Shielded or twisted wire for yaw and pitch servos and where to ground the servos

[RV14_TD]

I'm installing yaw and pitch servos in RV10. Vans specifies shielded wire for data connection to the servos. Dynon's installation manual (figure 140) shows twisted pair(s). Q: is there any practical reason to use a shielded wire there?
Another Q: Would it be OK to ground locally the servos or is it better to run a ground wire to the front of the aircraft

 

[Rhino]

Shielded or twisted wire is for noise reduction. Twisted wires usually suffice, which is why Dynon uses them, but shielded wire does a better job, which is probably why Vans specifies it. It's your choice. You can err on the side of caution and use shielded wire, or stick with twisted. Every plane is different, so no one can guarantee you a level of performance for either method.

The ground is kind of the same scenario. You could probably get away with grounding it locally, but you reduce the chance of noise if you use a central ground. You should absolutely not ground it locally if the shield for your data cable is grounded at the servo. Ground loops are discussed more here:

Ground Loops and Noise in Aircraft Systems

Quite a few forum threads and articles have at various times addressed ground loops, and their potential to cause noise or other interference in your avionics system. Simply put, a ground loop occurs when different paths to ground are used. The difference in resistance, or impedance, on those...

forum.flydynon.com

The big problem with servo wiring is, once the wing is closed up on most aircraft, you can't go back in to fix problems later without major effort. That's why I always err on the side of caution with wing wiring. But it's your plane. Only you can make the final decision as to which path to follow.

 

[RV14_TD]

Thx a lot!
If I would choose the shielded wires for the servos, where should I ground the shield?
At the servo, at the front grounding point, both?

 

[airguy]

Vans may specify shielded for the autopilot servos, but I think thats just a generic "catch-all" for good clean signals to the device. You will notice, if you look, that there is no such thing as a servo with Vans logo on it - so Vans cannot really say what is, or is not, needed for any particular servo. In this case I think I would pay attention to the people who actually MADE the device, and, you know, might actually know something about it...

Just sayin'...

 

[Rhino]

Look at the link I posted. You should have a common ground point somewhere. You may have separate common ground points for your power and your audio/data system. For the DSUB cable, I would ground the shield to the network hub, which in turn should be grounded to your common ground point.

 

[Rhino]

Vans may specify shielded for the autopilot servos, but I think thats just a generic "catch-all" for good clean signals to the device. You will notice, if you look, that there is no such thing as a servo with Vans logo on it - so Vans cannot really say what is, or is not, needed for any particular servo. In this case I think I would pay attention to the people who actually MADE the device, and, you know, might actually know something about it...

Just sayin'...

I tend to agree. But in places like the wing, where you can't easily open it up to fix problems, I prefer to err on the side of caution. But every builder decides their own path.

You could satisfy both methods, and run twisted wires inside a shield ground.

 

[RV14_TD]

Vans may specify shielded for the autopilot servos, but I think thats just a generic "catch-all" for good clean signals to the device. You will notice, if you look, that there is no such thing as a servo with Vans logo on it - so Vans cannot really say what is, or is not, needed for any particular servo. In this case I think I would pay attention to the people who actually MADE the device, and, you know, might actually know something about it...

Just

Vans specifically refers to Dynon servos in several instruction manuals
i.e. OP 58-14

 

[PaulSS]

Dynon servo kit does not have shielded cables.

 

[Rhino]

That's what airguy was pointing out.

 

[Carl_Froehlich]

What I did on all four builds:
- Four #22 conductor shielded wire for the four network connections.
- One single strand #20 wire for power.
- One single strand #20 wire for ground.
- One single strand #22 wire for autopilot disconnect.

Similarly I did a four #22 conductor shielded wire for the ADS-b receiver (mounted aft near the ADS-b antenna):
- Two wires for the serial connection.
- One wire for power.
- One wire for ground.

Carl

 

[airguy]

I built all my own cables as well, using twisted pair instead of shielded for the network connections, no issues in just over 3000 hours and 4 different airplanes.
I'm not thinking there is a real need for shielded over twisted pair, as long as you keep your data cables separated from heavy power and radio coax.

 

[Rhino]

I'm not thinking there is a real need for shielded over twisted pair...

I'm not either. That was just a joke.