So, asteroidbook345 was asking about a landing gear system that operates like real airliners, with correctly sequenced landing gear and gear door operation.
Let's start with an example.
(please disregard the landing gear on the far side, it's using a different setting.)
Now, time for math.
Let's start with "smooth function"
What is smooth function?
Smooth function means the output will follow the input at a set speed.
If your input changes from 0 to 1 instantly, and have smooth(input, 1), it will take one second for the output of the smooth function to actually go from 0 to 1, although input has been 1 the first instance.
in 1.9.202, landing gear input is 1 when gear down, -1 when gear up.
Without smooth function, the LG input will jump from 1 to -1 instantaneously, disallowing us to make any sequential programming.
However, with smooth function, we can make LG output something that "smoothly" changes from 1 to -1 in a duration. With that, we can set a series of mechanisms to be triggered with different value of Smooth(LG), thereby giving us a sequential landing gear operation.
Check this: (I am more used for -1 = down, 1 = up, so I will use "-LandingGear" for funky tree inputs for the rest of this tutorial)
Landing Gear Value for plain LG input:
(all mechanisms triggered at the same time, no sequence)
Landing Gear Value for smooth function: (Smooth(- LandingGear, 0.25))
-1>>(2 sec)>>-0.5>>(2 sec)>>0>>(2 sec)>>0.5>>(2 sec)>>1
(Landing Gear retracting)>>>>>>>>(Gear Bay Door Closing)
This is the basic principle we can use.
For this example, all bay doors are left open when the gear is down. This will be referred to as "WWII style" operation hereon.
(all gear/door actuators in this tutorial use 1 input for "up/closed" position and 0 for "down/open" position)
For this example, the inputs are like these:
Landing Gear Actuator:
clamp01(smooth(- LandingGear, 0.25) + 1)
Gear Door Actuator:
clamp01(smooth(- LandingGear, 0.25))
The clamp function effectively disables all the function changes beyond 0 and 1, so for the landing gear, the movement started when smooth function reaches -1 (which is, immediately after "Gear Up" command), and ends its movement when the function reaches 0 (i.e. 4 seconds after the command). There are additional 4 seconds of smooth function input, but they are disabled by the clamp function.
On the other hand, for the gear doors, it starts moving when the smooth function reaches 0 (i.e. 4 seconds after command; right after gear fully retracted), and stops moving when the function reaches 1 (i.e. the finish point). The smooth function started changing right after the gear up command, but the first 4 seconds are disabled by the clamp function.
And that's what the result looks like:
Landing Gear retracts for the first 4 seconds, then the last 4 seconds are for landing gear closing.
Now, I can hear some people yelling "but this is not what we saw at the beginning!"
Yeah, right, we have to start with something simple, right?
Now, for modern airliner-ish landing gear doors (hereafter referred to as "modern type"), the sequence is more complicated.
Modern airliners have two sets of bay doors, the smaller one ("auxiliary bay door") will remain open when the gear is down to allow the gear to protrude the gear bay, while the bigger one ("main bay door") will only open when the gear is in transition to allow passage.
So, the sequence for a modern type landing gear will be like this:
main open>>gear retraction>>>main and aux door close
Which means we need three different inputs, gear, main door, and auxiliary door.
Their funky tree input are as follows:
Landing Gear Actuator:
clamp01(smooth(- LandingGear, 0.25) + 0.5)
Main Door Actuator:
clamp01(2 * abs(smooth(- LandingGear, 0.25)) - 1)
Auxiliary Door Actuator:
clamp01(2 * smooth(- LandingGear, 0.25) - 1)
For the gear actuator, instead of being triggered between smooth function -1 and 1, it is now being triggered between -0.5 and 0.5, to empty up space for both 0.5~1 ranges for door operation.
For the Main door, since its behaviour is symmetric on either side of zero input, I used an absolute function to make its output symmetric. It will be triggered when the smooth function is between absolute value of 0.5 to 1. The 2x multiplier at the front means it has to open/close twice as fast, since unlike the WWII style, in modern style operation you only have 0.5 input range for gear doors.
For the Auxiliary door it is almost the same as the main door, except that it does NOT close when the smooth input is on the minus (gear down) side. It will wait until the gear is fully retracted, and use the last 0.5 input range to close the doors.
This aircraft has been uploaded for reference
Link to the Ju-288G-2 Landing Gear demonstrator