Most of the understanding about the aerodynamics of helicopter airfoil sections comes from two-dimensional wind-tunnel testing. However, there are some special considerations that must be understood and appreciated when attempting to interpret the two-dimensional characteristics of airfoil sections. These issues are especially important when comparing the relative performance of different airfoils. Usually in in 2-d testing, the test airfoil (wing) is made to fully span one dimension of the wind tunnel, i.e., across the height or width of the test section. This has the effect of making the wing appear to be of high aspect ratio. The pressures around the airfoil are usually measured by instrumentation at the mid-span where three--dimensional effects and interference effects as a result of the wind-tunnel walls are much smaller. The test airfoil may also be placed in a 2-d insert, as shown below. This reduces the span of the test airfoil, while still maintaining nominally 2-d flow.
Note that no matter what aspect ratio is used for the wing, three-dimensional separation will ultimately occur. If this three-dimensionality is severe, then it will almost certainly affect the airfoil characteristics at the mid-span measuring station. Thus, some knowledge of the three-dimensional stall development on the test airfoil is always essential, especially when comparing the aerodynamic behavior of different airfoils near maximum lift. As shown by various workers over the years, two-dimensional flow is extremely difficult to attain on any wing configuration with any wing aspect ratio when operating near maximum lift. The figure below shows an airfoil specimen being tested in a 2-d insert. The flow is rendered visible by surface oil flow in which titanium dioxide power has been disolved; this gives a white flow pattern on the black airfoil. The photo below shows the flow at the leading-edge of the airfoil (looking downstream). Note the region of laminar flow is terminated by a laminar separation bubble, which is evident from the accumulation of oil in a narrow band spanwise along the airfoil.This "bubble" is broken in places by small specks of undesolved particles of titanium dioxide powder that causes premature transition to a turbulent boundary layer flow. Downstream of the laminar separation bubble, the boundary layer is fully turbulent. The surface shear stress decreases toward the trailing edge, so the boundary layer ultimately begins to separate. For this airfoil, the stall mechanism was by means of the trailing-edge stall mechanism - that is the progressive movement of the turbulent trailing-edge separation point forward toward the leading-edge for increrasing angle of attack.
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