In the simplest form - one straight main strut to which the tire is attached:

tag your main strut, say l_gear.  adjust the axis so that the axis is at the point around which you want your gear to rotate.

MAke your tire part - l_tire a child of l_gear, placing it at the "bottom" of l_gear.

Select l_gear as your current part.

Go into animation mode (Shift-F5)

Your gear down is at keyframe=0.  Set your first keyframe at keyf=0 (Animation->Set key Frame)

go to keyframe 100 either press "n" 100 times, or Shift-n 10 times)

rotate l_gear 90 degrees (Transformation->Rotate then on the z-axis type "90", click on whole part).  OR on parts Properties, type 90 in the Zaxis rotation box.

Set keyframe.

Note that visually you should also have seen l_tire "following" the animation.

Get out of animation mode, save, and spawn the model - hopefully you have an animated l_gear.

Skyhawk. Just a thought. Make sure you based your project on an aircraft with retracting gear. This is defined in the Aircraft.cfg [contact_points] entries. This example is from the default Baron. The entries in red define the extension & retraction times in seconds. If the values are 0 or 0.0 the gear is fixed & will not retract.

[contact_points]
//GEAR
point.0=1,  0.82,  0.00, -3.96, 1600, 0, 0.633, 40, 0.42, 2.5, 0.90, 0,152, 180
point.1=1, -8.04, -4.79, -3.96, 1600, 1, 0.886,  0, 0.50, 2.5, 0.85,  2, 152, 200
point.2=1, -8.04,  4.79, -3.96, 1600, 2, 0.886,  0, 0.50, 2.5, 0.85, 3, 152, 200

gear DOWN is keyframe 100 (or before)

gear UP is keyframe 0 (or a bit later)


What the keyframes mean is that from time tick (keyframe 0 to keyframe 100 will take 3 seconds.  If you set your first keyframe (gear up) at keyframe 33 and it's extended completely by keyframe 66, the visual sequence takes only 1 sec ... this way you can animate gear doors,etc within the 100 frames.

open doors - keframes 0-33
gear extends keyframes 33-66
doors close - keyframes 66-100

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