Aerodynamics: The Outlook for the Future
In previous chapters, we have seen how the foundations of the aerodynamics were established and the developments were made in a little more than a century in this discipline in relation to the Aerospace Engineering applications. The progress is still continuing thanks to the advances made in wind tunnel and flight test measurements as well as the remarkable improvements achieved in computational means implemented in numerical simulations.
The knowledge provided by the classical aerodynamics is sufficient to determine the aerodynamic performances of the high aspect ratio wings at low subsonic speeds and the low aspect ratio wings at supersonic speeds. On the other hand, as the speed or angle of attack increases and/or the aspect ratio decreases, we need modern concepts for aerodynamic analysis. The increase in cruise speeds causes unsteady fluid-structure interaction because of unavoidable elastic behavior of high aspect ratio wings, and it also causes the wing to reach critical Mach numbers because of compressibility effects at high subsonic speeds. The three dimensional aeroelastic analyses of such wings can be done with reasonable computational effort because of advances made in modern aerodynamics. In addition, the design of supercritical airfoils, which has the geometry to delay the critical Mach number, has made the high subsonic cruise speed possible for the civilian and military aircrafts with wings having high aspect ratio, low sweep, low induced drag and high L/D for almost more than a quarter of a century.
During the last quarter of the twentieth century, the numerical and experimental studies performed for predicting the extra lift caused by the strong suction of a separated flow from the sharp leading edge made the design and construction of the planes with delta wings which are highly maneuverable at high angles of attack possible. At higher angles of attack the wing rock may occur depending on the sweep angle. The recent studies emphasize the effect of the leading edge sharpness or roundness on the wing rock phenomenon.
One of the ultimate and ambitious aims of the aerospace industries is to design and construct very fast vehicles which are to take the vast distances between the major cities on earth in a few hours. The research and development branches of
U. Gulfat, Fundamentals of Modern Unsteady Aerodynamics, 307
DOI: 10.1007/978-3-642-14761-6_9, © Springer-Verlag Berlin Heidelberg 2010
major aerospace companies have been conducting research to design a fast aerospace plane which can travel a distance equivalent of the half of the earth circumference in a couple of hours. All these designs are based on the sustainable hypersonic flight at upper levels of atmosphere. The concept of ‘wave-rider’ which was introduced more than half a century ago has become hot again because of its considerably high L/D values for sustainable hypersonic flight. The continuous hypersonic flight, on the other hand is possible only with powerful engines based on the supersonic combustion of fuels with very high heating capacities. The sustainable supersonic combustion, once thought to be out of question because of being unstable, first became possible under laboratory conditions since 1990s, and then were tested on small unmanned hypersonic vehicles for short durations after the introduction of flame control devices which can provide controls over time intervals less than a millisecond. However, so far most of the attempts made in sustainable hypersonic flight tests have failed. Since the costs of these tests are too high, to reduce the risk of failure it is necessary to go through intense and time consuming studies. In order to have most risk free tests, it is necessary to start with an adequate data base for the relevant flight conditions. This, naturally, requires large data base exchange among the countries which allocate substantial budgets for their aerospace development programs.
The advances made, during last two decades, in research and development indicate that the interest in aerodynamics is in two opposite directions. The first direction is the steady or unsteady flow analysis for very small sized objects, which may even operate indoors at low Reynolds number and at moderate to high angles of attack. The second one is the aerothermodynamics of the large sized aerospace vehicles which can cruise at very high altitudes with very high speeds.
The design and construction of unmanned light small sized air vehicles fall under the first direction mentioned above. Shown in Fig. 9.1 are comparative positions of the flying objects, ranging from very small to large, on a graph represented as the relation between the flight Reynolds number and the mass as modified from Mueller and DeLaurier (2003).
The small unmanned air vehicles are to fly and operate in Laminar flow regime as seen from Fig. 9.1. The flight of birds, however, occurs in laminar to turbulent transition. Both the small planes and the large jumbo jets flying in subsonic regimes function totally in turbulent flows. Shown in the left corner of Fig. 9.1, the flying insects, with their mass being less than a gram, generate lift and propulsion with flapping wings. In a hovering flight of insects, the free stream speed is zero; therefore, the maximum wing tip speed is taken as the characteristic speed for determining the Reynolds number. The flapping frequency of the wings is quite high for the considerably small wing span which makes the tip velocity still to yield a laminar flow. The flapping of wings for a hovering flight either occurs in a symmetrical forward and backward fashion with respect to a horizontal plane, or asymmetrical upstrokes and downstrokes with respect to an almost vertical plane (Wang 2005). In the first type of flapping the lifting force of the profile provides the hover, whereas in the second kind of flapping the hover is maintained with the drag generated by the profile. In addition, the experiments show that there is a
Fig. 9.1 Mass versus Reynolds numbers for the flying objects varying from very small to very large
sufficient lifting force generated by the wings flapping with amplitudes larger than their chords.
The sustainable forward flight with wing flapping is possible if the Reynolds number based on the free stream speed is larger than a critical value. Actually, for a thin airfoil at an effective angle of attack less than the dynamic stall angle, the product of the reduced frequency with the dimensionless plunge amplitude, kh, plays also an important role to get a propulsive force, Fig. 8.31, adapted from Gulcat (2009). The empirical criteria, in a laminar flow regime, to obtain a propulsive force with flapping becomes: log10(Re)*kh > 0.72, where Re is the Reynolds number based on the free stream speed. Below this value, negative propulsion is created. At higher angles of attack, where there is a strong leading edge vortex formation at very slow free stream velocities, the criteria to generate a propulsive force are based on the Reynolds number expressed, independently from the free stream speed, in terms of the frequency x and the airfoil chord c reads as: Re = xc2/v > 50 (Wang 2005). The first criterion is useful for cruising of the micro air vehicles, whereas the second criterion is helpful during the transition from hover to forward flight.
The purpose of defining a criterion for the sustainable flight conditions is to design, construct and operate small size air vehicles mainly capable of hover and/ or fly forward with flapping wings as is done in nature. In this respect, the principal aerodynamic challenge in Micro Air Vehicle design is recently stated, in the conclusions and recommendation section of NATO TR-AVT-101 publication, as the search for the greater robustness; namely, gust tolerance, maneuverability, and more predictable handling quantities such as capacity to hover or even perch rather than the pursuit of greater efficiency! (TR-AVT-101).
The second direction in aerodynamic research is the design of very large and very fast aerospace planes which operate in high altitudes. Obviously, because of compressibility, heating and the chemical decomposition of the air at very high speeds, the multidisciplinary concepts from thermodynamics and the chemistry must also be considered. Shown in Fig. 9.2 is the historical and comparative development of the air vehicle range, speed and the cruising altitudes adapted from Kuchemann (1978).
The air vehicles shown in Fig. 9.2 travel their indicated ranges R, which are expressed in terms of the earth’s diameter D, at about same time duration with cruising at given Mach numbers. At the upper right corner of Fig. 9.2, the ‘wave rider’ concept takes its position as the future aerospace plane to cruise at hypersonic speeds. The necessary steps to be taken with specific consideration to aerothermochemistry to develop such hypersonic planes are described in a paper by Tincher and Burnett (1994). In their work, they further study the capabilities of such a plane to maneuver with assistance of the gravity in the atmosphere of a planet while making interplanetary travel in the future. The research related to the hypersonic aerodynamics made in Europe and USA during last two decades is published under the title of ‘Sustainable Hypersonic Flight’ in AGARD-CP-600. The national and/or multinational aerospace programs mentioned in this conference proceedings, however, are either continuing with delay or postponed or even canceled due to budgetary restrictions at the start of the new millennium. The more up to date version of Fig. 9.2 is given by Noor and Venneri (1997) in their book ‘Future Aeronautical and Space System’ published in AIAA series. In their work, the design and performance characteristics of single or multistage, faster than 12 Mach planes, which can orbit in the outside of our atmosphere, are provided. In this context, at Mach numbers less than 12, only the sub-orbital flights in the upper
atmosphere seem to be possible. In this context, the most recent review of the challenges and the critical issues concerning the reliability of a computational data and the limitations of the experimental data for hypersonic aerothermodynamics is provided in the extensive summary by Bertin and Cummings (2006).
Speculative and overall predictions based on the various sources about the future aerospace projects as well as on the different scientific endeavors are provided by physicist Kaku (1998), who is a renowned Popular Science writer, in his recent book on visions (Kaku). As a futurologist, Kaku’s predictions on the future extends to the end of the twenty-first century wherein he sees the realization of projects related to even interstellar travel, which will increase our level of civilization to type I civilization according to the classification of civilizations defined by Nicolai Kardeshev.
At the beginning of this millennium an abominable act of terror committed with four hijacked midsize jetliners shocked the whole world and changed the direction of research and development in the western world drastically. This change, mainly concerning national security, affected the research areas in many disciplines as well as the direction of research in aerodynamics. The necessity of developing MAVs functioning outdoors as well as indoors have become significant in operations related to the security of humankind for many years to come (TR-AVT – 101). In this context, the unsteady aerodynamic tools are not only applied to analyze propulsive forces for aerial vehicles but also for the possible presence of explosive trace detection at the human aerodynamic wake (Settles 2006) for aviation security applications in a nonintrusive and reliable manner.
The last but not the least of many applications of unsteady aerodynamics is the studies of power extraction from an aerohydrodynamically controlled oscillating – wing for the purpose of clean energy generation. The possibility of producing energy from sailing ships or from tethered power generators flying in the global jet streams may increase the available energy densities one order of magnitude higher than the current energy densities available with conventional windmilling techniques on the surface of earth or power from rivers and tides (Platzer and Sarigul – Klijn 2009).
s2 = t — 2(Mx + R)