1: Function of flight vehicle structural members
- Page ID
- 95292
Function of flight vehicle structural members
The purpose of this chapter is to present a brief description of aircraft structural members and their function.
A structure may be defined as any assemblage of materials that is intended to sustain loads. It is important to recognize that structures are made from materials, and that the history of structures follows the development of materials and the development of tools to fabricate the materials. Ashby (1992) details a systematic approach to material selection in mechanical design, and the manufacturing processes required produce the functional shape of a design. The evolution of the airframe, for example, is tied closely to the introduction of materials and cost-effective means for their fabrication. Early aircraft were constructed of wire-braced wood frames with fabric covers. Currently, advanced composite materials are very attractive for weight-sensitive structures, like aircraft, because of their high stiffness-to-weight and strength-to-weight ratios. There is an interesting and rich history of the evolution of aircraft structures, but for the sake of brevity it is not presented here. Instead, the interested reader is referred to the textbook by Curtis (1997). Curtis details the history of fixed–wing aircraft structures from 1903 to modern aircraft.
In this text analytical methods are developed for the response and failure of the primary structural components of aircraft. The primary structure of a flight vehicle consists of the components that are necessary to sustain design ultimate flight and ground loads. Failure of the primary structure causes catastrophic collapse and loss of control. For aircraft the primary structure consists of the wings, fuselage, tail, and landing gear. Forms of construction are space trusses/frames, monocoque and semimonocoque.
Space truss/frame
A truss structure fuselage is often used in lightweight aircraft. See figure [fig1.1]. It consists of wood or steel tubes with a fabric covering providing aerodynamic shape. Members in a space truss are subject to axial forces, and members of a space frame are subject to axial forces, shear forces, and bending moments. The fabric covering does not add much to the overall stiffness of the structure.
Monocoque and semimonocoque constructions
Most flight vehicle structures are thin shells with the cover skin providing the aerodynamic shape. Monocoque refers to a shell without supporting stiffening members, whose origin is from the twentieth-century Greek word “mono” meaning alone, plus the French “coque” meaning shell. See figure [fig1.2] The wall of a monocoque structure has to be strong enough to resist bending and compressive and torsional loads without buckling. The challenge in monocoque design is maintaining strength within allowable weight limits. Another difficulty with monocoque structure is how to design it to accommodate concentrated loads such as engine mountings and wing-fuselage interface, which may require the incorporation of formers (frames) and bulkheads. For large cross-sectional dimensions the skin of a monocoque structure must be relatively thick. A more efficient type of construction is one which contains stiffening members that permit a thinner skin. Also, stiffening members can be used to diffuse concentrated loads into the skin. A stiffened thin-walled shell is called semimonocoque. A semimonocoque body structure and wing structure are shown in figure [fig1.3]. Both the body structure figure[fig1.3](a) and the wing structure in figure [fig1.3](b) have longitudinal stiffening members and transverse stiffening members supporting thin skins.
Longitudinal members are called longitudinals, stringers, or stiffeners. Longerons are longitudinal members having a large cross section. Longitudinal members act with the skin to resist applied bending and axial loads. Transverse members in a body structure are known as frames, rings, and if they cover most of the cross section they are called bulkheads. Pressure bulkheads cover the entire cross section. Frame members maintain cross-sectional shape and are used to distribute concentrated loads to the skin.
In a wing the longitudinal member is called a spar, and it consists of the spar web and spar cap. The spar cap act with the skin to resists axial and bending loads applied to the wing. The skin and the spar web develop shearing stresses to resist torsion and transverse shear due to bending. Transverse members in a wing are called ribs, and they act to maintain the airfoil shape. Ribs act with the skin and longitudinals in resisting circumferential loads due to pressurization.
Longitudinal and transverse members also function to divide the skin into smaller panels to increase the buckling strength. (See Example [ex11.5] on page .)
Additional components of a wing are shown in figure [fig1.4]. The internal wing structure consists of spars, ribs, and stringers. The external wing structure is the skin. Ribs are also used in ailerons, elevators (flaps), fins, and stabilizers. In a fixed-wing aircraft, the spar is the main structural member connected to the fuselage at its root and running spanwise to the tip of the wing. It bends in transmitting the lift due to flight loads acting on the wing. The flight loads acting on a wing not only cause bending, but a significant amount of torsion/ twisting of the wing as well. The skin and shear webs form closed cells in a wing, and torsion is resisted by shear stresses developed in the wall of these cells.
A semimonocoque fuselage structure for a transport aircraft is shown in figure [fig1.5]. The skin is stiffened by longitudinal stringers, spaced six to ten inches apart, which function to increase the buckling strength of the skin and resist fuselage bending loads. Transverse frames maintain the shape of the fuselage and are typically spaced twenty inches apart (Young, 2011).
Rocket structure
A full-scale rocket consists of a launch vehicle and payload. There are four major systems in a full-scale rocket: the structural system, the payload system, the guidance system, and the propulsion system. The structural system includes the cylindrical body, the fairings, and any control fins. The payload is the entire spacecraft suchas a , experiment, or whatever else is being lifted into space. When a spacecraft is to be launched by an expendable booster, a booster adapter, or a launch-vehicle adapter, structurally links the spacecraft to the launch vehicle. The payload and its structure is protected by a fairing. Also, refer to the configuration of the Atlas I launch vehicle shown in figure [fig18.1] on page . Atlas I consists of an expendable booster and an expendable second stage.
The cylindrical body of the launch vehicle, or frame, has a thin skin to reduce weight. Engine thrust is the dominate load that causes compression in the rocket parts. The buckling resistance of the thin skins is increased under compression loading by a grid of internal stiffening members attached to the skins similar to those shown in figure [fig1.3](a). The buckling loads for axially compressed cylindrical shells in experiments are significantly less than the buckling load determined from an analysis of the perfect structure. Imperfection in the shell geometry is main the reason for the discrepancy between theory and experiment Refer to the discussion at the end of article [sec10.2.1] on page . The buckling knockdown factor (KDF) has been introduced to reduce the buckling load predicted by the analysis of the prefect structure to aid in the structural design (Hilburger, 2018).
Ashby, M. F., Materials Selection in Mechanical Design. Oxford: Pergamon Press, 1992.
Curtis, Howard D., Fundamentals of Aircraft Structural Analysis. Jefferson City, MO: Richard D. Irwin, a Times Mirror Higher Education Group, Inc. Company, 1997, pp. 1–34.
Hilburger, M. W., “On the Development of Shell Buckling Knockdown Factors for Stiffened Metallic Launch Vehicle Cylinders.” Presented at the 2018 AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Kissimmee FL, AIAA 2018-1990. Washington, DC: American Institute of Aeronautics and Astronautics, 2018.
Young, Richard, “Fuselage Design 101: Basic Terms and Concepts.” Presented at the NTSB Airplane Structural Integrity Forum, Washington, D. C., September 21, 2011.