Trimming outputs

Introduction

This actually comes from working with RC yachts. So I start with an explanation for those not familiar with RC yachts. Others may skip to the next section.

Most RC yachts have two sails, a mainsail and a jib. Each is attached to a boom at the bottom. The boom has one end attached to something solid, and the other end free to swing with the wind, to the limits determined by a line called a sheet. The length of the sheet - and hence the angle the sail will take in the wind - is controlled by a servo or winch that you control remotely.

Generally, the sheets for both sails are controlled by a single servo, as the two sails having the same angle works fairly well. However, the ability to trim the two sails independently can result in better performance. So some classes of model yacht racing allow for a one servo per sail. You still want one stick to control both sheets, but each servo to have a different trim setting.

The goal

While there are instructions on the web for modifying a stock transmitter to allow a single stick to control both the jib and mainsail sheets, with each having it's own trim, the results seem less than optimal. The control stick runs diagonally, and you wind up losing the other direction for that stick, meaning you can't control the rudder and sheets with one stick.

Given a modern computerize transmitter, you can easily have one stick control two output channels. With deviationTx, you can also have the two channels have separate trims. In particular, you can have the ability to have one trim that trims both sails and a second that only trims one, just trim one of the two sails without trimming the other, or independent trims for each sail.

Trimming outputs

The default settings for the trims in deviation are to trim inputs. However, these can also be set to trim arbitrary inputs, or even outputs.

To get started, create an empty model, and set it to four channels. We're going to assume the protocol order is Elevator, Aileron, Throttle and Rudder. Go to Model Config > Mixer, click on the Simple button next to ELE, and change the input to THR. This will make the output of channels 1 and 3 follow the throttle stick, and the elevator stick is ignored. You can check this on the Transmitter > Channel monitor page.

We'll now create the three trim models discussed above, with three simple changes.

Second servo output trim

Right now, the throttle trim buttons will trim both channels, and the elevator trim buttons. like the elevator stick, do nothing. Let's fix this. Go to the Model Config > Trims page, and select the ELE button, and then change the Input to Channel 1, or whatever channel the elevator stick output went to originally. Now the Elevator trim buttons will change channel 1 but not channel 3. The Throttle trim buttons still change both.You can verify this on the Channel Monitor page.

Single trim

Next, go to Model Config > Mixer, and select the Simple button next to THR. Change the type in the top row to Complex, and then select Trim to change it to No Trim. This disables the Throttle input trim on this channel, so there is no way to trim the Channel 3 output. Channel 1 is still trimmed by either the Throttle trim or Elevator Trim buttons.

Split trims

Now let's enable the Throttle trims for just channel 1, not both 1 and 3. Go back to Model Config > Trims, and select the THR button. Change the input from THR to Channel 3 or whatever channel originally got the THR output. Now the throttle trim just controls Channel 3, not both channels, and the elevator trim controls channel 1. Note that this happens even though the Channel 3 mixer has trim disabled!

This change will work if the Channel 3 mixer was not changed as in the Single Trim section above.