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# SPIRAL DIVERGENCE:

Table of Contents:

  • What Is Spiral Divergence?
  • Example – Spiral Divergence
  • Recovery From Spiral Divergence

What Is Spiral Divergence

Spiral divergence or spiral instability is a condition where an aircraft is directionally very stable, but laterally very unstable. It is characterized by low angle of attack and high airspeed. When the lateral equilibrium of the aircraft is disturbed by a gust of air and a sideslip is introduced, the strong directional stability tends to yaw the nose into the resultant relative wind while the comparatively weak dihedral lags in restoring the lateral balance. 

Due to this yaw, the wing on the outside of the turning moment travels forward faster than the inside wing and, as a consequence, its lift becomes greater. This produces an overbanking tendency which, if not corrected by the pilot, results in the bank angle becoming steeper and steeper. At the same time, the strong directional stability that yaws the aircraft into the relative wind is actually forcing the nose to a lower pitch attitude. A slow downward spiral begins which, if not counteracted by the pilot, gradually increases into a steep spiral dive. Usually the rate of divergence in the spiral motion is so gradual the pilot can control the tendency without any difficulty. 

Many aircraft are affected to some degree by this characteristic, although they may be inherently stable in all other normal parameters. This tendency explains why an aircraft cannot be flown “hands off” indefinitely. A spiral divergence is not a type of spin because neither wing is stalled. In a spiral divergence, the aircraft will respond conventionally to the pilot’s inputs to the flight controls. 

Example – Spiral Divergence

For example, let’s consider a large finned aircraft with no dihedral. In this case, when the aircraft is in a bank and side-slipping, the side force tends to turn the plane into the relative wind. The outer wing travels faster, generates more lift, and the airplane will roll to still a higher bank angle. No lateral stability is present to negate this roll. The bank angle increases and the airplane continues to turn into the sideslip in an ever-tightening spiral.

spiral_divergence

Much research has gone into the development of control devices (wing leveler) to correct or eliminate this instability. The pilot must be careful in application of recovery controls during advanced stages of this spiral condition or excessive loads may be imposed on the structure. Improper recovery from spiral instability leading to inflight structural failures has probably contributed to more fatalities in general aviation aircraft than any other factor.

Since the airspeed in the spiral condition builds up rapidly, the application of back elevator force to reduce this speed and to pull the nose up only “tightens the turn,” increasing the load factor. The results of the prolonged uncontrolled spiral are inflight structural failure, crashing into the ground, or both. Common recorded causes for pilots who get into this situation are loss of horizon reference, inability to control the aircraft by reference to instruments, or a combination of both.

Recovery From Spiral Divergence

A diving aircraft has more kinetic energy (which varies as the square of speed) than when straight-and-level. To get back to straight-and-level, the recovery must get rid of this excess energy safely. The sequence is: Power all off; level the wings to the horizon or, if horizon has been lost, to the instruments; reduce speed using gentle back-pressure on the controls until a desired speed is reached; level off and restore power. The pilot should be alert to a pitch up tendency as the aircraft is rolled to wings level.

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