Hermann- check the tracking on the blades. Bring the throttle up as much as possible without going into a hover and look from the side. If one blade is higher than the other, adjust by bringing the low blade up to the other with the linkage. Don't lower the high blade because that will put the blade into negative pitch. The tape is probably causing additional drag and since you have one dinged blade you are probably getting more resistance. Check the thread of bdavison's blade build if you haven't read it yet, and also bladecprepair.com, lot's of great information.