Sunday 25 June 2023

Simplified INAV

How to simplify INAV model setup and start flying sooner.

The key to simplicity is having a plane that is trimmed and balanced well mechanically, and not starting with the confusing AUTO modes. With INAV 6.1.1 the default tuning PIFFs (stabilisation parameters) will be OK for most normal performance planes.

Auto Launch, Auto Tune, Auto Trim and Missions will greatly complicate your first INAV experience. It is much simper to get the model flying well first then add the fancy stuff later.

This article is intended for fixed wing pilots who know how to mechanically set up a model for correct throws, trims and CG. 

Start with an easy to fly model that has adjustable push rods.

Important first step 

Before you install the flight control board, adjust the pushrod lengths and connection holes to give reasonable throws. This can be done with a servo checker or using your radio and PWM receiver. 

Aim for reasonable throws using 100% servo travel because INAV manual mode uses 100% weight and 30% expo by default

Then fly the model to check aileron and elevator trims and CG placement. To adjust trims you need to land and adjust the pushrod lengths mechanically without touching the radio trims.  

Radio trims should never be used in INAV because they will be ignored in stabilised modes but active in manual mode.

Doesn't have to be perfect and it's OK if the plane is a little too agile with these full throws, as long as it is flyable.

Once the trims are close to correct you can enable "Continuously trim servos" in the Configuration page. This will continuously save fine adjustments to the servo midpoints for level flight in Manual.

Essential INAV Modes

The only modes you need to set up initially are ACRO, RTH, MANUAL and ANGLE 

ACRO is the default INAV mode and is active if no other mode is selected. In ACRO the plane is stabilised against any un-commanded rotations, like a side gust of wind. The model will tend to hold its attitude but respond normally to your stick inputs. ACRO is arguably the best general flying mode.

MANUAL is a mode that has to be selected, it is not the default mode. No mode means ACRO, not Manual. MANUAL is used to check trim and CG balance, and for safety if something is wrong with other modes. Experienced pilots may prefer to fly in MANUAL mode.

ANGLE is the fully self levelling stabilised mode. It is a mode by itself, but it is also active when any of the nav or GPS modes are used. In this simplified INAV setup it is also used to check board pitch trim. Model airplanes usually need a few degrees of nose-up angle of attack to maintain level flight. This trim setting can be found in the PIDS page - Mechanics Tab - LEVEL TRIM (deg). I usually start with +4 degrees then check if the plane is rising or descending in ANGLE mode and adjust as required.

Flying in ANGLE mode may feel odd to experienced pilots because you have to hold the sticks at the angle you want to fly at and it will self level when you centre the sticks. 

RTH mode uses GPS data to automatically fly the model back to the home location. It can also be set as the Failsafe action, to bring the model back home if RC signal is lost. In the INAV Failsafe screen choose RTH.

The above modes are all you need for a basic setup

Non-essential INAV modes to add once you have a working model

NAV LAUNCH is fun to play with but I prefer to launch normally with full control. It is easy to muck up the switch sequence and cause a failed launch. 

CRUISE and LOITER modes are useful for FPV flights but not essential.

AUTO TUNE / AUTO TRIM are not required if your model is mechanically trimmed with correct CG.

Mode switches

It is best to have your starting switch positions with no modes selected, which means the board will always start in ACRO

Here are my modes for the simple setup

CH 5 (2 position) Nothing - ARM (essential for ELRS receivers)

CH 6 (3 position) Nothing - ANGLE - MANUAL

CH 7 (2 position) Nothing - RTH

Add the fancy modes later

CH 8 (3 position) Nothing - NAV CRUISE - NAV LOITER

Note that when all switches are in the "Nothing" range the board will be in ACRO 


Check the control surfaces are responding correctly to stick movements. Do the High 5 check.

Check the control surfaces are responding correctly for stabilisation. Switch to Angle Mode and check Left wing lifted makes left aileron raise and right aileron go down, tail lifted makes elevator raise.

ACRO Throws

Check the control surface throws in ACRO Mode. They may be too small for sufficient control. Ideally they should be about 80% of the Manual Mode throws. If the throws are too small go to the PID Tuning page and increase the FF parameter for Roll and Pitch, then check throws again. If there is no difference between ACRO and MANUAL Mode throws reduce the FF parameters. 

This will ensure you have enough control to launch in ACRO Mode and some headroom for stabilisation.

First Flight adjustments

On the first flight I will launch in ACRO and fly a few circuits to make sure the plane is flying OK. 

Switch to ANGLE mode. Take note of whether the model holds altitude or climbs or descends. If you haven't entered anything in the Fixed Wing Level Trim window then the plane will most likely descend.

I usually start with +4ยบ and adjust more or less from there.

Launch again, climb to about 50m, fly out a bit then try RTH. Your model should turn and fly back to the arming site and circle above you at about 50m altitude and radius of 75m.

If that all works then you are ready to continue your INAV adventure.

INAV trouble-shooting checklist

What to check if your INAV setup is misbehaving

1. Radio setup

The model setup must have no mixing, no rates, no expo, no flight modes and no trims. Just 100% inputs for the first 4 channels. 

The plane-type selection, mixing, rates and expo are set up in INAV, not the radio.

GOLDEN RULE - Don't use an existing model setup and edit it for INAV. Make a fresh blank model in your radio to ensure there are no leftover mixes, trims, logical switches or overrides.

Configure your model like this -

Ch1 - 100% Aileron 

Ch2  - 100% Elevator 

Ch3 - 100% Throttle

Ch4 - 100% Rudder

Ch5 - usually Arming switch (specially for ELRS)

Ch6 Ch7 Ch8 Ch9 etc - Mode switches

The same model works for flying wings, conventional planes, twins, V-Tails and even quads

Connect your receiver to the designated Serial RX UART on the FC and connect the FC to your computer. 

Check that the channel bars in the receiver page move to the right when the 4 sticks are moved up and right.

If a channel bar moves the wrong way, invert the channel in your radio. This makes sure the stabilisation will work in the correct direction.

Note that the Roll Pitch Yaw Throttle bars do not match the channel order of Ch1 Ch2 Ch3 Ch4. As long as the stick inputs give the correct response all is good.

2. Receiver connection

If the channel bars don't move at all when you move the sticks - 

Check receiver is actually bound correctly to the INAV model in your transmitter. Use a servo/signal checker if in doubt. 

Check the receiver is powered on. Not all boards provide power to the receiver via the USB plug. Some need a battery connected to the FC.

Check your receiver is actually producing a serial signal like SBUS, iBUS or CRSF. Some receivers need to be switched from PWM to Serial output. 

Check the receiver is plugged in to the correct UART on the board and this UART is set to Serial RX in the Ports page. 

SBUS receivers often connect to UART2, with SBUS signal on the RX2 pin or the designated SBUS pin.

For ELRS receivers check Receiver RX is connected to UART TX and Receiver TX is connected to UART RX

Check the correct Serial Receiver Provider is selected in the Receiver page - CRSF for ELRS and TBS, or SBUS for FrSKY

3. Control surface movement

If a control surface moves in the wrong direction when you move the sticks - 

For control surfaces on a normal plane you can invert the channel in the Outputs page 

For control surfaces with mixed inputs, like Elevons or V-Tails, reverse the Weight in the INAV Mixer page for the offending control surface mixer line. For example - If the left elevator on this V-Tail is going down instead of up, change the Stabilised Pitch weight from 50 to -50 for S3

If you need to reduce the throws, enter a lower number for the weight in the INAV mixer or Outputs page

4. GPS

If the GPS icon is greyed out or red -

Check the GPS is connected to the correct UART.

Look in the Ports page for the UART with GPS selected.

Check "GPS for telemetry and navigation" is turned on in the Configurations page

Connect the GPS to the recommended GPS UART (or any spare UART) and select GPS as the sensor on that UART in the PORTS page

Normal UART connection - G to G, V to 5V, RX to TX, TX to RX

Compass connection is not required for fixed wing INAV.  GPS can provide all the required heading data. A poorly calibrated compass will cause problems.

Check the GPS has power. Some boards can power the GPS through the USB, others may require the battery to be connected

Once it is connected correctly and powered up the GPS icon (top of configurator window) should turn blue and the Total Messages number (GPS page) should start counting up.

Now it's just a matter of placing the GPS with a clear view of the sky and antenna facing up, and waiting for satellites to be acquired. This can take from a few minutes to over 10 minutes if this is the first connection.

5. Connections

If you are still having problems it's time to go over your connection again.

Check your receiver, ESC, GPS and servos are plugged in where they are meant to be and the right way around. Look at the wiring diagrams on the product page.

Things that often catch me out are -

SBUS-Out from the FrSKY X8R and X6R receivers are in different places.

Changing from SBUS to ELRS on the SpeedyBee F405 Wing, also have to change from UART2 to UART1. 

Layout of the pins on the Matek F405 Wing is quite different to many other Matek boards.

Also check for bent pins and solder bridges.

6. Correct firmware target

Check on the product page for the correct firmware target for your board. 

The target name is sometimes not obvious. For example for the Matek F405 Wing firmware target is MatekF405SE. 

With incorrect firmware, some functions may work OK but no all.

7. ESC constantly beeping

If your ESC beeps continuously it either means your receiver is not connected or the ESC needs calibration.

Make sure you have the correct protocol selected for your ESC in the Outputs page. If in doubt leave it at Standard.

ESC calibration in INAV

Connect the ESC and motor to the board

Remove the prop! Disconnect the flight battery

Go to the Outputs page

1. Slide the "I understand the risks..." button to the right

2. Move the Master slider to the maximum 

3. Connect the flight battery and wait for the ESC calibration beep

4. Slide the slider to the minimum and listen for the ESC calibration done beeps.

Test the motor spin up by carefully raising the Master slider just a tiny amount

8. Motor will not arm

If a GPS is connected then at least 6 satellites and 3D position lock are required for arming.

If you are not using GPS then the Failsafe setting must be Land or Do Nothing, because RTH will prevent arming.

If all the control surfaces are working but the motor won't arm, look along the bottom of the INAV window for Arming flags or error messages. 
9. Servo voltage

It is safest to leave the servo BEC voltage at 5V for widest compatibility. If you have changed to 6V or 8V on the servo BEC make sure your servos can handle it.

Check that all your servos are functional. One burned out servo will stop all of them working.