MECHANICS of FLUIDS LABORATORY - Mechanical Engineering

Experiment II
For a number of wings, lift and drag data
vary only slightly with Reynolds number and
therefore if lift and drag coefficients are graphed
as a function of Reynolds number, the results are
not that meaningful. A more significant
representation of the results is given in what is
known as a polar diagram for the wing. A polar
diagram is a graph on a linear grid of lift
coefficient (vertical axis) as a function of drag
coefficient. Each data point on the graph
corresponds to a different angle of attack, all
measured at one velocity (Reynolds number).
Referring to Figure 11.2 (which is the
experimental setup here), the angle of attack α is
measured from a line parallel to the chord c to a
line that is parallel to the free stream velocity.
If so instructed, obtain lift force, drag force and
angle of attack data using a pre selected velocity.
Allow the angle of attack to vary from a negative
angle to the stall point and beyond. Obtain data
at no less than 9 angles of attack. Use the data to
produce a polar diagram.
Analysis
Lift and drag data are usually expressed in
dimensionless terms using lift coefficient and drag
coefficient. The lift coefficient is defined as
L f
C L
=
ρV 2 A/2
where L f is the lift force, ρ is the fluid density, V
is the free stream velocity far upstream of the
wing, and A is the area of the wing when seen
from a top view perpendicular to the chord
length c. The drag coefficient is defined as
D f
C D
=
ρV 2 A/2
in which D f
is the drag force.
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