Designing a transmitter setup for F3F

[See also discussion thread on BARCS forum]

I love F3F, it's fast, competitive, and fantastic fun. The format is very simple, yet to do well requires skill, concentration, and many hours of practice.

F3F is also pretty demanding of the radio control system. On one hand, the transmitter should be simple to operate in the heat of competition. On the other hand, it should allow the pilot to make key adjustments safely,  without having to land each time.

In this post I'll review the main features to look for in a transmitter and describe my approach to programming advanced sets like the Multiplex mc4000 and Futaba 12FG. I'll also describe some useful functionality which I've implemented on my mc4000.

The focus of the article will be on features and ergonomics (in the broadest sense) rather than the details of programming.

Making adjustments often means landing the model first

Transmitter Checklist
OK, let's look at the kind of stuff your F3F transmitter needs to support, and why.

Wing  servo mixing
Different manufacturers have different ways of specifying F3X support, however, it really all boils down to eight key mixes for the wing servos:

• Roll
• Aileron stick -> aileron servo
• Aileron stick -> flap servo
• Butterfly (brakes)
• Spoiler stick -> aileron servo
• Spoiler stick -> flap servo
• Snapflap
• Elevator stick -> aileron servo
• Elevator stick -> flap servo
• Camber control
• Flap lever -> aileron servo
• Flap lever -> flap servo

The best way to check for these mixers is to borrow a tx and start programming. For example, the Spectrum DX-7 only has seven of the requisite eight mixers, which isn't obvious until you actually try programming it.

Tail mixers
Similarly, five mixers are needed for the v-tail:

• 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')

Enough about mixers, let's move on to...

Flight modes

Flight modes are essential for F3F. They enable the pilot to switch quickly between different sets of settings for trim, travel, differential etc. Flight modes are usually selected via a three position switch, so they need to be mutually exclusive. An easy way to ensure this is to map them to temporal phases of flight e.g. 'launch', 'speed', and 'landing'. Flight modes are available on most mid-range sets.

High end sets usually offer more than three flight modes. A fourth flight mode is often used in F3F for 'reflex', as an alternative to the normal 'speed' mode (this is explained in more detail later). Obviously with four flight modes, two switches will be required for selection. In order to resolve conflicts when more than one switch is used, flight modes on the mc4000 and 12FG are assigned priorities.

Mixer interlocks
Different flight modes require different mixers to be active. Ideally as you switch flight modes, activation of the appropriate mixers will be performed automatically. If your tx does not support this (and most mid-range sets don't), then you will need to define separate switches to activate the mixers. This is bad though - fiddling with redundant switches only serves to addle your brain when you should be concentrating on flying!

Adjustment sliders
Fiddling with the programming menus whilst flying is something we've all done, but it's not a good idea for obvious reasons. One solution is to program your Tx so you can adjust key settings directly, via spare knobs and sliders. Doing this can radically speed up the trimming process, and also reduce wear and tear on the model, by reducing the need to land each time.

Ideally, your transmitter will have three or four sliders assignable to useful things like snapflap volume, differential etc. In practice, this facility is only available on high end radios, and with varying degrees of usefulness.

Aileron Differential Suppression
In order to maintain roll response during the landing approach, it should be possible to cancel aileron differential as the brakes are deployed. Some radios provide this by default. Others (like the mc4000) require you to program it explicitly.

Key features of my Multiplex mc4000 setup
You could say this post has been a while in coming - I've been flying F3F with the mc4000 since 2001, and have been refining the setup ever since. My goals have been pretty consistent during this period:

1. Minimise the number of switch operations during a competition flight
2. Enable key settings to be adjusted in flight

The main features of the current setup are as follows:

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

Hedgehog-o-phobia
I prefer a minimalist control layout :-)

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

Notes

• Tx is 'mode two', i.e. aileron and elevator on the right-hand stick.
• Although there appear to be four sliders, only the outer sliders are functional on an mc4000 - the inner sliders are just place-holders. 
• The rudder and spoiler trim levers have been reassigned as adjusters for aileron diff and snapflap volume.

Flight modes
For my F3F models, I used to use three flight modes. However, having discovered the wonders of flight mode priorities (also available on the Futaba 12FG), I now use four flight modes controlled by two switches. It may sound complicated, but it's actually very simple as only one of the switches needs to be operated during the flight.

The four flight modes are:

• "Landing" - landing
• "Normal" - speed run
• "Reflex" - speed run, with reflex
• "Launch" - launch and thermal seeking

Flight modes are controlled by switches T1 (3-position) and SW2 (2-position). The up and down positions of switch T1 select Landing, and Launch flight modes respectively. The middle position (for the speed run) selects either Normal or Reflex depending on the position of SW2.

• T1 - up : LANDING
• T1 - middle: slaved to SW2
• SW2 - off: NORMAL
• SW2 - on: REFLEX
• T1 - down: LAUNCH

Reflex mode is typically selected via SW2 before the flight and then forgotten. During the actual flight, the pilot need only operate the main mode switch (T1). Furthermore, the pilot only needs to flick the switch twice  during the whole flight - once before entering the course (launch to normal/reflex), and again for the landing approach (normal/reflex to landing).

On the mc4000 and Futaba 12FG, flight modes are assigned 'priorities', careful choice of which enables the two switches to interact in the manner described above. 

Mixers
My mc4000 is programmed so the correct mixers are automatically switched in according to the flight mode. The mixers are as follows (for each flight mode):

• "Landing"
• spoiler enabled
• snapflap off
• camber preset off
• reflex off
• "Normal"
• spoiler off
• snapflap enabled
• camber preset off
• reflex off
• "Reflex"
  
• spoiler off
• snapflap enabled
• camber preset off
• reflex enabled
• "Launch"
• spoiler off
• snapflap off
• camber preset enabled
• reflex off

Restoring snapflap movement when reflex is enabled

Something which has cropped up on the forums is how to retain maximum snapflap movement when reflex is enabled. I've done this quite simply on the mc4000, by varying the amount of reflex according to the position of the elevator stick. With the elevator at neutral, there is maximum reflex. With the elevator stick fully back, reflex falls to zero, allowing the full effect of any snapflap to be restored.

In-flight adjustment via sliders
Up to four key parameters can be adjusted using sliders and trim levers. These are as follows:

1. Aileron differential
2. Snapflap volume
If you have 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. I've therefore programmed the mc4000 so that these two adjustments are assigned to the left hand trim levers.

3. Snapflap Expo

Now for something rather special - in flight adjustment of snapflap exponential. Together with snapflap volume adjustment (see above) it enables comprehensive adjustment of this key mixer. As far as I know snapflap expo is not available out of the box on any transmitter. I implemented it on the 4k by creating a mixer with two elevator inputs, one linear and the other exponential (using a 13-point curve). The two curves are blended, with the result weighted to one or other curve according to the position of the slider (slider F in the photo).

4. Camber
If you look at the control layout (above), slider E is unused. This could have been assigned to another adjustment, e.g. camber deflection in Launch mode.

Summary so far
Let's take stock and judge whether my goals have been achieved:

Minimise pilot workload
Yes, a complete F3F competition flight may be flown using the two sticks and a single 3-position switch, with just three switching operations during a typical competition flight.

Facilitate trimming
Partially. With four in-flight adjustments, I've made the most of the physical controls available on the mc4000. However it would be nice to have more adjustments. A bank of say eight sliders on the front face of the tx would be perfect, allowing virtually all trimming to be accomplished without landing. (the CG adjustment might pose a problem, but no doubt somebody will solve that!). The result would resemble an audio mixing desk - rather appropriately I think.

mc4000 mixer schematic
For mc4000 owners only, below is a schematic showing the mixer configuration on my setup. Note that it uses the maximum of five multimixes per model - this program is stretching even the mc4000's capabilities.

Mixer schematic for the Multiplex mc4000

Programming this setup takes some time, conversely making adjustments on the field is very easy. All the important mixer curves - e.g. spoiler compensation - are adjustable from a single menu point, even where multiple servos are involved. This also makes it easy to experiment with multi-point (5,7,9 or 13) point curves - nine points are useful for spoiler compensation, and all thirteen are needed for the snapflap expo adjustment feature.

More info on advanced mc4000 programming technique in the Multiplex Clinic. One can only hope the forthcoming 4000 replacement offers similar flexibility, but with a more modern programming interface.

Other RC systems
Some time ago, I reviewed the Futaba 12FG and tested it out by programming my Sting, with the same goals  in mind as when programming my 4000. The upshot is that the 12FG is capable of reproducing most of the functionality described above, with the exception of snapflap expo on a slider [update 9 Feb 2012 - see discussion thread for snapflap expo solution].

You can read my review of the 12FG here.

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 with no problems. However being a cheap set, 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. The pilot can adjust up to two settings in flight, using dedicated rotary knobs. You can also define your own mixers and curves. There are a couple of annoyances though, chief of which are the 'global' mixers, and the limited programmability of the 3-position switches.

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