If the center of gravity of the airplane is behind the aft limit approved for spinning, any spin may prove to be unrecoverable except by using some special spin-recovery device such as a spin-recovery parachute specially installed in the tail of the airplane which was offered for aircraft starting in the mid-1930s; or by jettisoning specially installed ballast at the tail of the airplane.
In the past, some airplanes displayed an unrecoverable spin in which the nose was higher, relative to the horizon, than in conventional spins. This is sometimes called a flat spin, although whether a flat spin is indeed unrecoverable depends on aircraft type and loading. The plane spins on its belly around the normal axis. The empennage will feel very light and loose. Depending on the aircraft, changing the rudder and aileron inputs or engine power settings may have little effect. There are a small number of accounts where pilots recovered from flat spins by loosening their restraint harnesses and leaning forward in an attempt to alter the position of the center of gravity.
Some World War II airplanes were notoriously prone to flat spins when loaded erroneously, such as the Bell P-39 Airacobra. The P-39 was a unique design with the engine behind the pilot's seat and a large cannon in the front. Without ammunition or a counterbalance load in the nose compartment, the P-39's center of gravity was too far aft to recover from a spin. Soviet pilots did numerous tests of the P-39 and were able to demonstrate its dangerous spinning characteristics. Bell then issued a recommendation to bail out if the airplane entered a spin. North American P-51 Mustangs with auxiliary fuel tanks not originally designed for the P-51 suffered from the same problem. Similarly, the Vought F4U Corsair was reputed to have appalling stall and spin recovery characteristics, even in the "clean" (no stores) configuration.
Modern fighter aircraft are not immune to the phenomena of unrecoverable spin characteristics. Although highly resistant to entering into a spin, once caught in one the Grumman F-14 Tomcat can exhibit a fast, flat spin from which it is nearly impossible to recover. Another example of a nonrecoverable flat spin occurred in 1963, with Chuck Yeager at the controls of the NF-104A rocket-jet hybrid: during his fourth attempt at setting an altitude record, Yeager lost control and entered a flat spin, then ejected and survived. On the other hand, the Cornfield Bomber was a case where the ejection of the pilot shifted the center of gravity enough to let the now empty aircraft self-recover from a spin and successfully land itself.
An airplane spin tends to flatten as it progresses because then its mass is distributed furthest from its center of rotation, as rotating objects tend to rotate about their axis of maximum rotational inertia. Aircraft have their maximum rotational inertia when spinning on their normal axis, i.e. flatly.
In purpose-built aerobatic aircraft, spins may be intentionally flattened through the application of power and aileron within a normal spin. Rotation rates experienced are dramatic and can exceed 400 degrees per second in an attitude that may even have the nose above the horizon. Such maneuvers must be performed with the center of gravity in the normal range and with appropriate training, and consideration should be given to the extreme gyroscopic forces generated by the propellor and exerted on the crankshaft.
Read more about this topic: Tailspin