What differentiates the entry procedures for spins and snap rolls?

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.

A snap roll isn't a roll. It's an "enhanced" spin, done horizontally.

A normal spin entered from level flight at 1G stall speed has an initial roll component along with the yaw component as the wing drops at the stall.

The same thing happens in a snap roll, except that the spin is entered with an accelerated stall, well above level flight stall speed and with power, with both the roll and yaw initiation enhanced with aggressive rudder and aileron input to get the horizontal auto-rotation roll-yaw going as fast as possible.

Because the snap is done in an accelerated stall, the stall can be broken immediately with forward stick, and aggressive opposite control inputs have to be used to overcome inertia to stop the snap roll. You initiate the left snap with the hard stick back/left and left rudder, and recover with hard stick forward/right and right rudder when about 3/4s of the way around.

You don't actually need aileron; airplane can snap roll quite nicely with no aileron, just rudder, it'll just be a bit slower. A pilot can enter a snap roll accidently just from mishandling, say pulling too hard in a turn and inducing an accelerated stall with the ball off center, and if the airplane has a sharp stall break, around you go, and before you've figured out what happened you're already right side up again.

If you hold the control inputs and keep the snap-roll going, you eventually arc down until you're snap-rolling vertically, or spinning the old fashioned way.

• Just a note -- snap roll can be performed in any direction, not only horizontal. There is even vertical downwards snap (9.9.10 in Aresti [fedach.cl/images/pdf/reglamento/… ], I had to look it up, but yes, it's there :) ). But the entry will be (in contrast to spin) from flying nose down already, while spin starts slow horizontal flight and nose drops first after the stall. Commented Mar 26, 2021 at 9:38

Exactly as John described, the main difference is speed. Or, more correctly, speed compared to what is necessary to maintain the current flight path.

Spin (at least as a competition figure) can be entered only from slow horizontal flight. Approaching 1G stall speed, airplane will eventually lose ability to generate enough lift for maintaining flightpath and while holding elevator all the way back, nose will drop in motion which is usually associated with "common" stall. You actually do not need any abrupt input to make it into spin (well, depends quite a lot on particular airplane, but generally it should be so). Just simply approach 1G stall speed with a bit out of coordination and keep pushing on the "wrong" pedal while nose is dropping down and ... done, you are spinning down. You can make transition more abrupt to make it look nicer, but it is not strictly necessary for entering the spin.

How much yaw- or roll-rotation you get depends on many parameters of particular airplane, c.g. and even duration of spin. There are airplanes (and c.g. positions in them) which will spin nose down with more roll-motion and some which will hold nose closer to horizon and performing more yaw-like rotation. And with number of turns (as speed builds up etc.) the characteristic can even change. Check flat-spin too.

In snap roll OTOH initial flight path is more-or-less maintained during the whole maneuver and you should not be anywhere close to stall-like dropping the nose. This does not need to mean high speed, if you fly, for example, snap at the top of looping (avalanche) where flight path asks for very low gee, or vertical snaps, you do not need much airspeed to maintain the c.g. trajectory.

What you need is an abrupt elevator input (so it will not change flight path but only AoA of wins without affecting overall movement through air too much). Actually, it is possible to perform negative snap from positive flight and vice versa (that is, to stall wing for snap roll in the opposite direction than it is generating lift for sustained flight when entering the maneuver). In this way, with help of rudder input you get one wing over its critical AoA and resulting lift difference will push airplane through the roll.

The Snap or Flick Roll certainly is a roll. The aircraft rotates around the longitudinal axis. This is by definition a roll.

It is just not an “aileron roll”, although some aileron can be used in the normal way to assist the “rudder roll”.

It differs from a spin in that neither wing is stalled prior to the application of rudder. When rudder is applied the resulting yaw stalls one wing only (or one side in a biplane). The resulting large difference in lift causes the rapid roll.

In a spin both wings (or sides) are stalled initially and the application of rudder (and other controls) varies how much more or less stalled the two sides are.

• It is the timing that makes for a good flick roll. The aeroplane is given a sudden increase in angle of attack with elevator but not quite enough to stall the wings. At that point full rudder will stall one side. The elevator should then be relaxed forward of neutral (in a positive flick) so you don’t lose a lot of energy. The rotation is increased by doing this. The flick is stopped by full opposite rudder at the right moment with only a tad more forward stick. Half flicks, which I have yet to try, require the technique to be modified. Commented Jan 26, 2023 at 4:52