Hi,
OK, the original blades for the Grace were just flat stock with a very negative pitch, the same designed used for windmill blades for pumping water. For running a mechanical water pump that was good because the speed was limited by the drag and it has good starting torque. However, that is not good for autogyros.

Your Clark Y provides fairly good lift but doesn't start well so the blades require a little shim on the trailing edge of the blade hub to get them started. Usually a couple layers of masking tape is all that is necessary. However, the SG6042 profile blade is much better because it can be mounted flat, with no shims, and provides more lift. If you have Clark Y blades made from wood, you can convert them to the SG6042 by sanding them a little thinner and sand a small area on the bottom the full length just in front of the trailing edge.

Phil