AERODYNAMICS

AERODYNAMICS

Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with the difference being that gas dynamics applies to all gases. Understanding the motion of air (often called a flow field) around an object enables the calculation of forces and moments acting on the object. Typical properties calculated for a flow field include velocity, pressure, density and temperature as a function of position and time. By defining a control volume around the flow field, equations for the conservation of mass, momentum, and energy can be defined and used to solve for the properties. The use of aerodynamics through mathematical analysis, empirical approximation and wind tunnel experimentation form the scientific basis for heavier-than-air flight.

WING

A wing is a surface used to produce lift for flight through the air or another gaseous or fluid medium. The cross-sectional shape of a wing is referred to as an airfoil. The word originally referred only to the foremost limbs of birds, but has been extended to include the wings of insects (see insect wing), bats, pterosaurs, and aircraft. The term is also applied to an inverted wing used to generate downforce in auto racing.



A common misconception is that it is the shape of the wing that is essential to generate lift by having a longer path on the top rather than the underside. While wings with this shape are always used in subsonic aircraft and sailing, symmetrically shaped wings can also generate lift by having a positive angle of attack and deflecting air downward. The symmetric approach is less efficient, lacking the lift provided by cambered wings at zero angle of attack. The source of this lift is a point of contention (see the talk page of this article), with various sources ascribing it to the Venturi effect (very similar to the Bernoulli effect), the Coanda effect, or even asserting that neither is relevant.

INCOMPRESSIBLE AERODYNAMICS

An incompressible flow is characterized by a constant density despite flowing over surfaces or inside ducts. A flow can be considered incompressible as long as its speed is low. For higher speeds, the flow will begin to compress as it comes into contact with surfaces. The Mach number is used to distinguish between incompressible and compressible flows.

Subsonic flow

Subsonic (or low-speed) aerodynamics is the study of inviscid, incompressible and irrotational aerodynamics where the differential equations used are a simplified version of the governing equations of fluid dynamics. It is a special case of Subsonic aerodynamics.

Mach number in the flow does not exceed 0.3 (about 335 feet (102m) per second or 228 miles (366 km) per hour at 60oF).

COMPRESSIBLE AERODYNAMICS

According to the theory of aerodynamics, a flow is considered to be compressible if its change in density with respect to pressure is non-zero along a streamline. This means that - unlike incompressible flow - changes in density must be considered. In general, this is the case where the Mach number in part or all of the flow exceeds 0.3. The Mach .3 value is rather arbitrary, but it is used because gas flows with a Mach number below that value demonstrate changes in density with respect to the change in pressure of less than 5%. Furthermore, that maximum 5% density change occurs at the stagnation point of an object immersed in the gas flow and the density changes around the rest of the object will be significantly lower. Transonic, supersonic, and hypersonic flows are all compressible.

Transonic flow



The term Transonic refers to a range of velocities just below and above the local speed of sound (generally taken as Mach 0.8–1.2). It is defined as the range of speeds between the critical Mach number, when some parts of the airflow over an aircraft become supersonic, and a higher speed, typically near Mach 1.2, when all of the airflow is supersonic. Between these speeds some of the airflow is supersonic, and some is not.

Supersonic flow

Supersonic flow behaves very differently from subsonic flow. Fluids react to differences in pressure; pressure changes are how a fluid is "told" to respond to its environment. Therefore, since sound is in fact an infinitesimal pressure difference propagating through a fluid, the speed of sound in that fluid can be considered the fastest speed that "information" can travel in the flow. This difference most obviously manifests itself in the case of a fluid striking an object. In front of that object, the fluid builds up a stagnation pressure as impact with the object brings the moving fluid to rest. In fluid traveling at subsonic speed, this pressure disturbance can propagate upstream, changing the flow pattern ahead of the object and giving the impression that the fluid "knows" the object is there and is avoiding it.

When the fluid finally does strike the object, it is forced to change its properties -- temperature, density, pressure, and Mach number -- in an extremely violent and irreversible fashion called a shock wave.



Supersonic flow behaves very differently from subsonic flow. The term supersonic is used to define a speed that is over the speed of sound (Mach 1). In dry air at 20 °C (68 °F), the threshold value required for an object to be traveling at a supersonic speed is approximately 343 m/s, (1,125 ft/s, 768 mph or 1,236 km/h). Speeds greater than 5 times the speed of sound are often referred to as hypersonic. Speeds where only some parts of the air around an object (such as the ends of rotor blades) reach supersonic speeds.

Hypersonic flow



The precise Mach number at which a craft can be said to be fully hypersonic is elusive, especially since physical changes in the airflow (molecular dissociation, ionization) occur at quite different speeds. Generally, a combination of effects become important "as a whole" around Mach 5. The hypersonic regime is often defined as speeds where ramjets do not produce net thrust. This is a nebulous definition in itself, as there exists a proposed change to allow them to operate in the hypersonic regime (the Scramjet).


COMPARISON OF REGIMES

Regime	Mach	Mph	km/h		General Plane Characteristics
Subsonic	<1.0	<768	<1,230		Most often propeller-driven and commercial turbofan aircraft with straight wings
Transonic	0.8-1.2	610-768	980-1,475		Sharp intakes; compressibility becomes noticeable; slightly swept wings
Supersonic	1.0-5.0	768-3,840	1,230-6,150		Sharper edges; tailplane is a stabilator
Hypersonic	5.0-10.0	3,840-7,680	6,150-12,300		Cooled nickel-titanium skin; highly integrated, small wings, see X-51A Waverider
High-hypersonic	10.0-25.0	7,680-16,250	12,300-30,740		Silica thermal tiles, blunt wings
Re-entry speeds	>25.0	>16,250	>30,740		Ablative heat shield; no wings; blunt capsule shape