Tuesday, 24 October 2017

iNav fixed wing tips

October 2017 - Betaflight is the most recent firmware for flight control boards (FCBs) and iNav is the GPS enabled
re-write of Betaflight which also caters for fixed wing models.

All the setup is done using the Chrome App called iNav Configurator. Works on PC and Mac as long as you can locate the correct drivers for the board you have. All the information on iNav is kept in the iNav Wiki on GitHub

I'm quite new to all this auto pilot stuff and there is a steep learning curve, so I thought I could pass on what I have learned so far.

So far I have experience with the Paris Sirius AIR3 F3 board with GPS from Multiwiicopter

and the Omnibus F4 V1 board and Ublox Neo M8N GPS module from Banggood.

The value of the AIR3 board is that it comes mounted in a gel damped housing with all the connectors and GPS sensor, and it's pre-loaded with the latest stable iNav firmware. Most of the tricky stuff is already done for you.

The AIR3 is an F3 board and doesn't have a built in OSD, but an addon OSD is available. The AIR3 OSD is configured using the MWOSD Chrome App, and partially by using your transmitter sticks.

If you're handy with a soldering iron and want to do it all yourself, you can follow Matt Ogbourne's video series on the Omnibus board setup. You will have to do all the soldering, firmware flashing and setup yourself. The Omnibus F4 does have a built-in OSD, which is configured in the iNav Configurator.

About Flight Control Boards
The connections to FCBs are a little more tricky than normal ESC/receiver/servo connections because FCBs are more sensitive to incorrect or fluctuating voltage supply. With voltage spikes, FCBs can shut down, brown out or reboot causing total loss of control of the model.

It is highly recommended that the board and servos are powered by an accurate and stable 5.0V BEC like the AIR3 iNav BEC. You need to make up a power distribution cable so the BEC can plug into the board, and power the servos separately. As the 5.0V is being supplied to the FCB by the BEC you must disconnect the 5V line (red wire) from the ESC. Not a good idea to have two power sources for the FCB. Then all the signal wires from the servos and ESC connect to the board. The ground wires can either connect to the BEC or the board.

Another recommended precaution is to plug a voltage smoothing capacitor into your FCB, like this 3300uF 25V from MultiWiiCopter. The capacitor connects to any spare output pin-set across the power and ground pins, and absorbs any voltage fluctuations. Can also be plugged into receivers for added safety.

If you want to live dangerously you can just plug the servos and ESC straight into the board. like you would with a receiver, but you are only one voltage spike away from a crash.

Transmitter setup
You do not use any mixing, rates, expo or trims in your transmitter.
Just program in 100% for each stick and whatever Mode switches you need.

My setup on the Taranis is:
Ch1 100 Aileron
Ch2 100 Elevator
Ch3 100 Throttle
Ch4 100 Rudder
Ch5 Switch E (Arming)
Ch6 Switch A (Air, Horizon and Angle modes)
Ch7 Switch D (Loiter and Return to Home modes)
Ch8 Switch C (Pass through and Launch assist modes)

iNav tips
There are three types of Modes available in iNav.

Full control modes

Passthrough Mode
Normal flying with no stabilisation, only the Expo value in iNav is used. If something is going wrong with the FCB or GPS switch back to this mode to take control.

Air Mode
Slightly stabilised, full control. Always active when no other mode is selected

Stabilisation modes 
These are Gyro and Accelerometer driven modes that rely on the board being level when the plane is in level flight attitude. Most planes fly level with the nose a few degrees up, so the board may need to be tilted down at the front, either in the mounting position or in iNav setup board alignment. Actually what we need is the board mounted level with the airfoil chord line a few degrees up. It varies with different wings and different flying speeds so to get this right you need to fly the plane and make appropriate adjustments to the board alignment in the field, without using Transmitter trims. See Plane Trimming Tips below.

Angle mode
Fully stabilised, bank and pitch angles limited. Can't do loops and rolls in this mode.

Horizon made
Starts off like Angle mode but allows greater control as the sticks are moved further.

Launch Assist Mode
This is a magic mode. Select Launch Assist, arm the board, advance the throttle to a good launch percentage (the motor will not spin up yet) throw the plane and the motor will spin up 0.5 sec after the throw acceleration is detected. The plane will climb for 5 sec then switch into any other mode you have selected, or until you move the sticks. I have used Launch Assist and RTL combined, so I can can launch and let the plane circle above without picking up the transmitter. A big crowd pleaser.

It's important to get the switch sequence right for Launch Assist to work.

GPS modes
These are GPS controlled modes, and they can be combined with the Stabilisation modes.

Altitude Hold
Does what it says, holds the flight path at the Altitude when selected. Very useful when combined with Angle or Horizon modes for level and stable flight paths.

Position Hold
The plane will circle around the point where the mode is selected, at the radius selected in iNav setup. Relies on a properly aligned FCB for altitude. Best to combine with Altitude Hold mode for a very useful Loiter mode.

Heading Hold
Uses rudder to hold the flight path to the heading when selected. Combine with Altitude hold for fixed heading and altitude flight path.

Return to Launch (RTL)
This is what it's all about. Enter this mode and the plane will fly back to where it was armed, at whatever altitude and throttle percentage you have setup in iNav, then circle overhead at the iNav setup altitude and radius (or even land automatically if you're game)

Failsafe RTL
For me this is the main reason for using iNav. Failsafe can be setup to go into RTL mode, meaning that if you lose radio contact with your plane it will fly back home.
On FrSKY X series receivers I set the failsafe to No Pulses, then in iNav - Failsafe RTL - Don't Land.

Important plane trimming tips
The correct way to trim your plane is to adjust the push rods and servo arms mechanically so it flies level and the control surface throws are OK with no trims or rates needed in the transmitter. Then you trim the FCB alignment, either physically or in iNav (using a laptop in the field), so the plane flies level in Angle or Horizon modes.
If you don't do this the plane will behave differently depending on which mode you're in. It might, for example, go nose up in Passthrough mode and nose down in Horizon mode. Of course GPS modes can mask any trim issues.

Here are all my iNav related videos on YouTube - iNav Playlist