Spinning

Spinning of an aircraft is a post-stall phenomenon (see [5]). An aircraft stall occurs in the longitudinal plane. Unavoidable manufacturing asymmetry in geometry and/or asymmetric load application makes one wing stall before the other. This creates a rolling moment and causes an aircraft to spin around the vertical axis, following a helical trajectory while losing height – even though the elevator has maintained in an up position. The vertical velocity is relatively high (i. e., descent speed on the order of 30 to 60 m/sec), which maintains adequate rudder authority, whereas the wings have stalled, losing aileron authority. Therefore, recovery from a spin is by the use of the rudder, provided it is not shielded by the H-tail (see Section 4.9). After straightening the aircraft with the rudder, the elevator authority is required to bring the aircraft nose down in order to gain speed and exit the stall.

Spinning is different than spiraling; it occurs in a helical path and not in a spiral. In a spiral motion, there is a large bank angle; in spinning, there is only a small bank angle. In a spiral, the aircraft velocity is sufficiently high and recovery is primarily achieved by using opposite ailerons. Spin recovery is achieved using the rudder and then the elevator.

There are two types of spin: a steep and a flat-pitch attitude of an aircraft. The type of spin depends on the aircraft inertia distribution. Most general-aviation air­craft have a steep spin with the aircraft nose pointing down at a higher speed, making recovery easy – in fact, the best aircraft recover on their own when the controls are released (i. e., hands off). Conversely, the rudder authority in a flat spin may be low. A military aircraft with a wider inertia distribution can enter into a flat spin from which recovery is difficult and, in some cases, impossible. A flat spin for transport aircraft is unacceptable. Records show that the loss of aircraft in a flat spin is pri­marily from not having sufficient empennage authority in the post-stall wake of the wing.

The prediction of aircraft-spinning characteristics is still not accurate. Although theories can establish the governing equations, theoretical calculations are not nec­essarily reliable because too many variables are involved that require accurate val­ues not easily obtainable. Spin tunnels are used to predict spin characteristics, but the proper modeling on a small scale raises questions about its accuracy. In partic­ular, the initiation of the spin (i. e., the throwing technique of the model into the tunnel) is a questionable art subjected to different techniques. On many occasions, spin-tunnel predictions did not agree with flight tests; there are only a few spin tun­nels in the world.

The best method to evaluate aircraft spinning is in the flight test. This is a rel­atively dangerous task for which adequate safety measures are required. One safe method is to drop a large “dummy” model from a flying “mother” aircraft. The model has onboard, real-time instrumentation with remote-control activation. This is an expensive method. Another method is to use a drag chute as a safety measure during the flight test of the piloted aircraft. Spin tests are initiated at a high altitude; if a test pilot finds it difficult to recover, the drag chute is deployed to pull the air­craft out of a spin. The parachute is then jettisoned to resume flying. If a test pilot is under a high g-strain, the drag chute can be deployed by ground command, where the ground crew maintains real-time monitoring of the aircraft during the test. Some types of military aircraft may not recover from a spin once it has been established. If a pilot does not take corrective measures in the incipient stage, then ejection is the routine procedure. FBW technology avoids entering spins because air data rec­ognize the incipient stage and automatic-recovery measures take place.

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