Dear "J..."

I'll try to uphold Steve's confidence! <G>

First off, in order for an autogyro rotor to "autorotate" the blades of the rotor must be producing lift with a minimum of profile drag. So symmetrical airfoils can be made to work, but there are, as Steve says, better alterrnatives.

The Clark-Y is a good airfoil. In fact a regular Clark-Y profile that is made a bit thinner than the "correct" pattern is probably a very good choice. I believe the regular Clark-Y has a max thickness of something like 11.72%. It has a zero lift angle of attack of -3.51 degrees.

The wind tunnel tests that Les Garber and I did a couple of years ago revealed that the SG6042 produced superior lifting performance. And flight tests seem to bear this out. This airfoil has a very slight undercamber toward the trailing edge. (The Clark-Y is flat from about 10% back from the leading edge all the way to the trailing edge.) The SG6042 has a zero lift angle of attack of -4.92 degrees and is 10% thick.

Using a Clark-Y profile that is 10% thick should give very good performance and the blade is easier to make since it doesn't have the slight undercamber.

Eric Sanders' Compufoil program will give you the profiles of these airfoils and you can manipulate the thickness too.

The wind tunnel information is summarized in an RC Modeler article published in the August 2001 issue pp 70-76.

Hope this helps.
Bill