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Servo and Radio Discussion
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You will need to use a decent servo. I have used both analog and digital servos with direct drive. Both work well. If you are interested in Direct Drive, then you are looking for the ultimate in precision and resolution. It would not make sense to go to the trouble of setting up a Direct Drive installation and then use a Futaba 3004 servo. This base line servo has a very large dead region (distance you have to move the stick off neutral before the servo moves), and low torque.

Direct Drive puts all the torque to the air, but you can still underpower a control surface. If you try Direct Drive and it does not feel rock solid, then the servo is not strong enough. The surface will “blow off” from where it is being commanded by the force of the airstream. The plane acts strange, much like the symptoms of a tail heavy CG.

The order of best resolution to worst goes as follows:

1) Digital servo with PCM receiver. This is as good as it gets!

2) Analog servo with a PCM receiver. Good precision, but the servo will “drift” a few degrees back and forth for a second after the transmitter stick is stopped. This is most noticeable at the center and end points.  The analog servo also does not hold a position very aggressively, and can be pushed several degrees off the commanded position.

3) Digital servo with an FM receiver. This setup is a waste of money. The servo fights hard to hold a level of precision that an FM receiver cannot provide. The position of the control surface will never be consistent with an FM receiver. Bump the stick in one direction and back to center and the control surface will stop in one place. Bump it the same way in the opposite direction and the surface will stop in a different place by a degree or two. This is FM drift. If the transmitter stick is moved very slowly through its travel, you will not see the control surface move smoothly, but rather it will “jump” in noticeably large increments. This is FM drift combined with the very high resolution of a digital servo.

4) Analog servo with an FM receiver. This is recommended over option 3. The analog servo’s lower resolution will actually “smooth out” the jumps mentioned above caused by the FM receiver.

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