Phase Three of the INAV Plane Tuning Masterclass contains just a few additional things that seem to be unimportant; but strongly impact the flight behaviour of your plane in any automatic flight mode. We will be tuning the INAV autopilot. This includes not only NAV WAYPOINT or RTH but also CRUISE and POSHOLD modes. Therefore it is essential for a reliable FAILSAFE situation. Before you start here, you need to have completed the Phase One and Phase Two tuning guides. Otherwise the navigation modes don’t have a solid base to work with.
Correct board alignment
INAV is very capable of correcting the plane’s unwanted behaviour and imperfections of the physical tune. But it still works the best, if we take advantage of correct aircraft mechanics, to match the base values of INAV. This way the software has less work to do, continuously making corrections, and can focus on actual navigational tasks. Tuning INAV autopilot completes the whole process, and makes INAV as reliable as it can be. Basically we need to make sure, that the Aircraft flies level and keeps altitude by itself, even without the help of INAV Navigation modes. To do that, we need to find the correct pitch angle for your craft at a given throttle value. There are two options how to do that, an easy and a more precise and fast way
First we try to find the correct pitch angle by using ANGLE and ALTHOLD together. You need to have the Pitch Angle and your Altitude shown in the OSD. Do not use NAV CRUISE Mode as this will take over throttle control. Throttle needs to be constant.
- Fly in ANGLE mode with ALTHOLD active.
- Set your throttle manually to the desired cruise throttle value and don’t change it.
- The plane now tries to maintain altitude and controls pitch automatically.
- INAV will hold a specific pitch angle to maintain altitude with minimal pitch correction.
- Watch your pitch angle in the OSD and estimate an average value if it fluctuates a bit.
- Land the plane and go to the Configuration Tab in Configurator
- Add the average OSD value to the current Pitch Alignment value
- Repeat the previous step until the pitch angle averages at 0°.
You can do a similar procedure but control the angle manually. This can be more precise as you don’t rely on any interference from the altitude PID controller.
- Fly the plane again in ANGLE mode but without ALTHOLD active
- Set your desired cruise throttle
- Watch the OSD for changes in altitude (use variometer item)
- Pull back on pitch carefully and try to find a spot where the altitude stays constant
- Check the pitch angle in your OSD and continue the same way as above.
WARNING: If you have installed your FC at 180° backwards, a positive pitch angle in the OSD will need a negative correction in PITCH alignment. If your FC is installed at a 90° or 270° YAW angle, you need to change the ROLL Alignment instead.
- At a 90° Yaw angle, you need to add the OSD value to the ROLL alignment.
- For 270° YAW Installation, you need to subtract the OSD value from the ROLL alignment.
After that, you will immediately recognise that ALTHOLD and POSHOLD will keep the altitude much more precisely than before.
Pitch to Throttle Ratio
The next stage in tuning INAV autopilot involves Pitch2Throttle, or P2T. The P2T ratio is essential for a stable flight in modes that have automatic throttle control. It is absolutely necessary to have this set up right for when you are in a failsafe situation. You are relying on the flight controller to fly the plane. So we must train it to fly your aircraft solo. If this value is set too low, it will likely cause a stall on every climb.
This value works in a very simple way but is extremely important. You can set it in the Advanced Tuning page below the max, min, and cruise throttle settings. You calculate the throttle change with the following formula:
Pitch Angle × P2T Ratio + Cruise Throttle.
Let’s assume you have a cruise throttle of 1400 and your plane pitches up at 10 degrees and has a P2T Ratio of 10
10 (degrees) × 10 (P2T) + 1400 (Cruise throttle) = 1500
So the motor will throttle up to 1500 on a 10 degree climb. If the plane is pitched down 20 degrees (-20)
-20 (degrees) × 10 (P2T) + 1400 (Cruise throttle) = 1200
Therefore the throttle will lower to 1200 on that 20 degree descent.
If the P2T ratio is too low, the plane will stall on the climb. This is because it cannot maintain enough airspeed. INAV will limit the throttle to the values set in min and max throttle.
Tuning the Pitch to Throttle Ratio
Using 3D Cruise
Tuning P2T in 3D Cruise is very easy with OSD but needs a bit trial and error. It is best to do this in NAV CRUISE mode with ALTHOLD (3D Cruise); because cruise has automatic throttle control already.
- Fly in NAV CRUISE mode with ALTHOLD active and check your GPS 3D speed (3D Speed needs to be set up in the OSD, not GPS Groundspeed)
- Pull back the PITCH stick to start a climb (by default the plane will climb at 200cm/s; you can change this in CLI by setting nav_manual_climb_rate)
- When you reach the maximum climb rate, watch your GPS 3D speed
- If the GPS 3D speed decreases, you need to raise the Pitch to Throttle ratio value
- However, if the GPS 3D speed increases a lot, lower the Pitch to throttle ratio
- If the speed stays the same or ideally slightly increases, the tune is complete.
- Repeat the steps till the GPS 3D speed stays the same or slightly increases on a climb.
In normal flight modes with ALTHOLD enabled, INAV uses nav_manual_climb_rate to set the rate of climb. By default this is set to 2m/s.
For fully autonomous navigation modes, like RTH or Waypoints, INAV uses nav_auto_climb_rate to set the climb rate. This is set at 5m/s by default.
You should set the manual climb rate to 5m/s too, to check if your plane can handle that climb speed or lower the auto climb rate down if you have a very large and low powered long range setup. Otherwise you might have still a too low Pitch to Throttle ratio when needed.
Another way to tune this is in ANGLE mode. Naturally, this mode uses manual throttle. For this you need throttle value and pitch angle activated in your OSD.
- Fly level first and set your desired cruise throttle manually with the throttle stick
- Slowly pull back the pitch stick and at the same time raise the throttle to climb
- Keep the pitch pulled all the way back if your plane can handle it and find a throttle value where your ground speed during climb matches your ground speed in level flight
- note down the needed throttle value and the pitch angle the plane has
- land the plane
You can then calculate the exact pitch to throttle ratio by the following formula:
P2T = ((Climb Throttle % - Cruise Throttle %) x 10) / Pitch Angle
INAV Plane Tuning Masterclass – Phase Three Conclusion
After these final adjustments, tuning INAV autopilot is now complete. Your plane should fly as good as possible in any condition. Do some intensive testing of navigation modes, especially RTH with automatic climb and do some POSHOLD tests. The plane should be able to keep height very precisely in calm conditions and maintain its speed when climbing or descending. At this point you can be very confident on long-range flights too, as FAILSAFE should work flawlessly. We hope you enjoyed out INAV Plane Tuning Masterclass. If so, please share this resource and help get everyone flying with INAV experiencing the best setup they can have.