The Analysis of Tabs on Aerodynamic Coefficients

1
The Analysis of Tabs on Aerodynamic Coefficients
Lorena Moreno, C.P. van Dam, D.
Engineering Department of Mechanical and
Aeronautical Engineering University of
California, Davis 95616
Abstract
The effects of translational tabs are being
studied to determine the effects on aerodynamic
load control on an airfoil. In the beginning
stages of this research, the complexity of the
problem was greatly simplified by using fixed
tabs of varying height and chord location on the
upper and lower surface of the S809 airfoil. At a
Reynolds number of 1,000,000, the lift and drag
analysis are done in an open circuit, low speed,
and low turbulence aeronautical wind tunnel at UC
Davis. Lift was measured using a force balance
under the test section, while drag values were
found using a traverse mechanism that moves a
pitot-static probe vertically and horizontally
across the test section of the wind tunnel.
Based on experimental testing, tabs on the upper
surface lead to a decrease of the coefficient of
lift, CL, up to separation of the flow where the
tabs become ineffective. There is also an
increase of the coefficient of drag, Cd. On the
lower surface there is a general increase in CL
as the flow remains attached at all angles of
attack however, the tab location survey showed a
reversal of trend for the tab location of 90
chord length from the leading edge where CL
decreases. The drag on the lower surface is
slightly increased with the tabs, yet at about an
8º angle of attack, the drag is reduced as the
tabs are positioned further away from the
trailing edge. Data analysis shows that for the
95 chord location and 1 chord tab height on the
lower surface there is a max increase of about
7.47 of the lift to drag ratio, CL/ Cd at 0º
angle of attack. Overall, the best location for
the design constraints of the translational tabs
on the lower surface is at 95 chord length from
the leading edge and tab height of 1.5 chord
length. On the upper surface a series of tabs
may be desirable. By inserting tabs at these
locations, there is more control of the flow on
the surface which can be applied to wind turbines
and airplanes.
Introduction
Results
The use of translational tabs on an airfoil is
being studied to learn its effects on aerodynamic
load control. For simplicity, fixed tabs of
varying height between 1 to 2 chord length of an
airfoil are being tested at different locations.
In comparison to the conventional bulky flap
devices currently used to control the aerodynamic
load on lifting surfaces, these micro-scale tabs
will offer a faster and potentially less
expensive method for controlling the flow over an
airfoil. Also, the tabs offer further benefits
since they may require less power to activate,
are substantially smaller in size and weight, and
do not require significant modifications in
materials or manufacturing compared to the
conventional lifting surface designs.
Applications may extend to load control of
airplanes, helicopters, rockets, missiles and the
improvement of power transfer in wind turbines.
To analyze the effects of translational tabs, the
problem was simplified by experimentally testing
small fixed brass L-brackets in the UC Davis
aeronautical wind tunnel to evaluate the
aerodynamic forces on the airfoil.
On the upper surface of the airfoil the
coefficient of lift, CL, decreases as the tab is
positioned closer to the leading edge and as the
tab height increases. Regardless of these
factors, there is no change in the maximum CL
(Refer to Figure 1). Depending on the tab
location, the tab loses its effectiveness after
separation in the flow at different angles of
attack. For the coefficient of drag, Cd, there
is a major increase in drag for a given tab
location as the height increases when compared to
the clean airfoil. This results in large drag
penalties at 40 and 60 chord lengths from the
leading edge. A series of tabs may be desirable
on the upper surface since the flow separates and
the tabs become ineffective at different angles
of attack for various tab locations.
Compensating for this separation may be to place
tabs at the different locations on the upper
surface and regulating which ones to use in
different situations. When one set of tabs may
be ineffective at one time, others can be used
instead for load control. On the lower surface,
the maximum CL is shifted upwards for tabs
located at the trailing edge and 95 chord length
from the leading edge (Refer to Figure 2). In
addition, increasing the tab height leads to both
an increase in effectiveness of the tab and an
increase in drag. Therefore, the closer to the
trailing edge of the airfoil on the lower
surface, the better control the tabs have on lift
and drag. Overall, the best location of the tab
was found to be at the trailing edge (TE)
however, the 95 chord length position on the
lower surface is the best compromise due to a
thickness requirement for placement of the
translational tabs. The lift to drag ratio,
CL/Cd, was used to compare the different tab
heights on the lower surface at the 95 chord
length location. On the lower surface, the
maximum CL/Cd for the baseline was found to be
85.53 at a 6 degree angle of attack. The values
of CL/Cd decreased for all tab heights however,
the 1.5 and 1 chord length heights were the
closest to the baseline and had equal maximum
values of 63.77. Overall, there was an increase
in CL/Cd for the 1.5 chord length compared to
the 1 chord length at the outer angles of
attack. Consequently, the best tab height found
for a tab placed at 95 chord length from the
leading edge was the height of 1.5 chord length.

Figure 1 CL vs. Alpha curve for tab location
study on upper surface.
Methods
The tabs were tested in a low speed open return
aeronautical wind tunnel at UC Davis with typical
test speeds ranging between 70mph and 120mph. It
is equipped with three SETRA 239 pressure
transducers that are used to determine the tunnel
velocity and the differential pressure for the
wake analysis. The test section has a
rectangular cross section of 33.6 in x 48 in and
a length of 12 ft. A force balance was used to
measure the aerodynamic forces, such as lift (L)
and drag (D), and also the moments. However, the
balance produces inaccurate drag measurements,
thus, drag values are determined using a straight
pitot-static pressure probe for the wake analysis
of the airfoil. A traverse system moves the
probe vertically and horizontally across the test
section of the tunnel to read pressure
measurements. On a typical test day, the wind
tunnel and model must be prepped before any data
is collected. The traverse mechanism is
calibrated to the center of the wind tunnel test
section in order to ensure accurate measurements
of the pitot-static probes position. The
turntable, in which the airfoil is mounted on, is
also adjusted and set to its zero position and
then calibrated using the wind tunnel data
acquisition software. Also, the S809 airfoil
surface is cleaned and a small brass L-bracket is
vertically positioned using double-sided tape.
Lift was measured at angles of attack ranging
between -4º and 20º at 1 degree increments while
drag values were determined at angles of attack
of -2º to 10º at 2 degree increments. To
determine the effects of the tabs, several
combinations of tab height and location were
tested on the airfoil. The height and location
were measured as a percentage of the 12 inch
chord length of the airfoil and were chosen based
on computational fluid dynamic results. On the
lower surface, tabs were positioned at the
trailing edge, 90 and 95 chord length. Tab
locations on the upper surface included 40, 60,
and 90 chord length from the leading edge and at
the trailing edge.
Figure 2 CL vs. Alpha curve for tab location
study on lower surface.
Conclusion
Through experiments in the wind tunnel, the
optimum tab locations and heights were found. On
the lower surface the CL vs. Alpha curve is
shifted and only one tab is needed for load
control on an airfoil. Due to design
constraints, the best location on the lower
surface to control the flow over the airfoil is
at 95 chord length from the leading edge with a
height of 1.5 chord length. In contrast, the
upper surface may require multiple tabs since the
flow separates at different angles of attack for
a given tab location. Since the tabs lose
effectiveness at different angles of attack, a
series of tabs can be used for a range of
operating conditions. Continuing research on this
project will involve 3-D tab testing in the wind
tunnel. Gaps will be placed between the tabs and
a variety of tab shapes will also be analyzed.
Special Thanks C.P. van Dam, Jonathon Baker,
Mercedes Piedra, the UC LEADS Program, and
Hewlett-Packard Roseville
Figure 3 Test section in Aeronautical Wind
Tunnel at UC Davis. The S809 airfoil at a 0º
angle of attack with pitot-static probe and fan
in background.