Both a spin and a snap roll are aerobatic manoeuvres involving stalling one of an airplane's wings earlier and more deeply than the other. Both are entered by pitching up sharply to stall the airplane, and applying a hard rudder input to yaw the airplane sideways and ensure that one wing stalls before the other, causing that wing to drop and the airplane to roll off to the side. What happens next differs between the two:
- In a spin, the increase in drag on the wing that stalls first causes the airplane to enter an extremely fast yaw (rotation about a vertical axis1) in the direction of said wing, keeping it more deeply stalled than the other wing; the high yaw rate centrifuges the airplane's mass away from the spin axis, preventing the airplane from rolling beyond the bank angle induced by the initial spin-entry yaw input.
- In a snap roll, it's the decrease in lift on the first-to-stall wing which is important (rather than the increase in drag), causing the airplane to enter an extremely fast roll (rotation about the aircraft's longitudinal axis) into the first-stalled wing; unlike with a spin, the airplane's inherent directional stability prevents the initiating yaw input of a snap roll from producing any persistent yaw.
Thus, the same type of entry procedure can apparently result in two wildly different outcomes: a spin, with an extremely-high yaw rate but no significant roll rate, or a snap roll, with an extremely-high roll rate but no significant yaw rate.
How does one differentiate a spin-entry procedure from a snap-roll-entry procedure? What determines whether the sudden pitch-up and hard rudder input produces a spin, or whether it generates a snap roll instead?
1: This axis usually is not coincident with the vertical axis of the airplane as a whole (that passing through the aircraft's center of mass), although it can be.