| Objective: ||Develop and demonstrate through flight tests reliable, high-fidelity MAV air data sensors coupled with a flow separation control scheme integrated with the aircraft flight control system.
|| Description: ||Research in response to interest in military and civilian applications of MAVs (here defined as aircraft with maximal linear dimension on the order of 10cm and below) has in recent times included 1) low Reynolds number aerodynamics, both experimental and computational, 2) MAV flight article design, and 3) development of enabling systems and sensors, such as high energy density batteries and robust navigation systems. Much controversy exists regarding whether progress in 2) is more dependent on 3) or 1); that is, whether the critical gaps in the aeronautical engineering of MAVs are more of systems integration or the underlying aerosciences.
The abstracted problem of MAV aerodynamics is closely related to fundamental issues in laminar-to-turbulent transition, rigorous description of vorticity transport, and coupling between nonlinearities in the structure and the flowfield that is, some of the most celebrated problems in all of continuum mechanics. The presence of massive separation with comparatively simple means of flow reattachment has attracted broad interest in MAV flow control. However, it is possible that MAV aerodynamics laboratory results, for example regarding the presence of large laminar separation bubbles (LSBs) for airfoils in low-turbulence wind tunnels, are of secondary importance for flight-article MAVs. Sources of noise, such as propeller wash, surface roughness, tip vortices for low-aspect-ratio wings, disturbances due to the ambient wind environment, and so forth, conceivably deemphasize the importance of two-dimensional (2-D) airfoil performance gauged in quiet environments, while also obviating the case for certain types of flow control. Meanwhile, for a very small aircraft that are bouncing around either due to gusts or its own maneuvering, the effective angle of attack and angle of sideslip are not easy to define and even more difficult to measure. If the state of the aircraft is not accurately known, then even a robust control system whether a straight flight control system, or, as recently investigated for larger UAVs a system of flow control for flight control, based on a model of the flow physics will likely fail. Furthermore, it remains to ascertain that flow relaxation time (upon actuation by the flow control system) is fast enough, relative to the aircraft flight dynamics; in other words, that the actuation bandwidth is adequate.
Definitive resolution of the above-mentioned controversy is predicated on experimentation in realistic environments: the flight testing of MAV with instrumentation and rigor normally associated with wind tunnels. For proof that a flow control scheme is both necessary (that is, the flowfield really is separated) and effective (it promotes rapid reattachment), one needs to know the full aircraft aerodynamic state, by which we mean the mutual interactions of time-dependent flowfields, aircraft aeroelastic deflections, the aircraft aerodynamic and inertial loads, the resulting flight dynamics, and to close the loop the effect on the flowfield.
The first portion of this topic is development of innovative, lightweight and high-precision air data sensors onboard a MAV flight article for measurement of quantities including the following: local shear stress (and thus the presence of flow separation), static pressure, vehicle angle of attack and angle of sideslip and local structural deflection of possibly very flexible lifting surfaces. One approach is by biomimetic inspirations for example, hair-like filaments projecting from flight surfaces, which would be sensitive to wall shear stress. The second portion is the development of methods to attenuate flow separation from MAV lifting surfaces, and the flight demonstration of such methods. Of course, the effect of the flow control shall be coupled into the aircraft flight control system.
|| ||PHASE I: Pursue feasibility-study concepts for (1) method of separation control relevant for low-aspect-ratio, low-Re wings in propwash and (2) air data sensors suitable to measure the MAV vehicle state in-flight. Explore how the flow control would use the air data sensor information as inputs.
|| || ||PHASE II: Conduct integrated prototype demo of flight and wind tunnel testing for 1) closed-loop separation control with sufficient bandwidth for flight control; 2) surface shear stress measurement for flow control system inputs; and 3) controlled flight, with effects of flow control on flight dynamics. Verify in flight that the flow control improves aircraft lift to drag ratio, gust tolerance, or both.
|| ||DUAL USE COMMERCIALIZATION: Military application: Air data sensors for MAVs and small aircraft in general, flight-testing purposes well beyond just flow control development. Military MAV applications include tagging and urban surveillance. Commercial application: Applications for aircraft flight testing in general are the same as for military. MAV applications themselves include search and rescue, pipeline and powerline inspection, and urban policing.
|| References: ||
1. Mueller, T. J., Ed., “Proceedings of the Conference on Fixed, Flapping and Rotary Wing Vehicles at Very Low Reynolds Numbers,” Notre Dame University, Indiana, June 5-7, 2000. Published as Vol. 195, Progress in Astronautics and Aeronautics, AIAA.
2. Ol, M., McAuliffe, B. R., Hanff, E. S., Scholz, U., Kaehler, Ch., “Comparison of Laminar Separation Bubble Measurements on a Low Reynolds Number Airfoil in Three Facilities", AIAA 2005-5149, 2005.
3. Engel, J., Chen, J., Chen, N.,and Liu, C. "Development and Characterization of an Artificial Hair Cell Based on Polyurethane Elastomer and Force Sensitive Resistors," The 4th IEEE International Conference on Sensors, Irvine, California, 31 October - 1 November, 2005.
5. SBIR Topic AF05-243, “Integration of Flight Control and Flow Control, http://www.acq.osd.mil/sadbu/sbir/solicitations/sbir051/af051.htm
|Keywords: ||MAV, micro air vehicle, low Reynolds number, flight testing, air data sensors, biomimetic, separated flow, flexible wing, flow control, flight control, shear stress|