Friday, 21 October 2011

Designing a transmitter setup for F3F

[updated 5 May 2019]

F3F presents an interesting challenge in terms of programming your transmitter. On the one hand, it must be simple to operate in the heat of competition. On the other hand, it should be possible to adjust lots of 'stuff' whilst flying the model, to aid setting up. Experience has convinced me that these are two critical - and sometimes conflicting - requirements.

Since I started flying F3F in the early 2000's, I've used a number of transmitters including a Futaba FF7 Super, MPX 3030, MPX 4000, MPX Royal Evo, MPX Cockpit SX, Futaba 12FG and - in the last 5 years - a transmitter using the OpenTx open source operating system. My goal has been to push each transmitter to its limits to achieve these goals.

In this post I'll review the main features to look for when choosing a transmitter for F3F. I'll then describe my approach to designing a setup, with particular emphasis on ergonomics - in the widest sense.

Simon Thornton prepares his model at the 2015 Welsh Open F3F

Transmitter checklist for F3F

In this section I'll review the mixers, flight modes and other features required for F3F. 

Mixers for wing servos

F3F - along with the other F3X classes - is pretty demanding in terms of mixing. I will define a 'mixer' in this context to mean a single interaction between a stick and a pair of control surfaces.
Using this definition, eight mixes are needed to drive the wing servos on an F3F model:
  • for Roll input:
    • Aileron stick -> aileron servo
    • Aileron stick -> flap servo
  • for Crow input:
    • Crow stick -> aileron servo
    • Crow stick -> flap servo
  • for Elevator input (elevator to camber mix, commonly known as Snapflap):
    • Elevator stick -> aileron servo
    • Elevator stick -> flap servo
  • for Flap input (direct camber adjustment):
    • Flap lever -> aileron servo
    • Flap lever -> flap servo
If your radio is short on mixers, you may decide you can discard a less important mix. Or else it may be time to upgrade.

Mixers for the v-tail

Similarly five mixes are needed to drive the v-tail servos:
  • Elevator stick -> tail servo (pitch)
  • Rudder stick -> tail servo (yaw)
  • Crow stick -> tail servo (trim compensation)
  • Flap lever ->tail servo (trim compensation) [useful but not essential]
  • Aileron stick -> tail servo (for 'coupled ailerons and rudder' aka. 'combi')
Check through the documentation of your radio to see how it handles these mixes.

Right, so that's mixers covered, let's move on to...

Flight modes

At their very simplest, flight modes allow the pilot to switch between different trim settings. However to be really useful, flight modes should also be able to activate mixers and switch between different control rates.

When designing your flight modes, remember that only one flight mode can be active at a time, so they must be mutually exclusive.  A good approach, therefore, is to base your flight modes on the temporal phases of flight. The logical ones for F3F are 'Launch', 'Normal', and 'Landing'. These conveniently map to a 3-position switch.

A fourth 'Reflex' flight mode is often used in high lift conditions. It's useful to think of Reflex and Normal as switchable alternatives.

Using this scheme, the pilot's F3F workflow is simple.
  • Before launch:
    - Use a 2-pos switch to preselect between Normal or Reflex modes (lift dependent!)
    - Select Launch mode, and launch the model
  • Just before diving into the speed run:
    - Select Normal /Reflex
  • Before landing:
    - Select Landing mode

Mixer interlocks

Each flight mode will need a particular combination of mixers.
  • Landing
    • crow enabled
    • snapflap off
    • camber preset off
    • reflex off
  • Normal
    • crow off
    • snapflap enabled
    • camber preset off
    • reflex off
  • Reflex
    • crow off
    • snapflap enabled
    • camber preset off
    • reflex enabled
  • Launch
    • crow off
    • snapflap off
    • camber preset enabled
    • reflex off

Aileron Differential Suppression

Aileron differential is used to counter adverse yaw in normal operation. However it's a two edged sword: when full crow brake is deployed, diff will reduce the roll response. The solution is to suppress aileron diff as the brakes are deployed ('aileron differential suppression').

Some radios have diff suppression built in (Multiplex Royal Evo/Pro/Profi). Others (e.g. Profi 4000, OpenTx) require you to program it explicitly. Note also that flap differential should not be suppressed, only aileron differential.

To further increase the roll response, you may wish to incorporate 'reverse differential', where the downgoing aileron has even greater travel than the upward aileron as full crow is approached.

Adjustment sliders

When trimming a new model, it's tempting to adjust the programming whilst flying, however, this is inherently unsafe for obvious reasons. Landing the model to make adjustments is better, however the ideal solution is to assign key adjustments to knobs and sliders. The result is quicker trimming and less wear and tear on the model.

Here are my suggestions for adjusters:

1. Aileron differential
Being able to adjust ail diff on the fly is useful for achieving axial rolls, important for F3F turns. I use the (redundant) rudder trim for this.

2. Snapflap volume
Snapflap volume is an essential adjustment for F3F. If possible assign it to your throttle trim or other 'sticky' control.

3. Snapflap Expo
In flight adjustment of snapflap expo is useful though not essential. I have programmed this on my Profi 4000 as well as the Taranis, and I understand that it can be cooked up on the Futaba 12FG as well. 

4. Snapflap deadband
Snapflap upper- and lower-deadband are very useful adjustments which I've relatively recently implemented on OpenTx using a Lua script. Both deadbands are adjustable via dedicated controls. I also have audio alerts enabled at 5% and 95% snapflap to aid tuning. On other systems, deadband can be implemented using curves, and these should be switchable in flight to enable comparison in real time.

4. Camber
Some positive camber is required in Launch mode. Some pilots like to play with camber actively, I prefer to have it on a switch, with a knob to adjust the preset value.

5. Crow compensation
An adjuster for max crow compensation is very useful to ensure a safe landing during that all important first flight. The mix should incorporate a curve for tuning the intermediate response.

Spec for an F3F setup

Putting all this together, here is the minimum spec I would recommend for F3F:
  • Four flight modes: Normal, Landing, Launch, Reflex.
  • In-flight adjustable aileron diff, snapflap volumesnapflap curve and camber.

Goals for an ergonomic interface

Here are three goals to keep in mind for a good user interface:
  1. Minimise the number of switch operations
  2. Place switches intelligently.
  3. Use appropriate controls (switches, sliders, knobs) for the mix adjusters.
Here are the layouts for my MPX 4000 and OpenTx transmitters:

F3F layout on Multiplex mc4000 

F3F layout: FrSky X9D (OpenTx).

F3F on mid-range systems

By now you may have the impression that only expensive transmitters are suitable for F3F. That's emphatically not the case - these days the power is in the software, not the hardware. In fact, some cheaper sets are remarkably good for F3F - the difference is in the amount of customisation available.

The FrSky Q X7 is a £100 radio running OpenTx. It makes an excellent F3F radio, when loaded with my F3F template.

Even the old Multiplex Cockpit SX has all the basic mixers required for F3F. It's comfortable to hold and easy to program. However it inevitably has some limitations: you can't activate flight modes and mixers via the same switch, you can't make adjustments except via the programming knob, you can't disable the crow, and you can't define your own mixers.

The Multiplex Royal Pro sits somewhere in between the Cockpit SX and the Multiplex 4000. You can adjust up to two settings in flight. You can also define your own mixers and curves. There are some annoyances though, chief of which are the horrid 'global' mixers, and the limited programmability of the 3-position switches.

The Futaba 12FG implements all the features of our ideal setup, except that snapflap expo is tricky to program (there is a discussion on the BARCS forum).


Hopefully this article will get you thinking about the wider challenges, as well as possible solutions, involved in programming an F3F model effectively.


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