Programmable Mixes

[econpatric]

I'm sure this question has been tackled before , but I have not been able to wrap my head around the reasoning and the process by which programmable mixes are used. Which surfaces are to be mixed together, and when is it appropriate to mix them? Can a person set these mixes to a switch so it can be turned on and off?

 

[gyro]

Which surfaces are to be mixed together

I think that depends on what you are trying to mix out. Think of a mix as a either combination of trims, or even as a "progressive trim".
Another way to think of mixes is to view them as an autopilot, that helps make some of the inputs for you when you tell it to do something.
I believe that the most common example of mixes for basic 3D flying, is "rudder to elevator" and "rudder to aileron". These combos are generally used to trim out the airplane for knife edge flight.

when is it appropriate to mix them?

when you have the plane doing something you don't want it do, and it'd be easier to fly the plane without having to constantly correct it.
If every time you turned to the right, your nose dropped, you'd want your autopilot to automatically include some nose up elevator everytime you turned. think of the computer mixing portion of your radio as an autopilot.

Can a person set these mixes to a switch so it can be turned on and off?

Absolutely! I have a DX8 radio, and it allows me to program the mixes to any number of switches, so that I can turn them on and off at a whim.

 

[njswede]

Mixes are used simply because no airplane is perfectly symmetrical and all airplanes (no matter what the designer tells you) have some form of of coupling. For example, let's say your elevator isn't perfectly centered. Whenever you deflect the elevator, you're going to apply uneven forces to the left and right side of the plane and it's going to want to roll. A good builder can minimize these effects by meticulous building, but it can never be completely eliminated. Other coupling effects stem for the basic design of the plane. For example, the rudder may not have an equal surface area above the center line compared to below the center line. This is going to create a tendency to roll when you apply rudder. And the list goes on.

So, in its most basic form, mixing can be used to eliminate these tendencies. For example, if your plane wants to roll left when you apply hard up elevator, you can mix in a couple of percent of right aileron to compensate for that. The result is a plane that flies truly perfect with no coupling. It's not easy to get it perfect (in fact, I'm still learning) and requires some practice and trial and error.

Which surfaces are mixed together? The answer is "any surface" and sometimes even the throttle. In the example above, we mixed elevator to aileron. Let's say your plane wants to pull towards the canopy when you're flying KE and applying a fair amount of rudder to keep it flying. In that case you can mix in some down elevator with the rudder to compensate for that.

And that brings is to the mixes that are controlled by switches. Let's say you want that rudder to elevator mix when you're flying KE, but you don't like how it affects upright flight. Well, then you can (on most radios) assign the mix to a button, so you can enable it when you're in KE and disable it otherwise.

So do you need mixes on your airplane? Well, do a couple of loops, turns and KE passes and check if you're completely free of coupling. If you are, you're truly lucky, but chances are that you have at least some of it. Figure out what axes overlap (e.g. pitch and roll) and dial in a mix that compensates for it.

 

[Steve_B]

Programming mixes is part of the process of trimming a plane (at least for those who use mixes). The mixes themselves are used to counteract 'coupling' that is inherant in just about every airframe. You can put mixes on switches if you want but for me there is no point. I just leave them 'on' all the time as there is no downside to having undesireble coupling mixed out.

So what is 'coupling' you are no doubt asking! Here is a response I wrote a few weeks ago to that question:

Coupling is an interaction between controls across the different axis of freedom. In principal each of the primary controls on a plane effect one axis of freedom:

•Elevator controls pitch
•Rudder controls yaw
•Ailerons control roll

In practice it's not quite that simple. What you find is that operation of one control has not only the desired effect (i.e. rudder causes yaw) but it has a secondary effect on one or both of the other axis. Taking rudder again as an example; when you apply rudder you often find that you get not only the desired yaw effect but the plane will also roll and pitch slightly.

Rudder is usually the ‘worst offender’ for cross coupling but aileron suffers from it too. It’s not unusual that when you apply aileron the plane not only rolls as you intended but it yaws in the direction opposite to roll direction (plane rolls right and yaws left). This is known as ‘adverse yaw’.

For some planes coupling is essential for proper control. Rudder and elevator control planes being the obvious example where we rely on the rudder to roll coupling in order to make the plane bank into a turn. Without coupling a rudder and elevator plane would be un-flyable. For aerobatic planes coupling is something we generally don’t want. Obviously having a plane roll when you wanted a yaw, or yaw when you wanted roll makes precision aerobatic flying more difficult. Knife Edge flying especially shows up any rudder coupling issues the model may have. A good designer of aerobatic plane will tweak things like the height of the wing and tail, dihedral angle and a bunch of other stuff in order to engineer out as much of this coupling as possible, but even on the best models there will always be some hint of coupling evident in some flying conditions.

Modern transmitters with computerised mixes can tweak out the coupling that remains. For instance, if when you apply right rudder in knife edge flight the plane rolls right and ‘tucks’ toward the belly then you would mix in some left aileron and some up elevator to come in automatically along with the right rudder. If mixed correctly should the plane will track perfectly straight in knife edge.

For adverse yaw coupling on aileron, the usual trick is to set some differential so that the up-going aileron moves a little further than the down-going one. This reduces adverse yaw and can help make rolls a little more axial. On non-aerobaitic planes some rudder might be mixed with aileron to counteract adverse yaw but this isnt suitable for aerobatic models.

 

[njswede]

Story of my life: Whenever I think I've posted a killer reply, Steve comes and posts an even better one.

 

[gyro]

Story of my life: Whenever I think I've posted a killer reply, Steve comes and posts an even better one.

get used to it... He actually needs to periodically download his brain to an SD card, otherwise he'd run out of space!

 

[ghoffman]

Well, one example is a rudder to elevator mix. When flying a knife edge (KE) most planes will pull to the canopy or landing gear depending on where the C/G is. In my case 4% rudder to elevator mix, makes the plane fly perfectly straight in KE.

 

[Steve_B]

Story of my life: Whenever I think I've posted a killer reply, Steve comes and posts an even better one.

Not sure if it was better.. just longer

If only I could translate the theory into practice

 

[dadstoysbg]

Also besides the coupling mixes you have some IMACC mixes used by a lot of the pilots. (1) Throttle to rudder. (2) Down line mix. In the advanced radios you can assign the different mixes to different flight modes.
A good example of coupling is my old Bob Truworthy Monster 12 Pitts. Because it had scale wings I had to have 24% up elev. to rudder mix to knife edge. On my H-9 Suk I only needed 3% mix for knife edge. Dennis

 

[Steve_B]

I guess throttle to rudder mix could be looked on as addressing a type of coupling.. throttle to yaw coupling?

Different flight modes sounds too complicated for me! I struggle enough just moving two sticks and occasionally flicking a rate switch