Friday, 21 October 2011

Designing a transmitter setup for F3F

F3F is the Formula 1 of slope racing. I've been an active competitor since 2002 and remain completely hooked (the fact that we fly in some of the most beautiful landscapes is just the icing on the cake!). During that time I've developed and refined the transmitter interface using two of the most flexible RC systems available - the Multiplex 4000 and OpenTx.

F3F is technically demanding. On one hand, the transmitter must be simple to operate so you can concentrate on the flying. At the same time, it should be possible to adjust certain key mixers without having to land the model. Experience has convinced me that these are two critical requirements!

In this post I'll review the main features to look for in a transmitter. I'll then describe my approach to ergonomics, using my Multiplex Profi 4000 and other radios as examples. The focus of the article will be on features and ergonomics rather than the details of programming.

Simon Thornton prepares his model at the 2015 Welsh Open F3F

Transmitter checklist for F3F

The first thing you'll need is a transmitter, and the more flexible the programming, the easier it will be to design a setup suitable for F3F.  In this section we'll review the mixing, flight modes etc. required.

Mixers for wing servos

Note that I will use the term mix in its simplest sense, that of one stick or lever interacting with one pair of control surfaces. A total of 8 mixes are needed to drive the wing servos of an F3F model.

  • for Roll input:
    • Aileron stick -> aileron servo
    • Aileron stick -> flap servo
  • for Spoiler input:
    • Spoiler stick -> aileron servo
    • Spoiler 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
R/C manufacturers can be a bit sloppy in the way they specify their glider features, and the devil is in the detail. The Spectrum DX-7, for example, has only seven mixes available for the wing servos, which isn't obvious until you actually try programming it. If your radio is short on mixers, you may decide you can discard one e.g. snapflap on aileron. Or else it may be time to upgrade!

Mixers for the v-tail

Four or five mixes are needed to drive the v-tail servos:
  • Elevator stick -> tail servo (pitch)
  • Rudder stick -> tail servo (yaw)
  • Spoiler 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')
OK that's mixers covered, let's move on to...

Flight modes

All decent radios offer flight modes. At their very simplest, flight modes allow the pilot to switch between different trim settings. At the next level, flight modes can also be associated with specific mixers and rates. This makes them paricularly useful for F3F since different phases of flight require quite different setups.

Flight modes are commonly selected by means of a three position switch, so it follows that they must be mutually exclusive.  A good approach, therefore, is to base your flight modes on the temporal phases of flight - for F3F the logical ones are 'launch', 'normal', and 'landing'.

A fourth 'reflex' flight mode is often used in high lift conditions. It's useful to think of Reflex and Normal as switchable alternatives. We'll see how to implement this later, using a two-position switch to select between one and the other.

Mixer interlocks

Each flight mode will require a particular combination of mixers to be active. So for example, in landing mode, you'll want to active the crow mix, but deactivate it in all other modes.

Ideally mixer activation will be performed automatically on change of flight mode. If your tx does not offer this, then you will need to extra switches to control the mixers - not a good idea though, as it increases the load on the pilot.

Adjustment sliders

When trimming a new model, it's tempting to adjust the programming whilst flying. However, this is just asking for trouble for obious reasons. A safer solution is to land, re-program, and relaunch. However, this is tiresome, and it's difficult to compare settings.

The best solution is assign key adjustments to dedicated knobs and sliders, so you can make adjustments safely whilst flying.The result is quicker trimming and less wear and tear on the model. This kind of thing can only be programmed on high end radios, however. We'll look at this in more detail later.

Aileron Differential Suppression

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

The solution is to suppress aileron differential as the brakes are deployed. Some radios provide this as a built-in feature (Multiplex Royal Evo/Pro/Profi). Others (e.g. Profi 4000, FrSky Taranis) require you to program it explicitly. Note also that flap differential should not be suppressed, only aileron differential.

Goals for an ergonomic interface

Before we progress, I propose three main factors for achieving an effective, ergonomic interface:
  1. Minimise the number of switch operations during a flight
  2. Intelligent placement of switches
  3. Enable key settings to be adjusted in flight
OK let's turn those goals into a feature list

Feature list for F3F model

  • Three flight modes
  • 'reflex' option
  • In-flight adjustment of:
    • Snapflap volume
    • Snapflap exponential
    • Aileron differential
    • Camber

Let's now look at how one might implement these features in a high end radio.

Switch placement

The fewer the switches, the less chance of pushing the wrong one. 

Location is also important - the most used switch should be easily accessible. Here's the F3F setup on my Profi 4000 (I'm right handed):

Simple control layout on my Multiplex mc4000 (click to enlarge)

Reassignment of trims

The rudder and throttle trim levers are superfluous for F3F. If your transmitter allows, it's a good idea to reassign them to adjust differential and snapflap volume, as in the Multiplex mc4000 example above.

Flight modes

Our reference setup uses four flight modes for LANDING, NORMAL or REFLEX, and LAUNCH. On my Profi 4000 and FrSky Taranis, these are selected via a 3-position switch, and a 2-position switch as follows:
  • 3-pos/up: mode = LANDING
  • 3-pos/middle: mode depends on 2-position switch
    • if 2-pos is off then mode = NORMAL
    • if 2-pos is on then mode = REFLEX
  • 3-pos/down:  mode= LAUNCH
During a competition, the pilot's workflow is simple.
  • Before launch:
    - Set the REFLEX option depending on wind conditions
    - Select LAUNCH mode
    - launch the model
  • Just before diving into the speed run: - Select NORMAL mode
  • Before landing:
    - Select LANDING mode

Flight mode priority

On some radios (e.g. MPX Profi 4000, Futaba 12FG, FrSky Taranis - but not the 'new' MPX Profi) the user can assign a priority for each flight mode. This powerful feature allows the above switching scheme to be implemented very easily, without the need for complex 'logical' switches.

Mixer activation

Most F3F models use four high level mixers: spoiler (aka. butterfly or crow), snapflap, camber preset, reflex. The state of each mixer will depend on the flight mode:
    • spoiler enabled
    • snapflap off
    • camber preset off
    • reflex off
    • spoiler off
    • snapflap enabled
    • camber preset off
    • reflex off
    • spoiler off
    • snapflap enabled
    • camber preset off
    • reflex enabled
    • spoiler off
    • snapflap off
    • camber preset enabled
    • reflex off
On the Profi 4000, Futaba 12FG and FrSky Taranis, the mixers can be enabled on change of flight mode.

In-flight adjustment via sliders

As mentioned, the ability to adjust mixers in flight is only available on high end radios, and even then some programming effort may be required. On my own Profi 4000 and FrSky Taranis, four key parameters are assigned to sliders and trim levers. Let's look at each of these key adjustments in detail:

1. Aileron differential
2. Snapflap volume
If you read Kevin Newton's splendid guide to Setting up a Racing Glider, you'll know that the two key adjustments for an F3F ship are (a) aileron differential and (b) snapflap volume. These two adjustments are assigned to the rudder and throttle trims on my 4000 and Taranis.

3. Snapflap Expo
In flight adjustment of snapflap exponential  is very useful. As far as I am aware, it's not available out of the box on any transmitter. However 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. Camber
If you look at the control layout for the mc4000 (above), slider E appears to be unused. In fact, I used to adjust the amount of camber deflection when in Launch mode. On the Taranis, I use a knob for this.

5. Spoiler compensation
My Taranis (but not the 4000) also has an adjuster for spoiler compensation.

Other RC systems

It's worth mentioning that you don't have to use high end radio gear for F3F. In particular the Multiplex Cockpit SX has all the required mixers. It's also very easy to program, and I've used it to fly my Sting. 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 spoiler, and you can't define your own mixers.

The Multiplex Royal Pro sits somewhere in between the Cockpit and the mc4000. 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).

Since first writing this post, I have also ported my full Profi 4000 setup to the FrSky Taranis.


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


No comments: