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Composite Repair Engineering Case Studies for U.S. Army Aerostructures

The U.S. Army fields a multitude of aircraft mission design series (MDS) developed by several different original equipment manufacturers with varying mission requirements and flight profiles. The structural analysis in this work assumes the materials, tooling, skillsets, and capabilities are organically available and proper at the repair location.

Figure 1. Potential composite applications for the U.S. Army’s fleet of H-64 Apache helicopters.

The U.S. Army operates and maintains several aircraft MDS to meet the warfighter’s multidomain mission. Aircraft fielded by the U.S. Army originate from multiple equipment manufacturers. These aircraft include rotary-wing configurations such as the AH-64D/E Apache, CH-47F Chinook, and H-60A/L/V/M Blackhawk aircraft which significantly vary in mission parameters and flight profiles. These aircraft contain structures made from a majority aluminum, steel, and titanium alloys which have dominated aircraft designs for much of the history of powered flight. However, the use of advanced composite material systems such as fiberglass, carbon, and aramid fiber reinforcement with high performance epoxy resins has steadily increased to optimize structural designs and improve mission capability.

Fiberglass was first developed in the 1930s, with carbon fiber materials subsequently developed in the 1980s. The utilization of polymer matrix composite material systems such as carbon-epoxy fabrics and aramid honeycomb core has significantly increased in civilian and military aircraft designs. This is a result of the significant increase in specific strength and stiffness capabilities and resistance to corrosion and fatigue that composites offer to the enterprise. While composites certainly are not new to aviation, their uses were primarily limited to secondary and lightly loaded structures such as covers, fairings, spoilers, and antenna. During the RAH-66 Comanche helicopter development effort, the U.S. Army made attempts to build a fiber-reinforced crashworthy airframe structure through the Advanced Composite Airframe Program (ACAP). The ACAP effort, conducted by Bell Helicopter and Sikorsky Aircraft, was intended to determine the feasibility of utilizing composites for fuselages and primary structure. The driver for this effort was to reduce weight, eliminate metal corrosion, build more tolerant battle damage structures, and reduce aircraft radar cross sections. While the program was cancelled in February 2004, the Comanche and ACAP programs demonstrated that composites would be acceptable for use on miliary aircraft. Additionally, these programs helped to set a precedence for future repairs of composite structures. The repair methods of that effort included standard wet layup, for smaller damage, and replacement of modular components for larger damage. Replacement of modular sections accomplished by removing smaller sub-sections of the structure and effectively splicing in a replacement part.

As technologies develop, aircraft mature, and strides are made in advanced materials development and analysis, aircraft have increased use of advanced composites within the aerostructure. These advanced materials are now being used in highly loaded structures, including aerodynamic profiles.

Evolving mission requirements led to the use of unmanned fixed wing aircraft, such as the MQ-1 Grey Eagle and RQ-7 Shadow, which employ carbon fiber composites as the bulk of the fuselage structure. Structurally critical applications include main rotor blade spars, bulkheads, fuselage skins, stringers, walkways, and stabilators. Additionally, it is suspected that future U.S. Army aircraft designs will have structures primarily constructed from advanced composite materials. The U.S. Marine Corps’ MV-22 Osprey tiltrotor aircraft has an airframe constructed primarily from carbon fiber reinforced epoxy material systems, which enables a more structurally efficient aircraft. The majority of the aircraft’s exterior is composite, in addition to the proprotor blades and many of the internal structural components. In order to increase payload, range, airspeed, and other critical mission capabilities, it is likely that the U.S. Army and aircraft manufacturers will begin incorporating and preparing to maintain a much greater number of these types of structures on future aircraft.

This work was performed by Bryan M. Steiner, Jared R. Peltier, Stephen J. Janny, and Brian A. Cerovsky for the U.S. Army Combat Capabilities Development Command. For more information, download the Technical Support Package (free white paper) below. FCDDAMR-2301

Topics:
Advanced composite materials Aeronautics Aerospace Aircraft Alloys Aviation Composite materials Manufacturing & Prototyping Materials Metals Metals Polymers Titanium alloys Vehicles
This Brief includes a Technical Support Package (TSP).
Document cover
Composite Repair Engineering Case Studies for U.S. Army Aerostructures

(reference FCDDAMR-2301) is currently available for download from the TSP library.

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    Magazine cover
    Aerospace & Defense Technology Magazine

    This article first appeared in the December, 2023 issue of Aerospace & Defense Technology Magazine (Vol. 8 No. 7).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document titled "Composite Repair Engineering Case Studies for U.S. Army Aerostructures" is a final report authored by Bryan M. Steiner, Jared R. Peltier, Stephen J. Janny, and Brian A. Cerovsky, covering the period from May to August 2023. It is published by the Systems Readiness Directorate of the U.S. Army Combat Capabilities Development Command and is approved for public release.

    The report focuses on the engineering and repair methodologies for composite materials used in U.S. Army aerostructures, specifically addressing the challenges and solutions related to composite repairs. It includes a series of case studies that illustrate practical applications of composite repair techniques, emphasizing the importance of maintaining the structural integrity and performance of military aircraft.

    Key sections of the report include detailed illustrations of various aircraft layouts, such as the H-64 Apache, H-60 Blackhawk, H-47 Chinook, and unmanned aerial vehicles like the MQ-1 Grey Eagle and RQ-7 Shadow. These illustrations provide a visual context for understanding the structural components that may require repair.

    The report outlines general maintenance practices, including field and depot maintenance procedures, and introduces the General MEC (Maintenance Engineering Change) Process, which is crucial for managing repairs effectively. It also discusses specific repair techniques, such as scarf repairs and bonded patch repairs, detailing the preparation, layup, curing, and final repair processes.

    Additionally, the document includes technical information on damage simulation and running load calculations, which are essential for assessing the impact of repairs on aircraft performance. The report emphasizes the significance of adhering to established repair configurations and guidelines to ensure safety and reliability.

    Overall, this report serves as a comprehensive resource for engineers and maintenance personnel involved in the repair of composite structures in military aviation. It aims to enhance the understanding of composite repair processes, promote best practices, and ultimately support the operational readiness of U.S. Army aircraft. The findings and methodologies presented are intended to be shared widely, contributing to the ongoing development of composite repair technologies in military applications.

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