Aerodynamics and Design for Ultra-Low Reynolds Number Flight

Chapter 3
34
Cl
0.60
0.50
0.40
0.30
0.20
0.10
0.00
-0.10
-0.20
-2 -1 0 1 2 3
α (deg.)
4 5 6 7 8
FIGURE 3.9 Lift curves for uncambered NACA 4-digit sections, Re=2000.
The decambering effect of the boundary layer is visualized by considering constant
velocity contours in the flow field. The area of reduced flow velocity is large and the
validity of defining a reference boundary-layer edge value is debatable. The contours
are chosen at a fixed fraction of the freestream velocity, low enough to be considered
within the boundary layer, providing a qualitative notion of the boundary layer geometry.
Several 0.2V contours are drawn for the NACA 0002 and NACA 0008 sections at
Re=6000 in Figure 3.10. Three angles of attack, 0.0, 2.0, and 4.0 degrees are shown.
These respectively represent the zero lift condition, the upper limit of the quasi-linear lift
range for the NACA 0008, and a point within the nonlinear range where trailing edge
separation comes into play. The boundary layer has little effect on the effective
geometry of the NACA 0002, but the thicker upper surface boundary layer of the NACA
0008 significantly decreases the effective camber of the airfoil.
NACA 0002
NACA 0004
NACA 0006
NACA 0008