Effect of Humidity (Moisture) on Density
The preceding paragraphs refer to air that is perfectly dry. In
reality, it is never completely dry. The small amount of water
vapor suspended in the atmosphere may be almost negligible
under certain conditions, but 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. In the first example, the pressure is needed at the
altitude for which density altitude is being sought. Using
Figure 4-2, select the barometric pressure closest to the
associated altitude. As an example, the 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.
Theories in the Production of Lift
In order to achieve flight in a machine that is heavier than air,
there are several obstacles we must overcome. One of those
obstacles, discussed previously, is the resistance to movement
called drag. The most challenging obstacle to overcome in
aviation, however, is the force of gravity. A wing moving
through air generates the force called lift, also previously
discussed. Lift from the wing that is greater than the force of
gravity, directed opposite to the direction of gravity, enables
an aircraft to fly. Generating this force called lift is based on
some important principles, Newton's basic laws of motion,
and Bernoulli's principle of differential pressure.
Newton’s Basic Laws of Motion
The formulation of lift has historically been an adaptation
over the past few centuries of basic physical laws. These
laws, although seemingly applicable to all aspects of lift,
do not explain how lift is formulated. In fact, one must
consider the many airfoils that are symmetrical, yet produce
significant lift.
The fundamental physical laws governing the forces acting
upon an aircraft in flight were adopted from postulated
theories developed before any human successfully flew
an aircraft. The use of these physical laws grew out of the
Scientific Revolution, which began in Europe in the 1600s.
Driven by the belief the universe operated in a predictable
manner open to human understanding, many philosophers,
mathematicians, natural scientists, and inventors spent their
lives unlocking the secrets of the universe. One of the most
well-known was Sir Isaac Newton, who not only formulated
the law of universal gravitation, but also described the three
basic laws of motion.
Newton’s First Law: “Every object persists in its state of rest
or uniform motion in a straight line unless it is compelled to
change that state by forces impressed on it.”
This means that nothing starts or stops moving until some
outside force causes it to do so. An aircraft at rest on the ramp
remains at rest unless a force strong enough to overcome
its inertia is applied. Once it is moving, its inertia keeps
it moving, subject to the various other forces acting on it.
These forces may add to its motion, slow it down, or change
its direction.
Newton’s Second Law: “Force is equal to the change in
momentum per change in time. For a constant mass, force
equals mass times acceleration.”
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Figure 4-4. Air pressure decreases in a venturi tube.
When a body is acted upon by a constant force, its resulting
acceleration is inversely proportional to the mass of the body
and is directly proportional to the applied force. This takes
into account the factors involved in overcoming Newton’s
First Law. It covers both changes in direction and speed,
including starting up from rest (positive acceleration) and
coming to a stop (negative acceleration or deceleration).
Newton’s Third Law: “For every action, there is an equal
and opposite reaction.”
In an airplane, the propeller moves and pushes back the
air; consequently, the air pushes the propeller (and thus the
airplane) in the opposite direction—forward. In a jet airplane,
the engine pushes a blast of hot gases backward; the force of
equal and opposite reaction pushes against the engine and
forces the airplane forward.
Bernoulli’s Principle of Differential Pressure
A half-century after Newton formulated his laws, Daniel
Bernoulli, a Swiss mathematician, explained how the pressure
of a moving fluid (liquid or gas) varies with its speed of
motion. Bernoulli’s Principle states that as the velocity of a
moving fluid (liquid or gas) increases, the pressure within
the fluid decreases. This principle explains what happens to
air passing over the curved top of the airplane wing.
A practical application of Bernoulli’s Principle is the venturi
tube. The venturi tube has an air inlet that narrows to a
throat (constricted point) and an outlet section that increases
in diameter toward the rear. The diameter of the outlet is
the same as that of the inlet. The mass of air entering the
tube must exactly equal the mass exiting the tube. At the
constriction, the speed must increase to allow the same
amount of air to pass in the same amount of time as in all
other parts of the tube. When the air speeds up, the pressure
also decreases. Past the constriction, the airflow slows and
the pressure increases. [Figure 4-4]
Since air is recognized as a body, and it is understood that
air will follow the above laws, one can begin to see how
and why an airplane wing develops lift. As the wing moves
through the air, the flow of air across the curved top surface
increases in velocity creating a low-pressure area.
Although Newton, Bernoulli, and hundreds of other early
scientists who studied the physical laws of the universe did
not have the sophisticated laboratories available today, they
provided great insight to the contemporary viewpoint of how
lift is created.
Airfoil Design
An airfoil is a structure designed to obtain reaction upon its
surface from the air through which it moves or that moves
past such a structure. Air acts in various ways when submitted
to different pressures and velocities; but this discussion
is confined to the parts of an aircraft that a pilot is most
concerned with in flight—namely, the airfoils designed to
produce lift. By looking at a typical airfoil profile, such as
the cross section of a wing, one can see several obvious
characteristics of design. [Figure 4-5] Notice that there is
a difference in the curvatures (called cambers) of the upper
and lower surfaces of the airfoil. The camber of the upper
surface is more pronounced than that of the lower surface,
which is usually somewhat flat.
NOTE: The two extremities of the airfoil profile also differ in
appearance. The rounded end, which faces forward in flight,
is called the leading edge; the other end, the trailing edge, is
quite narrow and tapered.
A reference line often used in discussing the airfoil is
the chord line, a straight line drawn through the profile
connecting the extremities of the leading and trailing edges.
The distance from this chord line to the upper and lower
surfaces of the wing denotes the magnitude of the upper and
lower camber at any point. Another reference line, drawn