4. Air Density

air density definition

mass of air per unti volume

fundamental gas equation

P V = R T

P = pressure in pascals

V = specific volume ( volume per unit mass )

R = gas constant for particular gas

T = Absolute Temp

Since for a unit mass of gas, density ρ is the reciprocal of volume V, therefore:

rho = P/RT

rho = P/RT

Density is directly proportional to Pressure

Density is inversly proportional to Temp

Dry air gas constant value

287 J/kg x K

Moist air density

Moist air is LESS DENSE that dry air

under similar conditions of pressure and temperature. This is because water vapour (H₂O, mol. wt. 18)

replaces heavier nitrogen (N₂, mol. wt. 28) and oxygen (O₂, mol. wt. 32) molecules

Factors affecting Air density

Three main factors affect air density

Altitude

Temperature

Humidity

AT a given pressure , density is

Inversly proportional to Absolute temp

1% rule for surface density

decrease of density by 1 % is produced by

A fall of pressure of 10hPa OR

An increase of temperature of 3 degree celcius OR

An increase in height of 300 ft

ISA density profile ?

In ISA, density decreases with height at all levels. The decrease at lower levels for 1000 ft is

approximately 3% of the value for any given level. This rule gives a good approximation up to 20,000

feet

density wrt to height at what height ½ ¼ 1/8

6km - ½

11km - ¼

17km - 1/8

Density of air at sea level is lowest & greatest where

lowest at Equator

greatest at poles

ABOVE 8KM REVERSAL occurs

desnity becomes greater at equator than poles (higher latitude)

piston engine vs jet engine perfomance at eqtr and poles ?

PISTON-ENGINE AIRCRAFT

(low cruise altitude): Operational efficiency greater at

HIGH LATITUDES

(poles) — denser air below 8 km.

JET AIRCRAFT

(high cruise altitude): Operational efficiency greater in the

TROPICS

(equator) — denser air above 8 km reversal.

Density Altitude definition

The altitude in the INTERNATIONAL STANDARD ATMOSPHERE (ISA) at which the PREVAILING DENSITY OCCURS

in the actual atmosphere. It is the pressure altitude corrected for non-standard temperature.

Desnity Altitude calculation

For every 1°C change in temperature, density altitude differs from pressure altitude by approximately 120ft

Aerodrome elevation: 2000 ft. QNH gives pressure altitude = 2000 ft. Temperature is 10°C above ISA

standard for that level.

Correction: 10°C × 120 ft/°C = 1200 ft addition

Density Altitude = 2000 + 1200 = 3200 ft

Aircraft will perform as if operating from a 3200-ft elevation aerodrome.

Density is usually expressed as

g/ cu m grams per cubic metre