*Kiddle Encyclopedia.*

**Aerodynamics** means studying how air (or gas) travels around something moving through it. Streamlining to reduce drag in vehicles is a major field in aerodynamics. Aircraft design is another. The study of gases that are not in motion is called aerostatics. Aerodynamics comes from Aero (Air), and Dynamic (Moving).

## Contents

## Aerodynamic Forces on Aircraft

One of the major goals of aerodynamics is to predict the aerodynamic forces on aircraft.

The four forces that act on a powered aircraft are lift (force), weight, thrust, and drag.

Weight is the force due to gravity and thrust is the force generated by the engine. Lift and drag are aerodynamic forces. Lift is defined as the aerodynamic force acting perpendicular to the direction of travel of the aircraft relative to the surrounding air, and drag is defined as the aerodynamic force acting parallel to the direction of travel. Lift is positive upwards and drag is positive rearwards.

## Aerodynamics in Other Fields

Aerodynamics is important in a number of applications other than aerospace engineering. It is a significant factor in any type of vehicle design, including automobiles. It is important in the prediction of forces and moments in sailing. It is used in the design of small components such as hard drive heads. Civil engineers also use aerodynamics, and particularly aeroelasticity, to calculate wind loads in the design of large buildings and bridges.

## Continuity Assumption

Gases are composed of molecules which collide with one another and solid objects. In aerodynamics, however, gases are considered to have continuous quantities. That is, properties such as density, pressure, temperature, and velocity are taken to be well-defined at infinitely small points, and are assumed to vary continuously from one point to another. The discrete, molecular nature of a gas is ignored.

The continuity assumption becomes less valid as a gas becomes more rarefied. In these cases, statistical mechanics is a more valid method of solving the problem than aerodynamics.

## Conservation Laws

Aerodynamic problems are solved using the conservation laws, or equations derived from the conservation laws. In aerodynamics, three conservation laws are used:

- Conservation of mass: Matter is not created or destroyed. If a certain mass of fluid enters a volume, it must either exit the volume or increase the mass inside the volume.
- Conservation of momentum: Also called Newton's second law of motion
- Conservation of energy: Energy is neither created nor destroyed. It may only change from one manifestation to another.

All aerodynamic problems are therefore solved by the same set of equations. However, they differ by the assumptions made in each problem. The equations become simpler as assumptions are made.

Note that these laws are based on Newtonian Mechanics, they are not applicable in Einsteinian Mechanics.

## Subsonic Aerodynamics

In a subsonic aerodynamic problem, all of the flow speeds are less than the speed of sound. This class of problems encompasses nearly all internal aerodynamic problems, as well as external aerodynamics for general aviation aircraft, model aircraft, and automobiles.

In solving a subsonic problem, one decision to be made by the aerodynamicist is whether or not to incorporate the effects of compressibility. Compressibility is a description of the amount of change of density in the problem. When the effects of compressibility on the solution are small, the aerodynamicist may choose to assume that density is constant. The problem is then an incompressible problem. When the density is allowed to vary, the problem is called a compressible problem. In air, compressibility effects can be ignored when the Mach number in the flow does not exceed 0.3. Above 0.3, the problem should be solved using compressible aerodynamics.

## Transonic Aerodynamics

Transonic aerodynamic problems are defined as problems in which both supersonic and subsonic flow exist. Normally the term is reserved for problems in which the characteristic Mach number is very close to one.

Transonic flows are characterized by shock waves and expansion waves. A shock wave or expansion waves is a region of very large changes in the flow properties. In fact, the properties change so quickly they are nearly discontinuous across the waves. Flow ahead of a shock wave is supersonic; flow behind a shock wave is subsonic.

Transonic problems are arguably the most difficult to solve. Flows behave very differently at subsonic and supersonic speeds, therefore a problem involving both types is more complex than one in which the flow is either purely subsonic or purely supersonic.

## Supersonic Aerodynamics

Supersonic aerodynamic problems are those involving flow speeds greater than the speed of sound. Calculating the lift on the Concorde is an example of a supersonic aerodynamic problem.

Supersonic flow behaves very differently from subsonic flow. The speed of sound can be considered the fastest speed that "information" can travel in the flow. Gas travelling at subsonic speed diverts around a body before striking it, it can be said to "know" that the body is there. Air cannot divert around a body when it is travelling at supersonic speeds. It continues to travel in a straight line until it reaches a shock wave and decelerates to subsonic speeds. Mathematically expressed, supersonic flow is hyperbolic while subsonic flow is elliptic.

Another example of the difference between supersonic and subsonic flow is the behaviour in a convergent duct (known as a nozzle in subsonic flow and a diffuser in supersonic flow). Subsonic flow in a convergent duct accelerates and supersonic flow decelerates.

## Hypersonic Aerodynamics

Hypersonic aerodynamics are characterized by viscous interaction phenomena, that is, the viscosity of the flow significantly affects the external flow, including shock waves. The shock waves are curved and chemically alter the surrounding air or gas, creating a partially ionized plasma with their high temperatures (caused in part by significant aerodynamic heating of the body). "Hypersonic" is typically considered to refer to the Mach 5 and faster region of aircraft speed, however some hypersonic phenomena can exist at speeds as low as Mach 3 (depending on the aircraft and the environment).