For airplanes, engine RPM must be above this threshold before the landing gear is raised. The plane will need to be above SetGearUpHeight off the ground, too.
To see Engine RPM, hit F8 to turn off radio tooltips (if you have them on), then hit ` (back-tick - left of the '1' key, aka the tilde) to bring up console window. Into the console window type:
Then press ` again to remove console window. You should see a load of internal stats on screen. Engine RPM is near the top of the bottom-right-most block of stats.
Engine RPM, and hence SetGearUpEngineInput values, are between -1 (reverse) and 1. A value of (around) 0 means the engine is stopped. Typical values are 0.7 for B17 and Corsair, and 0.8 or 0.83 for the rest.
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