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Pilot's Handbook of Aeronautical Knowledge
Principles of Flight
Theories in the Production of Lift

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




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 colder 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 does contribute.

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 conveyed
but the affect of humidity can be determined using online
formulas. In the first case, the pressure is needed at the
altitude for which density altitude is being sought. Using
Figure 3-2, 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 (http://www.srh.noaa.gov/
elp/wxcalc/densityaltitude.html) for density altitude, enter the
22.22 for 8,000 feet in the station pressure window. Entering
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 site (http://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

Newton's Basic Laws of Motion

The formulation of lift has historically been the adaptation
over the past few centuries of basic physical laws. These
laws, although seemingly applicable to all aspects of lift,
do not answer 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 best
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."

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.