What are Flight Modes?

Published by Mr. D on

With traditional model aircraft, you have a receiver; in to which you plug your servos and throttle control. So the movements you make on the transmitter are directly output to the control surface. With a fight controller there are considerably more options. So what are flight modes and what do they do?

This page accompanies my video What are Flight Modes? It is a part of my Flight Controller Therapy? series on YouTube. Have more flight controller questions? You may find the answers here.

Different types of flight modes

Flight mode is a term, often used to generalise different types of control available for the model. They can be broken down in to these different types:

Flight Modes

are used to control the aircraft, based on the pilot’s transmitter inputs

Navigation Modes

are autonomous instructions to the aircraft

Tuning Modes

enable the pilot to trim or tune the model, while it is flying

Flight Modes

This is where I’ll explain the four base flight modes of iNav. They are Manual, Acro, Horizon, and Angle. I have also included the closest matching flight modes for Ardupilot and Vector above the descriptions. They are ordered from most control by the pilot, to more control by the flight controller.

Manual

[Ardupilot – Manual | Vector – Stabilization Off]

Manual, previously known as pass through, is an unstabilised flight mode. It takes the signals from the transmitter and passes them, almost, straight to the servos. However, there are a couple of things that can modify the signal. However, these are iNav implementations of the traditional transmitter features of rates and expo. Manual is the closest that you can get to just plugging in to a receiver.

Expo, short for exponential, makes the centre of the gimbal less sensitive. You still have the full movement of the control surface. But the centre part moves slower compared to the outer, fully deflected part.

Rates in this instance are not the same as the rates in the PID controller. Rates here is a limit of the control surface.’s movement. By default, the manual rate is set to 100, which is 100 percent. Reducing this number reduces the throws of the control surface.

Acro

[Ardupilot – Acro| Vector – Gyro Stabilize]

Acro (sometimes called Rate; more often, wrongly, called Airmode) is a stabilised flight mode. The idea of the stabilisation is to help make the flight smoother. This may help the model feel more locked in or help combat wind. Acro is the default flight mode in iNav. This means that if no flight modes are selected, Acro is used.

With stabilised flight modes, you do not directly control the model. Instead, you tell the flight controller what you would like the model to do. It then evaluates what the best action to take to achieve that. iNav does this with a thing called the PIFF controller. The PIFF controller will be explained in another article, as it would make this simple article quite large.

Horizon

[Ardupilot – Stablize | Vector – 2D Mode]

Horizon grows on Acro’s stabilisation, but adds self levelling to the control. This is the first flight mode that when you let go of the controls, the flight controller will attempt to get the model level in both pitch and roll. This flight mode still allows complete rotations in all directions.

Angle

[Ardupilot – Fly-by-wire A| Vector – no similar flight mode]

Angle is a progression further from Horizon. Not only does it have stabilisation and self-levelling. But, it now has angle limits on the pitch and roll that you can go to with the model.

What about Airmode?

Airmode is not a flight mode. When Airmode was first introduced, it was activated on the modes page. A lot of the time, it was the only thing visibly active. This, and its name having mode in its name, probably lead to the misconception that it is a flight mode. When no flight modes are selected in BetaFlight or iNav, the Acro flight mode is enabled. So Airmode was modifying Acro in these cases.

Airmode is now set by a switch in the configuration page. This permanently enables Airmode. This means that it’s active on Acro, Horizon, and Angle flight modes when enabled.

What Airmode actually does is modify the current flight mode. It increases control of the model at low throttle.

Navigation Modes

Navigation modes automate some or all of the flight to achieve a desired purpose.

Altitude Hold

[Ardupilot – Fly-by-wire B | Vector – No matching flight mode]

Altitude hold maintains the current altitude of the model. Like a lot of the navigation modes, it is based on the Angle flight mode. So, making sure that Angle is well set up is paramount for successful navigation mode operation. When this mode is enabled, you can freely move left and right. You can also change the target altitude using the pitch control.

Cruise

[Neither Ardupilot or Vector have a matching flight mode]

Cruise keeps the model flying in the same heading. Again, it is based on the Angle flight mode. You can change the cruise heading with the roll or yaw controls. You have free control of the pitch

3D Cruise

[Ardupilot – Fly-by-wire B | Vector – No matching flight mode]

3D Cruise combines Altitude Hold and Cruise, to make a navigation mode that locks on to a heading and altitude. You can adjust the heading with the roll and yaw controls. You can also adjust the altitude with the pitch control.

Position Hold

[Ardupilot – Loiter | Vector – Loiter]

Based on Angle, the Position Hold navigation mode will bank the aircraft over to circle over the fixed position in the sky. The radius of these circles can be set in iNav Configurator. You can change a setting in the CLI to allow you to change the direction of the loiter circle using the yaw control. This command is:

set fw_loiter_direction = YAW
save

Return to Home

[Ardupilot – Return to Launch | Vector – RTH]

Return to Home will attempt to fly the model back to the home position. Once at the home position, the model will hold position above it. RTH is a pretty configurable flight mode, including the ability to:

  • Fly back home using different altitude settings:
    • AT_LEAST – Uses the RTH altitude as the minimum altitude
    • AT_LEAST_LINEAR_DESCENT – This works in the same way as AT_LEAST. Except that if you are above the RTH altitude, it will slowly descend during the RTH with the aim being to reach RTH altitude when the model arrives at home
    • EXTRA – Adds the RTH altitude to the current altitude to get the altitude to fly at
    • MAX – Will fly back at the highest altitude recorded over the flight
  • Set an altitude to loiter down toward when at home.
  • Choose whether to climb before or after turning once RTH is activated.

Return to home will be discussed in more detail in another article.

Launch Mode

Launch mode assists getting the model in the air. It is a fairly hefty subject. So instead of rewriting the book, I will just pass you on to a great guide, written by Marc Hoffman. Learn how to implement launch mode here.

Tuning modes

Some flight controller firmware has modes available to help tune the model in flight. Here are a couple of examples.

Auto-trim

[Ardupilot – TRIM_AUTO | Vector – Use transmitter trims]

Auto trim attempts to trim the model so that it flies straight and level with hands off the sticks. It adjusts the midpoints of the servos based on you keeping the model flying straight and level during auto-trim. Check the offset in the outputs page. If it is over 100 in either direction (< 1400 or > 1600), it is recommended to physically adjust the servos.

Auto-tune

[Ardupilot – AUTOTUNE | Vector – Gain pot]

Auto-tune attempts to tune the PID/PIFF controller for optimum performance for your particular model. When you use Auto-tune, it should be performed in the Acro flight mode. You should fly the model normally to get the best tune. By normally, it means how you would expect the fly the model. So if you are mostly cruising at a low throttle, tune doing that. If you are flying fast, with lots of agility, tune doing that. There is a great guide to tuning your model here.

I hope this was helpful

Thank you for reading this article. I hope you found it useful. There are more articles on this site that may answer more questions. I also have a YouTube channel at www.youtube.com/c/MrDFallingwithstyle with content like this. A like and subscribe on there would be greatly appreciated.

This article has helped 213 people. For more useful things, please check out my YouTube Channel.

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