VStabi Pioneer of Flybarless

EVO Firmware V8.2: Micro Heli Optimization Guide

With the current version 8.2 of the EVO firmware, micro helicopters receive exceptionally good support. Since these helis come with a few unique characteristics, I would like to share some tips on how to handle them.

The Difference

In micro helis, a motor is used at the tail instead of a variable-pitch tail rotor. This motor provides the necessary thrust to compensate for the main rotor's torque. However, since motors cannot reverse their direction of rotation very quickly (stop + reverse start), the tail thrust is limited in one direction once the tail motor reaches its minimum thrust. As a result, you can barely get any control authority in that direction.

The Solution

To produce at least a brief burst of thrust in the unloading direction, the main motor can be used to quickly build up some torque, thereby assisting the tail motor. We call this feature TTE (Tail Torque Extension). This means the main motor partially takes over the tail's torque control.

The Problem

With this solution, the main motor becomes part of the control loop that governs the tail. Consequently, the previous relatively sluggish and smooth regulation of the main rotor RPM is no longer sufficient. Instead, it is crucial to gain extremely fine control over the main motor's torque in order to integrate it meaningfully into the control loop.

The Consequence

The demands on the main motor's ESC (Electronic Speed Controller) increase enormously. Furthermore, there is a very strong dependency on how the ESC(s) are parameterized and controlled. You will rarely find a setup that works generically; special adjustments are always required. Therefore, this V8 version includes specific settings for components like XLPower or Logo ESCs. The motors, propellers, and blades used also play a major role!

Recommendation

To largely avoid issues, we recommend using the presets that we have flight-tested. All of these unique characteristics have been taken into account there. This applies in particular to the setup of tail parameters such as TTE sensitivity and Preload. You can easily make personal adjustments to things like the Control Response.

Pro Tips

If you do want to fine-tune things yourself, here is a breakdown of the effects and what you need to look out for:

Tail Gain (Tail Sensitivity)

The gain value is scaled significantly higher than before. A gain of 25 corresponds roughly to 80 in the previous software version (only for motorized tails!). Therefore, always start with low values, such as 10, and work your way up. Values from 25 onward already yield very good tail performance. Too much tail gain leads to wagging/shaking in a hover, or heavy tail slapping under high motor loads. Too little tail gain causes control delays, especially under load (e.g., tictocs with simultaneous tail input).

TTE (Tail Torque Extension)

This essentially determines how strongly the tail demand influences the main motor—i.e., how much it helps with torque management. A starting value of 15 is appropriate; good tails reach 40–45. This value depends on the tail gain: the higher the gain, the stronger its effect on the main motor. Values that are too low result in sloppy tail stops, while values that are too high lead to a permanent drift of the entire tail, even in a hover.

Preload

Preload is a function that proactively increases torque during critical phases in order to generate thrust in time. This results in more pleasant and precise tail starting and stopping behavior. If there is too little preload, the tail stops very hard in one direction but rather softly in the other. Too much preload leads to unnecessary RPM spikes. The starting value is 0, and you can adjust it to 10–20 depending on personal preference.

Pitch Correction

This accounts for the RPM fluctuations on the main rotor caused by the effects mentioned above. With a setting of 0, no correction is performed. If you provoke RPM fluctuations (for example, with a jerky pirouette), you can observe whether the heli climbs or descents. If it climbs, increase the value until it no longer climbs but holds its altitude. You can test this in upright or inverted flight. Ultimately, you will likely end up with values between 8 and 20, depending on your base RPM.

The Governor (RPM Regulator)

The governor also has an impact on the tail, as it now has the task of catching the RPM that was raised by the torque injection. It ensures that the rise/fall remains minimal and that the RPM is quickly and seamlessly brought back to the set value. The output range must be set correctly for TTE to work flawlessly; meaning, the ESC must operate within a medium throttle percentage (opening range) to have the headroom to react to torque demands in both directions.