Tailspin - Modes

Modes

The US National Aeronautics and Space Administration (NASA) has defined four different modes of spinning. These four modes are defined by the angle of attack of the airflow on the wing.

During the 1970s NASA used its spin tunnel at the Langley Research Center to investigate the spinning characteristics of single-engine general aviation airplane designs. A 1/11-scale model was used with nine different tail designs.

Some tail designs that caused inappropriate spin characteristics had two stable spin modes – one steep or moderately steep; and another that was either moderately flat or flat. Recovery from the flatter of the two modes was usually less reliable or impossible. The further aft that the center of gravity was located the flatter the spin and the less reliable the recovery. For all tests the center of gravity of the model was at either 14.5% of Mean Aerodynamic Chord (MAC) or 25.5% of MAC.

Single-engine airplane types certified in the normal category must be demonstrated to recover from a spin of at least one turn, while single-engine aircraft certified in the utility category must demonstrate a six turn spin that cannot be unrecoverable at any time during the spin due to pilot action or aerodynamic characteristic. NASA recommends various tail configurations and other strategies to eliminate the flatter of the two spin modes and make recovery from the steeper mode more reliable.