I haven't found two differently designed planes that use the same
control input to get into one. The plane I do this with the most
consistently is my Staudacher. I put the plane into a vertical climb at
full throttle to altitude. I then bury the sticks in the top left
corners, full throttle, left rudder, left ail, down elevator. The plane
does one odd tumble, then does one horizontal flip, then enters the
knife edge spin and stays there until I move the sticks. With this
plane, this method is 100% fool proof. Note that I am not using the
extreme 3D throws in this maneuver, but about 30 degrees throw on
elevator and a little more on the rudder. At full extremes, I can't do
it. It just tumbles oddly.
Wont work that way on my Hots. I do a hammerhead, and when the plane is
horizontal to the ground, I hit down elevator and throttle, followed
half a second later with the rudder and aileron inputs mentioned above.
Works most of the time. Once you do it once with your plane, you'll be
able to repeat it soon afterward. Your consistency will go up once you
get a feel for what it wants.