For airplanes, the engine's RPM must be below this threshold before the plane's landing gear is lowered. The plane will need to be below the SetGearDownHeight, 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 tilde) to bring up console window. In the console window type:
Then press ` again to remove console window. You should see many internal stats on screen. Engine RPM is near the top of the bottom-right-most block of stats.
Engine RPM, and hence SetGearDownEngineInput values, are between -1 (reverse) and 1. A value of (around) 0 means the engine is stopped. Values used in BF 1942: 0.4 for the B17 and Corsair, 0.55 for all the rest.
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