Peter Flick

Design and application of compliant mechanisms for morphing aircraft structures

Morphing aircraft structures can significantly enhance air vehicle performance. This paper highlights ongoing work to design novel compliant mechanisms that efficiently morph aircraft structures in order to exploit aerodynamic benefits. Computational tools are being developed to design structures that deform into specified shapes given simple actuator inputs. In addition, these synthesis methods seek to optimize the stiffness of the structure to minimize actuator effort and maximize the stiffness with respect to the environment (external loading). These tools have been used to study two different types of morphing systems: (i) variable geometry wings and (ii) high-frequency vortex generators for active flow control. Several case studies are presented which highlight the design approach and computational and experimental results of these morphing aircraft systems.

Flight testing of Mission Adaptive Compliant Wing

This paper describes flight test results of a “Mission Adaptive Compliant Wing” (MACWing) variable geometry trailing edge flap in conjunction with a natural laminar flow airfoil. The MAC-Wing technology provides lightweight, low power, variable geometry reshaping of the upper and lower flap surface with no seams or discontinuities. In this particular program, the airfoil-flap system is optimized to maximize the laminar boundary layer extent over a broad lift coefficient range for endurance aircraft applications. The expanded “laminar bucket” capability allows the endurance aircraft to significantly extend their range (15% or more) by continuously optimizing the wing L/D throughout the mission. The wing was tested at full-scale dynamic pressure, full scale Mach, and reduced-scale Reynolds Numbers on Scaled Composites’ White Knight aircraft. Test results confirmed laminar flow regime up to approximately 60% chord for much of the lift range. Analysis and test results suggest significant fuel savings, weight savings and a higher control authority. Preliminary drag results, future aerodynamic applications and vehicle performance projections are discussed.

Closed-Loop Stall Control System

A closed-loop, stall sense and control system was demonstrated on a morphing airfoil. The FlexSys, Inc. Mission Adaptive Compliant Wing was modified to accept a Boeing Co. dielectric barrier discharge actuator panel in a location immediately upstream of the trailing-edge morphing flap, and hot-film sensors were installed on the model surface. A signal analysis algorithm, developed by Tao Systems, Inc., was applied to the hot-film signals to detect separation and trigger activation of the dielectric barrier discharge actuators. The system was successfully demonstrated in the U.S. Air Force Research Laboratory Phillip P. Antonatos Subsonic Aerodynamics Research Laboratory wind-tunnel facility, and an improvement in lift of about 10% was observed at Mach 0.05 (chord Reynolds number 9 x 10 5 ) under closed-loop control and a turbulent boundary-layer state. Actuator effectiveness was demonstrated up to Mach 0.1, but must be extended to Mach 0.2-0.3 to enable a practical stall control system for takeoff and approach of large aircraft. It may be possible to obtain that level of performance by optimizing the actuator locations and input waveforms.

Closed-Loop Stall Control on a Morphing Airfoil Using Hot-Film Sensors and DBD Actuators

A closed-loop, stall sense and control system was demonstrated on a morphing airfoil. The FlexSys, Inc. Mission Adaptive Compliant Wing (MACW) was modified to accept a Boeing Co. dielectric barrier discharge (DBD) actuator panel in a location immediately upstream of the trailing-edge morphing flap, and hot-film sensors were installed on the model surface. A signal analysis algorithm, developed by Tao Systems, Inc, was applied to the hot-film signals to detect separation and trigger activation of the DBD actuators. The system was successfully demonstrated in the AFRL SARL wind tunnel facility, and an improvement in lift of about 10% was observed at Mach 0.05 (chord Reynolds number 9× 10) under closed-loop control and a turbulent boundary layer state. Actuator effectiveness was demonstrated up to Mach 0.1, but must be extended to Mach 0.2–0.3 to enable a practical stall control system for takeoff and approach of large aircraft.

Structural Modal Control and Gust Load Alleviation for a SensorCraft Concept

SensorCraft is an Air Force Research Lab (AFRL) concept for a high flying vehicle that will be capable of providing greatly increased Intelligence, Surveillance, and Reconnaissance (ISR) capabilities. Part of the technology SensorCraft will require is high aerodynamic and structural efficiency to accomplish its mission goal of long range and sustained presence. To achieve a long loiter time, it will have a light weight structure with a high aspect ratio wing that carries much of the fuel mass. Hence, the structural modes will be in the same frequency range as the rigid body modes and will be strongly coupled with them. Wing stresses will need to be reduced and the gust load contribution will need to be minimized to keep the structural bending loads low along the span of the wing. This paper documents the results of a wind tunnel test at NASA Langley in the Transonic Dynamics Tunnel of a gust load alleviation system for a SensorCraft concept. The wing was fixed to the wall of the tunnel and the 4 trailing edge and 1 leading edge control effectors were used to simultaneously control first and second bending as well as control the pitching moment at the balance attachment.

Unsteady Aerodynamic Observable for Gust Load Alleviation and Flutter Suppression

The leading-edge stagnation point (LESP) location was used to measure and control unsteady aerodynamic forces generated by a exible wing in response to gust and control actuation. The LESP location measured with sub-millisecond response at a number of span stations on the AFRL SensorCraft model installed in NASA’s Transonic Dynamics Tunnel (TDT) is shown to highly correlate, albeit with a phase dierence, with the structural response obtained with conventional strain gages and accelerometers. The results indicate that the LESP could be used as an aerodynamic observable for closed-loop ight control applications. Preliminary results from utter experiments conducted in the TDT tunnel will be shown. The role of unsteady aerodynamic observables for ight control and aerodynamic eciency will be discussed.

Autonomous Flight Control Development on the Active Aeroelastic Wing Aircraft

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. Wing control surfaces are also being deflected to provide maneuver load control during pitching maneuvers. Simulation of the AAW control laws shows acceptable flight performance using this new control surface scheme. Time histories of these simulations will be presented to demonstrate the performance capability of the AAW aircraft while maintaining structural loading within pre-specified limits. Although structural loading is not currently being used as a feedback to the controller, the control law design was developed with load constraints as one of the driving parameters. Flight testing for Phase II should begin in the fall of 2004. At that time, we will be able to verify that the control laws work as they did in the simulations. An example of how this technology can be used would be to design future aircraft with good performance and maneuver load control (MLC) but without horizontal tails. The design space could also be opened up to thinner wings, higher aspect ratio wings, or morphing wings. The goal is not to provide a stiff wing for conventional control, but to allow optimization of the structure to make use of more flexible wings. Control inputs would be the same as for conventional aircraft, but rolling and pitching moment can be achieved through a variety of autonomous control surface inputs, including the wing as a control surface in and of itself. This program was developed under contract from the Air Force Research Laboratorys Air Vehicles Directorate with flight test development sponsored by NASA-DFRC.