Modern aircraft tend to be more reluctant to spin compared to older designs, however it is not impossible for them to spin. Mishandling the controls in turns, stalls, and flight at minimum controllable airspeeds can put even the most reluctant aircraft into an accidental spin. Proficiency in avoiding conditions that could lead to an accidental stall/spin situation, and in promptly taking the correct actions to recover to normal flight, is essential. An aircraft must be stalled and yawed in order to enter a spin; therefore, continued practice in stall recognition and recovery helps the pilot develop a more instinctive and prompt reaction in recognizing an approaching spin. Upon recognition of a spin or approaching spin, the pilot should immediately execute spin recovery procedures.

A well understood spin behavior is vital to a trainer aircraft. Therefore, Spin Testing forms an important part in the Flight Test program for such an aircraft.

Basics

A spin is an aggravated stall that typically occurs from a ‘full stall’ occurring with the aircraft in a yawed state and results in the aircraft following a downward ‘corkscrew‘ path. As the aircraft rotates around a vertical axis, the outboard wing is less stalled than the inboard wing, which creates a rolling, yawing, and pitching motion. The aircraft is basically descending due to gravity, rolling, yawing, and pitching in a spiral path. The rotation results from an unequal AOA on the aircraft’s wings. The less-stalled rising wing has a decreasing AOA, where the relative lift increases and the drag decreases. Meanwhile, the descending wing has an increasing AOA, which results in decreasing relative lift and increasing drag.

A spin occurs when the aircraft’s wings exceed their critical AOA (stall) with a sideslip or yaw acting on the aircraft at, or beyond, the actual stall. An aircraft will yaw not only because of incorrect rudder application but because of adverse yaw created by aileron deflection; engine/prop effects, including p-factor, torque, spiraling slipstream, and gyroscopic precession; and wind shear, including wake turbulence. If the yaw had been created by the pilot because of incorrect rudder use, the pilot may not be aware that a critical AOA has been exceeded until the aircraft yaws out of control toward the lowering wing. A stall that occurs while the aircraft is in a slipping or skidding turn can result in a spin entry and rotation in the direction of rudder application, regardless of which wingtip is raised. If the pilot does not immediately initiate stall recovery, the aircraft may enter a spin.

Entry to Spin

In the entry phase, the pilot intentionally (or accidentally) provides the necessary elements for the spin.

The standard entry procedure for a spin is similar to a power-off stall. During the entry, the pilot should slowly reduce power to idle, while simultaneously raising the nose to a pitch attitude that ensures a stall. As the aircraft approaches a stall, smoothly apply full rudder in the direction of the desired spin rotation while applying full back (up) elevator to the limit of travel. Maintain the ailerons in the neutral position during the spin procedure.

Variations in entry procedure, including inverted flight can be considered for testing the spin susceptance of the aircraft.

Phases of a Spin:

  • Incipient Phase:
    • The transition from a stall to a full spin is called the incipient phase
    • During this phase, the aerodynamic forces are not balanced and the aircraft is more or less tumbling.
    • A further distinction between the Post Stall Gyration (PSG) and the incipient phase of the spin is that the angle of attack is continuously above the stall angle of attack for the aircraft in the incipient phase of the spin. Whereas during a PSG, the angle of attack may intermittently be less than stall angle of attack.
  • Developed Phase:
    • The developed phase begins when the aerodynamic and inertial forces and moments are approaching equilibrium.
    • The spin, although chaotic looking from inside the cockpit, has been established and it is possible to identify the spin mode
    • Recovery procedures are now necessary to break the spin
    • A fully developed spin is one in which the trajectory has become vertical and no significant change in the spin characteristics is noted from turn to turn.
  • Recovery Phase:
    • During the recovery phase, controls are applied to stop spin, recover from the unusual attitude and maintain straight and level
#Picture: FAA Flying Handbook(2016)

  • In a spin, one or both wings are in a stalled condition, if both are stalled one wing will be in a deeper stall condition than the other. 
  • The wing that stalls first will drop, increasing its angle of attack and deepening the stall. Both wings must be stalled for a spin to occur.  The other wing will rise, decreasing its angle of attack, and the aircraft will yaw toward the more deeply-stalled wing.
  • The difference in lift between the two wings causes the aircraft to roll, and the difference in drag causes the aircraft to yaw.
  • The yaw rotation is dominant in characterizing a spin.
  • To a pilot, the recognition of a sustained (though not necessarily steady) yaw rate is probably the most important visual cue that a spin is occurring. Even though roll rate and yaw rate are often of nearly the same magnitude, the pilot still ordinarily recognizes the spin because of the yaw rate.

Spin Modes and Descriptive Modifiers

  • Sense: Erect (Upright, +ve AoA), Inverted (-ve AoA)
  • Attitude: Extremely Steep, Steep, Flat
  • Rate: Slow, Fast, Extremely Rapid
  • Oscillations: Smooth, Mildly Oscillatory, Oscillatory, Highly Oscillatory, Violently Oscillatory

Recovery

Recovery is defined as the transitional event from out-of-control conditions to controlled flight, that point at which the angle of attack is below stall AoA and no significant uncommanded angular motions remain.

Recovery Types to be Considered for Testing

  • Standard/PARE Recovery- (Power to Idle >Ailerons Neutral>Rudder opposite>Elevator Forward)
  • Simultaneous
  • Rudder-Only
  • Elevator-Only
  • In-Spin Aileron
  • (Parke-)Müller-Beggs Emergency Recovery- (Power to Idle>Hands off from the stick> opposite rudder until rotation>Neutralize rudder )
  • All Controls Free
  • All Controls Neutral (Out-of-Control Recovery)

As a rough estimate, an altitude loss of approximately 500 feet per each 3-second turn can be expected in most aircraft in which spins can be demonstrated for training.

Notes:

  • Controls which are effective in normal flight may be inadequate for recovery from the spin unless sufficient consideration has been given to this problem in the design phase.
  • Recovery from inverted spins is generally easier than recovery from upright spins, since it is more likely that the rudder is in undisturbed airflow (most aircraft recover from an inverted spin as soon as the controls are neutralized).
  • A flat spin is characterized by a near level pitch and roll attitude with the spin axis near the CG of the airplane. Recovery from a flat spin may be extremely difficult and, in some cases, impossible.
  • A Post-Stall Gyration (PSG) is an uncontrolled (random or not random) motion around one or more aircraft axes following departure in the post-stall flight regime (MIL-F-83691B). A basic rectangular wing trainer is unlikely to exhibit the random motions of a swept-wing, fuselage-loaded military aircraft, and will often go straight into an autorotation after departure from the linear flight regime, if the aircraft is stalled with pro-spin control inputs. However, the a predominant yaw is to be present in order for it to count as a spin, and therefore all non-random, non-yawing motions, including rolling departures, snap rolls, and tumbles are included in the definition of PSG. A spin can start predominantly as roll, and substantial yaw develops only later.
  • The Spiral mode is an autorotation mode similar to a spin. The center of rotation is close to the center-line of the aircraft but the aircraft is not stalled. Many aircraft and gliders will not spin at forward CG locations but will spiral. Many aircraft will enter a spin but the spin will become more vertical and degenerate into a spiral. It is important to note that when the spin transitions into the spiral the airspeed will increase as the nose goes down to near vertical. The side forces on the aircraft build very rapidly and recovery must be effected immediately before exceeding the structural limits of the airplane.


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