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AERODYNAMICS
is all about
CREATING CHANGES
IN AIR PRESSURE
To create LIFT (or a positive upward
force to counter the pull of GRAVITY),
the wing must move air molecules out of their
normal at rest position. The wing will need to
be slightly tilted upward at the leading edge.
This is called a positive angle of attack. As a
wing passes through the air it is dividing a
massive sea of molecules. The leading edge of
the wing is the dividing point. What happens to
these molecules that are disturbed as the wing
passes through them will have an effect on the
wing.
If air molecules are pushed together their
collective air pressure will be greater. If
they are spread apart, their air pressure will
be lowered. Higher pressure under the wing and
lower pressure above it will cause the wing to
be lifted toward the lower pressure side, and
will tend to lift upward.
The molecules that pass below the wing are
pushed downward toward the rest of the sea of
molecules below it. To a very slight extent,
this packs the molecules a little closer
together which is another way of saying that it
slightly increases the air pressure just below
the wing. Because air (made up of billions and
billions of molecules) is very springy, the
actual higher pressure created is very, very
slight. What is even more significant is what
is happening on the top of the wing.
Since the wing is slightly tilted with the
leading edge higher than the trailing edge (a
positive angle of attack), the molecules that
are just above the wing will have to rush
downward to fill the void that would have been
created above it as it moves forward. These
molecules are being spread apart as they rush
downward away from the other molecules at rest
above them. Since molecules that are spread
apart have a lower pressure than molecules that
are packed together, there is a lower pressure
area established above the wing. This makes the
relatively higher pressure air under the wing
push it toward the lower pressure air above it.
This is LIFT and can be used to overcome GRAVITY that is trying to pull the
airplane downward.
The faster the wing moves forward, the more air
molecules it moves through. This will cause a
greater change in the air pressure both above
and below the wing. The greater this difference
in pressure is, the greater the LIFT will
be.
However, to move these air molecules out of
their normal at rest position takes energy. We
could relate this energy to DRAG. The
way that the air molecules are made to move out
of their normal position will affect the
efficiency of the wing and the overall DRAG of the wing. Newtons First Law of Physics
states that a body at rest will remain at rest
until acted upon by an external force. Even
these little air molecules follow this rule.
Our wing will force them out of their position.
The quicker and the further that we make them
move, the more energy that it takes. The
highest performance results in disturbing the
air molecules the minimum amount necessary to
get the desired results. Or in aerodynamic
terms, we try to minimize the DRAG.
Changing the shape or the angle that a wing
penetrates through the air will change the
position of the air molecules around it,
creating higher or lower pressure areas.
If the wing is tilted even higher at the front
edge than before (and the airplane continues
going forward in the same direction and at the
same speed), it will displace more air molecules
causing higher high pressure under the wing and
lower low pressure above it. This will create
greater LIFT and allow the airplane to be
heavier to go in the same path. However, it
will have greater DRAG because it is
displacing more air molecules and making them
move further, which takes more energy. So our
goal is to keep the wing at as small an angle
possible to obtain just the minimum LIFT that will be necessary to fly our airplane and
minimize the DRAG and the THRUST requirement. The heavier the plane is, the more LIFT that it needs and therefore the
higher DRAG that it will have and will
also require more THRUST to propel it at
the speed necessary to keep it flying. The
lighter your plane, the less LIFT it
needs to battle GRAVITY, and the less THRUST it needs to battle the decreased DRAG. A light airplane will make it
easiest to win competitions!
The ANGLE OF ATTACK is the angle of the
wing in relation to the air that it is
penetrating through (in our case its flight
path). The key to making this airplane fly
maximum duration is to force it to fly on both
wings, the primary wing up front and the
secondary wing in the back (Horizontal
Stabilizer). By forcing the tail to carry a
portion of the airplanes weight it decreases
the amount of weight that the wing has to
carry. Wings obtain lift when angled slightly
upward at the wings leading edge. This is
called a POSITIVE Angle of Attack. This
angle combined with the speed of flight
determines the lift. The greater the Angle
of Attack the greater the Drag. This angle
changes throughout the course of the flight.
Our goal is to balance the Angle of Attack and
the speed of the flight in such a way that we
minimize drag and therefore reduce the power
necessary to keep our plane aloft. With less
power necessary, the rubber motor can be thinner
(and therefore longer at 2 gr.) and allow the
propeller to turn longer. There are a number of
factors that will influence this important Angle
of Attack, especially the models weight and the
Center of Gravity.
The propeller is nothing more than a rotary
wing. Instead of going in a straight line,
the blades (each one is a wing) go around in a
circular path. As described in the wing
paragraph, these blades are pushed through air
molecules (in this case we are using the stored
energy in our wound up rubber band to turn the
propeller). The molecules that go under the
rear face are pushed backward and are packed
just a little bit closer together than they were
at rest creating a tiny bit higher pressure.
The molecules going over the top will be sucked
toward the forward surface of the propeller
blade causing them to spread apart from the
other molecules beyond them, creating a lower
pressure area. Therefore, the propeller is
pulled forward, toward the lower pressure. This
is EXACTLY the same as with the wing that is
traveling in a straight line. The wing had to
be tilted slightly higher at the leading edge to
create LIFT, as does the propeller. Each
blade needs to needs to be tilted to a Positive Angle of Attack to create THRUST.
In propeller terminology this angle is called
the Pitch. THRUST is just LIFT turned roughly 90 degrees to point it forward.
The more angled the blades are (at a given
speed), the greater the pressure difference will
be between the front of the blade (lower
pressure) and the back (higher pressure). The
problem is that it will now take more force to
keep the propeller turning the same speed. This
means that we would need a thicker, stronger
rubber band.
There is another way of increasing THRUST (or horizontal lift). If the propeller blades
are twisted to a lower pitch (not as great an
angle), they will not displace as many air
molecules each time they go around. This means
that they will have lower DRAG as well as
lower LIFT for each revolution.
But the propeller now will turn faster (with the
same force from the rubber motor) and maybe go
through even more total air molecules per
given period of time. This would mean that you
now have greater THRUST! The problem is
that you are now using up the winds of your
rubber motor faster, and may run out before you
have flown long enough to win your competition.
One solution could be to reduce your rubber size
since thinner rubber has less power and will
cause the propeller to turn slower, extending
the propeller run time. But if the propeller
now turns too slowly, it will not develop enough THRUST to keep your plane flying at the
end of the flight. So this competition is all
about balancing the power of the stored energy
in the rubber motor with the ability of the
propeller to develop enough THRUST to fly
your airplane for the longest duration.
This brings up the shape of the propeller
blade. A cambered surface is an efficient low
speed airfoil. This curved shape more gradually
accelerates the air molecules into their new
positions (both pushing them together for higher
pressure and spreading them apart for lower
pressure). As mentioned before, the Pitch is the angle of the blade. But for the
highest efficiency, the pitch should
gradually change from the Root (near the center) to the Tip. As the
propeller turns, the outside edge (the tip) goes the greatest distance because it is traveling around the greatest
circumference. As you progress toward the
center of the propeller it will be traveling a
shorter distance for every revolution because it
will move a shorter distance (smaller
circumference). Therefore, to move an
equivalent amount of air for each revolution,
the pitch (angle) should be greater near
the center and less near the tip. Most
commercially available propellers are designed
that way to increase their efficiency. This
constantly changing pitch across the length of
the blade is called a helical pitch. There is a
simple means of accomplishing this type of
helical pitch with a simple cylindrically
cambered piece of plastic. By tilting the
axis of the blade cut about 15 degrees
(tip forward) to the axis of the cambered
plastic sheet, you will yield a coarser
pitch (more angled) at the center and a flatter
pitch (less angled) at the tip. This is
what you want for greater efficiency. Slightly
changing the angle of this axis will affect a
change in pitch from root to tip.
There is also a bit of an art in experimenting
with the outline of the blade. Most
aerodynamicists will agree that an elliptical
shaped tip (smooth rounded with a varying
radius) is the most efficient, as it minimizes
unnecessarily swirling air molecules around the
tip because of the higher pressure air being
sucked toward the lower pressure air. This is
something that will occur with a square cut
tip. However, the faster moving outer portion
of the blade is where some of the most valuable
lift (thrust) is created. Especially with
smaller diameter propellers, it may be necessary
to utilize a wider airfoil all the way to the
tip to create the desired thrust. This would
mean keeping a square cut or at least a very
blunt tip. The highest efficiency wings are
long and thin. This would be my choice with our
propellers too, except that the rules regulate
the maximum length of the propeller (keeping
them much shorter than optimum).
The width of each blade is another area to
experiment. A wider blade (with a given camber)
will displace more air with each revolution but
could be set to a lesser pitch. Generally this
approach would favor a thicker rubber band. A
narrower blade could displace less air with each
revolution and therefore have less drag, but it
would turn faster and could use a thinner rubber
band that could hold more winds. Not only are
these generalities food for thought, but the
width that you choose could also be applied
gradually across the length of the blade. Maybe
designing your blade to be wider in the middle
then gradually thinning toward to tip would be
good? Maybe very wide all of the way to the
root? Maybe a svelte high aspect ratio (sleek
and slim) using a very thin rubber motor? You
decide. Look at all of the propellers that you
can find to see if you can figure out what might
work best for your plane.
Experimentation will rule! More than ever, this
year you can control your destiny in this event!
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