Hello,
I have 4 Futaba 3010 servos in my GS WM P-51D. I am using one for each flap and one for each aileron. I have large Dubro control horns and my clevises are installed one inch from the center of the output shaft on the servo. The clevises are Sullivan and are a nice snug fit into the servo and wing control horns. These servos are new with the plane and have about 20 flights +/- on them.
I have some free play at the output of each servo. Measured about .050" at the clevis for the servo end. The servo arm is tight on the servo. I can see the output has some slight looseness or backlash. The .050" is measured at the end of the servo arm and this becomes about 1/8" at the end of the control surface (aileron or flap).
Is this normal or do I have some wear in the servo geartrain. One flap has slightly less play than the other 3 control surfaces which has me thinking that there are varying degrees of wear taking place. This plane is fairly fast and I am concerned about control surface flutter. I fly a large pattern and may not hear the flutter until it is too late. Any thoughts or ideas would be appreciated. Thanks.

Any 'play' is a bad thing, but there has to be a little for the geartrain to work properly. It does sound like your servos' gears are a bit loose. You might try replacing the gears on one of the servos to see if it tightens up a bit.