Properties of air Flashcards
Why is the proportion of each of the constituent gases of air not important aerodynamically
All have equal value for the way in which they add to the mass of air
What is oxygen needed for
To sustain the power produced by the engine and the life of the pilot
How is density calculated
The total mass contained in a given volume
Why is density important
Determines how well an aircraft and its engine can perform, as an aircraft climbs the density of air around it decreases and the wings of aircraft become less able to generate lift, at the same time the power output of the engine reduces
What is the easiest way to calculate density
By using pressure and temperature
What is pressure
The total force acting on a given area
Units of pressure
n/m-2 or Pa
What is atmospheric pressure usually measured in
hPa
Relationship between pressure and density
The higher the density the higher the pressure
What is tempreture
A measure of how much energy the molecules of a substance have or how fast they vibrate
In masses of air with the same temperature, that with the ______ pressure has the highest density
Highest
How far does the atmosphere extend from the earths surface
500 miles
Why is the air nearest the earths surface the densest
The weight of the air in the outermost reaches bears down on the air nearer the surface and compresses it
What happens to pressure, temperature, and density in the troposphere
They all decrease with altitude
What does ISA standard for
International standard atmosphere
What is the ISA
A hypothetical set of atmospheric conditions, which represents an average of the conditions experienced worldwide
ISA initial sea level pressure
1013.2 hPa
ISA pressure decrease rate
Pressure decreasing with altitude at a rapid rate initially around 1 hPa every 30ft to 5,000 ft and at a lower rate thereafter
ISA initial sea-level temperature
+15C
ISA temperature decreasing rate
Decreasing at a uniform rate of 1.98C/1000ft up to 36,090 ft
ISA initial sea-level density
1.225kg/m3
ISA density decreasing rate
Decreasing with altitude at a rapid rate initially and a lower rate thereafter
What will a high temperature indicate
Low density and poorer performance
What will a low temperature indicate
High density and better performance
What will a high pressure indicate
High density and better performance
What will a low pressure indicate
Low density and poorer performance
What is density altitude
The altitude in the ISA which has the same density as the actual altitude you are trying to measure
How is the density altitude worked out
Enter the temperature and pressure into the aircraft’s performance chart, use the navigation computer to work it out or do a manual calculation
The hypothetical ISA is based on ___ air
Dry
Range of average humidity in the air
20-80%
How does humidity affect the density
Moist air is less dense than dry air as water molecules have less mass than the molecules of dry air it has replaced
What is viscosity
The tendency for molecules of fluid to stick together and cling to any solid surface they come in contact with
In masses of air with the same pressure, that with the ______ temperature has the highest density
Lowest
