David E. Parekh

Aerodynamic Flow Control over an Unconventional Airfoil Using Synthetic Jet Actuators

Control of flow separation on an unconventional symmetric airfoil using synthetic (zero net mass flux) jet actuators is investigated in a series of wind tunnel tests. The symmetric airfoil comprises the aft portion of a NACA four-digit series airfoil and a leading edge section that is one-half of a round cylinder. The experiments are conducted over a range of Reynolds numbers between 3.1 × 10 5 and 7.25 × 10 5 . In this range, the flow separates near the leading edge at angles of attack exceeding 5 deg. When synthetic jet control is applied near the leading edge, upstream of the separation point, the separated flow reattaches completely for angles of attack up to 17.5 deg and partially for higher angles of attack. The effect of the actuation frequency, actuator location, and momentum coefficient is investigated for different angles of attack. The momentum coefficient required to reattach the separated flow decreases as the actuators are placed closer to the separation point. In some cases, reattachment is also achieved when the actuators are placed downstream of the stagnation point on the pressure side of the airfoil

MODIFICATION OF LIFTING BODY AERODYNAMICS USING SYNTHETIC JET ACTUATORS

The control of separated flow on an unconventional airfoil using synthetic jet actuators was investigated experimentally. A symmetric airfoil based on the aft portion of a NACA four-digit series airfoil with a cylindrical leading edge was used in the experiment. The tests were conducted at Rec=3(10)5. For a>5°, the flow separated from the airfoil surface. Applying synthetic jet control near the leading edge, upstream of the separation point, reattached the separated flow fixangle of attack up to 18°. The effect of control location and amplitude was investigated for different angles of attack. Hot wire measurements in the nearwake of the airfoil revealed a transient passing of vortices associated with the transition from separated to reattached flow on the airfoil.

Active Flow Control on the Stingray Uninhabited Air Vehicle: Transient Behavior

The application of leading-edge separation control on an uninhabited air vehicle (UAV) with 50-deg leading-edge sweep is investigated experimentally in a full-scale close-return wind tunnel using arrays of synthetic jet actuators. Active flow control is used to enhance vehicle control at moderate and high angles of attack for takeoff and landing activities or gust load alleviation. The surface-mounted synthetic jet actuators are operated in various waveforms where the carrier frequency is at least an order of magnitude higher than the characteristic shedding frequency of the UAV. Actuation yields a suction peak near the leading edge, however, while excitation with a sinusoidal waveform results in a sharp suction peak near the leading edge; pulse modulation yields a larger and wider suction peak. The flow transients associated with controlled reattachment and separation of the flow over the UAV are investigated using amplitude modulation of the actuation waveform by measuring the dynamic surface pressure at different locations on the upper surface of the UAV’s wing. Phase-locked measurements show that the transients, associated with the onset of reattachment and separation, at high angles of attack are accompanied by the shedding of large-scale vortical structures and oscillations of the surface pressure. However, no oscillations are observed when the baseline flow is attached.

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.

ACTIVE FLOW CONTROL APPLICATION ON A MINI DUCTED FAN UAV

A new class of Unmanned Aerial Vehicle (UAV) is investigated, based on the flying ducted fan concept. The vehicle uses a mechanically simplified control approach employing synthetic jet active flow control in lieu moving control surfaces or articulated rotor blades. As a result, the UAVs propulsion and control systems are simplified to a single moving part, a fixed pitch propeller. This mechanical simplicity makes the application of active flow control particularly attractive for development of mini and micro UAVs. The active control of flow separation over a lifting surface as a means of discrete state aerodynamic control briefly approached from the theoretical standpoint, then explored from an experimental standpoint. Effectiveness of a synthetic jet active flow control technique for generating the intended aerodynamic force changes on a ducted stator vane is explored and validated by experiments on a 3-D duct model. Positive results of these experiments lead to development of a full-scale (2-3 Kg. G.W.) proof of concept dynamic model on which the application of flow control was successfully demonstrated.

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.