It is possible to program a ‘speedĀ  mix’ for radio controlled flying wings with a computer radio. But before I explain, some basic trimming theory (as I understand it) is in order .

The Theory

All aircraft are trimmed to fly at a particular speed. In full size gliders, we adjust the attitude to get the speed we want, then trim accordingly so the glider wants to ‘stay there’. If for some reason the aircraft becomes displaced from this attitude and speed (eg turbulence raises the nose briefly or a ham fisted clod at the controls doesn’t know what he’s doing etc) the glider will, if left to it’s own devices, return to the original speed it was trimmed for.

[singlepic id=50 w=320 h=240 float=right]All of my models: powered, glider, conventional tail or flying wing exhibit this behaviour, some better than others. Second to this, the throttle becomes a means to control climb or descent. If an aircraft is trimmed well for slow speed flight and full throttle is applied you will soon find yourself in a climb. The opposite holds true too. An aircraft trimmed for fast flight will soon find itself in a descent if power is cut. The aircraft is converting the engine’s energy into height or the aircraft’s height back into forward motion in an effort to maintain a constant airspeed.

The Problem

I have never seen this demonstrated so well as on my Swift Flying Wing. I generally have it trimmed for a nice slow flat glide. This makes launching and landing easy. It also means the swift is ready for mucking about at ground level, or full throttle climbs and leisurely descents. This becomes a problem when I want to move between fast & furious and low & slow in the one flight. It’s a pain to use the conventional trim tabs or buttons on the radio. Especially on the computer radio where it means carefully counting ‘beeps’ to make sure I return the trim to the original position for landing.

The Solution

Eventually this idea came to me out of the blue. It happened when I was on the dunny which is where all my best thinking happens. I have no idea whether this method is already commonplace but I’m pretty pleased with myself for figuring it out.

The solution is so simple, simply mix a spare (switched channel) on your radio to the elevator channel! Obviously not a 100% mix otherwise you would have permanent ‘full up’ or ‘full down’ elevator. In my case I found that 12% worked well. Here’s what I did with my Futaba 6EXPA.

  1. Enter the radio’s programming screen by holding ‘mode’ and ‘select simultaneously.
  2. Press ‘mode’ until you reach the ‘PMIX’ screen.
  3. Use the +/- selector to change ‘PMIX’ to ‘ON'[singlepic id=131 w=320 h=240 float=center]
  4. Press ‘select’ then use the +/- selector to choose the mix rate. I found 12% works well for the swift.[singlepic id=128 w=320 h=240 float=center]
  5. Press ‘select’ again and use the +/- selector to choose the ‘master’ channel. I used Ch5 which is commonly used for the landing gear.[singlepic id=129 w=320 h=240 float=center]
  6. Press ‘select’ again and use the +/- selector to choose the ‘slave’ channel. This needs to be whatever channel your radio uses for elevators. Mine uses Ch2 and I believe that’s standard for all radios.[singlepic id=130 w=320 h=240 float=center]
  7. Hold ‘select’ and ‘mode’ simultaneously to exit the programming screen.
  8. Now toggle the Ch5 switch to move the elevons to their uppermost position and retrim to their original neutral point.
  9. That’s it! Now you will have one elevon ‘centre’ position for slow flying just like before and with the flick of a switch a second ‘centre’ position for crazy fast flying where the model doesn’t have a permanent tendency to climb. Each new ‘centre’ position is only a few degrees (if that) or millimetres apart but it makes a big difference on a flying wing. It’s just like changing gears!