Please help with retract servo setup. - RC Groups
Thread Tools
Oct 11, 2006, 09:34 AM
Registered User

Please help with retract servo setup.

Hereís the question. How do you change the limits on a ĎRetractí servo? I have 3 HS-75s and the travel is too much for the retract lever. The end point adjustment on my JR6000 Tx does nothing to limit the travel. I know the EPA is working because if I connect a HS-81 to the same channel, I can limit its motion. Even if I connect to the inner-most hole on the servo wheel, itís too much travel. Iíve never set up retracts before. Would using a bell crank with different ratios on the input & output levers work? Thanks for any help.
Sign up now
to remove ads between posts
Oct 11, 2006, 03:04 PM
Registered User
There's no way to change the travel on HS-75 (without opening the servo). It travels 170 degrees, period. The only thing you can do to limit the distance a control rod connected to the servo arm will travel is to change the distance from the center the control rod is connected. In general, to set up a retract servo you measure the distance the retract mechanism requires from one stop to another. Then choose a servo arm which has holes which are the same distance apart. For example, if your retract mechanism requires 20mm of travel, choose holes on the servo arm which are 20mm apart from each other or 10mm apart from the center. For the HS-75, the red servo har has the holes marked in terms of mm from center. Just choose the appropriate holes. If the innermost holes are still to much travel, drill new ones closer in.

Here's an example. The inner holes are 20mm apart:
Oct 11, 2006, 03:42 PM
Registered User
Thanks thunder1. The info and pics are a big help. I think another problem is that the center of the servo pivot is not directly in line with the retract push rod. Will have to look at repositioning the servo. Thanks again!!
Oct 11, 2006, 08:29 PM
Use the 4S Luke
feathermerchant's Avatar
Do this - Hold the pushrod above the center of the servo wheel. Move it all the way to retract and lock the gear. Make a mark where it appears from under the wing sheeting. Move the rod so the gear is down and locked. Mark it again same way. The distance between the marks is the travel you need to run the gear. Find a pair of holes in the servo wheel/arm that are a little farther apart and try those. You can do one side at a time.
Oct 11, 2006, 11:33 PM
Eye Drather Beef Lying
ElectRick's Avatar
Originally Posted by RickAvery
Thanks thunder1. The info and pics are a big help. I think another problem is that the center of the servo pivot is not directly in line with the retract push rod. Will have to look at repositioning the servo. Thanks again!!
I'm assuming you are doing a mains retract setup here.

If I understand your comment correctly, you don't want either individual pushrod to wind up directly over the center of the servo arm while retracted, because they will interfere with each other and bind. The ideal scenario at 'gear up' is both pushrods side by side, nearly touching each other, centered as a pair over the output arm screw. The pushrods being configured this way gives you a sort of mechanical uplock action, because the pushrod cannot be mechanically moved when it's pulling directly across the servo arm, or nearly so.

You also want to make sure there is no binding anywhere in the linkage at either end of travel.

Oct 11, 2006, 11:43 PM
Deletedfor proving Nauga wrong
Because of the 180 deg rotation of the retract servo, it is common to put a SMALL bend in each pushrod to allow for some clearance of the arm when the rods are both "pulled"

If you use the EZ-connectors it takes just a little bend to give the clearance needed. Ball links work as well as the EZ connectors, but are a little harder to adjust. If you use clevices, you will need to carve up a large round wheel to make things work.

Choose the holes on the servo arm that are separated by the exact distance the retracts specify for thier travel. Set the retracts at full "push" position (normally retracts up), and lock the pushrods in position on the servo. Operate the retracts and they should lock down perfectly.

Its all in setup... and choosing the correct holes on the arm.
Oct 12, 2006, 12:38 AM
Registered User
Thanks to all for your input. Your time and comments are a great help! The servos are being used on a P-38, 1 servo per retract so having to deal with the binding issues from two push rods is not a problem. Because of all of your input, I'm now inclined to think that the placement of the servo will have to be moved and then some of your techniques to determine the correct travel can then be applied. Any other tips or comments greatly appreciated. Thanks again.
Oct 12, 2006, 04:10 AM
Deletedfor proving Nauga wrong
Note that if you don't use a retrat servo, you should split the power for the retracts from the RX battery (not a bad idea even with the retract servos)

Take a servo extension or Y and clip the red lead at about 1 inch from the RX end. Splice in a switch harness to the red and black leads to provide power to the servo(s) the red lead being clipped will prevent the RX from powering the retracts, but the - and signal line will still have the servo respond (when the retract battery switch is on)

This way, if there is a problem with the retracts that drains the battery, it won't be your RX battery. You can land with the retracts up with little damage. Its pretty hard to prevent damage if the RX loses power.
Oct 12, 2006, 07:26 AM
Use the 4S Luke
feathermerchant's Avatar
OK another solution to the accidental draw due to retract servo binding-
I power my retracts from teh same UBEC that powers the rest of the radio gear.
Before the first flight of the day, I connect my Wattmeter between the battery and ESC. I run the gear up and down. If there is any amp draw after the gear stops, I know something is binding so I fix it.
I have had the gear hang in flight. Not fully retracted. But the UBEC was happy to supply the servo with whatever current it drew plus run the other radio gear.
Oct 12, 2006, 12:27 PM
Deletedfor proving Nauga wrong
UBEC pulling from the motor battery ... the motor stops before you run out of RX power, the airplane is gonna tell you to land. (prop stops)

I'm one of the "odd birds" that does strange things such as powering the RX from a separate battery from the motor battery, because sometimes I play around with thermals after the prop stops. It can be an interresting challenge to see how long you can keep "heavy metal" flying without the motor running. But playing that game and powering the RX from the motor batt... is not a good idea.
Oct 12, 2006, 01:17 PM
Use the 4S Luke
feathermerchant's Avatar
I hear you. The plane is the H9 P-51. It won't be flying 1 min past LVC!
I fly the bigger planes on a timer anyway. Never hit LVC.
Let's do some math. Motor pulls ~1000W. Servos, receiver and UBEC what 20W?
So the motor battery will never notice the rc gear. The real issue is if the UBEC quits or the battery goes open you have an 8lb freeflight. OTOH, if you are using a separate battery and it quits or the switch fails, same thing.
There is a dual UBEC option and you could power one from the flight battery and one from another battery.....
Oct 12, 2006, 02:50 PM
Deletedfor proving Nauga wrong
I do like to cover all the potential failure modes... You can't 100% guaantee everything will work right every flight. But you can cut your chances of failures taking out the whole system.

Also... I have more flight time using glow... so I tend to think of RX setups in terms of glow power.
Oct 14, 2006, 12:34 AM
Speed Demon
GregG's Avatar
One way I've found to drive both main retract linkages the same distance yet have a great range of adjustment is to use a separate bellcrank. Drive the retracts directly with a bellcrank and drive the bellcrank from the retract servo. You could then use one of the adjustable servo arms on the retract servo to ultra fine tune the throw.

The pic below shows a parkflyer with very limited room and this type of linkage setup. The servo wheel would have been too wide and the wire linkage would have stuck out if I tried to drive the retracts directly unless the servo was exposed outside the wing. In this instance the retract driving wire linkage was hidden.

Click on the picture for the full sized image

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Please help with hh gyro setup! whitty7628 Micro Helis 3 Dec 28, 2006 11:19 PM
Discussion please help with brushed foamy setup for casual flying Crash n' Sniff Power Systems 5 Jan 11, 2006 11:54 PM
Please Help With Retract Door Mechinisims wannabe Fuel Jet Talk 5 Oct 17, 2005 08:14 PM
Please Help With 8U Radio Setup For CCPM Chasvo Electric Heli Talk 1 Jul 18, 2004 02:35 PM
Question Please help with Eco 8 Setup sjgusmc21 Electric Heli Talk 4 May 08, 2004 05:20 AM