Atmosphere Flashcards
Density
Term used to represent the number of molecules and how closely they are packed into any given volume of a substance
What effects air density
Temperature and pressure
Temperature effect on air density
As temp increases (and air isn’t contained) it’s molecules will have more KE meaning they will move further apart and so air density decreases
Pressure effect on air density
As pressure increases density also increases due to denser air having more molecules that collide with a surface and hence more pressure is generated
ISA
Is a hypothetical set of atmospheric conditions that represents an average of the conditions experienced worldwide
ISA pressure
- sea level = 1013.2hPa
- decreases at a rapid rate up to 5000ft (about 1hPa/30ft) and a lower rate thereafter
ISA Temperature
- sea level = +15C
- decreasing a uniform rate of 1.98C/1000ft up to the tropopause (which is set in the ISA as 36,090ft)
ISA density
- sea level = 1.225 kg/m^3
- decreases with altitude at a rapid rate initially and a lower rate thereafter
Altitude bands in which atmospheric pressure and density are reduced to 75%, 50%, and 25% their normal sea level values
- 75% = 8,000 to 10,000ft
- 50% = 18,000 to 22,000ft
- 25% = 34,000 to 41,000ft
Density altitude
The altitude in the ISA that has the same air density as actual altitude, so if actual conditions differ from ISA then density altitude will be different from actual altitude by an amount depending on how much prevailing pressure and/or temp conditions differ from ISA
What does density altitude measure
Aircraft performance
How does humidity effect density altitude
As humidity increases, density decreases due to water molecules having less mass than dry air molecules so moist air is less dense than dry air therefore with a high humidity the density at a pilot’s location is equivalent to a higher altitude in the standard atmosphere. Thus the term “high density altitude.”
How is best aerodynamic and engine performance achieved
- High pressure
- low temp
- dry air (low humidity)
Viscosity
Is the tendency for the molecules of a fluid to stick together and cling to any solid surface they come in contact with, it plays an important part in aerodynamics. When air moves across a perfectly flat plane surface the layer of molecules in immediate contact with the surface will be brought to a standstill because of viscosity, successive layers of air above the bottom layer will be slowed by decreasing amounts until the point is reached where the effect of viscosity is no longer felt. The layer of retarded air is called the boundary layer.
Boundary layer flow
Beginning at the outer edge of the boundary layer the velocity of the airflow is progressively decreased until it is brought to a stop at the surface. The flow within the boundary layer exists in 2 forms - laminar flow and turbulent flow
