Kline-Fogelman Airfoils - Figures and References
If the KF airfoil is new to you and you're interested in a little of the history of it, this wiki page has a good summary of all the details:
This post will be updated from time to time and has figures of the variations on the Kline-Fogelman airfoil (Kline-Fogelman modified or KFm) as they have been and are being used by many RC builders. The figures for the various KFm airfoils include a comment about the recommended wing thicknesses expressed as a percentage of the root chord.
Using the KFm3 as an example, the recommended thickness is 9-12%. That would be the recommended range for wing thickness at the thickest point and the thickness is a percentage of the root chord.
The KFm3 wing has two steps, the first or 50% step would be at the thickest part of the wing, and the 9-12% distance would be the distance from the wing bottom to the top of the 50% step.
On a KFm3 wing with with an 8"/203mm chord the thickness would be 9-12% of 8"/203mm. And that point would be at the first step. And 9-12% of the 8"/203mm measurement means that the wing thickness at the first step could be in the range of 0.72-0.96" or 18.3-24.4mm.
To continue the example, if the KFm3 wing were built with 6mm thick FFF foam, there would be three layers of foam at the first (50%) step and two layers at the second (75%) step. And the wing thickness would be 18mm at the first step and 12mm at the second step.
Those percentages and the resulting step heights are approximate and can vary, your materials really decide exactly what you will wind up with for wing thickness and step heights.
The attachments include Rich Thompson's excellent comparison study of the KFm1 to KFm4 airfoils to Symmetrical, Clark-Y, and Flat Plate airfoils.