When are Servo extensions/ runs to long

[thisguy65]

I have a freewing F-18c that I am re-wiring to by pass the blue box

The leads for the wings need 24” extensions (aileron and flaps). Lead off servo is 6” + a 4-6” y cable to bring the wings servos to one channel for ailerons and flaps. Rudder and elevator will have close to 40-42” distance.

While do all this I couldn’t help but think, am I setting my self up for failure by having such long leads.

Logically With the blue box I could run extensions from the box up to the front of the nose and still have the same run, if not longer.

So this brought up the questions, is there a general rule for when extensions/ runs of servo leads are too long? My extensions are the thicker gauge wire to tone down the resistance, it’s more of a “make me feel better” kinda thing.

[fhhuber]

The 2.4 ghz systems are not as sensitive to the long leads issue as 72 mhz.

A long wire can act sort of as an antenna, absorbing the RF signals of all of the radio systems operating nearby. With 72 mhz the frequency of the absorbed signal could feed back to the RX and scramble the RF decoder. We used small circuits between the servo wires and the RX t filer this out. The circuit also boosted the pulse signal to the servo, making up for the resistance of the long wire.

I made my circuits using an inverter gate IC chip, inverting the signal twice. With 8 gates on the chip and each pair of inversions being able to serve 1 channel for 2 servos on a Y, 1 chip could serve 4 channels for up to 6 ft between RX and servo.

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We learned a better way was to put one RX in each wing and 1 or 2 in the fuselage, just feeding power to the multiple RXs using a single battery. With just long twisted pairs of power wires, we were not feeding the inducted interference to the decoder.

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2.4 ghz does not seem to induce the feedback to the decoder and the 2.4 gh RXs seem to resist the issue from being a problem if people are flying 72 mhz nearby.

ALL radio systems are subject to issues due to bad connections between RX and servo. That breaks up the pulses fed to the servo resulting in random and unpredictable servo movements.

[xviper]

If you're choosing to eliminate that blue box, the actual wire run shouldn't be any longer than what's there already. Think about it, from the wing board, which you'll eliminate, you'll get rid of that multi-wire cable that goes to the board. If you replace all that with extensions directly to a "Y", then to the RX, the actual length can be the same, assuming you pick extensions of the appropriate length (24" seems excessive to me - the actual distance from the wing board to the RX is way shorter than that). As for the wiring from the tail of the plane, those wires are already there. You simply unplug them from the board, put on a "Y" for the elevators and hook up directly to the RX. The "Y" should be no longer than the existing run from the board to the RX. Same for the rudder and nose steering, if you want them "Y'd" together. You need no "Y" if you plug tail rudder directly and the steering directly to a separate channel for retract up steering deactivation. If you select your extensions carefully, you may actually shorten the servo wire run. But like what's already been posted, length of servo wire run isn't a concern anyway.

PS, I measured from the furthest tail servo (elevator) to where my RX is as about 25". I'm not sure where you get 42". And like I said, those wires are already there.

[thisguy65]

Yes from a logical stand point didn’t matter but when increasing a rather smooth wires problem coukdjust covering my bases:)

25” extension + 4” lead + 12” extension to get to the nose. Rx is setup like Hellnatgnofury on the nose



EDIT: Thinking about this again, could put the rx close to where the blue box was and still have access to adjust the pots. This would lessen the wires dramatically. Wings would be 12”. No extensions needed at that point other than few y harness. But may be hard to see the pots on the rx

[xviper]

Yes from a logical stand point didn’t matter but when increasing a rather smooth wires problem coukdjust covering my bases:)

25” extension + 4” lead + 12” extension to get to the nose. Rx is setup like Hellnatgnofury on the nose

I see. My RX is behind the battery, very close to the blue box.

[thisguy65]

I see. My RX is behind the battery, very close to the blue box.

Going to see how much room I still have if push the rx back further. What I didn’t like is the esc/ battery wires being on top/close to the rx. With the stuff removed should clean up the airframe nicely.

[fhhuber]

Pulling most of the functions off of the blue box, and eliminating the (very nice...) single plug units at the wing roots, will eliminate several connections in each servo lead between RX and servo. Each connection is a potential failure point and bypassing all of the blue box and wing root connections is a trouble-shooting step in finding out why a servo is acting strange.

I tend to just use the blue box or multifunction board for lights and retract&door sequencing.