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As the cost and complexity of modern aircraft systems increases, emphasis has been placed on model-based design as a means for reducing development cost and optimizing performance. To facilitate this, an appropriate modeling environment is required that allows developers to rapidly explore a wider design space than can cost effectively be considered through hardware construction and testing. This wide design space can then yield solutions that are far more energy efficient than previous generation designs. In addition, non-intuitive cross-coupled subsystem behavior can also be explored to ensure integrated system stability prior to hardware fabrication and testing. In recent years, optimization of control strategies between coupled subsystems has necessitated the understanding of the integrated system dynamics.

The Air Force Research Laboratory Deployment and Sustainment Division (AFRL/HES), in coordination with Arthur D. Little, Inc., has undertaken a system-of-systems approach to defining and designing aerospace ground equipment (AGE). This method is usually used to take advantage of the open architecture nature of ne and developing electronics through modular design. This paper discusses how we applied it across the electronic, mechanical, and structural aspects of AGE to research and develop a new concept to meet the burgeoning needs of the 21st century Air Force.

As part of the DoD/NASA Lithium-Ion and More-Electric Aircraft (MEA) development programs, in-house life-testing and performance characterization of lithium-ion batteries of sizes 1-20 amp-hours (Ah) were performed. Using AC impedance spectroscopy, the impedance behavior of lithium-ion cells with respect to temperature, cycle number, electrode, and state-of-charge was determined. Cell impedance is dominated by the positive (cathode) electrode, increases linearly with cycle number, and exponentially increases with decreasing temperature. From cell performance testing, we have seen the cell behavior is extremely sensitive to the ambient temperature. Preliminary battery performance results as well as AC impedance and life cycle test results are presented below.

This paper describes the development, operation and testing of an across-gimbal ambient thermal transport system (GATTS) for carrying cryocooler waste heat across a 2-axis gimbal. The principal application for the system is space-based remote sensing spacecraft with gimbaled cryogenics optics and/or infrared sensors. GATTS uses loop heat pipe (LHP) technology with ammonia as the working fluid and small diameter stainless steel tubing to transport 100–275 W across a two-axis gimbal. The tubing is coiled around each gimbal axis to provide flexibility (less than 0.68 N-m [6 lbf-in] of tubing-induced torque per axis) and fatigue life. Stepper motors are implemented to conduct life cycling and to assess the impact of motion on thermal performance. An LHP conductance of approximately 7.5 W/C was demonstrated at 200 W, with and without gimbal motion. At the time this paper was written, the gimbal had successfully completed over 500,000 cycles of operation with no performance degradation.

As technology for both military and civilian aviation systems mature into a new era, techniques to test and evaluate these systems have become of great interest. To achieve a general understanding as well as save time and cost, the use of computer modeling and simulation for component, subsystem or integrated system testing has become a central part of technology development programs. However, the evolving complexity of the systems being modeled leads to a tremendous increase in the complexity of the developed models. To gain confidence in these models there is a need to evaluate the risk in using those models for decision making. Statistical model validation techniques are used to assess the risk of using a given model in decision making exercises. In this paper, we formulate a transient model validation challenge problem for an air cycle machine (ACM) and present a hardware test bench used to generate experimental data relevant to the model.

A highly modified F/A-18 aircraft is being used to demonstrate that aeroelastic wing twist can be used to roll a high performance aircraft. A production F/A-18A/B/C/D aircraft uses a combination of aileron deflection, differential horizontal tail deflection and differential leading edge flap deflection to roll the aircraft at various Mach numbers and altitudes. The Active Aeroelastic Wing program is demonstrating that aeroelastic wing twist can be used in lieu of the horizontal tail to provide autonomous roll control at high dynamic pressures. Aerodynamic and loads data have been gathered from the Phase I AAW flight test program. Now control laws have been developed to exploit aeroelastic wing twist and provide autonomous flight control of the AAW aircraft during Phase II. Wing control surfaces are being deflected in non-standard ways to create aeroelastic wing twist and develop the required rolling moments without use of the horizontal tail.

Side-facing seats are present in a variety of aircraft. During impact, these seats load the occupants in a different manner than typical forward-facing seats, namely the occupants are exposed to a lateral impact. In order to minimize injury during a crash, it is necessary for the occupants to prepare themselves and be situated in a position for maximum protection. In an effort to understand occupant initial position in a side-facing seat, a 3-D rigid-body model was developed of a side-facing seat configuration with three occupants, using the Articulated Total Body (ATB) program. The occupants were seated side-by-side in webbed troop-style seats, and each occupant was restrained by a lap belt. Three different initial occupant positions were studied, and each of the three occupants in a given simulation were seated in the same position. A 10 G lateral pulse with an approximate duration of 200 ms was applied to the vehicle.

The Engine Supplier Base Initiative (ESBI) is a joint Air Force/Industry cooperative agreement aimed at achieving affordable precision investment cast airfoil and large structural components for man-rated gas turbine engines. The ESBI program will obtain these goals through the implementation of business and technology improvements with specific focus on increased product quality and reduced cycle time. This program has brought together competitors in the business to solve sector wide problems. This paper presents the framework of the teaming approach as well as results achieved in quality and cycle time improvements through technical and business process improvements.

A two-port fuel-injection (PFI) system is added to a Rotax 914 four-cylinder spark-ignition engine to allow two fuels of different reactivity to be injected simultaneously in order to vary the fuel octane number during engine operation. Engine performance using the dual-fuel PFI system is compared to that using injection of primary-reference-fuel (PRF) blends via a single-PFI system for fuel octane ratings of 50, 70, and 87 octane. The on-the-fly octane control of dual-PFI system is found to control fuel-octane well enough to produce maximum indicated mean effective pressure (IMEPn) results within ± 2% of single-PFI PRF IMEPn results. IMEPn is compared among dual-PFI blends from 20 to 87 octane, neat n-heptane, neat JP-8, and JP-8/isooctane blends. Maximum IMEPn for these fuels is established for the Rotax 914 engine operating from 2500 to 5800 rev/min.

Due to the instabilities that may occur in power systems with regulated loads such as those used in military aircraft, ships, and terrestrial vehicles, many analysis techniques and design methodologies have been developed to ensure stable operation for expected operating conditions. However, many of these techniques are difficult to apply to complex systems and do not guarantee large-displacement stability following major disturbances such as faults, regenerative operation, large pulsed loads, and/or the loss of generating capacity. In this paper, a design paradigm is set forth guaranteeing large-displacement stability of a power system containing a significant penetration of regulated (constant-power) loads for any value of load power up to and including the steady-state rating of the source. Initial investigations are performed using an idealized model of a dc-source to determine the minimum requirements that ensure large-displacement stability.

Modern military engines desire both the fuel efficiency of high-bypass turbofans and the high specific thrust of a low-bypass turbofan. Using traditional engine architectures, performance and efficiency are in conflict, so an engine is usually designed to best meet requirements for its primary mission. While the concept of a variable cycle engine is not new, recent advances in engine architecture technology suggest that adding a second bypass stream to a traditional turbofan can provide significant benefits. This “third stream” (the core flow being the primary stream and the inner bypass being the second stream) airflow can be independently modulated so that engine airflow demand can be matched with the available inlet flow at a variety of operating points, thereby reducing spillage drag. Additionally, the third stream air provides a valuable heat sink for cooling turbine cooling air or dissipating other aircraft heat loads.

The two-phase flow modeling is done using the level set method to identify the interface of vapor and liquid. The modifications to the incompressible Navier-Stokes equations to consider surface tension, viscosity, gravity and phase change are discussed in detail. The governing equations are solved using finite difference method. In the present work, investigations on the effect of thermal conductivity and latent heat of vaporization of liquid on heat transfer in a 44 µm thick liquid film containing vapor bubble with droplet impact is investigated. The importance of thermal conductivity and latent heat of vaporization of liquid on heat transfer is identified. The variation of heat flux with thermal conductivity and latent heat is plotted. The computed liquid and vapor interface, velocity vector and temperature distributions at different time instants are also visualized for better understanding of the heat removal.

Two compact intercoolers are designed for the Rotax 914 aircraft engine to increase engine power and avoid engine knock. A study is performed to investigate the effects of unsteady airflow on intercooler performance. Both intercoolers use air-to-liquid cross flow heat exchangers with staggered fins. The intercoolers are first tested by connecting the four air outlets of the intercooler to a common restricted exit creating a constant back pressure which allows for steady airflow. The intercoolers are then tested by connecting the four air outlets to a 2.4 liter, 4 cylinder engine head and varying the engine speed from 6000 to 1200 RPM corresponding to decreasing flow steadiness. The test is performed under average flight conditions with air entering the intercooler at 180°F and about 5 psig. Results from the experiment indicate that airflow unsteadiness has a significant effect on the intercooler's performance.

As aircraft continue to increase their power and thermal demands, transient operation of the power and propulsion subsystems can no longer be neglected at the aircraft system level. The performance of the whole aircraft must be considered by examining the dynamic interactions between the power, propulsion, and airframe subsystems. Larger loading demands placed on the power and propulsion subsystems result in thrust, speed, and altitude transients that affect the aircraft performance and capability. This results in different operating and control parameters for the engine that can be properly captured only in an integrated system-level test. While it is possible to capture the dynamic interactions between these aircraft subsystems by using simulations alone, the complexity of the resulting system model has a high computational cost.

The aircraft electromechanical actuator (EMA) cooling fan is a critical component because an EMA failure caused by overheating could lead to a catastrophic failure in aircraft. Fault tree analysis (FTA) is used to access the failure probability of EMA fans with the goal of improving their mean time to failure (MTTF) from ∼O(5×104) to ∼ O(2.5×109) hours without incurring heavy weight penalty and high cost. The dual-winding and dual-bearing approaches are analyzed and a contra rotating dual-fan design is proposed. Fan motors are assumed to be brushless direct current (BLDC) motors. To have a full understanding of fan reliability, all possible failure mechanisms and failure modes are taken into account. After summarizing the possible failure causes and failure modes of BLDC fans by focusing on each failure mechanism, the life expectancy of fan ball bearings based on a major failure mechanism of lubricant deterioration is calculated and compared to that provided in the literature.

An experimental study is performed to investigate the possibility of relaxing the octane requirement of a Rotax 914 engine equipped with a pre-chamber jet ignition system. A pre-chamber jet igniter with no auxiliary fuel addition is designed to replace the spark plug in cylinder two of the test engine and is evaluated across engine speeds ranging from 2500 to 5500 RPM. Experiments are performed across both normally aspirated and boosted configurations using regular 87 AKI gasoline fuel. Normally aspirated results at 98 kPa manifold absolute pressure show a 7-10° burn rate improvement with the jet ignition combustion system. Tests to determine the maximum load at optimal combustion phasing (no spark retard) are then conducted by increasing boost pressure up to maximum knock limits.

Rotating Heat Pipe (RHP) technolog y is being developed for high speed (>20 krpm) regimes of electric motor/generator cooling. The motivation for this research is the potential application of the high speed RHPs for the thermal management of advanced rotating electrical machines. The passive nature and relatively simple features of this device are attractive for the removal of waste heat from the rotors of electric machines. Interesting air-cooling experimental results of two high speed RHPs designed, fabricated and tested at AFRL are presented here. Emphasis is made on external heat removal concepts useful for cooling the RHP condenser in order to be successful in promoting this technology to real world problems.

Next generation aircraft will require more electrical power, more thermal cooling, and better versatility. To attain these improvements, technologies will need to be integrated and optimized at a system-level. The complexity of these integrated systems will require considerable analysis. In order to characterize and understand the implications of highly-integrated aircraft systems, the effects of pulsed-power, highly-transient loads, and the technologies that drive system-stability and behavior, an approach will be taken utilizing integrated modeling and simulation with hardware-in-the-loop (HIL). Such experiments can save time and cost and increase the general understanding of electrical and thermal phenomena as it pertains to aircraft systems before completing an integrated ground demonstration. As a first step toward completing an integrated analysis, a dynamometer “drive stand” was characterized to assess its performance.

Aircraft power demands continue to increase with the increase in electrical subsystems. These subsystems directly affect the behavior of the power and propulsion systems and can no longer be neglected or assumed linear in system analyses. The complex models designed to integrate new capabilities have a high computational cost. Hardware-in-the-loop (HIL) is being used to investigate aircraft power systems by using a combination of hardware and simulations. This paper considers three different real-time simulators in the same HIL configuration. A representative electrical power system is removed from a turbine engine simulation and is replaced with the appropriate hardware attached to a 350 horsepower drive stand. Variables are passed between the hardware and the simulation in real-time to update model parameters and to synchronize the hardware with the model.

This paper presents the concept of a dual latent heat sink for thermal management of pulse heat generating electronic systems. The focus of this work is to verify the effectiveness of the concept during charging through experimentation. Accordingly, custom components were built and a prototype version of the heat sink was fabricated. Experiments were performed to investigate the implementation feasibility and heat transfer performance. It is shown that this heat sink is practicable and helps in arresting the system temperature rise during charging (period of pulse heat load).