Scientific Laws of Motion and Energy
Sir Isaac Newton in 1687, published his, Philosophia
naturalis principia mathematica where
he articulate the laws governing the movement
of matter through space. These three laws are:
- An object at rest tends to stay at rest
and an object in motion tends to stay in
motion in a straight line at constant speed
unless acted upon by an external, force.
- The rate of change of momentum of a body
is proportional to the resultant force acting
on the body and is in the same direction.
- To every action (force applied) there is
an equal and opposite reaction (equal force
applied in the opposite direction).
The conservation of energy Law first stated
by Julius Robert Mayer in 1842 teaches that energy
can neither be created nor destroyed. It only
changes its form. This principle is implicit
in Newton’s laws, though not actually stated
by him.
For our purpose we could limit these laws and
paraphrase them as follows:
- Inertia is universal and lasting. In a
fluid, so are external forces.
- Big heavy objects (airfoils) have more impact
than small light objects (fluid molecules),
but all objects have some impact.
- Energy is transferred but not lost. Transfer
less energy and you can fly farther.
I want to emphasize the second part of paraphrase
2; these laws apply to any object whether
it is a solid object or a fluid object. Airfoils
(solid objects) interact with fluids (air and
water) in accordance with these physical laws.
Airfoils and fluid molecules both are objects
and they act upon one another. Fluid molecules
at the airfoil surface are acted upon by two
external forces. The first is the airfoil itself
which collides with the fluid and moves it aside,
the second force is the pressure of the surrounding
air; or more accurately, the difference in pressure
between the air at the airfoil surface and the
air further out. As air attempts to bounce off
the airfoil in a new straight line it is forced
to turn and follow the airfoil curve because
there is greater pressure away from the surface
than directly on the surface.
Aerodynamics simplifies the explanation of fluid
motion by assuming that air is incompressible
until it reaches the speed of sound, and that
water is always incompressible. This simplification
is based on the premise that pressure differences
are too small to matter so they are ignored.
However, we all experience differences in air
pressure, slight though these differences may
seem. For instance, when you close a door in
a room, curtains in front of an open window move
away from or toward the window. This is because
air pressure in the room is changed momentarily
causing air to move in or out of the window to
equalize the pressure difference. You don’t
have to close the door at the speed of sound
to see these effects. They happen even with a
gently closed door.
Stand in a flowing river and you can feel the
pull it exerts on your legs. There is more pressure
on the upstream side of and less on the downstream
side. If the flow is stopped, or you float along
with the flow the pressure is equalized and you
no longer feel the tugging pull of the water.
Wave a flat object through the air near a smooth
water surface. You will see small ripples on
the water. Your motion causes the flat object
to alter air pressure which in turn alters the
water pressure.
These examples are for very small effects, but
the force of a fluid grows exponentially with
the square of the velocity. Wind causes waves
to form on water. Air moving at 112 kilometers
per hour (70 miles per hour) has the force of
a hurricane.
Airfoils moving continuously, at an angle-of-attack
through fluid act as pumps. Fluid is forced away
from the airfoil at the windward side, and drawn
toward the airfoil at the lee side. Consider
an airplane wing; fluid is both drawn down and
forced down in a continuous motion. The wing
is sitting on a higher pressure cushion of air.
Or if you prefer, the wing is sucked upward by
the lower pressure air above. Either way, it
is the difference between pressure below and
pressure above that keeps the wing from responding
to gravity.
Nature tries to equalize pressure differences
on the two sides by having fluid from above move
downward or fluid from below forced upward at
the airfoil trailing edge. Flow moving up from
the windward side fills the lower pressure area
on the lee side. Flow on top is able to continue
in a straight line diverging away from the lee
surface. That is, upper flow is separated from
the trailing edge.
Fluids bend around the surface of a passing
airfoil because there is pressure from surrounding
fluid forcing the closer fluid molecules to stay
on the surface. Fluid molecules want to follow
Newton’s first law of motion and continue
in a straight line but that would create a vacuum
at the surface. We have all heard the saying, “nature
abhors a vacuum”. The molecules at the
surface are moved aside by collision with the
airfoil surface. At the same time, they are made
to follow the airfoil surface by pressure of
the surrounding air molecules. If they did depart
from the airfoil surface (separate) it would
create a vacuum that must be filled from somewhere
else. The only other place replacement fluid
can come from is the higher pressure windward
side.
Bicambered™ Airfoils and Newton's Laws -
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