated tube. Additionally, the 
braid is loaded with longitudinal (axial) fibers or yarn over a portion of 
the circumference on the inside of the intended curvature. In coordination 
with the bias angle of the braided fibers, the longitudinal fibers are 
tensioned upon inflation, maintaining the desired curvature. 
Optionally, fibers in tape or other form can be applied longitudinally to 
the outside of the curvature of an inflated, curved beam as described 
above. The addition of longitudinal fibers opposite the axial fibers 
included in the braid increases the bending stiffness of the beam for 
those applications where this is needed. 
The fibers which give burst-resisting strength to the tube are braided in a 
design which determines the curvature of a braided tube. The principle 
governing the curve formation is that a braided tube of bias angle greater 
than 54.7 deg. will grow in length (and shrink in diameter) as it is 
pressurized until the elongation reduces the bias angle to 54.7 deg, at 
which point no further elongation will occur with additional pressure. 
With the inextensible fibers added to one side of the braid, the tube 
tends to bend instead of elongating. 
The radius of curvature R is determined by the trellising of the fibers 
which allows the surface of the outside of the curve to elongate an amount 
e (dimensionless, e.g. inches per inch). Where d is the diameter of the 
tube, the radius of curvature of the tube is determined by 
EQU d/R=e 
With no loading of axial fibers, thus a balanced braid, the bias angle 
determines the elongation of the surface on the outside of the curve. The 
amount of elongation, while depending primarily on the bias angle, depends 
also on the elasticity of the liner and other factors. The amount of 
elongation increases approximately in proportion to the amount that the 
bias angle exceeds 54.7 degrees. 
This discussion outlines the principles of how the curve is developed. It 
is also possible to express these principles in terms of equations that 
are useful for engineering. However, since the liner elasticity and other 
factors also affect curvature, in practice, short sections are constructed 
to verify each braid design. 
The axial fibers are placed over less than 60 degrees of the tube 
circumference. Too wide a "stripe" results in non-uniform loading of 
stress within the fibers, i.e., stress concentration at the edges of the 
strip. 
With the same braider and mandrel setup one can vary the final shape of the 
beam by controlling the braid bias angle. A bias angle of 54.7 deg. will 
produce a straight beam. Higher bias angles produce curvature that is 
progressively tighter as the bias angle increases. Thus, bias angle can be 
varied over the length of the finished beam. 
The single disadvantage in the above-described curved beam construction is 
that the stiffness against bending is less than that for a beam made on a 
curved mandrel with a full axial braid. Where this weakness might be 
critical, it can be overcome by the bonding along the outside surface of 
the curve of a tape made of high modulus textile material, while the beam 
is inflated and curved to the design shape.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
While an almost unlimited combination of tube diameters, curvatures and 
pressures are possible in curved, inflated, tubular beams, the preferred 
embodiment is presented in FIG. 6 as a tube 1 of 12 3/4inches diameter and 
60-foot length configured as an inflated arch functioning as a supporting 
element for a large tent having an interior width of 30 feet and height of 
24 feet. Other embodiments of the concept could apply to other sizes of 
arch, to flying wing structures and to any structure where an inflatable, 
tubular element is of use. This particular design is inflated to 50 psi. 
The ends and the valving for inflation are not indicated. Any number of 
means of valving the gas and of sealing the ends of the beam are possible. 
As seen in FIG. 1, a length of the elastomeric tube 1, a portion of the 
inflated tubular arch of FIG. 6, is made of a Urethane film 2 of 0.010 
inch thickness. The braided bias fibers 3 are 1140 denier Kevlar 49 yarn 
with a tensile strength of approximately 55 pounds each, there being 
eleven ends on each of 144 carriers. The axial fibers 4 are two plies of 
7100 denier Kevlar 49, each having a strength of approximately 688 pounds. 
The axial fibers 4 are distributed over eight inches of the circumference 
in 15 braid locations. Their total strength is 10,320 pounds. (Sufficient 
strength to support the tube fully buckled at 100 psi.) 
The bias fibers 3 are braided at an angle of 57 degrees over the full 
length of the beam 1, except for an 18-inch section, the peak of the arch, 
6 in FIG. 6, in the center which is braided at 62 degrees, a high bias 
angle. The peak of the arch 6, the center section, in FIG. 6, with the 
high bias angle causes the tight curvature that forms the peak of the arch 
6. The axial fibers 4 extend uniformly throughout the length of the tube, 
supporting the curvature as controlled by the braid bias angle. 
FIG. 2 shows a cross section of the inflated tube 1. The elastomeric inner 
layer 2 is covered with the reinforcing fibers, the bias braid 3 and the 
axial fibers 4. Elastomeric can also be deposited within all of the fibers 
surrounding the inner layer 2 as a way of protecting them and keeping them 
in place. The deposited elastomer can be made tough enough such that it, 
instead of the inner tube, can function as the gas barrier. 
FIG. 3 clarifies the orientation of the fibers by showing a view where the 
tube 1 has been split parallel to the axis along the outside of the tube's 
curve and the tube 1 has been laid open. Thus, the braided fibers 3 are 
seen with the axial fibers 4 interwoven through the central area of the 
braided fibers 3. 
FIG. 4 shows the cross section of an inflated, curved tube 5 with its 
elastomeric inner tube 2 with a second layer of axial fibers 4 and braided 
fibers 3 for added stiffness. The outer braid layer is bonded to the tube 
only after the tube is inflated to its design shape. 
FIG. 5 shows the cross section of an inflated, curved tube 7 with its 
elastomeric inner tube 2 and with reinforcing tape 8 bonded opposite to 
the axial fibers for added stiffness. The tube is inflated to its design 
shape at the time the tape is added to the construction. 
In any given curved beam the bias angle of the uninflated weave varies with 
the design curvature at a particular region along the beam. While specific 
design parameters for a curved, inflatable tubular beam have been 
presented as the preferred embodiment, the concept is not limited by such. 
Also, for any given span and load and curvature the actual final setting 
of the design parameters is determined by the building of test pieces of 
tube. From this the final design evolves. It is the control of the bias 
angle in the braid along with the added axial fibers, both to achieve a 
design with the right strength and shape, which is the invention. 
The placement of a gas barrier for the inflated beam can be inside the 
fibers.