Allen J. Lockyer

Qualitative assessment of smart skins and avionic/structures integration

Current military aircraft employ multiple single function antennas installed at different locations to provide communications, navigation and identification (CNI), electronic warfare and radar and weapon delivery in the .15 to 18 GHz frequency bands. The smart skins concept, wherein several antennas are integrated into one (or a few) multifunction apertures conformal to the outer geometry of the aircraft, promises considerable benefits. These include extended antenna coverage, efficient use of aircraft realestate, quick installation and replacement and structural weight savings. However, to realize these payoffs, several disparate technical and operational issues such as development of multifunction apertures, integration of the radiating elements and repackaging the electronics into load-bearing structure, antenna isolation and resource management, and tolerance to low velocity impact damage, need to be resolved. Potential payoffs and the technical challenges of smart skins implementation and avionics repackaging is discussed in quantized transitional states from black box avionics traditional packaging to structurally integrated avionics of the future. Qualitative assessments of related smart skin technologies and risk reduction approaches, which could transition the technology to current and future aircraft, are proposed, and preliminary cost estimates presented.

Power systems and requirements for integration of smart structures into aircraft

Electrical power distribution for recently developed smart actuators becomes an important air-vehicle challenge if projected smart actuation benefits are to be met. Among the items under development are variable shape inlets and control surfaces that utilize shape memory alloys (SMAs); full-span, chord-wise and span-wise contouring trailing control surfaces that use SMA or piezoelectric materials for actuation; and other strain-based actuators for buffet load alleviation, flutter suppression and flow control. At first glance, such technologies afford overall vehicle performance improvement. However, integration system impacts have yet to be determined or quantified. Power systems to support smart structures initiatives are the focus of the current paper. The paper has been organized into five main topics for further discussion: (1) air-vehicle power system architectures – standard and advanced distribution concepts for actuators, (2) smart wing actuator power requirements and results highlighting wind tunnel power measurements from SMA and piezoelectric ultrasonic motor-actuated control surfaces and different dynamic pressure and angle of attack; (3) vehicle electromagnetic effects (EME) issues, (4) power supply design considerations for smart actuators featuring the aircraft power and actuator interface, and (5) summary and conclusions.

Acoustic emission testing on an F/A-18 E/F titanium bulkhead

An important opportunity recently transpired at Northrop Grumman Corporation to instrument an F/A - 18 E/F titanium bulkhead with broad band acoustic emission sensors during a scheduled structural fatigue test. The overall intention of this effort was to investigate the potential for detecting crack propagation using acoustic transmission signals for a large structural component. Key areas of experimentation and experience included (1) acoustic noise characterization, (2) separation of crack signals from extraneous noise, (3) source location accuracy, and (4) methods of acoustic transducer attachment. Fatigue cracking was observed and monitored by strategically placed acoustic emission sensors. The outcome of the testing indicated that accurate source location still remains enigmatic for non-specialist engineering personnel especially at this level of structural complexity. However, contrary to preconceived expectations, crack events could be readily separated from extraneous noise. A further dividend from the investigation materialized in the form of close correspondence between frequency domain waveforms of the bulkhead test specimen tested and earlier work with thick plates.

Air vehicle integration issues and considerations for CLAS successful implementation

Over the last decade, Northrop Grumman Corporation under internal and DoD funding, and others, have been working on integration of RF antennas into load-bearing aircraft structures. This multidisciplinary effort, collectively referred to as Conformal Load-bearing Antenna Structures (CLAS), requires concurrent consideration of structural and antenna performance issues and has involved a team consisting of avionics, structures, material, and manufacturing expertise. From the published articles to date it could be argued that the technology has had some spectacular success in its initial stages but not much has been published about the issues raised by CLAS that would still need to be addressed and solved for final technology inclusion in an operational air-vehicle. Presented are some key results from the Air Force Research Laboratorys (AFRL) Smart Skins Structures Technology Demonstrator (S3TD) program that while funded from the Air Vehicles Directorate looked at the total picture of integration from a multidisciplilnary standpoint. Issues related to airframe integration are also discussed that need further study and evaluation before CLAS can be sanctioned as a viable future DoD technology. Such topics, in no particular order of priority are 1) airframe CLAS panel location, 2) airframe configuration issues, 3) EMI/lightning issues, and 4) repair issues and supportability, 5) panel design enhancement, risks, and issues.

Adaptive smart wing design for military aircraft: requirements, concepts, and payoffs

New developments in smart structures and materials have made it possible to revisit earlier work in adaptive and flexible wing technology, and remove some of the limitations for technology transition to next-generation aircraft. Research performed by Northrop Grumman, under internal funding, has led to a new program sponsored by ARPA to investigate the application of smart structures and materials technologies to twist and adapt and aircraft wing. Conceptual designs are presented based on state-of-the-art materials, including shape memory alloys, piezoelectrics, and fiber optic sensors for incorporation in a proposed smart wing design. Plans are described to demonstrate proof-of-concept on a prototype 1/10 scale -18 model that will be tested in a wind tunnel for final validation. Highlights of the proposed program are summarized with respect to program objectives, requirements, key concept design features, demonstration testing, and smart wing technology payoffs and risks.

Application for smart skin technologies to the development of a conformal antenna installation in the vertical tail of a military aircraft

Recent developments in smart skins technology at Northrop Grumman have paved the way toward incorporating avionics communication functions, previously provided by blade antennas, into the vertical tail of a military aircraft. Radio frequency communication link ranges can be significantly improved by structurally integrating the antenna radiating element into the tail region. Excitation of the large vertical tail surface improves radiation efficiency in the VHF-FM, VHF-AM, and UHF frequency bands. Analysis shows use of the whole tail region as an antenna would provide the best gain and coverage, however, confining the antenna design to the end cap region alone, also significantly enhances performance compared to blade installations. Near term technology applications to retrofit aircraft with minimum perturbations to the existing tail design are therefore possible. Multidisciplinary aspects of the application approach are discussed under the subheadings (1) antenna design, (2) structures and materials, (3) manufacturing, and (4) weight assessment along with the resolution of key technical road blocks. Finally, recommendations for further work necessary to transition the application to a production aircraft are discussed.

Power systems and requirements for the integration of smart structures into aircraft

Electrical power distribution for recently developed smart actuators becomes an important air-vehicle challenge if projected smart actuation benefits are to be met. Among the items under development are variable shape inlets and control surfaces that utilize shape memory alloys (SMA); full span, chord-wise and span-wise contouring trailing control surfaces that use SMA or piezoelectric materials for actuation; and other strain-based actuators for buffet load alleviation, flutter suppression and flow control. At first glance, such technologies afford overall vehicle performance improvement, however, integration system impacts have yet to be determined or quantified. Power systems to support smart structures initiatives are the focus of the current paper. The paper has been organized into five main topics for further discussion: (1) air-vehicle power system architectures - standard and advanced distribution concepts for actuators, (2) smart wing actuator power requirements and results - highlighting wind tunnel power measurements from shape memory alloy and piezoelectric ultrasonic motor actuated control surfaces and different dynamic pressure and angle of attack; (3) vehicle electromagnetic effects (EME) issues, (4) power supply design considerations for smart actuators - featuring the aircraft power and actuator interface, and (5) summary and conclusions.

Prototype testing and evaluation of a structurally integrated conformal antenna installation in the vertical tail of a military aircraft

Further proof-of-concept development for structurally integrating communication antennas in the vertical tail of a military aircraft at Northrop Grumman is presented. Bread board testing on a full scale dual tail aircraft mock-up of a structurally integrated multifunction tail tip antenna, in the VHF-FM, VHF-AM, and UHF-AM frequency regimes, has confirmed earlier simulation results, where it was suggested that smart skin installation electrical performance gain and radiation characteristics might compare favorable to conventional dorsal deck mounted blade installations. Scale model, and eventually full scale ground mock-up testing encouraged further development leading to fabrication of a preliminary flight test of a smart skin tip demonstration article. A low cost flight test program in the VHF SINCGARS band (30 to 88 MHz) has illustrated that structural integration, fabrication and manufacturing issues can be addressed for full feasibility with minimum penalties despite the hostile vibro-acoustic, moisture and electromagnetic environment. Salient features of the engineering technical design effort and recommendations for future concept development are discussed.