Mars Airplane Airfoil Design with Application to ARES

Flight in the Mars environment presents a unique design challenge for Mars airplane airfoils. A family of airfoils with varying camber was developed for application to the Aerial Regional-scale Environmental Survey (ARES) Mars Scout airplane design. A point of diminishing returns was observed where additional camber no longer increased maximum lift. The selection of specific airfoils from this family was based on evaluation of the desired characteristics at various locations on the wing. The current wing design for the ARES aircraft uses sections with moderate camber which provide a good compromise between high-lift capability and cruise performance. If future analysis determines that more capability is needed during the pullout maneuver, airfoils with higher maximum lift coefficient may be selected from the family. INTRODUCTION The concept of sending an airplane to Mars to perform regional-scale scientific surveys (over several hundred kilometers) has drawn interest for over a quarter century. Reference 1 provides a detailed historical account of several published Mars airplane design efforts. Because of the competitive nature of opportunities for Mars exploration, other efforts toward design and technology development have been made without archival publication. Notable among those have been activities throughout the 1990’s in the Space Projects Division at NASA Ames Research Center in collaboration with the Naval Research Laboratory and David Hall Consulting, Inc., and at the NASA Jet Propulsion Laboratory in collaboration with AeroVironment, Inc. The 1999 NASA Research Announcement (NRA) for a Mars Micromission Airplane concept stimulated activities at NASA Langley, Ames, and Dryden, as well as a wide range of other design efforts. While most of those efforts are unpublished, considerable qualitative design experience has been accumulated within the interested community. * Research Engineer; Associate Fellow AIAA. † Aerospace Engineer; Senior Member AIAA. ‡ Senior Research Scientist. The recent (and ongoing) development of an airplane for the Aerial Regional-scale Environmental Survey (ARES) mission, a proposed Mars Scout mission in 2007-2008, has fostered a substantial airfoil design effort. To support the early configuration development and initial performance assessment of the ARES airplane, a series of airfoils was designed with a range of thickness and camber. This “family” of airfoils allows trades to be made between various performance attributes of the ARES aircraft. This paper describes the design of this family of airfoils and the design considerations that led to the selection of specific airfoils for the ARES aircraft. The expected flight conditions and performance requirements of the airplane are discussed, followed by a description of the general design philosophy, computational design tools, and procedures. A brief description of the design and characteristics of each airfoil in the family is presented, along with the effects of design parameters such as camber, thickness, and boundary layer transition on performance at these flight conditions. Finally, the wing design issues that led to the selection of specific airfoils for the ARES airplane are discussed. Computational validation of the airplane design has been performed with high-fidelity 2-D and 3-D NavierStokes simulations. Some experimental validation of computational predictions is available for similar airfoils, while experimental validation of the ARES airfoils and vehicle performance is planned for the near future. DESIGN CHALLENGES AND OBJECTIVES Mars Flight Conditions The atmospheric environment on Mars presents unique challenges for the design of aircraft. Table 1 gives representative surface atmospheric conditions from the Mars Global Reference Atmosphere Model (MarsGRAM). Actual atmospheric conditions may vary 20% or more from these values due to location, season, and time of day. 2nd AIAA "Unmanned Unlimited" Systems, Technologies, and Operations — Aerospac 15 18 September 2003, San Diego, California AIAA 2003-6607 This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. 2 American Institute of Aeronautics and Astronautics Table 1. Representative Mars Surface Atmospheric Conditions Parameter Value percentage of Earth sea level condition Temperature 220 °K 77% Density 1.25x10 kg/m 1% Pressure 530 N/m 1/2% Speed of Sound 225 m/s 66% The low temperature and extremely low atmospheric density, combined with the relatively small size of aircraft that can be delivered to Mars, lead to flight conditions characterized by a combination of high subsonic Mach numbers and very low Reynolds numbers. The requirement for subsonic flight arises from the scientific objectives and instrument capabilities. Atmospheric sampling instruments expect to make insitu measurements of atmospheric gases. Acquiring gas samples at supersonic speeds would require the samples to pass through a shock at the probe intake where the sudden rise in temperature and pressure could introduce spurious changes in the atmospheric chemistry. In addition, surface-imaging requirements restrict flight speed. Unless rotating optics are used to track the surface, image-blurring constraints limit the flight speed at a given altitude. Altitude may in turn be limited by image resolution requirements as well as aircraft performance capability. The allowable weight of a given airplane is proportional to MCL, where the lift coefficient, CL, is reduced by some margin below CLmax to allow for gusts, maneuvers, robust flying qualities, and performance uncertainties. The M term arises from the dynamic pressure, 1/2γP∞M. Within the subsonic regime, the achievable CLmax increases in proportion to the PrandtlGlauert parameter, (1-M) . The strong dependence of allowable wing loading on Mach number from both of these effects suggests that higher Mach numbers will allow heavier, more capable airplanes. This trend is limited by the onset of transonic flow with shock– boundary layer interactions that drastically reduce CLmax. Figure 1 provides a notional illustration of these trends. For unswept wings, the maximum lifting ability is generally achieved with M=0.6 to 0.7. For the 1999 Micromission studies, airplanes were designed to fly with wing chord Reynolds numbers of about 50,000. In comparison, the current ARES Mars Scout airplane is designed to fly with a wing chord Reynolds number of about 150,000–200,000. While it is routine to operate low-speed model aircraft at these Reynolds numbers (and even significantly lower) on Earth, their performance capabilities at these conditions are diminished significantly compared to larger aircraft. Figure 2 illustrates the effect of Reynolds number on the performance of a typical model airplane airfoil. For this particular airfoil at a Reynolds number of 150,000, there is a modest increase in drag but little degradation of maximum lift compared to higher Reynolds numbers. At 100,000 Reynolds number, drag is increased further and maximum lift capability is diminished. At 60,000 Reynolds number, there is substantial reduction of Clmax as well as an additional increase in drag. A general conclusion from this trend is that while the size constraints imposed by the Mars Micromission concept presented an extremely challenging airplane design problem, the substantially larger ARES Mars Scout aircraft enables a much more robust and capable airplane. The low Reynolds number still presents a design issue, but one that can be managed with careful design and testing. The unique combination of low Reynolds number and high subsonic Mach number poses a combination of issues and benefits that are discussed later in this paper. Mission and Operational Requirements In addition to the unusual flight conditions, mission and/or operational requirements add further difficulties to the wing design problem. To maximize the size and usefulness (payload, endurance, etc.) of the aircraft, most proposed configurations are deployed from a folded condition in a container designed for atmospheric entry. The first flight phase after deployment from the entry aeroshell is then a pullout maneuver from a very steep descent angle at high altitude. With very low dynamic pressure, the airplane quickly accelerates to the point where the lifting capability needed for the pullout is diminished by buffet onset at high Mach number (see Figure 1). Even if a stabilizing drogue chute is used to prevent excessive Mach numbers, this maneuver requires as much lift as possible at an extreme flight condition of high Mach number and low Reynolds number. This maneuver most likely establishes the constraint on maximum aircraft weight and becomes the critical design flight condition for assessing airfoil and wing performance. Depending on the folding/packaging scheme chosen, the vehicle itself may pose an additional requirement on the airfoil design. The desire for simple folding arrangements often limits horizontal tail size and moment arm. It may well be that a wing with more camber may be capable of higher maximum lift coefficient, but may produce so much pitching moment