Effects of Atmospheric Pressure,
Temperature, Altitude, and Density of air on Aircraft Performance
The Atmosphere
Before investigative the fundamental
laws of flight, several basic facts & fundaments must be considered, namely
that an aircraft operates in the air. Therefore, it is imperative to understood
the properties of air that affect the control and performance of an aircraft.
The mixtures of air in the
earth’s atmosphere are composed mostly of nitrogen and oxygen. Air is
considered a fluid because like other fluids it also fits the definition of a
substance that has the ability to flow or assume the shape of the container in
which it is enclosed. If the container with air inside is heated the pressure
increases and if cooled pressure decreases.
Depending on the available
information about the atmosphere, a flight's success or even its ability to
take off may be decided. The preparation of flight plans takes into account a
wide range of specific information, including the different elements of the
earth's atmosphere, variations in temperatures and pressures at various
altitudes above the planet, the characteristics of the weather encountered by
aircraft while in flight, and many other specifics.
Pressure Changes with Altitude
The average daily air pressure at
sea level is 14.7 pounds per square inch (14.7 psi).
With increasing altitude,
atmospheric pressure drops because atmospheric pressure at every altitude is a
function of the weight of the air above it. It seems to reason that the overall
weight of the air above a region at 15,000 feet would be lower than that at
10,000 feet.
Temperature doesn't drop uniformly with Altitude
According to studies of the
atmosphere, the temperature does not drop uniformly with height; rather, it
gets continuously colder up to a height of about 7 miles, after which the rate
of temperature change dramatically slows down and nearly stays constant at 55°
Centigrade (218° Kelvin) up to about 20 miles. The temperature then starts to
rise, reaching a maximum of 77° Centigrade (350° Kelvin) at 55 miles above sea
level. Following that, it continues to rise, reaching a height of 250 to 400 miles
and a temperature of 2,270° Centigrade (2,543° Kelvin). Without the protective
layer of the atmosphere, a man or any other living thing would be cooked on the
side facing the sun and frozen on the other from the 50 mile level above.
The effects of atmospheric
temperature, altitude, and density of air on aircraft performance are covered
in the following paragraphs.
Density
A substance's density is its
weight per unit of volume. Air may be compressed because it is a mixture of
gases. When two similar containers of air are both under half the pressure, the
air in the higher-pressure container weighs twice as much as the air in the
lower-pressure container. The air in the other container is twice as dense as
the air under the higher pressure. For an equal weight of air, the area that is
under a larger pressure only takes up half the space of that area that is under
a lesser pressure.
The following principles apply to
gas density:
- Density varies in direct proportion to pressure. If pressure increases, density will also increase with proportion.
- Density varies inversely with the temperature. If the temperature increases, density also decrease.
A mass of hot air is less dense
than a mass of cool air, and air at high altitudes is less dense than air at
low altitudes.
The aerodynamic performance of an aircraft with the same horsepower is impacted by density changes. An aircraft can travel more quickly when the density is low at a high altitude than when the density is high at a low altitude. This is due to the fact that air provides less resistance to the aircraft when there are fewer air particles per unit of volume.
Density Altitude
The air density would be equal to
the indicated air density at the point of observation at the density altitude,
which is the altitude in relation to typical atmospheric conditions. The air
density is specified as a height above mean sea level, or in other words, the
density altitude. The pressure altitude may also be thought of as the density
altitude when it has been heated to a non-standard temperature.
The density altitude will rise as a result of an increase in temperature, a decrease in atmospheric pressure, and, to a much lesser extent, an increase in humidity. The density altitude at a certain site may be much greater than the true altitude in hot and humid weather.
When evaluating an aircraft's
aerodynamic performance in various weather scenarios, the density altitude is
employed in aviation. Changes in air density can also affect the lift produced
by the aircraft's airfoils and the relationship between its indicated airspeed
(IAS) and true airspeed (TAS). The density and make-up of the atmosphere also
have an impact on the power the aircraft's engine produces.
Humidity
Humidity is a measurement of the
amount of water vapour in the atmosphere. Usually invisible to the unaided eye,
water vapour is the gaseous form of water. Humidity can foretell the occurrence
of rain, dew, or fog. At higher temperatures, more water vapour can be absorbed
into the atmosphere.
- The weight of water vapour in a unit volume of air is known as absolute humidity.
- The percentage of actual moisture in the air to the amount of moisture it would hold if it were saturated at the same temperature and pressure is known as relative humidity.
The density of the air varies
inversely with the humidity, assuming constant temperature and pressure. The
air density is lower on moist days than it is on dry days. For this reason, an
aircraft requires a longer runway for takeoff on damp or rainy days than it
does on dry days.
By itself, water vapour weighs approximately five-eighths as much as an equal amount of perfectly dry air. When air contains water vapor, it is not as heavy as dry air containing no moisture.
Also Read
0 Comments