Benefits of Lower Angles-of-Attack
Quite often airfoils are operating at low angles-of-attack
that produce much less lift and where need for
drag reduction is very important. Typical situations
are wings and propellers of high speed aircraft,
or boat rudders and keels that are cruising/not
maneuvering, much of the time.
The rear hump is a favorable pressure gradient
that smoothes the otherwise highly turbulent
flow at center and toward the trailing edge of
the airfoil. Drag reduction from reduced turbulence
can be greater than that provided by laminar
flow along the front half of traditional low
drag airfoils. Also,laminar flow is an illusive
quality that is often prevented by dirt or moisture
on the airfoil. The Bicambered™ advantage
is always available, not being affected by dirt
or moisture.
Experts claim that once turbulent, flow stays
turbulent. The molecules once disturbed cannot
be put back in line. This is true, but separation
can be eliminated and turbulence can be reduced
so fluid disturbance and energy exchange is kept
to a minimum.
Additional Benefits of Bicambered™ Airfoils
Noise produced by airfoils is caused by turbulent
flow that separates somewhere along the upper
surface of the airfoil. In extreme cases, this
separation creates swirling eddy currents that
can move much faster locally than the airfoil
itself is traveling. When these flows reach critical
speed, (mach 1, the speed of sound), they can
produce loud cracking sounds like the crack of
a whip, or bark of a propeller, or the pocketta-pocketta
sound of a helicopter. Below mach 1, airfoils
make lesser noises; humming, swooshing, whistling,
etc. which can be annoying. By reattaching flow
at the back of the airfoil and providing a much
shorter down slope for separation to develop,
Bicambered™ airfoils can reduce decibel
levels and sound carrying ability substantially.
How It Works—A Summary Overview
Just as any other object, air and water molecules
are subject to the laws of physics. If they
strike another fluid molecule, or some other
object, they bounce off taking a new direction.
Part of their energy is imparted to the object
struck. In turn they absorb some of the energy
of the other object. It is like a multitude
of billiard balls careening off one another.
If you have many billiard balls in a straight
line and you strike the first in line squarely,
only the last in line moves though the energy
has been transmitted through all of them.
The analogy applies to fluids in that they do
not actually bounce off a moving airfoil; rather
they cling to the airfoil surface sending the
energy of the encounter outward to adjoining,
then further out fluid molecules.
Parts of the airfoil surface are moving away
from the surrounding air. Here the surface pressure
is a partial vacuum and adjacent fluid is forced
toward the surface by higher pressure from the
adjoining fluid.
Fluid flows from the highest pressure area to
the lowest. If pressure is higher on the surface
moving toward the fluid than on the side moving
away it attempts to move from the first to the
second filling in the area of lowest pressure
with turbulent, contrariwise movement.
To counter this tendency the pressure holding
the fluid to the airfoil must be greater than
the fluid’s inertia, i.e. its tendency
to travel in a straight line. The concave surface
at the top of a Bicambered™ airfoil creates
a much lower pressure that keeps upper flow
attached.
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