Bicambered™ Airfoils and Newton’s
Laws
This brings us to the answer why Bicambered™ airfoils
are superior at keeping fluid flow attached.
The concave space in the top, lee surface increases
the vacuum effect of the lee side. The lee side
flow has to move down into the concave to fill
the potential vacuum. This downward motion further
decreases pressure at the airfoil surface. The
windward side high pressure flow cannot fill
this vacuum because it is blocked from moving
into the center lee section by the rearward hump
or trailing edge thicker portion. Only fluid
from above can fill this vacuum so the lee side
flow is very effectively ‘sucked’ down
into the concave. The higher the angle of attack
the more the upper flow is redirected downward.
Further back, it encounters the hump of the back
section. This upward slope is a favorable pressure
gradient that smoothes flow and reduces boundary
layer turbulence enabling flow to stay attached
on the short trailing edge down slope.
It might help to think of the double airfoil
analogy. The first airfoil redirects flow so
it can more easily follow the second airfoil.
If there is trailing edge separation, it is
limited to the last downward slope of the upper
surface of the airfoil. In aerodynamic parlance, ‘the
rear adverse pressure gradient is much shorter
than the adverse pressure gradient of a single
cambered traditional airfoil. Traditional airfoils
have an adverse pressure gradient of about 40%
to 65% of chord. The bicamber allows this gradient
to be less than 25%. Airfoils with 14% final
adverse pressure gradient have been used with
amazing success.
It is well established that insect wings can
operate at incredible angles-of-attack (50° or
more without stalling). Insect wings consistently
operate at lift values unattainable with ordinary
man-made airfoils. Insect wings have veins and
pleats similar to the humps and concaves of Bicambered™ airfoils.
Likewise the Bicambered™ airfoil will operate
at angles-of-attack well above the 16° to
22° maximum angle-of-attack attainable with
ordinary airfoils.
Why Concave on Both Top and Bottom?
The concave portion of the lower, windward surface
provides benefits as well. First of all it gives
the airfoil any manner of internal curve, referred
to as camber. It allows a flat camber with symmetrical
airfoils. It allows for a constant gentle camber
as is true for the best traditional airfoils.
In addition, the concave allows fluid to flow
upward before being again directed downward by
the rear lower hump. Pressure is increased in
the lower concave while the hump at the back
section impedes flow toward the trailing edge
and up to the lee side.
The overall effect of the Bicambered™ airfoil
is to create lift coefficients measured at
more than 2.5 with very low drag coefficients.
This value is comparable to the lift of a bumble
bee wing. Ordinary airfoils have maximum lift
coefficients of around 1.6.*
Bicambered™ airfoils improve airflow by
using the back half of the airfoil more effectively
than do traditional airfoils. Whether you think
of it as connecting two smaller airfoils together,
or providing a second favorable pressure gradient
to improve flow on the back half of the airfoil,
the benefits are clearly, more attached flow,
a stabilized boundary layer, significant reduction
of lee surface pressure and increased windward
surface pressure. Greater pressure difference
produces more lift; less turbulence yields less
drag and less noise.
In summary, at high angles-of-attack, Bicambered™ airfoils
can deliver roughly 60% more lift than ordinary
airfoils and do so with greatly reduced drag.
* Abbott & von Doenhoff, Theory of Wing Sections, Appendix IV, NACA 23012. Benefits
of Lower Angles-of-Attack -
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