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Joshua Boeldt, Mitchell Burmeister, David Davis, Kyle Patt, Adam Mathiak
Mr. Kuehl
Pre-Calculus
13 January 2005
Aerodynamics
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, 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
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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 well-known law, although it is often misapplied to things other than motion.
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 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
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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 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.
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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 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. 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
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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 body and create an effective shape
much different from the physical shape of an object. Therefore, the flow condiditions 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 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,
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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 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/HowPlaneFly.html>
NASA.com. 2 November 2005. Beginners Guide to Aerodynamics. 17 December 2005.
<http://www.grc.nasa.gov/www/k-12/airplane/Idrart.html>
NASA.com. July 2000. Gas Properties Definitions. 15 December 2005
<http://www.grc.nasa.gov/www/k-12/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