Thank you. That is the formula that I was looking for and I've printed it out for later this evening. I was told by a (very) knowledgeable person that the tip speed should be somewhere near, but below, the speed of sound, generally around 18,000 fps. He told me that if the blades were not starting to howl that the tip speed was not fast enough for efficient and clean cutting. Some of you may laugh at this, as it sounds a bit anal, but I'm just the messenger and I don't know either way. Right now, though, I believe I need to get my engine rpm higher as it is running about 300rpm too low. Then again, am I really going to realize any noticeable difference? I guess I'll find out.