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By: Joshua Boeldt, Mitchell Burmeister, David Davis, Kyle Patt, and Adam Mathiak
What is aerodynamics? The word comes from two Greek words:
aerios, concerning the air, and dynamis, which means force.
Aerodynamics is the study of forces and the resulting motion of
objects through the air. Humans have been interested in aerodynamics
and flying for thousands of years, although flying in a heavier-than-air
machine has been possible only in the last hundred years.
Aerodynamics affects the motion of a large airliner, a semi truck, a
beach ball, or a kite flying high overhead. Even, the curveball thrown
by big league baseball pitchers gets its curve from aerodynamics. One
of the best objects to study with aerodynamics is airplanes, which are
most effected by physical forces and the air. Since aerodynamics is so
influential in our lives, it is imperative to understand how forces affect
motion through the air.
Newton’s laws are the first and most basic rules applied to
motion over space. The laws are the backbone of mechanical physics,
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as we now know it. The first of these laws was a concept originally
credited to Galileo. It states that an object moving in a straight line,
with no outside forces acting upon it, will continue to travel in a
straight line. This concept is more commonly referred to as inertia.
The flip side of this law states that if an object is stationary, it will
remain stationary, until acted upon by an outside force. Newton’s
second law is much less well known than his other two. Newton’s
second law of motion states that the sum of all forces acting on an
object are equal to the object’s mass, multiplied by its acceleration.
This means that if an object is moving at a constant speed, the sum of
all the outside forces acting upon that object equal zero; forces of
equal magnitude must be pushing in opposite directions. According to
Newton, when the sum of all the forces is not zero, then the object
must be accelerating. The law also proves that acceleration is directly
proportional to the force applied on the object, but inversely
proportional to the object’s mass. Therefore, the greater the mass, the
slower an object accelerates, but the greater the force, the faster the
same object accelerates. Newton’s third law of motion is another wellknown law, although it is often misapplied to things other than motion.
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His third law states that for every force there is an equal and opposite
force. This means that if a person was to push on a wall, the wall
would push back on the person with a force equal in magnitude, but
opposite in direction to the force of the push the person is applying. As
a side note, this rule also states that for every force in the universe,
there must be another force canceling the first out. Therefore, one
force cannot exist alone; there will always be at least two forces
acting on an object.
Lift and drag (Figure 1, Pg. 8) are both aerodynamic forces that
affect the flight of a powered airplane. The lift over drag ratio, or L/D
ratio, is very important to consider when designing an airplane. Lift
directly opposes the weight of an airplane and keeps it in the sky. Lift
is generated when the air is turned by the solid wing, as described in
Newton’s third law of action and reaction. On a plane’s wing, the upper
and lower parts of the wing generate lift because they both contribute
to the turning of airflow. While lift is the force that keeps the airplane
flying, drag opposes this force. Drag is a mechanical force that is
generated by contact of a solid body (airplane) and a liquid or gas, in
this case air. Drag can be referred to as aerodynamic friction. As the
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air molecules move over the wing, drag is created. The smoother a
surface is, the more drag is reduced, and the L/D ratio is increased.
Lift and drag correlate even more directly through a force called,
“induced drag.” Induced drag occurs because at wing tips where
airflow is distorted span wise resulting in an air drag behind the wings.
Lift and drag correlate directly and are extremely important to
consider in the aerodynamics of an airplane.
There are two main theories about how lift is created. First, at
the front of a wing the air "splits", and recombines at the rear of it. If
the top of a wing is longer than the bottom, air travels faster over the
top of a wing than over the bottom. According to the Bernoulli Effect,
the quicker the air is moving, the lower the pressure. This means that
there is lower pressure on the top than the bottom. This causes air on
the bottom to try to move upward, pushing the wing upward with it
(Figure 4, Pg. 9). On the other hand, the angle of attack of the wing
pushes the air downward. This causes the air to leave the wing with
more downward velocity than it started with. Since there needs to be
an upward movement to counteract this downward movement, the
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wing creates lift. This second theory is commonly called, “Newtonian
lift,” because it is based on Newton’s third law.
The most accurate explanation of lift comes from combining the
two theories. As air moves faster over the top of the wing than over
the bottom, a near 'vacuum' is created over the top of the wing. Air
from the bottom of the wing tries to go upward pushing the wing
upward with it, which is the Bernoullian lift. However, air on the top of
the Bernoullian vacuum is also moving downward into the vacuum. An
effect, known as the Coanda Effect, assists this downward movement.
The Coanda Effect (Figure 2, Pg. 9) states that a stream of air has a
tendency to stick to a surface. At the rear of the wing, due to the
Coanda Effect, the air attempts to follow the curvature of the wing
downward, resulting in a net downward momentum change in the air.
Also, at the front of the wing, the air is forced to separate and the air
below the wing transfers its upward momentum to the wing. The
unbalanced momentum is transferred into the wing creating lift. The
combined result of the two theories is on page 9, Figure 5.
The two remaining forces that act upon an airplane (Figure 1, Pg.
8) in flight are weight and thrust. These two forces are opposites of
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the previous forces, lift and drag. Weight is the downward force acting
on the plane as a result of gravity. Weight refers to the heaviness of
the plane, which causes the airplane to fall towards earth’s surface.
Thrust, usually by mechanical means, is the force that acts in
opposition to drag. In an airplane the thrust comes from propellers or
jet propulsion. When a plane is said to have reached its cruising
altitude, this means that the plane is now at a consistent height and
maintaining a constant speed. This means that the two pairs of
aerodynamic forces will be equivalent so that thrust equals drag and
lift equals weight. In order for an airplane to “take off” the airplane
must generate more thrust to over come drag, and there must be a
greater amount of lift to surpass the weight. While an airplane is
landing, however, the opposite is true. The thrust must be safely
reduced to a level less than the amount of drag, and the lift must be
reduced to be less than the weight.
All matter is made from atoms, which are made up of protons,
electrons, and neutrons. Individual atoms can combine with other
atoms to form molecules. Oxygen and nitrogen, which are the major
components of air, occur in nature as diatomic (two atom) molecules.
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Under normal conditions, matter exists as either as a solid, a liquid, or
a gas. Air is, obviously, a gas. In any gas, there are a very large
number of molecules that are only weakly attracted to each other and
are free to move about in space. With gases, we can investigate the
large scale action of the gas as a whole. Scientists refer to the large
scale motion of the gas as the macro scale and the individual
molecular motions as the micro scale. Some phenomena are easier to
understand and explain based on the macro scale, while other
phenomenon are more easily explained on the micro scale. Macro
scale investigations are based on things that we can easily observe
and measure. But micro scale investigations are based on rather
simple theories, because we cannot actually observe an individual gas
molecule in motion. Overall, macro scale and micro scale
investigations are just two views of the same thing.
As an object moves through the air, the viscosity (stickiness) of
the air becomes very important. Air molecules stick to any surface,
crating a layer of air near the surface, called a boundary layer. This
principle is called the Coanda Effect, and it changes the shape of the
object. This boundary layer may even lift off or “separate” from the
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body and create an effective shape much different from the physical
shape of an object. Therefore, the flow conditions in and near the
boundary layer (Figure 2, Pg. 8) are often unsteady, which means that
they change in time. The boundary layer is very important in
determining both the drag and lift of an object.
As an object moves through the air, the compressibility of the air
also becomes important. Air molecules move around an object as it
passes through the air. If the object passes at a low speed (under 200
mph) the density of the fluid remains constant. But for higher speeds
some energy of the object goes into compressing the fluid, moving the
molecules closer together and changing the air density. This alters the
amount of the resulting force on the object. This effect is more
important as speed increases. Near and beyond the speed of sound
(about 700 mph) shock waves are produced that affect both the lift
and drag of an object.
Knowing the main factors that affect aerodynamics, the way to
make an object aerodynamic is simply to reduce the drag. As stated
earlier, thrust is the force used to overcome drag, but in order to
reduce the amount of energy required to produce thrust, an object’s
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drag must be lowered. Reducing the amount of energy required to
produce thrust is essential for conserving fuel and saving money. In a
car, a more aerodynamic shape results in better gas mileage (requires
less thrust), and this means spending less money on gasoline. On any
object, drag is reduced by decreasing the amount of air flow
resistance created by the object (Figure 3, Pg. 9). For example, on an
airplane, any edges on the plane are rounded in order to improve
aerodynamics. Whenever there is less contact between the leading
edge of an object and the air, there will be less resistance. However,
drag can also be decreased on other parts of the object. Shortly into
flight, the airplane’s landing gear will be retracted to reduce the
amount of surfaces in contact with the air. This also gives the main
body of the airplane a smooth, seamless surface. So an object is said
to be aerodynamic when the drag is reduced to a minimum. In other
words, there is very little resistance on the object from the air. Simply
put, the less drag and air resistance disrupting the object’s motion, the
more aerodynamic the object will be as it moves.
The study of how forces act upon an object in motion is crucial
because everything that moves is influenced by aerodynamics. This
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means that Newton’s laws of motion are the guidelines for how the
forces will act. The four main forces that affect an airplane’s ability to
fly are lift, drag, thrust, and weight. If planes flew in vacuums there
would be no need to be concerned with the properties of air itself,
however, because planes fly through the atmosphere, they are also
greatly affected by the air particles. Finally, the four main forces that
act upon an airplane along with the air itself all influence flight, and
they are summed up in aerodynamics.
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Bibliography
Clark, John. Matter and Energy: Physics in Action. New York: Oxford
University Press, 1994.
Lehrman, Robert. Physics The Easy Way. United States of America:
Barron’s Educational Series, 1998.
Suplee, Curt. Everyday Science Explained. Willard, OH: R.R. Donnelley
and Sons, 1998.
Aeromuseum.com. How does an airplane fly? 17 December 2005.
<http://www.aeromuseum.org/Education/Lessons/HowPlaneFly/Ho
wPlaneFly.html>
NASA.com. 2 November 2005. Beginners Guide to Aerodynamics. 17
December 2005. <http://www.grc.nasa.gov/www/k12/airplane/Idrart.html>
NASA.com. July 2000. Gas Properties Definitions. 15 December 2005
<http://www.grc.nasa.gov/www/k12/cdtemp/airplane/gasprop.html>
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Lift
Thrust
Weight
Figure 1
Air Flow over Surface
Figure 2
Drag
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Figure 3
Figure 4
Figure 5