Novel Design Methodology and Development of Composite Landing Gear Struts for an Unmanned Aircraft

Abstract

One of the most important and crucial systems of all the aircraft is its landing gear, which enables the aircraft to taxi and take off while on ground and ensures a secure and safe landing of the aircraft after the successful flight. This system of the aircraft generally, accounts for only 2.5 to 5 percent of the overall weight of the aircraft. The main function of the landing gear system is to absorb kinetic energy during landing and taxiing, to the extent that the minimum load is transferred to the airframe. With the advancements in the field of composite materials, focus of the designers of the landing gear systems has been shifted from conventional metallic landing gear struts to composite landing gear struts, owing to their high strength to weight ratio, fracture toughness and corrosion resistance properties. A variety of composite materials are available in the market and selection of the appropriate composite material for the development of a landing gear strut is an active area of research in the aviation industry. In this research work, a selection methodology of fiber reinforced composite material for a retractable main landing gear strut of a given unmanned aircraft (up to 1600 kg mass) has been proposed. Ashby material selection and Cambridge Engineering Selector (CES)® approaches were used for preliminary selection of materials. After confirmation of market availability of the recommended materials, Standard Modulus (SM) carbon fiber, E-glass fiber/epoxy, S-glass fiber/epoxy were considered for analysis. For the design and analysis of a main landing gear strut, maximum landing loads for one point and two-point landing conditions were calculated using FAA FAR 23 airworthiness requirements. Materials were categorized computationally based on them strength-to-weight ratio and the Tsai-Wu failure criterion. As a result, landing gear strut composed of T700 carbon fiber/epoxy qualifying the Tsai-Wu failure criterion, and having a maximum strength-to-weight ratio, was recommended for development afxx ter experimental validation. In the next phase, experimental validation of the selected material against the landing loads of the designed landing gear strut was required before undertaking the full-scale prototype manufacturing. It is worth mentioning that determination of suitable testing and qualification procedures for fiber reinforced polymer matrix composite structures are also an active area of research due to their increased demand, especially in the field of aerospace. Therefore, a generic qualification framework for composite based main landing gear strut of an unmanned aircraft is also proposed in this research work. For this purpose, a landing gear strut composed of selected T700 carbon fiber/ epoxy material was analyzed for the given aircraft. Computational analysis was performed on ABAQUS CAE® to evaluate maximum stresses and critical failure modes encountered during one point landing condition, as defined in UAV Systems Airworthiness Requirements (USAR)and Air Worthiness Standards FAA FAR Part 23. A three-step qualification framework including material, process and product-based qualification was then proposed against these maximum stresses and failure modes. The proposed framework revolves around the destructive testing of specimens initially as per ASTM standards D 7264 and D 2344, followed by defining the autoclave process parameters and customized testing of thick specimens to evaluate material strength against the maximum stresses in specific failure modes of main landing gear strut. Once the desired strength of the specimens was achieved on the basis of material and process qualifications, qualification criteria for the main landing gear struts were proposed, which can not only serve as a testing criteria of the landing gear struts during mass production, but also can give confidence to undertake the full scale prototype manufacturing of main landing gear struts using qualified material and process parameters. In the final phase, manufacturing of main landing gear struts was undertaken. For this purpose, a mold was developed for autoclave curing of the full-scale prototype. Both the main landing gears were manufactured in two autoclave cycles by following all the processes developed in this research work. Specimens required for qualification of the main landing gear struts, as defined in the qualification framework, were also cured in the same autoclave cycles. Desired strength as defined in the qualification framework was successfully achieved through destructive testing of these specimens. The proposed and manufactured main landing gear struts are ready and recommended for installation on aircraft subject to their drop test qualification. In parallel, a study on oleo-pneumatic nose landing gear strut was also performed, which is presented as appendix to this thesis.