Keep Your Plane Flying Level Without Manual Trim Adjustments
For those of you who fly with the FrSky Taranis, or a radio utilizing the OpenTX Platform, I want to share a quick little function to add to your electric fixed wing models that will automatically adjust elevator trim proportional to the speed of your model. Are you constantly having to change your elevator trim to keep your model flying level? If so, this Quick Tip is for you.
Normally, the best technique to trim an RC model’s pitch, so that it maintains level flight at all speeds, is to adjust the motor’s thrust angle. That is why most properly designed RC models have a few degrees of down thrust built-in. After a few flights and maybe an adjustment or two to the thrust angle, you can usually correct any wild changes in pitch caused by changes in speed. By doing this, your plane is put into mechanical trim.
Your radio also has a digital trim which allows you to fine tune your model’s behavior. If your model is out of mechanical trim; however, using digital trim won’t work well because every time throttle position is changed, elevator trim will be affected.
Unfortunately, on a recent Crash Test Hobby Pinata I built, after trial and much error, I couldn’t get its mechanical trim right. Because the Taranis is such a programmable radio, I decided to compensate for my Pinata’s bad behavior by creating a mix that would add a small amount of down trim as the throttle increased. My solution worked, but it was complicated, and could only be tweaked by going into the radio settings and adjusting a curve.
I posted my mix on a Tarainis Forum to ask for a better way and someone responded with this SIMPLE function. I didn’t write this myself. I wish I thought of it but I didn’t. I also wish I could properly credit the person who helped me, but I can’t, so I’m simply passing his solution along because it worked so well for me.
What it Does
This Quick Tip will show how to create a simple mix, that can be adjusted in flight using your radio’s elevator and throttle trim, that will keep your plane flying level at all throttle positions.
- It will only work for fixed wing aircraft, powered by an electric motor.
- Quick changes in throttle will cause your models to become slightly out of trim until its speed catches up.
How It Works
- Trim your model to fly level at cruise speed.
- Advance to full throttle and adjust your throttle trim up or down until your plane flies level.
- Return to cruise speed and set your elevator trim to neutral.
- Advance your throttle back to full and adjust your throttle trim once more till you are happy.
- Cut your throttle and adjust elevator trim up or down until your plane glides level.
That is it. Once it is set, this function will proportionally adjust elevator trim to your throttle setting. Your throttle trim switch is now your high speed pitch trim and your elevator trim switch is your low speed pitch trim.
I prefer to use OpenTX Companion on my computer, so if you haven’t already done so, download and install it now. For instructions on how to install OpenTX Companion, visit OpenTX University. The screenshots below were created using OpenTX 2.1.9 firmware. If you have installed another version, your screens may be slightly different.
Step 1: On the Inputs tab, select your Throttle Channel, In my case [I3]Thr and select No for Include Trim. In my case, I am using my Advanced Throttle Safety, so I’ve selected NoTrim for both Throttle Input lines.
Step 2: On the Mixes Tab, select an unused channel and create a two line mix.
On the first line, set Name to Trim and Source to [I3]Thr. Weight and Offset should both be set to -50.
On the second Mix line, set Source to TrmT, Weight to 100, and Multiplex to MULTIPLY.
Step 3: Add the new mix to your Elevator Channel. In my case, my Pinata is a Flying Wing using Deltawing Mixing, so I have to add it to both wings.
Add a line under your Elevator MIx. Set Name to Trim, Source to CH11, and Weight to 12. This is a good starting point. It will be the Maximum +/- value that you can set using the Throttle Trim Switch. If your plane needs only a small amount of correction, decrease this number for more resolution in the switch.
I hope you find this Quick Tip beneficial. It certainly was for me. This Variable Proportial Elevator Trim Function is simple to program and even simpler to use. I’ve shown you how to use it for Elevator Trim. Some pilots who fly models with large props may find this tecnique beneficial for their Rudder Trim as well. Let me know below how you like and use it.