RC Groups

RC Groups
    Electric Plane Talk
        Separate battery for a retract servo? How?

#1 tim hooper May 04, 2003 04:47 PM

Separate battery for a retract servo? How?

I'm thinking about fitting retracting u/c to my next proposed project.

I keep hearing horror stories about stalled servo's draining all the current from the main batteries, so is there a diode or sumfink so I could use a separate small cell pack just to power the u/c.

Just disabling the BEC won't work as the stalled servo scenario will still kill the radio gear.

Any thoughts before Bri shifts this thread elsewhere?


#2 Tony Oliver May 04, 2003 05:22 PM

1 Attachment(s)
Is this what you want Tim?


#3 Paul Willenborg May 04, 2003 06:08 PM


Is the project big enough to carry an honest-to-gosh retract servo? One of their big advantages is that they "time out" if stalled and turn themselves off. Although I still think a seperate battery is a good idea.

At least this was true in the past. These days it seems to be hard to find anything but the most superficial information about servos.


#4 Andy W May 04, 2003 07:44 PM

Re: Separate battery for a retract servo? How?

Originally posted by tim hooper
Any thoughts before Bri shifts this thread elsewhere?


#5 Magna May 04, 2003 08:20 PM

Do you really need to use retract servo? assuming that travel is properly set and no binding?

What happen if you use normal std servo. You just basically swing the servo arm from one end to the other, does this really need retract servo? WHats the differences?

#6 sluufuut May 04, 2003 08:42 PM


I have installed retracts on my 1/2A P-51 conversion using a Hitec HS-85BB. The advantage here is the ability to set EPA to eliminate the binding/stall. I am pleased with this setup.

#7 Paul Willenborg May 04, 2003 08:59 PM

>>...the ability to set EPA to eliminate the binding/stall.<<

>>...assuming that travel is properly set and no binding?<<

All well and good until one leg gets a little bent when it hits a bump or the plane swerves badly on takeoff. Then it binds and 30 seconds later the BEC goes into thermal overload and the plane crashes. A single stalled servo may draw several amps.

Do you cycle your landing gear and check them after EVERY landing? You'd better.

The difference is (or was) that a retract servo knows it should only operate for a short time and turns itself off even if jammed badly. It's a shame there are not any micro retract servos. They would not be difficult to manufacture.


#8 gkamysz May 04, 2003 09:50 PM

Retract servos have a geneva mechanism that locks the servo into the end of it's travel. At this point the motor is turned off and you can not move the ouput arm regardless of how hard you try. The motor will not turn back on until it is commanded to go to the other extreme of travel. Retract servos are two position, 180 degree travel servos and have mechincal locks in these positions.

I don't think there are actually any retract servos that shut down after a given time period, not the one's I've played with anyhow.


#9 Paul Willenborg May 04, 2003 10:57 PM


The old Futaba low profile (136G?) retract servo would shut off after just a few seconds if stopped in an intermediate position. Pretty sure the equivalent JR was the same. I'm not sure about current versions.

#10 KillerWatt May 04, 2003 10:58 PM

You could install in the retract wiring, a slo-blow fuse rated at about double the amp draw for the normal servo involved in the retract system.... at least under any abnormal current condition, the blown fuse should auto-disconnect from the flite pack any let the other chans work normally........... kw

#11 leccyflyer May 05, 2003 03:07 AM

I'm not moving this anywhere- lots of useful info in there, the diagram has found it's way straight onto my hard disk.

Hoops with retracts???? It'll be rivets next, you mark my words ;)

#12 Tony Oliver May 05, 2003 07:04 AM

Battery system
What I forgot to say was that I used this system in my F117 - our field is rough, and there's every chance that even a touch-and-go will end with a non retracting retract system.

The added weight is negligible - a second 500mah rx pack is more than sufficient. I used a 280mah pack with no problems.


#13 mkirsch1 May 05, 2003 09:20 AM

All you have to do is disconnect the red wire going from the retract servo to the receiver.

Here's what I would do: Get a short quality Y-harness, a standard switch harness, and your retract battery. At the receiver end of the Y-harness, extract the center pin from the servo plug by carefully lifting the plastic catch with an exacto knife. Insulate this pin with heat shrink tubing to prevent any shorts. Now, just connect the battery and switch harness to the one leg of the Y-harness, and the retract servo to the other.

#14 GregG May 05, 2003 01:51 PM

1 Attachment(s)

One of these days I'm going to have to learn how to do line drawings on the PC. For now please forgive my crude manual drawing.

The way I've tackled this problem before on my Twin Bushwacker used one switch to operate a 4.8V pack for the RX and a 6V pack for the retracts. The switch opened and closed the negative lead in the radio system on both packs. The four channels of flight control were operated as you would normally do them. The retract servo on the other hand got the + from the 6V pack and the - and signal wires from the retract channel on the RX. Hope this makes sence.

#15 William A May 05, 2003 02:22 PM

Greg, try this.


All times are GMT -5. The time now is 05:01 PM.