Feb 7,2017
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By LUIS COMENDADOR

The matter exists in three different states according to the magnitude of the bonding bonds between their molecules these are: solid, liquid and gaseous.

The three parameters that define a fluid are density, pressure and temperature.

Density is the mass per unit of volume. ρ=m/v

In a fluid the density can vary from one point to another, then it does not make sense to speak of the density of a set of points.

There are some fluids which the density varies a lot (compressible) and others in which it remains practically constant (incompressible). An example of Incompressible is water and compressible is air.

The compressibility of the air only rises from certain speeds ( better said Mach Number)

**The atmospheric density decreases exponentially with height:**

The molecules of the gases are in constant randome movement, due to that it has a kinetic energy which is revealed with an internal energy called temperature. It is messured in kelvin

**T (° K) = t (° C) +273.15**

In the ICAO international standard atmosphere the temperature decreases 2 degrees every 1000 feet

It is the force per unit area. The pressure attached to a point is called a static pressure.

P=F/S ( P= pressure, F = force, S = surface)

1 atmosphere of pressure = 1013.2mb = 760mmHG.

**The atmospheric pressure decreases exponentially with height :**

There is an equation that relates the pressure, temperature and density. It is called the Equation of the perfect gas

**P * v = nRT**

P-> Pressure , v-> Volume , n-> molecular mass , R -> is the universal constant of the perfect gases = 0.082 (atmosphere * liter) / (mol * ° K) , T -> temperature.

For us the formula shown above is not very useful because we need to relate , pressure, temperature and density. So we transformed it in a way we like. As density (ρ)=m/v , and n is a mass, we can modify the equation of perfect gases like this P = nRT/v which is the same as** P=ρRT**. Now we have a relation among them.

**P=ρRT**

The origin of the calculations of an aircraft behaviour is the International Standard Atmosphere. So we are interested in how it changes referred to the I.S.A atmosphere

The equation of perfect gases realated to I.S.A is:

The relation referred to I.S.A is:

We rename our variables like this :

The result is:

**δ = σ * θ **