Friday, 21 October 2011

Designing a transmitter setup for F3F

F3F models are amongst the most demanding in terms of programming. Over the years, I've programmed a variety of radios for this task. These include the Futaba FF7 Super, MPX 3030, MPX 4000, MPX Royal Evo, MPX Cockpit SX, Futaba 12FG and FrSky X9D+ running OpenTx. More recently, on a FrSky X20 running Ethos.

In this post I'll review the main features to look for when purchasing a transmitter. I'll also describe my approach to designing a setup, with particular emphasis on ergonomics.

Simon Thornton prepares his model at the 2015 Welsh Open F3F


Experience has shown that there are three critical requirements for a great F3F setup:
  • The transmitter must have all the mixing required
  • The setup must be simple to operate in flight
  • Key settings should be adjustable in flight, for quick trimming

Transmitter checklist for F3F

Okay, so let's start off by reviewing the mixers, flight modes and other features required for F3F. 

Mixers for wing servos

F3F 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
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. If a radio is short on mixers, one option is to discard the less important mixes. Or else it may be time to upgrade!

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

Flight modes

At their very simplest, flight modes allow you to switch between different sets of trim settings, in particular for elevator trim. However to be really useful, flight modes should also be able to activate the particular mixers and control rates.

It may sound obvious, but only one flight mode can be active at a time.  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, we can derive a simple workflow for flight:
  • Before launch:
    - Choose either the Normal or Reflex option, depending on conditions.
    - Activate Launch mode and launch the model
  • Just before diving into the speed run:
    - Activate Normal/Reflex 
  • Before landing:
    - Activate Landing mode

Mixer interlocks

Each flight mode will require a particular combination of mixers to be enabled, shown in bold/italic below:
  • 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

Crow brakes and Landing mode

It's a good idea to have a separate flight mode for crow brakes, in order to ensure a consistent aerodynamic configuration during their use.  I call this 'Landing' mode. In landing mode, camber or reflex should be inhibited, so that the brakes always start from a zero camber baseline.

On older radios, a switch may be needed to activate Landing mode. With OpenTX and other modern operating systems, Landing mode may be activated automatically as the crow lever is moved away from the idle position.

Aileron Differential Suppression

Aileron differential is used to counter adverse yaw in normal operation. However differential is a two edged sword: when full crow brake is deployed, any differential will reduce the roll response. 

To counter this, diff should be suprressed gradually as the brakes are deployed.  Some radios have diff suppression built in (Multiplex Royal Evo/Pro/Profi). Other radios (e.g. Profi 4000, OpenTx) require it to be programmed explicitly. 

Note also that only aileron differential should be suppressed. Flap differential should remain constant.

To further increase the roll response, you may wish to incorporate 'reverse differential', where the downgoing aileron’s movement is further increased 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. Landing the model to make adjustments is better, however it's not very convenient. 

A better solution is to use spare transmitter controls. The result is quicker trimming and less wear and tear on the model.

Some applications:

1. Aileron differential
Being able to adjust  diff on the fly is useful for turning your 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. Or a switch, to provide two or three levels of snapflap.

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 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 essential to ensure a safe landing during that all important first flight. The mix should incorporate a 'compensation curve' to define the response at intermediate crow values.

Spec for an F3F setup

Putting all this together, here's a typical specification for an F3F setup:
  • Four flight modes: Normal, Landing, Launch, Reflex.
  • In-flight adjustable aileron diff, snapflap volumesnapflap curve and camber.

Goals for an ergonomic interface

Some goals to keep in mind for a good user interface:
  1. Minimise the number of switch operations
  2. Place switches intelligently.
  3. Use the appropriate type of control (switch, slider or knob) for each adjuster.
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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