From the SIFA IRC chat in 1997

(Adrian) I'm looking at the subobject parameters in the editor
right now, I'll give an explanation...
(Adrian) OK - Object of course is the filename of the
subobject you previously created.
(Adrian) Rotate is the axis about which the object rotates,
and the number after it is the 'animation parameter'.
(Adrian) For instance, the number 12 means the current angle
of the landing gear.
(Adrian) There is also a parameter that specifies the angle of
the propellor, and one for the speed of rotation of the
propellor.   With these we could say 'Rotate the propellor
according to this parameter, until it's going too fast, and
then switch to this flickery thing that looks like a
propellor going fast instead'
(Adrian) In the editor you use 'Cond' in the subobject dialog
box to say that if a certain condition is met, switch to
the 'Else' subobject.
(Adrian) Having checked, I can now tell you that parameter
'10' is the propellor angle and parameter '9' is the
propellor speed.
(Adrian) You specify these 'parameters' in the subobject
dialog box.
(Adrian) First you create a seperate object for the gear or
prop.  Then you load the main object (the plane in this
case) and somehow attach the subobjects.  It'll take me a
while to remember how to do it
(Adrian) Actually I re-learned how to put subobjects on - you
just go to Point/Line mode click on SubObj and select the
point you want to attach the subobject to.  You then get
the fileselector to specify the file containing the object
you want to attach.  I suppose everyone already knew
that...