INAV VTOL Tailsitter

INAV 7.1 now has VTOL capability, thanks to INAV dev Shota Hyashi 

With Mixer Profile switching and a Transition mode we can now change between horizontal and vertical board alignments on the fly.

It is not yet perfect, lots of experimentation with PIDs and rates is required to make each different model work smoothly.

INAV VTOL doc - https://github.com/iNavFlight/inav/blob/master/docs/VTOL.md

Tailsitter initial setup tips

At this stage (INAV 7.1.2 June 2024) Tailsitter is not available as a platform choice so you need to enter this CLI command - Set tailsitter_orientation_offset = ON

This allows the board orientation to be horizontal for Mixer Profile 1 and vertical for Mixer Profile 2 with a 45º pitch forward transition mode.

Start with a normal fixed wing INAV setup. This will be Mixer Profile 1 / PID Profile 1.

Turn on "PID Profile will use same index...." so that the mixer and PIDs will match when switching Profiles.

Change to Mixer Profile 2 and PID Profile 2 and choose Quad X as the Mixer Preset.

Again turn on "PID Profile will use same index...." 

Later we will need to alter the Quad X mixer settings depending on the fixed wing model we are using. 

I am testing the AtomRC Theer wing and AtomRC Penguin.

In the Modes page - Configure Mixer Profile 2 and Profile Transition Modes using a channel which is operated by a 3 position switch.

Low channel value enables Profile 2 (VTOL), middle value adds the Transition Mode, high value switches to Profile 1 (level flight).

OK that covers the basics for all INAV tail sitters.

Now we need to make changes depending on the particular model, and play with the PIDs 

AtomRC Theer Tailsitter settings 

13th June 2024 - My Theer Tailsitter is now working OK for takeoff, hover, transition and level flight. I am having problems switching back to vertical and remaining stable. 

Profile 1 - use a normal twin motor flying wing mix

Profile 2 - Use Quad X mix like this screenshot, but make the changes below

In VTOL flight Motors give thrust and roll control.

Elevons give pitch and yaw control

Delete motors 3 and 4

Change Motor Roll values to 1, -1. Adjust directions as required to give the correct stabilisation direction..

Change Motor Pitch and Yaw values to 0, 0

Add elevon servo mixes and adjust directions and weights as required

This is the resulting Tailsitter mix

Initially the Theer did not have enough pitch stability action to keep it vertical so I added more elevon area. 

The preset Quad PIDs are incorrect for servo control so it's best to use values closer to Airplane PIDs for pitch and yaw, then adjust from there. 

For more elevon action in Angle mode I increased the pitch P value until the servos started shuddering, then backed off a bit. 

Then I increased the Level Pitch P value for more aggressive Angle mode pitch stability. 

set mc_p_level = 40 (0 to 255) Default value is 20

Here is my latest Theer VTOL Diff_All file Use with caution.

AtomRC Penguin Tailsitter settings (Not yet tested)

Profile 1 - normal mix for a twin motor plane without rudder

Profile 2 - Quad X but needs the changes below

In VTOL flight Motors give thrust and roll control,

elevator gives pitch control and ailerons give yaw control.

Delete motors 3 and 4

Change Motor Roll values 1, -1. Adjust directions as required to give the correct stabilisation direction.

Change Motor Pitch and Yaw values to 0, 0

Add elevator and aileron servo mixes and adjust directions and weights as required

Try increasing the Level Pitch P value for more Angle mode pitch stability

set mc_p_level = 40 (0 to 255) Default value is 20

In VTOL mode Elevator controls Pitch as usual but Ailerons change to Yaw

My YouTube VTOL Playlist 

WARNING - The INAV VTOL code is still being developed and is not yet complete. 

My tailsitters are still fairly unreliable and I haven't added GPS yet.  More tweaking is required. 

Taranis Q X7 Buddy Box Setup

How to connect two FrSKY TaranisQ X7 radios for shared Instructor and Student control

The Instructor and Student radios are connected via the DSC sockets using a 3.5mm stereo audio cable (TRS style) 

Link to 3.5mm TRS cable

Note that RadioMaster radios require different cables. 3.5mm TRRS (Smart phone style) for "Old" radio and TS for "New" radios. Link to RadioMaster Trainer Cable information

FrSKY TaranisQ X7 trainer setup

Instructor radio (Master)

In the Model Setup page - Set Trainer Mode to Master/Jack

Mixer page (Adjust weights and expo as required. Best to be the same as in the Student radio)

Ch1 100 Ail E30

Ch2 100 Ele E30

Ch3 100 Thr

Ch4 100 Rud

In Special Functions page - Select a switch, Select TRAINER and STICKS and tick box

Bind Instructor radio to the receiver

In Radio Setup  > TRAINER page

Ail := 100 CH1

Ele := 100 CH2

Thr := 100 CH3

Rud := 100 CH4

Student Radio (Slave)

In the Model Setup page

Turn the Internal RF module OFF

Set Trainer to Slave/Jack

Ch Range 1 to 8 (just has to be more than 4)

Mixer Page (Adjust weights and expo as required)

Ch1 100 Ail E30

Ch2 100 Ele E30

Ch3 100 Thr

Ch4 100 Rud

Check direction of throws on both radios and adjust or invert channels as required

Best Radios for INAV 2024

This advice applies to non INAV models as well. The only difference is you would use receivers with PWM outputs for servos rather than a Serial output for a flight control board

INAV only requires a very simple model setup in the radio, just the 4 control channels plus a few mode switches. 

You can get by with just about any radio system including RadioMaster, FrSKY, FlySKY, Futaba and Spektrum as long as the receiver can output a serial signal using a protocol like iBUS, SRXL2, SBUS or CRSF.

My preference is for radios that use the EdgeTX operating system and ExpressLRS RF protocol. EdgeTX is the most flexible and programmable operating system and ExpressLRS gives the most secure and long range RF link.

Spektrum and FlySKY operating systems are less flexible and more locked down than EdgeTX.

Below are my top suggestions for a good INAV radio in 2024

Note: EdgeTX, INAV and ExpressLRS are all RC Community developed OpenSource projects so they are often updated and  will require a little more research and experimentation to master than traditional locked down systems like Spektrum.

ExpressLRS Radios

1. RadioMaster Boxer ELRS  (or Boxer 4in1 with ELRS RF Module) 

The Boxer is a very reasonably priced radio with plenty of switches. One momentary, one locking, two 2 position, two 3 position and one 6 position switch as well as two pots. It has a full sized JR RF module bay. The only things missing are sliders on the sides. There is a large battery bay for 2S LiPos.

2. Zorro and TX16S are good options too, although I am not a fan of the colour touch screen version of EdgeTX on the TX16S. 

My daily radio is actually the Zorro 4in1 with Ranger ELRS RF module. The game controller form factor is not to everyone's taste but I like it. It uses 18350 Li Ion batteries which are hard to find and have less capacity. But I like the side sliders for panning cameras and also crow braking on gliders. 

The TX16S would suit someone who wants all the bells and whistles including a large colour touch screen. But I find it unnecessarily big and heavy.

3. FrSKY TaranisQ X7 with external BetaFPV or RadioMaster ELRS RF Module

This is FrSKYs cheapest radio and it was my favourite for many years. I love the rotary selector dial and overall feel.

It only binds with FrSKY ACCESS and D16 receivers unless you add an external ELRS RF module.

ELRS Receivers

Any ELRS receiver can be used with any ELRS radio or module

RadioMaster RP1, RP2, RP3, RP4TD ELRS or any of the PWM receivers with Serial output enabled

BetaFPV - ELRS Nano, Lite, SuperD and Micro PWM or SuperP PWM with Serial output enabled

ELRS gear has amazing range, at least 5km even with the nano receiver versions. Or you can think of it as very secure RF link for short range even with obstacles. The diversity receivers like SuperD and RP4TD extend the link security even further for extended long range or close range penetration.

SBUS Radios

1. RadioMaster Boxer 4in1 

All RadioMasters radios come in different versions. CR2500 for one chip multi-protocol (Mainly used for RadioMaster and FrSKY receivers), 4in1 with 4 chip multi-protocol, or ELRS.

2. Zorro and TX16S are also good options. ELRS external module can be added

3. FrSKY TaranisQ X7 (only compatible with FrSKY ACCESS and D16 receivers)

4. FrSKY Tandem and Twin radios (Higher quality hardware but more expensive and limited to FrSKY receivers)

SBUS Receivers

FrSKY X6R, RX6R

RadioMaster R81, R86C

FlySKY iA6B using iBUS or SBUS

My YouTube Playlists to help with the learning curve. It's worth the little extra effort of watching a few videos on the relevant subjects

RADIOMASTER Playlist

INAV Playlist

ExpressLRS Playlist

OpenTX / EdgeTX Playlist

ExpressLRS for Planes

ExpressLRS is an Open Source RF system with amazing long range and low latency. Flight distances of more than 10km are easily achievable using basic ELRS gear and low power transmission.

It was originally designed to work with multi rotors and flight control boards and uses the CRSF serial receiver protocol.

More recently PWM receivers have been released which allow normal servo connection direct to the
receiver without using an FCB. This makes ELRS a viable system for normal line-of-sight planes and gliders.

However it is essential to understand that ELRS is quite different to the traditional RF protocols like FrSKY ACCST / ACCESS or Spectrum DSMX. To achieve the low latency and solid connection over long range the ELRS devs have optimised the data packets to be as short as possible and included safety checks. 

Below are some of the compromises required for ELRS to function properly

Setup selections are made using the ELRS Lua script on your radio, and some of the default options must be changed for fixed wing models.

Channel resolution

To keep data packages small only the essential data is sent with every packet. This means with some Packet Rates not all channels are full resolution.

In the ultra low latency modes Ch 1 to 4 are full resolution, while Ch 5 is only 1 bit (low or high) and all other channels act like 6 position switches.

However more recently, lower frequency Packet Rates have been added, which do give full resolution on all channels and are recommended for fixed wing use. 

Best settings for fixed wing models are:

Packet Rates 100 Full or 333 Full with Switch mode on WIDE.

However Ch5 should not be used for anything but arming, even though it can be full resolution.

Channel 5 requirement

To function safely ELRS needs to know when the model is armed or flying. So it looks at the data from Ch5, which is sent with every data packet, to check the PWM value.

Ch5 needs to be in the "High" state, or PWM value above 1500ms, to show that the model is armed or flying. 

When Ch5 is high the buttons on your ELRS RF module are locked to avoid accidental mid air changes, which could cause a loss of control. 

Dynamic Power only works when Ch5 is High. Dynamic Power continuously adjusts the RF transmitting power to the minimum required.

So Ch5 should be set as the arming or throttle cut switch or set as high, and not left unassigned or used for control surfaces or throttle. This applies to any model, even if they don't require arming, including motorless gliders.

Here is a great explanation by RC-Soar - https://rc-soar.blogspot.com/2023/08/an-introduction-to-elrs-including.html

ExpressLRS on GitHub - https://www.expresslrs.org/quick-start/getting-started/