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.

December 1, 2023

Army Combat Capabilities Development Command, Redstone Arsenal, Alabama

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