Spin aerodynamics
Spin = stall aggravated by yaw: one wing more stalled than the other, so aircraft autorotates (automatic, self-sustaining roll) (TP 1102).
Mechanism (TP 1102): wing making less lift near stall drops.
| Wing | Angle of attack | Result |
|---|---|---|
| Downgoing | GREATER | Loses more lift (no self-restoring, unlike ordinary angles); more stalled; drag rises sharply, adds yaw, pushes AoA up further |
| Upgoing | REDUCED | Stays less stalled; more lift; accentuates roll |
Direction (TP 1102; TC sample exam Q21):
- Stall aggravated by yaw autorotates toward the dropping wing.
- Incipient spin follows the turn: stall in descending left turn gives incipient left spin.
Motion (TP 1102):
- Roll, yaw, pitch together on helical (corkscrew) path about vertical axis.
- Pitch flat to steep.
- Forward and vertical speeds both comparatively low.
Stages (TP 1102):
| Stage | Span | Detail |
|---|---|---|
| Incipient | Stall and start of rotation until spin axis vertical or nearly vertical | Flight path swings horizontal to vertical; rotation rate builds to fully developed rate, typically 4 to 6 seconds (about two turns) in light aircraft |
| Fully developed | Near-vertical descent | Attitude, angles, motion repetitious and stabilized turn to turn; held by balance of aerodynamic and inertia forces and moments |
Incipient AoA (TP 1102):
- About half-turn: aircraft pointed almost straight down, but AoA usually already above the stall (inclined flight path).
- Near end of first turn: nose may come back up while AoA keeps increasing.
Forces and speed (TP 1102 Ex.13, Ex.14):
- Forces: somewhat above normal, relatively steady.
- Airspeed: low, at or about stalling speed.
Entry needs a full stall, not a nose-down attitude (TP 1102):
- AoA is key; high nose-up attitude not necessary.
- Can start from descending, level, or climbing attitude, or from an accelerated stall.
- Wing must be FULLY stalled, else likely a skidding spiral of increasing airspeed, not a spin.
Trap: assuming a spin needs a steep nose-down start. AoA, not pitch attitude, triggers it; entry possible from level or climbing flight.
References
- TP 1102 (Aeroplane Flight Training Manual), Ex. 13 Spinning: Autorotation, The Incipient Stage, The Fully Developed Stage, Entry
- TC sample exam Q21 (TP 13014E)
- TP 1102 Ex. 14 Spiral (airspeed in a spin is constant and low)