Hak-Tae Lee

Flutter Suppression for High Aspect Ratio Flexible Wings Using Microflaps

Miniature trailing edge effectors (MiTEs) are small flaps (typically 1% to 5% chord) actuated with deflection angles up to 90 degrees. Because of their small size, these devices provide the opportunity for high bandwidth control. The present study considers the use of many such control surfaces to increase the flutter speed of a high aspect ratio flexible wing. A finite element plate model is used to model the structural dynamics and an unsteady panel method provides the aerodynamic loads. Experimental flutter testing shows good agreement with the numerical stability analysis. The MiTE is modelled by a single panel element at the trailing edge with varying boundary conditions at its collocation point. In spite of the complex viscous aerodynamics of the MiTEs, the panel model proved to be adequate in simulating the steady and unsteady behavior. The use of these effectors for control is complicated by their nonlinear characteristics. Since the actuator is only effective at high deflection angles, it is only deflected in one of three positions: up, down, and neutral. The design of a nonlinear feedback controller has been performed using numerical optimization. Introduction The Gurney flap is a small (typically 1% ∼ 5% chord) flap used to increase the maximum lift of an airfoil section. It was developed and applied to racing cars by Robert Liebeck and Dan Gurney in 1960’s, although similar devices were employed in World War II aircraft such as the P-38 ∗Doctoral Candidate, Department of Aeronautics and Astronautics, AIAA Student Member †Professor, Department of Aeronautics and Astronautics, AIAA Fellow ‡Doctoral Candidate, Department of Aeronautics and Astronautics, AIAA Member Copyright c ©2002 by authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. and F8-F. Numerous wind-tunnel tests and numerical computations have been conducted on both single element and multi-element airfoils with Gurney flaps. These studies confirm that despite their small size, Gurney flaps with deflections near 90 degrees can increase maximum lift and the lift produced at a given angle of attack. Liebeck explained this effect, produced by a short region of separated flow directly upstream of the flap, with two counterrotating vortices downstream that effectively modify the trailing edge Kutta condition. This was verified to be correct for time averaged flow by flow visualizations and CFD simulations. In the present work, we consider the use of devices similar to Gurney flaps, not to increase maximum lift, but to provide high bandwidth, robust control. Miniature Trailing edge Effectors (MiTEs) are small movable control surfaces at or near the trailing edge, deflected to large angles to produce control forces and moments that may be used for flight control or structural mode control. The current study, begun in 1998, deals with the use of such actuators for aeroelastic control. MiTEs have distinct advantages over conventional control surfaces: High bandwidth actuation can be achieved due to their small size and inertia, enabling their use for flight control or for higher frequency structural mode control with significantly reduced power requirements. Spanwise variation and interdigitated deflections can produce rolling, pitching, and yawing moments, as well as the control of specific structural modes. Because the surfaces are deflected in a discrete manner (up, down, or neutral), no active servo-feedback is required, eliminating the expense of accurate, high-rate servo actuators and enabling a large number of these effectors to be fabricated at a low cost. The use of a large number of small, simple effectors also makes the system faulttolerant. The application of MiTEs for aeroelastic control is demonstrated here by designing an active control system that can suppress the flutter of a flexible wing. High aspect ratio flexible wings are of interest

Computational Investigation of Airfoils with Miniature Trailing Edge Control Surfaces

Miniature trailing edge effectors (MiTEs) are small flaps (typically 1% to 5% chord) actuated with deflection angles up to 90 degrees. The small size, combined with little required power and good control authority enables the device to be used for high bandwidth control. Numerous experiments and computational simulations have been conducted for two dimensional airfoils with miniature flaps. However the three-dimensional effects of these devices haven’t been extensively investigated. The present study examines the steady three dimensional aerodynamics of MiTEs using incompressible Navier-Stokes flow solver. Preliminary contents • Spanwise lift distribution Preliminary computation shows that the influence of a single MiTE is widely distributed along the entire span of a wing. Spanwise lift distribution will be investigated. • Spanwise length of a individual actuator Since MiTEs do not usually encompass full span, it is necessary to investigate the effect of the spanwise length, in addition to the height. The influence of the length of MiTE on the control effectiveness will be examined. • Gap If more than two MiTEs are actuated separately, Gaps are required between the MiTEs, especially when the wing is flexible. Changes in the control effectiveness and drag due to the gaps between the actuators will be investigated. Doctoral Candidate, Department of Aeronautics and Astronautics, AIAA Student Member Professor, Department of Aeronautics and Astronautics, AIAA Fellow 1 2nd AIAA Flow Control Conference 28 June 1 July 2004, Portland, Oregon AIAA 2004-2693 Copyright

Closed-Form Takeoff Weight Estimation Model for Air Transportation Simulation

Takeoff weight is an important parameter for computing accurate aircraft trajectories. Most systems used for simulating air traffic and for providing air traffic management decision support use takeoff weights that depend only on the aircraft type. This paper proposes a closed-form algorithm for estimating takeoff weights based on flight plan and aircraft performance data. The algorithm is derived by combining the constantaltitude-cruise range equation with the weight estimation procedure commonly used in aircraft design. The model first determines whether the payload is limited by payload capacity, maximum takeoff weight, or fuel tank capacity, and then calculates the takeoff weight accordingly. The model is verified against manufacturer provided payload range diagrams for a jet and a turboprop aircraft. Accurate and fast takeoff weight estimation with negligible computational overhead will enhance large scale air traffic simulations by improving the accuracy of trajectories and fuel burn estimates. Improvement in trajectory and fuel burn estimates will benefit the assessment of noise and emissions as well as improve the accuracy of automated conflict detection and resolution algorithms.

Interaction of Airspace Partitions and Traffic Flow Management Delay

The interaction of partitioning the airspace and delaying flights in the presence of convective weather is explored to study how re-partitioning the airspace can help reduce congestion and delay. Three approaches with varying complexities are employed to compute the ground delays. In the first approach, an airspace partition of 335 high-altitude sectors that is based on clear weather day traffic is used. Routes are then created to avoid regions of convective weather. With traffic flow management, this approach establishes the baseline with per-flight delay of 8.4 minutes. In the second approach, traffic flow management is used to select routes and assign departure delays such that only the airport capacity constraints are met. This results in 6.7 minutes of average departure delay. The airspace is then partitioned with a specified capacity. It is shown that airspace-capacity-induced delay can be reduced to zero at a cost of 20 percent more sectors for the examined scenario. While the first two approaches investigate the upper and lower bounds in terms of delay and number of sectors, the third approach investigates the tradeoff between the number of sectors and the delay by re-applying the traffic flow management using the re-partitioned sectors. In this approach, the weather constraints are reflected in the sector partitions, and the delay is shared between airspace and airports. The solutions discovered by this approach are 6.9 minutes of average delay with a 312 sector configuration and 8.1 minutes of delay with a 253 sector configuration. Results show that a sector design that is tailored to the traffic and weather pattern can reduce delay while reducing the number of sectors at the same time. However, airspace partitioning can only address the delays caused by airspace congestion. Even in the presence of convective weather, the airport capacity constraint causes the majority of the delay.

Two Dimensional Unsteady Aerodynamics of Miniature Trailing Edge Effectors

Miniature trailing edge effectors (MiTEs) are small flaps, typically around 1% chord, actuated with deflection angles up to 90 degrees. Their small size, combined with low required power and good control authority, enables the devices to be used for high bandwidth control as well as conventional attitude control. However, some of the aerodynamic characteristics of these devices are complex and poorly understood. This paper describes time accurate computations that were performed using the INS2D flow solver to resolve unsteady characteristics including transient response and vortex shedding phenomena. The frequency response was studied to identify the dynamics of moving MiTEs, an important aspect of these new control devices.

Benefit Assessment of the Precision Departure Release Capability Concept

A Precision Departure Release Capability concept is being evaluated by both the National Aeronautics and Space Administration and the Federal Aviation Administration as part of a larger goal of improving throughput, efficiency and capacity in integrated departure, arrival and surface operations. The concept is believed to have the potential of increasing flight efficiency and throughput by avoiding missing assigned slots and minimizing speed increase or path stretch to recover the slot. The main thrust of the paper is determining the impact of early and late departures from the departure runway when an aircraft has a slot assigned either at a meter fix or at the arrival airport. Results reported in the paper are for two scenarios. The first scenario considers flights out of Dallas/Fort Worth destined for Hartsfield-Jackson International Airport in Atlanta flying through the Meridian meter-fix in the Memphis Center with miles-in-trail constraints. The second scenario considers flights destined to George Bush Intercontinental/Houston Airport with specified airport arrival rate constraint. Results show that delay reduction can be achieved by allowing reasonable speed changes in scheduling. It was determined that the traffic volume between Dallas/Fort Worth and Atlanta via the Meridian fix is low and the departures times are spread enough that large departure schedule uncertainty can be tolerated. Flights can depart early or late within 90 minutes without accruing much more delay due to miles-in-trail constraint at the Meridian fix. In the Houston scenario, 808 arrivals from 174 airports were considered. Results show that delay experienced by the 16 Dallas/Fort Worth departures is higher if initial schedules of the remaining 792 flights are kept unaltered while they are rescheduled. Analysis shows that the probability of getting the initially assigned slot back after perturbation and rescheduling decreases with increasing standard deviation of the departure delay distributions. Results show that most Houston arrivals can be expected to be on time based on the assumed zero-mean Normal departure delay distributions achievable by Precision Departure Release Capability. In the current system, airport-departure delay, which is the sum of gate-departure delay and taxi-out delay, is observed at the airports. This delay acts as a bias, which can be reduced by Precision Departure Release Capability.

Interaction of airspace partitions and traffic flow management delay with weather

The interaction of partitioning the airspace and delaying flights in the presence of convective weather is explored to study how re-partitioning the airspace can help reduce congestion and delay. Three approaches with varying complexities are employed to compute the ground delays. In the first approach, an airspace partition of 335 high-altitude sectors that is based on clear weather day traffic is used. Routes are then created to avoid regions of convective weather. With traffic flow management, this approach establishes the baseline with per-flight delay of 8.4 minutes. In the second approach, traffic flow management is used to select routes and assign departure delays such that only the airport capacity constraints are met. This results in 6.7 minutes of average departure delay. The airspace is then partitioned with a specified capacity. It is shown that airspace-capacity-induced delay can be reduced to zero at a cost of 20 percent more sectors for the examined scenario. While the first two approaches investigate the upper and lower bounds in terms of delay and number of sectors, the third approach investigates the tradeoff between the number of sectors and the delay by re-applying the traffic flow management using the re-partitioned sectors. In this approach, the weather constraints are reflected in the sector partitions, and the delay is shared between airspace and airports. The solutions discovered by this approach are 6.9 minutes of average delay with a 312 sector configuration and 8.1 minutes of delay with a 253 sector configuration. Results show that a sector design that is tailored to the traffic and weather pattern can reduce delay while reducing the number of sectors at the same time. However, airspace partitioning can only address the delays caused by airspace congestion. Even in the presence of convective weather, the airport capacity constraint causes the majority of the delay.