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旋翼機(jī)飛行手冊(cè) ROTORCRAFT FLYING HANDBOOK

時(shí)間：2011-04-05 11:32來源：藍(lán)天飛行翻譯 作者：航空 點(diǎn)擊：次

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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.
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.

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 amount of water vapor the air can hold. This amount varies with temperature; warm air can hold more water vapor, while colder air can hold less. Perfectly dry air that contains no water vapor has a relative humidity of 0 percent, while saturated air that cannot hold any more water vapor, has a relative humidity of 100 percent.

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