Supersonic Aerodynamics Pdf

[Kristy Suzuki]

Everyaircraft has aerodynamic characteristics. But there are additional considerations that have to be made when flying supersonic. This guide breaks down the major components of supersonic aircraft design and shows our approach to the designs for Overture and XB-1.

Our aerodynamicists have to minimize this effect through tailored design of the aircraft from nose to tail. There are a number of ways engineers tailor aircraft design to meet the supersonic wave drag challenge; here are a few examples.

The swept wing is much less susceptible to stalls compared to common subsonic airliners because of vortices (swirling sections of air; see image below) that are generated at the leading edge of the wing and boost lift. This extra boost, however, increases drag once again, which must be assessed in the design process to ensure adequate performance when Overture is flying near an airfield.

We also need to consider our community impact. As we improve our aerodynamics at low speeds, we generate less noise from the airframe (as it moves through and impacts the air) and the engines, as they adjust to overpower drag. The upshot is that Overture will sound just as quiet as a typical airliner when it takes off from your local airport.

At any single point in the sky, the type of materials we use (e.g. aluminum vs. carbon composite) have an effect on aerodynamic performance. The materials and structure define how the vehicle changes shape at different speeds and altitudes. These small shape changes alter the aerodynamic properties of the vehicle and thus its performance.

This is accounted for in the design process through iteration of materials, structural layout, and aerodynamic performance. The first thing we do is define the shape of the vehicle in the most critical area where it will fly: supersonic cruise altitude and speed. Then, we solve for other operating contexts (subsonic flight, taxiing, etc.) and assess them for acceptable performance.

Balancing all these concepts and turning them into a working airplane design is no small task. The production design of Overture was the result of 26 million core-hours of simulated software designs, five wind tunnel tests, and 51 full design iterations.

Early design stages use fast computational methods to understand broad trends in aircraft design. As the process matures, increasingly expensive computational methods are used to predict the performance of a design. Only a few of the most promising designs can be used in the highest fidelity computational fluid dynamics (CFD) runs or wind tunnel testing.

Wind tunnel testing can have different goals. These could include calibrating computational prediction methods, exploring different design trades like tail size or aileron span, obtaining control power increments to ultimately be used in a flight simulator, and obtaining critical performance data like lift and drag.

We work with model makers to manufacture a scaled model of a design and then take that model to a wind tunnel facility to test. Different wind tunnel facilities specialize in different things. They can:

The last step is to compare our wind tunnel tests to computational results. This allows us to augment simulated physics with experimentally measured physics to increase the accuracy of the aircraft models predictions

Modern aerodynamics only dates back to the seventeenth century, but aerodynamic forces have been harnessed by humans for thousands of years in sailboats and windmills,[3] and images and stories of flight appear throughout recorded history,[4] such as the Ancient Greek legend of Icarus and Daedalus.[5] Fundamental concepts of continuum, drag, and pressure gradients appear in the work of Aristotle and Archimedes.[6]

In 1799, Sir George Cayley became the first person to identify the four aerodynamic forces of flight (weight, lift, drag, and thrust), as well as the relationships between them,[11][12] and in doing so outlined the path toward achieving heavier-than-air flight for the next century. In 1871, Francis Herbert Wenham constructed the first wind tunnel, allowing precise measurements of aerodynamic forces. Drag theories were developed by Jean le Rond d'Alembert,[13] Gustav Kirchhoff,[14] and Lord Rayleigh.[15] In 1889, Charles Renard, a French aeronautical engineer, became the first person to reasonably predict the power needed for sustained flight.[16] Otto Lilienthal, the first person to become highly successful with glider flights, was also the first to propose thin, curved airfoils that would produce high lift and low drag. Building on these developments as well as research carried out in their own wind tunnel, the Wright brothers flew the first powered airplane on December 17, 1903.

During the time of the first flights, Frederick W. Lanchester,[17] Martin Kutta, and Nikolai Zhukovsky independently created theories that connected circulation of a fluid flow to lift. Kutta and Zhukovsky went on to develop a two-dimensional wing theory. Expanding upon the work of Lanchester, Ludwig Prandtl is credited with developing the mathematics[18] behind thin-airfoil and lifting-line theories as well as work with boundary layers.

As aircraft speed increased designers began to encounter challenges associated with air compressibility at speeds near the speed of sound. The differences in airflow under such conditions lead to problems in aircraft control, increased drag due to shock waves, and the threat of structural failure due to aeroelastic flutter. The ratio of the flow speed to the speed of sound was named the Mach number after Ernst Mach who was one of the first to investigate the properties of the supersonic flow. Macquorn Rankine and Pierre Henri Hugoniot independently developed the theory for flow properties before and after a shock wave, while Jakob Ackeret led the initial work of calculating the lift and drag of supersonic airfoils.[19] Theodore von Krmn and Hugh Latimer Dryden introduced the term transonic to describe flow speeds between the critical Mach number and Mach 1 where drag increases rapidly. This rapid increase in drag led aerodynamicists and aviators to disagree on whether supersonic flight was achievable until the sound barrier was broken in 1947 using the Bell X-1 aircraft.

Understanding the motion of air around an object (often called a flow field) enables the calculation of forces and moments acting on the object. In many aerodynamics problems, the forces of interest are the fundamental forces of flight: lift, drag, thrust, and weight. Of these, lift and drag are aerodynamic forces, i.e. forces due to air flow over a solid body. Calculation of these quantities is often founded upon the assumption that the flow field behaves as a continuum. Continuum flow fields are characterized by properties such as flow velocity, pressure, density, and temperature, which may be functions of position and time. These properties may be directly or indirectly measured in aerodynamics experiments or calculated starting with the equations for conservation of mass, momentum, and energy in air flows. Density, flow velocity, and an additional property, viscosity, are used to classify flow fields.

Flow velocity is used to classify flows according to speed regime. Subsonic flows are flow fields in which the air speed field is always below the local speed of sound. Transonic flows include both regions of subsonic flow and regions in which the local flow speed is greater than the local speed of sound. Supersonic flows are defined to be flows in which the flow speed is greater than the speed of sound everywhere. A fourth classification, hypersonic flow, refers to flows where the flow speed is much greater than the speed of sound. Aerodynamicists disagree on the precise definition of hypersonic flow.

Compressible flow accounts for varying density within the flow. Subsonic flows are often idealized as incompressible, i.e. the density is assumed to be constant. Transonic and supersonic flows are compressible, and calculations that neglect the changes of density in these flow fields will yield inaccurate results.

Viscosity is associated with the frictional forces in a flow. In some flow fields, viscous effects are very small, and approximate solutions may safely neglect viscous effects. These approximations are called inviscid flows. Flows for which viscosity is not neglected are called viscous flows. Finally, aerodynamic problems may also be classified by the flow environment. External aerodynamics is the study of flow around solid objects of various shapes (e.g. around an airplane wing), while internal aerodynamics is the study of flow through passages inside solid objects (e.g. through a jet engine).

Unlike liquids and solids, gases are composed of discrete molecules which occupy only a small fraction of the volume filled by the gas. On a molecular level, flow fields are made up of the collisions of many individual of gas molecules between themselves and with solid surfaces. However, in most aerodynamics applications, the discrete molecular nature of gases is ignored, and the flow field is assumed to behave as a continuum. This assumption allows fluid properties such as density and flow velocity to be defined everywhere within the flow.

The validity of the continuum assumption is dependent on the density of the gas and the application in question. For the continuum assumption to be valid, the mean free path length must be much smaller than the length scale of the application in question. For example, many aerodynamics applications deal with aircraft flying in atmospheric conditions, where the mean free path length is on the order of micrometers and where the body is orders of magnitude larger. In these cases, the length scale of the aircraft ranges from a few meters to a few tens of meters, which is much larger than the mean free path length. For such applications, the continuum assumption is reasonable. The continuum assumption is less valid for extremely low-density flows, such as those encountered by vehicles at very high altitudes (e.g. 300,000 ft/90 km)[6] or satellites in Low Earth orbit. In those cases, statistical mechanics is a more accurate method of solving the problem than is continuum aerodynamics. The Knudsen number can be used to guide the choice between statistical mechanics and the continuous formulation of aerodynamics.

3a8082e126