RE: Hitec 5645 servos, Are they good servos?
It's the same servo as a 645 non-digital. Those don't center worth a darn.
All the 5645 is--is a digital circuit board. That's got nothing to do with the actual POT that centers the servo. I wouldn't use them if you expect any type of precision.
I'm a JR guy, so I'll stop telling the truth about those Hitecs now. Don't want to hurt anybodys feelings.
You want a servo that centers? Buy a JR or Futaba.
You don't need digitals on that airframe. It's only 74" And you don't need metal gears either. Thats a "trend" thats caught on in recent years. Do you know what the guys were flying on their 35% birds 15yrs ago? Standard nylon geared servos. They just used 4 of them on a rudder and 2 of them on an elevator.
If I was going to rig that 74" plane with a gas engine, I'd put JR4721 coreless servos on it. They are 120oz torque and will center everytime. Use a 6v battery and you got a 150oz coreless servo thats just about bulletproof on a smaller airframe like yours.