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Spins cannot occur without stalls. To enter a spin, a pilot would hold the elevator up and let speed bleeds off. This is how you'd approach a stall. Just as the stall starts to develop, apply full rudder; now the world is spinning around you.

Now, how does one enter an inverted spin? How do you stall an aircraft while it is inverted?

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    $\begingroup$ do a half-roll first and then do the same steps? $\endgroup$ – ratchet freak Sep 19 '15 at 13:15
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    $\begingroup$ @ratchetfreak except stick forward? $\endgroup$ – Simon Sep 19 '15 at 13:17
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When inverted, cut power. As speed bleeds off, gradually move stick forward keep your flight level, the nose will come up quite a bit over the horizon. It is important not to start sinking here as that will prevent the speed coming down low enough. When the stick hits the forward stop, kick in full rudder either way.

Enjoy!

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    $\begingroup$ What can also be done if the aircraft has sufficient pitch authority is to fly it into an upright stall, but to push full forward suddenly just before the wing stalls. When inverted, keep pushing and add rudder. $\endgroup$ – Peter Kämpf Sep 19 '15 at 15:54
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    $\begingroup$ Yes, that is also a fun variation. kind of a spin inversion. Something similar can happen when you intend to break an upright spin but hang on to opposite rudder too long and push stick forward a little to much. If you do not understand what is happening here going from upright spin to inverted spin you might think you are still spinning upright and the spin recovery did not work. This confusion is slightly less fun. The rule is, look forward over the cowling, not up through the canopy to see which way you are spinning. $\endgroup$ – Wirewrap Sep 19 '15 at 17:12
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    $\begingroup$ Some aircraft exhibit unusual spin characteristics whereby the aircraft can transition multiple times from erect to inverted and vice versa, including reversal of direction, even with controls released. Not all spins are characterised by stable rates of rotation. Some are highly oscillatory and can also depend on entry conditions such as tail slide entries and power settings above idle thrust. $\endgroup$ – busdrivingtupperware Sep 21 '15 at 14:36
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The easiest way to enter is the same as for an upright spin.

Cut engine. Bleed off speed. Keep altitude by pulling gradually. When buffeting indicates approaching stall, push stick decisively forward to mechanical stop and rudder pedals to one side. Same as you would enter a negative snap roll.

This is a clean and gentle entry, with a minimum radius, keeping the assigned box. Spectators do not recognise positive or negative (inverted) spin. But for the pilot, recognizing the direction of turns and recovery action is quite a bit easier.

Indications here apply for symmetrical, fully aerobatic airplanes.

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    $\begingroup$ Welcome to Aviation.SE! Nice answer. $\endgroup$ – kevin Aug 20 '17 at 19:57
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So two things you need for a spin entry, as you mentioned, are a stalled wing and large enough yaw rate. If you stall the wing inverted (meaning pushing the control column forward, instead of pulling in a classic upright stall) it really doesn’t matter at what attitude you achieved this two requirements. But the method used in trainings is as @Wirewrap explained in his answer, because it is less oscillatory and a fully developed spin can be achieved in a couple of turns.

A word of caution though; inverted spins can be very disorienting and are prone to wrong recovery inputs by the pilot. So they should not be attempted without proper training by an instructor (like anything in aviation). Good news is it’s really difficult to find yourself in one, accidentally. There is almost no situation in normal flight requiring you to apply negative g and rudder at the same time (except maybe while recovering from upright spin). I think that’s why it's not demonstrated even at CFI trainings.

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