Aeronautics and astronautics – Aircraft structure – Fuselage and body construction
Reexamination Certificate
2001-04-10
2004-07-27
Johnson, Blair M. (Department: 3634)
Aeronautics and astronautics
Aircraft structure
Fuselage and body construction
Reexamination Certificate
active
06766984
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a reinforcing support structure for aircraft structural panels of the skin-stiffener type, and more particularly to stiffeners mounted on aircraft panels.
BACKGROUND OF THE INVENTION
Over the past several years aerospace design engineers have been challenged to address ever higher payload requirements. This has occurred in parallel with weight reduction and other structural efficiency goals that have driven the industry. Ever present safety and structural integrity demands have led the industry to look for more efficient and effective structural members and configurations that are more robust while addressing weight concerns.
Generally speaking, a stiffened aircraft panel's load-carrying capability is directly related to the type and strength of the stiffener typically installed on one side of the panel. The current approach within the industry to meet higher competitive demands has ranged from resorting to sophisticated alloys including extrudable aluminum and other alloys. It has also included such approaches as using specialty fiber reinforced composite structures, and increasing the depth of conventional stiffener designs as well as the yield strength of the material used in making the stiffeners. In order to appreciate the uniqueness and novelty of the current invention, a better understanding of the current state of the art in addressing the above airframe requirements follows. The first, and most common, approach taken by the industry in addressing the higher requirements has been to make conventional stiffeners out of heavier or higher strength materials. These traditional stiffener designs include the C-channel stiffener and Z-shaped stiffener, as well as the hat-shaped stiffener. Heavier gauges such as 0.055 inch min. (17 gauge) to 0.070 inch min. (15 gauge) material are now common. The use of thicker material has not only lead to greater tooling and handling costs, but also as will be shown, has had the effect of creating other major problems simultaneously.
The aircraft structural panel including any attached stiffeners is a system of parts interacting with each other as they are acted upon by combinations of pressures and in-plane as well as bending loads. Currently, stiffened-panel aircraft structures found in the wings, empennage, and fuselage of aircraft are typically constructed using aluminum or fiber reinforced composite skins with aluminum stiffeners and titanium members, or stiffeners using fiber reinforced composites.
These panels may also be supported by metal stiffeners to provide greater support as the stiffened panel system sustains bending moments as well as in-plane and pressure forces during service. However, an incompatibility occurs when relatively thick, stiffer sections, i.e., stiffeners made of 0.55 inch min. to 0.070 inch min. material are joined or fastened to thinner, less stiff sections, i.e., stringers made of 0.023 inch min. to 0.038 inch min. material. The area where these two sections are joined is an area of load transfer and thus of relatively higher stress. The reason for this is that the stiffer section resists conforming to the deformation of the less stiff stringer sections as loads are increased. The result is that one part of the system, the stringers, try to slide relative to another part of the system, the stiffener. This may result in early failure of the system, such as by buckling of the stiffener or of the stringer. This is due to in-plane compressive loads that result from the constraint that the stiffener imposes on the adjacent members as the loads are increased. Because of the increased stress at the joining area, manufacturers have been forced to modify parts of the stiffened panel to offset this effect. For example, because the use of heavier stiffeners increases the shear load through the joining area or fasteners, especially on the outer extremes of the panel width (near the panel edges), heavier panels and mounting members have had to be introduced. Still another approach to alleviate the problem has been to use additional shear ties or fasteners. This has been implemented in an attempt to reduce the high local in-plane compressive stresses that the heavy stiffeners may impose on the panel skin. However, this approach is undesirable because by increasing the number of parts, it increases the complexity and cost of the system.
This approach requires still heavier stiffeners, since the stiffener failure risk is somewhat reduced when it acts as an independent component rather than as part of a fully integrated system. Another drawback to additional shear ties is that it requires substantially more parts and installation time.
The second approach generally taken by the industry is to make the current hat-shaped and C-channel stiffeners deeper and out of thinner, yet higher yield strength material. This offers the advantage of reducing in-plane stress as noted above while at the same time increasing bending stiffness due to the deeper configuration. However, this approach has major disadvantages.
First, the thinner materials used in these traditional stiffener configurations make these stiffener sections more susceptible to edge stress concentrations. The conventional C-channel, Z-shaped, and hat-shaped stiffeners have a “blade edge.” This edge is very susceptible to imperfections in the sheet material along this edge as well as to damage during manufacture, shipping/handling and installation. These imperfections along the blade edge become stress concentration points or focal points at which failure of the stiffener can initiate. A more detailed description of this failure initiation follows.
Even the most perfect, smooth edge of the conventional stiffener will experience a very localized point of high stress gradient due to the characteristic edge stress concentration associated with open sections under bending loads.
Thus, initiation of an edge “bulge” or “crimp” on a perfect smooth edge is nothing more than the creation of an edge imperfection that is large enough to grow or “propagate” easily. It is significant that this stress concentration may be made worse by the presence of any relatively small local edge imperfections, even those on the order of size of the thickness of the stiffener material itself.
These imperfections near the edge can be in the form of edge notches, waviness (in-plane or out-of-plane), local thickness variations, local residual stress variations, or variations in material yield strength. Where multiple imperfections occur together, they may all compound together to further increase the stress concentration effect, and thus lower the load level at which failure is initiated. Thus, the existence of any edge imperfections in a conventional stiffener has the effect of enhancing an already established process of failure initiation.
Second, all the above conventional stiffeners, when manufactured out of relatively thin sheet materials are more susceptible to buckling due to the reduced thickness. Buckling is an instability in a part of the stiffener associated with local compressive or shear stresses. Buckling can precipitate section failure of the stiffener. This in turn causes a stress concentration in the adjacent panel skin near the buckled stiffener section, which may cause the stiffened panel to fail.
Finally, some thinner conventional stiffeners can experience “rolling” when placed under load. Rolling may be caused when the shear stresses within the stiffener result in a net torque about the centroid of the thin walled cross-section thus causing the cross-section to twist, possibly making the stiffener unstable. Another cause of rolling is the curvature of the panel itself that is induced by in-plane or pressure loads that are imposed upon the stiffened panel. Some airframers have increased the cross-sectional length of the flange furthest from the panel skin of the conventional C-channel stiffener in their attempts to solve the rolling problem, but have been met with only marginal improvement. This is because the increased
Browning & Bushman P.C.
Icom Engineering Corporation
Johnson Blair M.
LandOfFree
Stiffeners for aircraft structural panels does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Stiffeners for aircraft structural panels, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Stiffeners for aircraft structural panels will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3251862