Aerodynamics and Design for Ultra-Low Reynolds Number Flight

2.2.4 Comparison with Experiment
There are a small number of relevant experiments in the literature that provide a
Chapter 2
reasonable basis for comparison with INS2d at the Reynolds numbers of interest. One
interesting result of the current work, to be discussed in greater detail in Chapter 3, that
is supported by experiment is an increase in attainable lift coefficient as the Reynolds
number is reduced. Thom and Swart [15] tested a small R.A.F. 6a airfoil model in an oil
channel and water channel at Reynolds numbers below 2000. They observed large
increases in lift coefficient at fixed angles of attack as the Reynolds number was reduced
from 2000 to almost one.
Validation of the computational analyses is difficult due to the almost complete absence
of experimental data at relevant Reynolds numbers. The Thom and Swart experiment is
based on a 1.24cm chord airfoil with manufacturing deviations from the R.A.F. 6a. This
small test piece was hand filed to shape, causing the measured geometry to vary across
the span. An exact validation is not possible due to the unknowns in the section
geometry, but comparison with computations for the R.A.F. 6 airfoil with a 256 by 64
grid show reasonable agreement with experiment. No coordinates for the R.A.F. 6a
could be located, but the R.A.F. 6 appears to be nearly identical. The results are shown
in Figure 2.3. The Reynolds number varies from point to point and ranges from Re=650
to Re=810. The computed drag is on average 7.5% lower than experiment, but the trends
in C d with angle of attack agree. Corresponding C l data is only given for α=10.0. The
computational result matches the experimental value of C l=0.52 within 3.0%.
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