Monday, August 20th, 2012 10:21 am GMT -7 Monday, August 20th, 2012 10:21 am GMT -7Monday, August 20th, 2012 10:21 am GMT -7
Airplane tail horizontal stabilizer elevator control surface

There are many ways of hooking up two servos to one elevator, but some work much better than others.




Description of the Problem

I got an email from a website visitor with a problem, asking if I could help. He had a new very fast flying RC model airplane. The maiden flight had gone great.

Others at the flying field looked over the model and recommended that he use two servos to drive the elevator instead of one. Looking for an easy solution, he decided to hook up the two servos using the transmitter’s V-tail feature.

Setting them up this way caused a problem. The two servos were now also being controlled by the rudder transmitter stick. I presume the model did not already have a rudder. Even worse, the email continued, moving the rudder stick caused the elevator halves to move the wrong way.

He had two questions for me. First, how could he reverse the direction that the rudder stick moved the elevator halves? Second, how could he turn off the rudder stick action entirely?

Not Ganged Servos

It was clear from his email that he was not using the two servos to drive one control surface. Ganged servos like these are found sometimes on giant scale models. There are additional issues that come up when you use a configuration like this. The key is to closely synchronize the movement of the two servos so they do not end up fighting each other as they move the control surface. Since this was not relevant to the writer’s questions, I won’t say anything more about it right now.

Reversing the Rudder

The fix for the reversed rudder action is easy. Just swap where the two servos are plugged in the receiver. That is assuming that the two servos move in the same direction when they are controlling the elevator action.

Disabling the Rudder

There are many ways to have a control stick not move its corresponding control surface. Setting the travel volume to zero in both directions is the most logical. You might be able to also set the exponential movement to zero percent. A similar effect could be achieved by using dual throws and setting the travel percentage to zero for both switch positions.

Convoluted Solution

Did my proposed solutions for disabling the rudder stick sound nutty? Good.

Calling the two elevator halves a V-tail is just bound to cause problems. Why? Because it is not a V-tail. You are telling the transmitter that you have a rudder, when in fact you do not. To fix it, then you have to tell the rudder stick not to do anything.

I guarantee you that next year this modeler will have completely forgotten about all the contortions he went through in setting up his model. There are better ways to do this, my friends.

Simplest Solution

By far the simplest solution would have been to use a Y harness and plug in the two servos to the elevator channel of the receiver. No transmitter programming is required. Quick, easy, and maybe even lighter.

Programmable Mix Solution

Using a Y harness may not be ideal. The biggest downside is that you lose the ability to adjust each servo separately. You may need to set the center position of each servo individually, for example.

Yes, I know that using a programmer for a digital servo would be a way around this limitation. But few of us do this, and I’m not sure I would recommend it, anyway.

A better solution than using a V-tail mix is to use a programmable mix. No, not all transmitters  have these. How you use them might be different on your transmitter, too. Here’s the general idea.

First, plug in the second elevator servo into a spare channel on the receiver. There might be restrictions on which channel you can use. The best bet is to use a channel labeled “aux”.

Second, tell the transmitter that you want to use one of the unused programmable mixes. Set the elevator stick to control this secondary channel. To start out with, you want a 100% direct relationship between the stick movement and the servo. You can adjust it later as needed.

This is a relatively simple, flexible, and logical solution to the problem.

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