Blake A. Moffitt

Design and Performance Validation of a Fuel Cell Unmanned Aerial Vehicle

This paper describes methods for design of an unmanned aerial vehicle which uses a proton exchange membrane fuel cell as its primary powerplant. The proposed design methods involve the development of empirical and physics-based contributing analyses to model the performance of the aircraft subsystems. The contributing analyses are collected into a design structure matrix which is used to map aircraft performance metrics as a function of design variables over a defined design space. An exhaustive search within the design space is performed to identify optimal design configurations and to characterize trends within the design space so as to inform lower-level design decisions. The results of the design process are used to construct a demonstration fuel cell-powered aircraft. Test results from the demonstration aircraft and its subsystems are compared to predicted results to validate the contributing analyses and improve their accuracy in further design iterations.

Comparison of Design Methods for Fuel-Cell-Powered Unmanned Aerial Vehicles

This paper presents two comparisons of design methods for fuel-cell-powered unmanned aerial vehicles. Previous design studies of fuel-cell-powered aircraft have used design methods that contain intrinsic assumptions regarding the design of a fuel cell powerplant and regarding the interactions between the powerplant and aircraft application. This study seeks to understand the effects of these design assumptions on the fuel cell powerplant structure and the aircraft performance. A design methods comparison is constructed by first developing a multidisciplinary modeling and design environment that is more general than the design processes proposed in literature. The design processes from previous studies can then be imposed on the more complete design environment to determine the performance costs and morphological changes caused by the design assumptions. In the first design study, results show that designing fuel-cell-powered aircraft using automotive-type fuel cell design rules leads to a low-efficiency powerplant and a low-performance aircraft in long-endurance and long-range unmanned aerial vehicle applications. The second design study shows that designing the aircraft powerplant using powerplant design criteria (such as specific energy) rather than aircraft design criteria (such as range) leads to suboptimal aircraft performance, especially for long-endurance unmanned aerial vehicle applications. The results of these studies show that the application-integrated design of aviation-specific fuel cell powerplants can significantly improve the performance of fuel-cell-powered aircraft for a variety of scales and missions.

Test Results for a Fuel Cell-Powered Demonstration Aircraft

A fuel cell powered airplane has been designed and constructed at the Georgia Insitute of Technology to develop an understanding of the design and implementation challenges of fuel cell-powered unmanned aerial vehicles (UAVs). A custom 448W net output proton exchange membrane fuel cell powerplant has been constructed and tested. A demonstrator aircraft was designed and built to accommodate this powerplant and the fuel cell powered aircraft has performed seven test flights to date. Test data show that the aircraft performance validates the models used for design and optimization and that the fuel cell aircraft is capable of longer endurance, higher performance test flights.

Design Space Exploration of Small-Scale PEM Fuel Cell Long Endurance Aircraft

Due to their high energy density, proton exchange membrane (PEM) fuel cell systems are becoming increasingly attractive as the primary powerplant for low-power, long-endurance aircraft applications. Although PEM fuel cell technology has been applied for automotive and stationary use, limited design and experimental work has been performed and documented for actual aircraft applications. In order to better understand the design and performance tradeoffs for PEM fuel cell powered aircraft, a high-level conceptual design study of small-scale long-endurance aircraft is performed. This study builds upon design lessons learned through the development and flight testing of a PEM-powered demonstrator aircraft designed and built by the Georgia Institute of Technology. The study focuses on identifying and exploring the concept design space appropriate for small unmanned air vehicles with ranges of up to 5000 km flying at low altitudes with endurances of up to 64 hours. A Quality Function Deployment is used in conjunction with a Matrix of Alternatives to define multiple competing aircraft configurations based on current advanced technologies in PEM fuel cells, hydrogen storage, electric propulsion, aircraft design, and structural materials. A baseline propulsion system consisting of a liquid cooled PEM fuel cell with compressed hydrogen storage powering multiple electric tractor propeller motors was chosen. The corresponding baseline aerodynamic configuration consisted of a high-aspect ratio tapered wing with multiple tractor propellers. Eleven design variables governing the powerplant, propulsion, and aircraft design were chosen and used as inputs to a combination of surrogate and physics based models that were solved using fixed point iteration. Using range, endurance, climb rate, and aircraft mass as metrics, the problem was optimized using a sequential unconstrained minimization technique (SUMT) with an extended interior penalty function using a simplex optimization search algorithm. Several design constraints were active at the optimal solutions for both range and endurance. Results showed that the design was primarily driven by design variables governing hydrogen storage. The analysis also showed that optimizing a design for energy density did not produce the best aircraft design for either long range or long endurance. With the same payload, aircraft optimized for range and endurance were much smaller and had better range, endurance, and climb performance than aircraft optimized for energy density.

Validation of Vortex Propeller Theory for UAV Design with Uncertainty Analysis

The emergence of the field of minito meso-scale unmanned aerial vehicle (UAV) design has generated renewed interest in propeller modeling, analysis and design. This paper presents a procedure for deriving the performance of an UAV-scale propeller from geometric measurements using commercially available airfoil modeling software and the vortex theory of airscrew propellers. Vortex theory formulations using the Prandtl tip loss factor as well as the Goldstein circulation function are presented and results are compared to wind-tunnel tests of UAV propellers. The effects of measurement and modeling uncertainties on the performance of the propeller are quantified and propagated through the algorithm using system sensitivity analysis.

Energy Management for Fuel Cell Powered Hybrid- Electric Aircraft

Many researchers have proposed hybridization of fuel cell powerplants for unmanned aerial vehicles with the goal of improving the aircraft performance. The mechanisms of this performance improvement are not well understood. This work poses the problem of deriving energy management strategies for fuel cell powered, hybrid fuel cell powered and internal combustion powered aircraft as an optimal control problem. Dynamic programming and sequential quadratic programming are used with reduced order dynamic models to solve for optimal energy management strategies and optimal flight paths for these aircraft. Results show that hybridization and flight path management does not improve the endurance of fuel cell powered aircraft for a fixed airframe design, as it can for internal combustion powered aircraft. During the aircraft design process, hybridization does allow the aircraft power constraints to be decoupled from the aircraft energy requirements, with beneficial results in an integrated aircraft design process.

Design Studies for Hydrogen Fuel Cell Powered Unmanned Aerial Vehicles

This paper presents a comparison of design methods for fuel cell powered unmanned aerial vehicle s. Previous studies of fuel cell powered aircraft have used design methods that contain intrinsic assumptions regarding the design of fuel cell powerplant and regarding the interaction s between the powerplant and aircraft application. This study seeks to understand the effects of these design assumptions on the powerplant structure and the aircraft performance of fuel cell powered aircraft. A comparison is constructed by developing a multidisci plinary modeling and design environment that does not contain the assumption suggested in previous studies. The design assumptions from previous studies can then be imposed on the more complete design method to determine the performance costs and morpholo gical changes caused by the design assumptions. In the first design study, results show that designing fuel cell powered aircraft using automotive -type fuel cell subsystem design rules leads to a low efficiency powerplant and a low performance aircraft in long -endurance and long -range UAV applications. The second design study shows that designing the long endurance aircraft powerplant towards maximum specific energy leads to suboptimal aircraft performance, especially for long -endurance UAV applications.

Flight Test Results for a Fuel Cell Unmanned Aerial Vehicle

Fuel cell -powered aircraft are an emerging and impo rta nt application of fuel cell and advanced aircraft technologies. This paper presents and analyzes test results for a fuel cell powered unmanned aerial vehicle. The aircraft under analysis is a fully functional technology demonstrator that uses a polymer ele ctrolyte fuel cell system fueled by compressed hydrogen . The tested performance of the fuel cell system, propulsion system and aircraft is critically evaluated using data from flight and laboratory testing.

Reducing Uncertainty of a Fuel Cell UAV through Variable Fidelity Optimization

As a result of their high specific energy, high efficiency, improved environmental performance, and rapid refuelability, polymer electrolyte membrane (PEM) fuel cells are increasingly being considered as primary powerplants for unmanned aerial vehicles (UAV’s). The combination of a highly constrained design space and a large amount of performance uncertainty complicates the design of fuel cell aircraft. This paper documents a method that improves the design of a fuel cell powered long-endurance (~24 hr) lowaltitude UAV by using multidisciplinary design optimization techniques to address the highly constrained design space, and systems sensitivity analysis (SSA) to estimate the performance uncertainly. The multidisciplinary analysis of the aircraft is decomposed into contributing analyses (CA’s) that are structured so as to simplify experimental validation and to improve the fidelity of the SSA. The contributing analyses are coupled into a design system matrix that is iteratively solved using Newton’s method to calculate aircraft performance metrics as a function of input design variables. An optimizer searches the design space to find the values of the design variables that maximize the endurance of the aircraft subject to several performance constraints. SSA is used to estimate the percentage of the overall uncertainty in the performance metrics based on the uncertainty of the contributing analyses and the uncertainty of the design input variables. Based on the SSA, CA’s with significant contribution to the total performance uncertainty are selected and laboratory experiments are then conducted to obtain data used to increase the fidelity of these CA’s through calibration or regression. The optimization and uncertainty propagation is then repeated to obtain an improved design. In this paper, several iterations of the SSA/Experimental Validation procedure were performed until a design was selected that was based on high fidelity CA’s modeling all components of the fuel cell propulsion system. The predicted performance of the aircraft in terms of endurance and climb rate is reported with the predicted uncertainty after each iteration in the design.

Validated Modeling and Synthesis of Medium-Scale Polymer Electrolyte Membrane Fuel Cell Aircraft

This paper describes a methodology for design and optimization of a polymer electrolyte membrane (PEM) fuel cell unmanned aerial vehicle (UAV). The focus of this paper is the optimization of the fuel cell propulsion system and hydrogen storage system for a baseline aircraft. Physics-based models, and experimentally-derived sub-system performance data are used to characterize the performance of each configuration within a design space. The results of aircraft synthesis and performance modeling routines are used to create response surface equations where tradeoffs among component specifications can be explored. Significant tradeoffs between fuel cell performance, hydrogen storage and aircraft aerodynamic and propulsion system design are presented. Validation and test results from a proof-of-concept fuel cell UAV propulsion system are presented. Validated models of the fuel cell and aircraft systems are used to predict the performance of fuel cell UAVs at the scale of the baseline aircraft.