I just purchased an ARF that requires one (1) each servo per gear. How are these servos electronically set-up? I'm new to mechanical retracts but not pneumatics. Please help.

Assuming you're using something like HS75's or RT88's as the servo's, there's no adjustment: they turn 180*. You put the servo arm/pushrod connection mechanism on each retract, then adjust the servo arm/pushrod to produce the required amount of travel to activate and lock the retract in the up/down position. Then you plug each servo into the end of a "Y" connector, and the other end of the "Y" into your retract channel. Each servo will rotate the same direction, so the only thing you really need to check before tightening up all the screws is that each both retracts are opening/closing when activated. If one is going the "wrong" direction, move it's connection to the other side of the servo arm.

Thank you for the information. Is it adviceable to run a seperate battery pack for the retracts?

If you can, yes that is a good idea.

Have the servos powered by that battery pack.

This way if there is binding or a problem with the retracts, they will not consume your flight pack's batteries.

Note that some retracts do NOT require "retract servos". It really depends upon your plane and TX.

Retract servos move to two somewhat fixed positions. This is great for retracts that use an interconnecting wire to drive two retracts off a single horn or horn/wheel. In some cases retract servos may be mandatory.

However retract servos not be necessary for retracts driven off a single servo, where you can control the end points and movement of the servos via a computer controlled TX.

I have various planes that use inexpensive standard non-retract servos on the retracts. All operate w/o problems and with newer TX's you can control the movement speed of the servos to get very realistic retract movement... You can't do this with retract servos.

That depends on a lot of things... which retracts, how big the wheels are you're retracting, and what else might be going on with the radio system when you've activated the retracts. Personally, I don't think it's necessary, provided your retracts and servos are adjusted properly. At the end points, retract servos should be drawing essentially no power. When I was testing with the retracts on my H9 Bluenose, just for fun I put a wattmeter on the battery, and a pair of RT88's drew about .7amp when operating. I had a pretty healthy 4 cell pack powering everything, and the voltage dropped down to around 4.6V for about 5 seconds. But if you have a older battery, and are flying 3D manouvers while activating the retracts, or you're going to do multiple touch and goes with the retracts cycling each time, it's conceivable that you could pull the voltage down enough to create a problem.

If I was flying a Spectrum 2.4M setup, I'd run a 6V battery just to avoid a potential voltage drop that might "brownout" the receiver. That will also move the retracts faster, and give you more torque on the servos. The other alternative would be to add a second battery running in parallel with the main receiver battery. That adds capacity, and is simpler than figuring out the circuitry necessary to power the retracts with a separate battery.

There's a slew of people here who advocate separate packs for retracts.

If your plane can afford the weight of a small pack for the retracts, why risk it?


But it's up to the pilot.

If you are as careful as FallDownGoBoom is, you wouldn't need to use a separate pack at all. His safety checks are sound and advisable.


However people who tend to ask this question tend not to be that experienced with all of this, so the safer course is better advice to the novice.

Gauge your own experience level to decide.

In the past I have only used "Robart" pneumatic retracts however the model has the mechanical retracts already installed. In order to install pneumatics, modifaction to the strut channel and whell wells would be necessary. "Robart" struts and wheel assemblies are not light. I thought I could attach the strut to the strut wire and modify the channel. Then the question arose could the retract servo, retract the weight of this assembl, either statically of dynamically? A pneumatic assembly would negate this issue.

Why not put one of the retracts in a vise, attach the strut and using a fish scale measure the amount of pull required to move the mechanical retract.?
Factor in 40% more for safety and see if that will be handled by your servo.

I suspect you will not have problems with most medium torque servos.

If the mechanical retract does not bind anywhere, very little force is required to move the retracts.

The mechanism locks, at either extreme, so as long as the servo holds the mechanism in place the mechanical retracts do not put strain on the servo.... only during servo movement.

What "gets" you with mechanical retracts, or any retract for that matter, are things like debris or metal particles that can lodge in the wrong place preventing arm movement or actuator movement.

I see some people claim that pneumatics are far more reliable, but IMHO if both are kept clean with free movement of the mechanism, operational reliability should be about the same.

I'll take your advise and conduct these tests. Better on the bench.

Hey Flycatch, in case you need to "reverse" a servo; check out [img]file:///C:/DOCUME~1/Admin/LOCALS~1/Temp/moz-screenshot.png[/img]<![if gte mso 9]><xml><w:WordDocument><w:View>Normal</w:View><w:Zoom>0</w:Zoom><woNotOptimizeForBrowser /></w:WordDocument></xml><![endif]><span style="font-size: 12pt; font-family: "Times New Roman";">http://www.youtube.com/watch?v=EK_FdRTenl0</span> When using standard off the shelf servos (not a reverse servo) and mounting requires the gears facing each other, you'll find this handy.

You do not need to reverse a servo. I've used the H-9 retracts in two different planes and the RT-88 servo works just fine as is

Flycatch:

One thing the might help this entire discussion would be if we knew what airplane, which retract servos, and which manufacturer's retracts are under consideration. When I framed my original answer it was aimed sort of at the generic mechanical retract environment. I agree with Mike, you shouldn't have to reverse a retract servo... but there's alway a "gotcha" that may come up.

Information can be either positive or negative. The retracts are already installed in this aircraft, they are OEM. The plane in question WM P-47D 1/6 scale. The overall weight of the plane fully loaded should between 11-13 lbs. Due to the weight factor I wanted to install pneumatic retracts but this will entail extensive modification.

On that plane even strong Mini-servos will do for the retracts... the clearance height is the only issue.

I'd use medium torque standard low profile servos to be safe however.


Rememebr that the servo moves the retract, but it does not hold it in place once locked.

Try pulling the rod to either extreme and you'll see that the retract will not move once locked.

If you set things up as the manual shows, you will not need to put each servo on a separate channel nor use a reverser.... even though the control arms have to move in seemingly opposite directions.

[link=http://www.theworldmodels.com/para/instruction/pdf/A213.pdf]Manual for this plane here..[/link]