Aeronautics and astronautics – Aircraft structure – Details
Reexamination Certificate
1997-04-04
2002-10-15
Jordan, Charles T. (Department: 3644)
Aeronautics and astronautics
Aircraft structure
Details
C244S03500A, C244S210000
Reexamination Certificate
active
06464171
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of supersonic aeronautics, and relates more specifically to a novel supersonic or hypersonic body section design incorporating one or more hollow channels in the body section that allow freestream air to flow through the channel(s) resulting in reduced drag and sonic boom.
2. Description of Related Art
A variety of supersonic and hypersonic vehicles are currently being studied for commercial and military applications. However, existing supersonic and hypersonic technology suffers from several drawbacks. For example, aerodynamic drag is substantial at supersonic and hypersonic speeds, and adds significantly to fuel expenses. Also, noise due to sonic booms has been found objectionable, in particular, by those living under flight paths and near airports.
The advantages of supersonic and hypersonic transport, however, justify continued research and development directed to overcoming these drawbacks. For example, the high speed civil transport (HSCT) aircraft is designed to cruise at a free stream Mach number of approximately M
∞
=2.4 and seeks to overcome the economic and environmental barriers that have limited the success of previous supersonic commercial concepts. Other supersonic flight vehicles of significant interest include single-stage-to-orbit (SSTO) and multi-stage launch vehicles, tactical and strategic hypersonic and supersonic missiles, hypersonic cruise aircraft and planetary entry vehicles. These vehicles are similar in that their range, payload mass fractions and economic feasibility are extremely sensitive to aerodynamic drag.
A discussion of the effects of drag reduction on such supersonic vehicles is given by Bushnell, D., Supersonic Aircraft Drag Reduction, AIAA Paper 90-1596, 1990. If the lift-to-drag ratio (L/D) of the HSCT at cruise is increased by just 10% there would be a significant positive impact on the economy and success of that vehicle. Proposed hypersonic vehicles such as the National Aerospace Plane and X-30 have not advanced due, in part, to diminishing projected payload margins and concerns regarding airbreathing engine capabilities. As pointed out in Bushnell, drag reductions allow lower fuel requirements and can lead to reduced operating costs, as well as reduced sonic boom and noise effects. Reviews of supersonic drag reduction techniques also are given by Hefner, J. N., and Bushnell, D. M., An Overview of Concepts for Aircraft Drag Reduction, AGARD Rep. 654, June, 1977, pp. 1-1 to 1-30; and by Jones, R. T., Aerodynamic Design for Supersonic Speeds, Adv. in Aero. Science, Vol. 1, 1959, pp. 34-52
A schematic of a typical drag breakdown, taken from Kuchemann, D., The Aerodynamic Design of Aircraft, Pergamon Press, Ltd., 1978, for supersonic vehicles (M
∞
=2.4) is shown. The vehicle drag coefficient is shown as a function of the product of &bgr;={square root over (M
∞
2
−1)} and the semispan to length ratio, s/l. It should be noted that vehicles with s/l lower than approximately 0.2 (i.e. vehicles whose wings are extremely swept) are impractical due to excessive required runway lengths. Note also that s/&bgr;l=1 corresponds to the case in which the Mach number normal to the wing leading edge is sonic. The drag on supersonic vehicles can be classified into three different categories: (1) skin friction drag, (2) drag due to lift, and (3) zero-lift bluntness (thickness-wave) drag. Skin friction drag is due to fluid viscosity and is a function of the total wetted surface area of the vehicle. Drag due to lift consists of induced drag and the component of wave drag which is a function of the inclination of the vehicle surfaces with respect to the freestream direction at a non-zero lift orientation. Finally, the zero-lift bluntness drag is the wave drag due to the thickness and bluntness of the leading and trailing edges of the vehicle in a zero lift orientation. The zero-lift bluntness drag (i.e. thickness-wave drag) increases rapidly with freestream Mach number and can be responsible for well over ⅓ of the total vehicle drag.
Linearized supersonic theory indicates that for an airfoil of a given thickness, the shape which gives minimum zero-lift bluntness drag is the sharp diamond airfoil. However, very sharp leading edges are not practical for a number of reasons: (1) very sharp leading edges are difficult and expensive to manufacture; (2) some blunting is required for structural strength, (3) the flow over wings with sharp leading edges is very susceptible to separation even at low angles of attack and flight speeds; and (4) the heat transfer to sharp leading edges at high supersonic Mach numbers is severe.
For hypersonic vehicles, heat transfer considerations often dictate the design of the nose and the leading edges. The heat transfer to such vehicles is most severe at stagnation points which occur on the leading edges and nose of the vehicle. Theoretical and numerical predictions of stagnation point heating have been developed by Fay, J., and Riddell, F., Theory of Stagnation Point Heat Transfer in Dissociated Air, J. Aero. Sci., Vol. 25, pp. 73-85, February, 1958, and are also described by Anderson, J. D., Hypersonic and High Temperature Gas Dynamics, McGraw-Hill, Inc., New York, 1989. Kemp and Riddell have developed an accurate semi-empirical relation for stagnation point heat transfer. Kemp, N., and Riddell, F., Heat Transfer to Satellite Vehicles Re-entering the Atmosphere, Jet Propulsion, pp. 132-137, February, 1957. Theoretical formulations, experimental data, and semi-empirical formulas all agree in the fact that stagnation point heat transfer is inversely proportional to the square root of the nose or leading edge radius, i.e.,
q
stag
∝
1
r
n
Thus, sharp leading edges (i.e., r
n
=0) experience extreme heat transfer, which may melt or otherwise damage the component. Therefore, the nose, leading edges of wings, tails, fins, struts, cowls, and other appendages of supersonic and hypersonic vehicles are blunted so that the heat transfer and structural loads will be manageable. With leading edge blunting, the simple diamond airfoil is modified. However, much of the wave drag experienced by these vehicles is due to leading edge blunting. Wave drag is responsible for approximately one-third of the total drag experienced by aircraft, atmospheric entry vehicles, missiles, and other vehicles in supersonic and hypersonic flight.
For vehicles which cruise at low supersonic Mach numbers, heat transfer considerations do not dictate the design of the wing leading edges. At subsonic, off-design conditions, such as takeoff, landing, climb and maneuvering flight, blunted leading edges are desirable so that flow separation is prevented. However, a blunted leading edge results in higher drag at supersonic cruise conditions relative to a wing with a sharp leading edge. Therefore, it has been found desirable that airfoils utilized in such applications be significantly blunted during subsonic maneuvering phases of flight, but perform more like sharp leading-edge airfoils at supersonic cruise conditions. The provision of such an airfoil is one object of the present invention.
Sonic boom has also been found to limit the application of supersonic and hypersonic transport. To date, the most successful strategies utilized for minimization of sonic boom stem from evaluation of the Whitman F-Function. Whitman, G. B., The Flow Pattern of a Supersonic Projectile, Communications on Pure and Applied Mathematics, Vol. V., 1952, pp. 301-348. See also Carlson, H. W., and Maglieri, D. J., Review of Sonic Boom Generation Theory and Prediction Methods, Journal of Acoustical Society of America, Vol. 51, 1972, pp. 675-685; and Middleton, W. D., and Carlson, H. W., A Numerical Method for Calculating Near-Field Sonic-Boom Pressure Signatures, NASA TN D-3082, 1965. The F-Function is based on the cross-sectional area distribution and the lift distribution of the generating vehicle. Modified linearized
Deveau Todd
Dinh T.
Georgia Tech Research Corp.
Haley Jacqueline
Jordan Charles T.
LandOfFree
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