sin αst

Figure 6.5: Definition of typical airfoil parameters
� The chord line is a straight line connecting the leading and trailing edges on the
airfoil
� The mean camber line is a line drawn halfway between the upper and the lower
surfaces
ROTOR
� The frontal surface of the airfoil is defined by the shape of a circle with the leading
edge radius. The centre of the circle is defined by the leading edge radius and a
line with a given slope relative to the chord line.
A NACA 4412 airfoil, which is an old design, used in e.g. Akron-Funk Model B aircrafts from
the 1930s [28, p. 61], is selected as the blade airfoil profile. The profile, shown on figure
6.6, is selected due to its performance characteristics and manufacturability. The high
pressure side of the profile is almost flat, which facilitates manufacturing by hand-carving.
This production method is assumed for the wooden blades, since it is available in all devel-
oping countries.
Figure 6.6: NACA 4412 airfoil
The NACA four-digit code defines the profile shape [10, p. 110]:
� 1st digit - maximum camber in percentage of the chord
� 2nd digit - location of maximum camber along chord line (from leading edge) in
tenths of the chord
� 3rd and 4th digit - describing maximum thickness of the airfoil in percentage of
the chord. The maximum thickness position of all four-digit airfoils amount to 30
% of the chord length.
Thus the NACA 4412 profile has a maximum camber of 4% located 40% of the chord from
the leading edge with a maximum thickness of 12% of the chord.
47