The atmosphere is an envelope of air that surrounds the Earth and rests upon its surface
Air is a Fluid:
Understanding the fluid properties of air is essential to understanding the principles of flight
Fluids generally do not resist deformation when even the smallest stress is applied, or they resist it only slightly
We call this slight resistance viscosity
Fluids also have the ability to flow
Just as a liquid flows and fills a container, air will expand to fill the available volume of its container
Understanding the fluid properties of air is essential to understanding the principles of flight
Viscosity:
Viscosity is the property of a fluid that causes it to resist flowing
The way individual molecules of the fluid tend to adhere, or stick, to each other determines how much a fluid resists flow
High-viscosity fluids are "thick" and resist flow
Low-viscosity fluids are "thin" and flow easily
Air has a low viscosity and flows easily while other fluids like oil or grease have a higher viscosity and do not flow as easy
Since air is a fluid and has viscosity properties, it resists flow around any object to some extent
Friction:
Bernoulli's Principle
Friction is the resistance that one surface or object encounters when moving over another
Friction exists between any two materials that contact each other
The surface of a wing, like any other surface, has a certain roughness at the microscopic level
The surface roughness causes resistance and slows the velocity of the air flowing over the wing [Figure 4-1]
Molecules of air pass over the surface of the wing and actually adhere (stick, or cling) to the surface because of friction
Air molecules near the surface of the wing resist motion and have a relative velocity near zero
The roughness of the surface impedes their motion
The layer of molecules that adhere to the wing surface is referred to as the boundary layer
Once the boundary layer of the air adheres to the wing by friction, further resistance to the airflow is caused by the viscosity, the tendency of the air to stick to itself
When these two forces act together to resist airflow over a wing, it is called drag
Pilot Handbook of Aeronautical Knowledge Microscopic Surface of a Wing
Pressure:
Pressure is the force applied in a perpendicular direction to the surface of an object
Often, pressure is measured in pounds of force exerted per square inch of an object, or PSI
An object completely immersed in a fluid will feel pressure uniformly around the entire surface of the object
If the pressure on one surface of the object becomes less than the pressure exerted on the other surfaces, the object will move in the direction of the lower pressure
Pressure Altitude:
Pressure altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.92 "Hg (1,013.2 mb) as measured by a barometer
An altimeter is essentially a sensitive barometer calibrated to indicate altitude in the standard atmosphere
If the altimeter is set for 29.92 "Hg SDP, the altitude indicated is the pressure altitude
As atmospheric pressure changes, the SDP may be below, at, or above sea level
Pressure altitude is important as a basis for determining airplane performance, as well as for assigning flight levels to airplanes operating at or above 18,000 feet
The pressure altitude can be determined by:
Setting the barometric scale of the altimeter to 29.92 and reading the indicated altitude, or;
Applying a correction factor to the indicated altitude according to the reported altimeter setting
Density Altitude:
SDP is a theoretical pressure altitude, but aircraft operate in a nonstandard atmosphere and the term density altitude is used for correlating aerodynamic performance in the nonstandard atmosphere. Density altitude is the vertical distance above sea level in the standard atmosphere at which a given density is to be found. The density of air has significant effects on the aircraft's performance because as air becomes less dense, it reduces:
Power because the engine takes in less air
Thrust because a propeller is less efficient in thin air
Lift because the thin air exerts less force on the airfoils
Density altitude is pressure altitude corrected for nonstandard temperatures, and is used for determining aerodynamic performance in the nonstandard atmosphere
As the density of the air increases (lower density altitude), aircraft performance increases; conversely as air density decreases (higher density altitude), aircraft performance decreases
A decrease in air density means a high density altitude; an increase in air density means a lower density altitude
Density altitude is used in calculating aircraft performance because under standard atmospheric conditions, air at each level in the atmosphere not only has a specific density, its pressure altitude and density altitude identify the same level
The computation of density altitude involves consideration of pressure (pressure altitude) and temperature. Since aircraft performance data at any level is based upon air density under standard day conditions, such performance data apply to air density levels that may not be identical with altimeter indications. Under conditions higher or lower than standard, these levels cannot be determined directly from the altimeter
Density altitude is determined by first finding pressure altitude, and then correcting this altitude for nonstandard temperature variations. Since density varies directly with pressure and inversely with temperature, a given pressure altitude may exist for a wide range of temperatures by allowing the density to vary. However, a known density occurs for any one temperature and pressure altitude. The density of the air has a pronounced effect on aircraft and engine performance. Regardless of the actual altitude of the aircraft, it will perform as though it were operating at an altitude equal to the existing density altitude
Air density is affected by changes in altitude, temperature, and humidity. High density altitude refers to thin air, while low density altitude refers to dense air. The conditions that result in a high density altitude are high elevations, low atmospheric pressures, high temperatures, high humidity, or some combination of these factors. Lower elevations, high atmospheric pressure, low temperatures, and low humidity are more indicative of low density altitude
Effect of Pressure on Density:
Since air is a gas, it can be compressed or expanded. When air is compressed, a greater amount of air can occupy a given volume. Conversely, when pressure on a given volume of air is decreased, the air expands and occupies a greater space. At a lower pressure, the original column of air contains a smaller mass of air. The density is decreased because density is directly proportional to pressure. If the pressure is doubled, the density is doubled; if the pressure is lowered, the density is lowered. This statement is true only at a constant temperature
Effect of Temperature on Density:
Increasing the temperature of a substance decreases its density
Conversely, decreasing the temperature increases the density
Thus, the density of air varies inversely with temperature
This statement is true only at a constant pressure
In the atmosphere, both temperature and pressure decrease with altitude and have conflicting effects upon density
However, a fairly rapid drop in pressure as altitude increases usually has a dominating effect
Hence, pilots can expect the density to decrease with altitude
If a chart is not available the density altitude can be estimated by adding 120' for every degree Celsius above the ISA:
For example, at 3,000' pressure altitude (PA), the ISA prediction is 9° C (15° C - [lapse rate of 2° C per 1,000' x 3 = 6° C])
However, if the actual temperature is 20° C (11° C more than that predicted by ISA) then the difference of 11° C is multiplied by 120' equaling 1,320
Adding this figure to the original 3,000' provides a density altitude of 4,320' (3,000' + 1,320')
Effect of Humidity (Moisture) on Density:
Since air is never completely dry, a small amount of water vapor is always present in the atmosphere
In other conditions humidity may become an important factor in the performance of an aircraft
Water vapor is lighter than air; consequently, moist air is lighter than dry air
Therefore, as the water content of the air increases, the air becomes less dense, increasing density altitude and decreasing performance
It is lightest or least dense when, in a given set of conditions, it contains the maximum amount of water vapor
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 holds more water vapor, while cold air holds less
Perfectly dry air that contains no water vapor has a relative humidity of zero percent, while saturated air, which cannot hold any more water vapor, has a relative humidity of 100 percent
Humidity alone is usually not considered an important factor in calculating density altitude and aircraft performance, but it is a contributing factor
As temperature increases, the air can hold greater amounts of water vapor
When comparing two separate air masses, the first warm and moist (both qualities tending to lighten the air) and the second cold and dry (both qualities making it heavier), the first must be less dense than the second
Pressure, temperature, and humidity have a great influence on aircraft performance because of their effect upon density
There are no rules of thumb that can be easily applied, but the affect of humidity can be determined using several online formulas
As an example, the pressure is needed at the altitude for which density altitude is being sought
Using [Figure 2], select the barometric pressure closest to the associated altitude
The standard pressure at 8,000 feet is 22.22 "Hg
Using the National Oceanic and Atmospheric Administration (NOAA) website (www.srh.noaa.gov/ epz/?n=wxcalc_densityaltitude) for density altitude, enter the 22.22 for 8,000 feet in the station pressure window
Enter a temperature of 80° and a dew point of 75°
The result is a density altitude of 11,564 feet
With no humidity, the density altitude would be almost 500 feet lower
Another website (www.wahiduddin.net/calc/density_altitude.htm) provides a more straight forward method of determining the effects of humidity on density altitude without using additional interpretive charts
In any case, the effects of humidity on density altitude include a decrease in overall performance in high humidity conditions
Performance figures in the aircraft owner's handbook for the length of takeoff run, horsepower, rate of climb, etc., are generally based on standard atmosphere conditions (59° Fahrenheit (15° Celsius), pressure 29.92 inches of mercury) at sea level
Where pilots may run into trouble when they encounter an altogether different set of conditions
This is particularly true in hot weather and at higher elevations
Aircraft operations at altitudes above sea level and at higher than standard temperatures are commonplace in mountainous areas
Such operations quite often result in a drastic reduction of aircraft performance capabilities because of the changing air density
It is not to be used as a height reference, but as a determining criteria in the performance capability of an aircraft
Air density and density altitude have an inverse relationship
That is to say, air density, which decreases with altitude, causes an increase in density altitude
The further effects of high temperature and high humidity are cumulative, resulting in an increasing high density altitude condition
High density altitude reduces all aircraft performance parameters
To the pilot, this means that the normal horsepower output is reduced, propeller efficiency is reduced and a higher true airspeed is required to sustain the aircraft throughout its operating parameters
It means an increase in runway length requirements for takeoff and landings, and a decreased rate of climb
Example: An average small airplane requiring 1,000' for takeoff at sea level under standard atmospheric conditions will require a takeoff run of approximately 2,000' at an operational altitude of 5,000'
A turbocharged aircraft engine provides provides sea level horsepower up to a specified altitude above sea level
Density Altitude Advisories:
At airports with elevations of 2,000' and higher, control towers and FSSs will broadcast the advisory "Check Density Altitude" when the temperature reaches a predetermined level
These advisories will be broadcast on appropriate tower frequencies or, where available, ATIS. FSSs will broadcast these advisories as a part of Local Airport Advisory
These advisories are provided by air traffic facilities, as a reminder to pilots that high temperatures and high field elevations will cause significant changes in aircraft characteristics
The pilot retains the responsibility to compute density altitude, when appropriate, as a part of preflight duties
All FSSs will compute the current density altitude upon request
Density Altitude Precautions:
Fly lighter, don't carry unnecessary baggage
Review POH for special procedures, like mixture position on takeoff, cruise, and landing (likely leaning until peak RPM)
Fly indicated airspeeds (ground speed will be faster)
Maneuver carefully, and conservatively
Anticipate eggagerated deceleration and effects of flight surfaces like flaps
Fly early in the day when temperatures, and therefore density altitude, are lowest
Recall density altitude effects lift surfaces, the propeller, and the engine all at once
