Bill Freeman wrote:

Paul,

Many of the props out there are using old fashioned
airfoils, with the Clark Y being the most common,
apparently.  The Clark Y is most famous for being the wing airfoil
for the Cub.  It is also interesting to point out that Mr. Taylor's
(the Cub designer) next design was the Taylorcraft, which despite
being side-by-side rather than tandem like the Cub and using the
same Continenal 65 as the Cub was substantially faster in cruise
due to Mr. Taylor discovering that the Clark Y was a *very*
draggy airfoil.  The T'craft does everything a Cub will do, except
being a lot faster (IIRC, roughly 10-12 mph faster - a LOT in
this class of A/C).

I did not have the coordinates for the Clark Y, but for comparison, I
have used a NACA 2412 which is visually similar to the Clark Y.
I compared the coeff of drag for three different airfoils at various Mach
numbers.  All these were for a 2.5 inch chord airfoil to represent the
tip section, with the Clark Y standin being 12% thick, and then two of
NACA's special prop airfoils, the 16-212 (12% thick) and the 16-206
(6% thick).  The difference is quite dramatic, even at relatively low
speeds.  Remember the Cub vs T'craft.

This is why many wood and composite props that use "conventional
looking" airfoils are pretty inefficient.  Most of the serious go-fast
guys have figured this out, and Klaus is making money off of it.  I'll be
providing the airfoils for Perry, so the blades for the prototype CS
prop should be much better than the std blades being built for many
wood and some composite props.

Bill

Mike, here are the coordinates and a gif to show you what the 16-206 looks like.
Bill is right. This does look like the airfoil Klaus is using.

Paul Lamar

 NACA 16 SERIES AIRFOIL M = 2.0, T/C =  6.0
      50.       50.

 0.0000000 0.0000000
 0.0010273 0.0018797
 0.0041050 0.0038848
 0.0092204 0.0060641
 0.0163526 0.0083174
 0.0254721 0.0106085
 0.0365416 0.0129143
 0.0495156 0.0152169
 0.0643407 0.0175004
 0.0809560 0.0197507
 0.0992932 0.0219542
 0.1192770 0.0240977
 0.1408253 0.0261684
 0.1638496 0.0281534
 0.1882551 0.0300399
 0.2139417 0.0318149
 0.2408037 0.0334654
 0.2687309 0.0349785
 0.2976083 0.0363416
 0.3273175 0.0375422
 0.3577362 0.0385685
 0.3887396 0.0394097
 0.4202001 0.0400558
 0.4519885 0.0404982
 0.4839742 0.0407302
 0.5160258 0.0407466
 0.5480115 0.0405375
 0.5798000 0.0400897
 0.6112605 0.0393934
 0.6422638 0.0384435
 0.6726826 0.0372398
 0.7023917 0.0357877
 0.7312692 0.0340979
 0.7591963 0.0321868
 0.7860584 0.0300764
 0.8117449 0.0277939
 0.8361505 0.0253715
 0.8591747 0.0228456
 0.8807230 0.0202567
 0.9007068 0.0176480
 0.9190441 0.0150649
 0.9356594 0.0125543
 0.9504845 0.0101634
 0.9634584 0.0079393
 0.9745279 0.0059279
 0.9836475 0.0041737
 0.9907796 0.0027197
 0.9958950 0.0016083
 0.9989727 0.0008837
 1.0000000 0.0006001



 0.0000000 0.0000000
 0.0010273 -.0022725
 0.0041050 -.0037977
 0.0092204 -.0052084
 0.0163526 -.0065028
 0.0254721 -.0076950
 0.0365416 -.0087986
 0.0495156 -.0098252
 0.0643407 -.0107845
 0.0809560 -.0116845
 0.0992932 -.0125317
 0.1192770 -.0133309
 0.1408253 -.0140856
 0.1638496 -.0147978
 0.1882551 -.0154682
 0.2139417 -.0160961
 0.2408037 -.0166796
 0.2687309 -.0172156
 0.2976083 -.0177003
 0.3273175 -.0181289
 0.3577362 -.0184962
 0.3887396 -.0187965
 0.4202001 -.0190245
 0.4519885 -.0191749
 0.4839742 -.0192430
 0.5160258 -.0192248
 0.5480115 -.0191107
 0.5798000 -.0188861
 0.6112605 -.0185393
 0.6422638 -.0180620
 0.6726826 -.0174501
 0.7023917 -.0167038
 0.7312692 -.0158277
 0.7591963 -.0148312
 0.7860584 -.0137283
 0.8117449 -.0125371
 0.8361505 -.0112795
 0.8591747 -.0099805
 0.8807230 -.0086678
 0.9007068 -.0073705
 0.9190441 -.0061185
 0.9356594 -.0049410
 0.9504845 -.0038661
 0.9634584 -.0029187
 0.9745279 -.0021200
 0.9836475 -.0014858
 0.9907796 -.0010250
 0.9958950 -.0007371
 0.9989727 -.0006077
 1.0000000 -.0006001


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