THE causes and nature of the spinning of an airplane, and measures for the prevention of and recovery from a spin, are discussed. Tests and analysis are said to have shown that spinning is a stable motion of rotation, and that the real dangers are in its instability. Recovery from a spin is held to be more important than prevention, as complete knowledge of means of recovery will lead to mastery of the whole phenomenon.
The spinning motion is a combination and balance of aerodynamic and purely dynamic forces and couples, asserts the author. Full-scale experiments prove beyond doubt that side-slip may be very pronounced in a spin, which changes considerably the rate of roll of the simple autorotational kind. The rolling of the wings leads to the establishing of a yawning couple which may become dangerous, tending to keep the craft in a spin because of the increased shielding of the tail surfaces.
The dangerous spinning tendencies of modern airplanes are stated to be due primarily to the trend of design toward high speed, heavy loading per square foot of wing area, short and deep fuselages, and consequent shielding of the tail surfaces at the high angles of attack that are ordinarily associated with spinning.
Some new disposition of the control surfaces so as to increase their effectiveness and particularly to prevent shielding of the rudder by the body is more important, the author believes, than increase in the size of the stabilizing and control surfaces.
Conceding that automatic front wing-slots prevent the possibility of an incipient spin and are a positive means of recovery from a normal spin, doubt is expressed of their successful use in recovery from a flat spin. It is suggested that it would be interesting to investigate the possibility of using automatic leading-edge slots on the horizontal tail-surfaces.
In conclusion, four characteristics of paramount importance in preventing an incipient spin are given, and five characteristics for assuring recovery are enumerated as desirable.
Discussers who narrate some of their experiments and experiences in spins agree with most of the statements made by Dr. Watter in his paper, including the assertion that very little definite knowledge of flat-spinning is possessed. Some maintain that the spin is an essential defensive maneuver in military aviation. “Flat spin” is thought by one to be an incorrect term for the peculiar dangerous spinning phenomena, as the attitude of the airplane may be either flat or at a very steep angle. The speed at which a pilot goes into a spin and the order in which he operates the rudder and stick is said to have much to do with the development of the spin, the general attitude of the plane, and the time of recovery. Modification of tail surfaces has overcome bad spinning characteristics in some airplanes but not in others. It has been found that any cellular arrangement probably will spin badly if conditions are right for it.
One discusser believes that an airplane that is easy to put into a spin is easy to recover, and vice versa. Inverted Frise balanced ailerons markedly reduced the flat-spinning tendency in one experiment. In another experiment, with the ailerons removed from the upper wings, the plane recovered from the spin in about one turn with reverse control. If the center of gravity of an airplane is far enough forward, a spin cannot be started at all. Increasing the area of the stabilizer and fin reduces the tendency to flat-spin but also reduces maneuverability of the plane. Another speaker found that the rudder is much the most important control surface in recovery from a spin, and thinks that in commercial aviation the ounce of prevention against getting into a spin is worth the proverbial pound of cure. In conclusion, Dr. Watter emphasizes that the problem is to find some positive control that will enable the pilot to bring any airplane out of any spin.