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 complicated AUTO modes. With INAV 6.1.1 the default tuning PIFFs (stabilisation parameters) will be good out of the box.

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

Start with a well behaved model that has adjustable push rods.

Important first step 

With the model in Manual Mode, or even before you install the flight control board, adjust the pushrod lengths and connection holes to give reasonable throws and level flight, without needing reduced rates and radio trims. Also establish the correct CG for stable level flight with all gear installed. 

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.

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.

RTH mode is great when you want the model to come back to you automatically or in a "loss of signal" failsafe situation. In the INAV Failsafe screen choose RTH.

MANUAL is a mode that has to be selected, it is not the default mode. 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 mode and 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.

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. 

The above modes are all you need for a basic setup

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

You can enable "Continuously trim servos" to automatically trim servo mid points.

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 and balanced for 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 ARM (essential for ELRS receivers)



Add the fancy modes later



Check the control surfaces are responding correctly to stick movements. 

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

Then switch to MANUAL to check roll and pitch trim and CG. Don't use the radio trims, because they will only work for Manual Mode.

If needed I will land and make trim adjustments by adjusting the pushrods, and CG adjustments by shifting the battery.

Then launch again and repeat until the plane is flying level without needing stick inputs. Don't touch the radio trims.

Use ANGLE to check LEVEL TRIM (deg)

Once the control surfaces are trimmed I will launch again and 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.

INAV trouble-shooting checklist

What to check if your INAV setup is misbehaving

1. Transmitter 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.

Don't use a model setup that you have used previously. Make a fresh blank model in your radio to ensure there are no leftover mixes, trims, logical switches or overrides.

INAV radio setup

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

Connected the receiver to the FC and connect to INAV configurator.

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

Check the receiver is powered on. Some boards provide power to the receiver via the USB, but some need a battery connected to the board. It is safe to connect the flight battery while the USB is connected, but remove the prop or disconnect the motor.

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 plugged in to the correct UART on the board and this UART is set to Serial RX in the Ports page. SBUS receivers usually connect to UART2 with SBUS signal on the RX2 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

I 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. 

Wiring on the Matek F405 Wing is quite different to many other Matek boards.

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. 

7. ESC constantly beeping

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

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