When comparing experimental and theoretical results, you should bear in mind, that neither wind tunnels nor computers perfectly represent the reality. As discussed above, wind tunnels have shortcomings like fixed walls, turbulence, noise and model quality, whereas theory is always based on a mathematical model of the real world - assuming this and neglecting that. A typical comparison is given below; at lower Reynolds numbers the discrepancies can be even larger.
Comparison of theoretical with experimental results.
The figure above shows a typical comparison
between both theoretical analysis methods and experimental
results at low Reynolds numbers. The experiment shows the strong influence
of a laminar separation bubble, which roughly doubles the drag
coefficient. The Eppler code cannot cope with laminar separation bubbles,
but issues a warning for all the calculated data points, saying that there
might be a separation bubble. Drela's XFOIL takes the laminar separation
bubble into account, but under predicts the drag. Thus both methods cannot
predict the drag values at Reynolds numbers below 200'000 if laminar
separation occurs, but hint the user that something might be happening.
My experiences with XFOIL indicate, that it tends to shift the polars to
higher lift coefficients and that the simple panel method (in conjunction
with the spline method) has some problems with leading edges, which often
results in jaggy velocity distributions even for perfectly smooth
airfoils.
It is very, very difficult to make a fair comparison of airfoils at low Reynolds numbers. On the one hand, numerical methods have serious shortcomings, but can be used to judge airfoils under comparable conditions. On the other hand, experiments are not only difficult and time consuming, but can also yield very different results even in the same wind tunnel, due to the three dimensional nature of laminar separation bubbles and spanwise variations in lift and drag. These variations can be much bigger than the differences between different airfoils, thus making a comparison of two dimensional polars, as measured by a single wake rake questionable.
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