The next page is where the actions on the controls will be mapped to servos. OpenTX does not have any predefined mixing functions that relate only to a particular model type or situation, it rather gives you a blank canvas you can build upon. The key to configuring a model on OpenTX is not to think about "activating the delta mix" like on certain radios, but rather to think about what you want your control on the model to do in response to an input on the radio's controls. The mixer is where all that "logic" gets entered.
The various channels are outputs, for example CH1 being the servo plug #1 on your receiver (with the default protocol settings). A channel without a mixer line will just center a servo that would be connected to it. Each mixer line connects one input to the channel it's on. Inputs can be: The 4 stick axes The 4 pots and sliders The heli mixer outputs (CYC1-3) A fixed value (MAX) The 8 physical switches The 32 custom (logical) switches The trainer port input channels (PPM1-8) Each of the radio's 32 channels, which allows using channels as a virtual functions for clarity (mix several inputs into one reuseable function, that can then be assigned to one or more channels). Note that the settings of the SERVOS page are NOT taken into account there. All inputs work on a -100% to +100% basis. Sticks, pots, channels, CYC sources, trainer inputs will vary proportionally within this range. 3-position switches will return -100%, 0% or +100%. 2-position switches (and logic ones) will return -100% or +100%. MAX is always +100%. If you want the servo connected to the #2 plug of your receiver to be controlled by the elevator stick, you will simply create a mixer entry on CH2 with Ele as source. Easy enough! There can be as many lines as needed on each channel, and the operation between each line can be selected. To create a new line, you would LONG press the ENTER key, and select insert before/after. By default all the lines on a same channel are added together, but a line can also multiply those before it, or replace them. For clarity, each line that is currently active and contributing to the channel's output will have its source displayed in bold. This can be very handy when many are present and to check switch functions.
For each mixer line, several parameters are available: A name can be entered for convenience The weight (in %) of the input can be set. This sets how much of the input control has to be mixed in. A negative value inverts the response. An offset on the input value can be added. A trim can be used, for sticks this is by default the trim associated to the stick, but can be chosen to be one of the other trims (for cross-trimming for example) or disabled altogether. For other inputs the trim defaults to OFF, but can of course be set to one if required. Either a differential value can be set (reduces response by the specified percentage on one side of the throw) or a curve (built-in or custom) can be assigned. When a custom curve is selected, a press of the MENU key will bring you to the curve editor. The modes the mixer line is active in can be selected (see D/Rs). A switch (physical or virtual) can be used to activate the mixer line. A sound warning (1, 2 or 3 beeps) can be set to play whenever the line is active. The Multpx setting defines how the current mixer line interacts with the others on the same channel. "Add" will simply add its output to them, "Multipl" will multiply the result of the lines above it, and "Replace" will replace anything that was done before it with its output. The combination of these operations allows creating complex mathematical operations. Response of the output can be delayed and/or slowed down with regard to the input change. Slow could for example be used to slow retracts that are actuated by a normal proportional servo. The time is how many seconds the output will take to cover the -100 to +100% range. As a little example, if you wanted to add some compensation on the elevator channel when you increase throttle, you would go through a simple path: What's the control surface I want this to act on? Elevator, which is connected to CH2. When do I want it to move? When I move the throttle stick, in addition to whatever would already be present (usually the elevator stick). So you would simply go on CH2, and insert a new line with Thr as source. Type would be Add as the compensation needs to be added to the "normal" elevator response. As the required compensation is likely small, you will dial in a small weight, maybe 5%. On the ground with motor disconnected, you will check the elevator compensates in the correct direction. If not, you'll invert the weight to -5%. You could then assign a switch, in order to be able to activate/deactivate it in flight to see if the amount of compensation is actually appropriate. If the correction is more complicated, you might want to assign and create a curve that matches what's required.