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DENSITY ALTITUDE
The density of the air directly affects the performance
of the helicopter. As the density of the air increases,
engine power output, rotor efficiency, and aerodynamic
lift all increase. Density altitude is the altitude above
mean sea level at which a given atmospheric density
occurs in the standard atmosphere. It can also be
interpreted as pressure altitude corrected for nonstandard
temperature differences.
Pressure altitude is displayed as the height above a
standard datum plane, which, in this case, is a theoretical
plane where air pressure is equal to 29.92 in. Hg.
Pressure altitude is the indicated height value on the
altimeter when the altimeter setting is adjusted to
29.92 in. Hg. Pressure altitude, as opposed to true altitude,
is an important value for calculating performance
as it more accurately represents the air content at
a particular level. The difference between true altitude
and pressure altitude must be clearly understood. True
altitude means the vertical height above mean sea level
and is displayed on the altimeter when the altimeter is
correctly adjusted to the local setting.
For example, if the local altimeter setting is 30.12 in.
Hg., and the altimeter is adjusted to this value, the
altimeter indicates exact height above sea level.
However, this does not reflect conditions found at this
height under standard conditions. Since the altimeter
setting is more than 29.92 in. Hg., the air in this example
has a higher pressure, and is more compressed,
indicative of the air found at a lower altitude.
Therefore, the pressure altitude is lower than the actual
height above mean sea level.
To calculate pressure altitude without the use of an
altimeter, remember that the pressure decreases
approximately 1 inch of mercury for every 1,000-foot
increase in altitude. For example, if the current local
altimeter setting at a 4,000-foot elevation is 30.42, the
pressure altitude would be 3,500 feet. (30.42 – 29.92 =
.50 in. Hg. 3 1,000 feet = 500 feet. Subtracting 500 feet
from 4,000 equals 3,500 feet).
The four factors that most affect density altitude are:
atmospheric pressure, altitude, temperature, and the
moisture content of the air.
ATMOSPHERIC PRESSURE
Due to changing weather conditions, atmospheric pressure
at a given location changes from day to day. If the
pressure is lower, the air is less dense. This means a
higher density altitude and less helicopter performance.
Density Altitude—Pressure altitude corrected for nonstandard temperature
variations. Performance charts for many older aircraft are based
on this value.
Standard Atmosphere—At sea level, the standard atmosphere consists
of a barometric pressure of 29.92 inches of mercury (in. Hg.) or 1013.2
millibars, and a temperature of 15°C (59°F). Pressure and temperature
normally decrease as altitude increases. The standard lapse rate in the
lower atmosphere for each 1,000 feet of altitude is approximately 1 in.
Hg. and 2°C (3.5°F). For example, the standard pressure and temperature
at 3,000 feet mean sea level (MSL) is 26.92 in. Hg. (29.92 – 3) and
9°C (15°C – 6°C).
Pressure Altitude—The height above the standard pressure level of
29.92 in. Hg. It is obtained by setting 29.92 in the barometric pressure
window and reading the altimeter.
True Altitude—The actual height of an object above mean sea level.
8-2
ALTITUDE
As altitude increases, the air becomes thinner or less
dense. This is because the atmospheric pressure acting
on a given volume of air is less, allowing the air molecules
to move further apart. Dense air contains more air
molecules spaced closely together, while thin air contains
less air molecules because they are spaced further
apart. As altitude increases, density altitude increases.
TEMPERATURE
Temperature changes have a large affect on density altitude.
As warm air expands, the air molecules move further
apart, creating less dense air. Since cool air
contracts, the air molecules move closer together, creating
denser air. High temperatures cause even low elevations
to have high density altitudes.
MOISTURE (HUMIDITY)
The water content of the air also changes air density
because water vapor weighs less than dry air.
Therefore, as the water content of the air increases, the
air becomes less dense, increasing density altitude and
decreasing performance.
Humidity, also called “relative humidity,” refers to the
amount of water vapor contained in the atmosphere,
and is expressed as a percentage of the maximum
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ROTORCRAFT FLYING HANDBOOK1(45)
