SPINS AND PROBLEMS OF SPIN RECOVERY

The motion of an airplane in a spin can involve many complex aerodynamic and in­ertia forces and moments. However, there are certain fundamental relationships regarding spins and spin recoveries with which all aviators should be familiar. The spin differs from a spiral dive in that the spin always involves flight at high angle of attack while the spiral dive involves a spiral motion of the airplane at relatively low angle of attack.

The stall characteristics and stability of the airplane at high lift coefficients are im­portant in the initial tendencies of the airplane. As previously mentioned, it is desirable to have the wing initiate stall at the root first rather than tip first. Such a stall pattern prevents the undesirable rolling moments at high lift coefficients, provides suitable stall

warning, and preserves lateral control effec­tiveness at high angles of attack. Also, the airplane must maintain positive static longi­tudinal stability at high lift coefficients and should demonstrate satisfactory stall recovery characteristics.

In order to visualize the principal effects of an airplane entering a spin, suppose the air­plane is subjected to the rolling and yawing velocities shown in figure 4.32. The yawing velocity to the right tends to produce higher local velocities on the left wing than on the right wing. The rolling velocity tends to increase the angle of attack for the downgoing right wing Car) and. decrease the angle of attack for the upgoing left wing Ca0- At airplane angles of attack below the stall this relationship produces roll due to yaw, damping in roll, etc., and some related motion of the airplane in unstalled flight. However, at angles of attack above the stall, important changes take place in the aerodynamic char­acteristics.

Figure 4-32 illustrates the aerodynamic characteristics typical of a conventional air­plane configuration, i. e., moderate or high aspect ratio and little—if any—sweepback. If this airplane is provided a rolling displace­ment when at some angle of attack above the stall, the upgoing wing experiences a decrease in angle of attack with a correspond­ing increase in CL and decrease in CD. In other words, the upgoing wing becomes less stalled. Similarly, the downgoing wing experiences an increase in angle of attack with a corre­sponding decrease in CL and increase in CD. Es­sentially, the downgoing wing becomes more stalled. Thus, the rolling motion is aided rather than resisted and a yawing moment is produced in the direction of roll. At angles of attack below stall the rolling motion is resisted by damping in roll and adverse yaw is usually present. At angles of attack above the stall, the damping in roll is negative and a rolling motion produces a roiling moment in the direction of the roll. This negative damping in roll is generally referred to as “autorotation."

When the conventional airplane is stalled and some rolling-yawing displacement takes place, the resulting autorotation rolling mo­ments and yawing moments start the airplane into a self-sustaining rolling-yawing motion. The autorotation rolling and yawing tenden­cies of the airplane at high angles of attack are the principal prospin moments of the conventional airplane configuration and these tendencies accelerate the airplane into the spin until some limiting condition exists. The stabilized spin is not necessarily a simple steady vertical spiral but may involve some coupled unsteady oscillatory motion.

An important characteristic of the more conventional airplane configuration is that the spin shows a predominating contribution of the autorotation tendency. Generally, the conventional configuration has a spin motion which is primarily rolling with moderate yaw. High directional stability is favorable since it will limit or minimize the yaw displacement of the spinning airplane.

The fundamental requirement of the spin is that the airplane be placed at an excessive angle of attack to produce the autorotation rolling and yawing tendencies. Generally speaking, the conventional airplane must be stalled before a spin can take place. This relationship establishes a fundamental prin­ciple of recovery—the airplane must be un­stalled by decreasing the wing angle of attack. The most effective procedure for the conven­tional configuration is to use opposite rudder to stop the sideslip, then lower the angle of attack with the elevators. With sufficient rudder power this procedure will produce a positive recovery with a minimum loss of altitude. Care should be taken during pullout from the ensuing dive to prevent excessive angle of attack and entry into another spin.

It should be appreciated that a spin is always a possible corollary of a stall and the self­sustaining motion of a spin will take place at

excessive angles of attack. Of course, a low speed airplane could be; designed to be spin – proof by making it stallproof. By limiting the amount of control deflection, the airplane may not have the longitudinal control power to trim to maximum lift angle of attack. Such a provision may be possible for certain light planes and commercial aircraft but would create an unrealistic and impractical limita­tion on the utility of a military airplane.

The modem high speed airplane configura­tion is typified by low aspect ratio, swept wing planforms with relatively large yaw and pitch inertia. The aerodynamic characteristics of such a configuration are shown in figure 4.32. The lift curve (Cl versus a) is quite shallow at high angles of attack and maximum lift is not clearly defined. When this type of airplane is provided a rolling motion at high angles of attack, restively small changes in CL take place. When this effect is combined with the relatively short span of this type airplane, it is apparent that the wing autorotation contribu­tion will be quite weak and will not be a pre­dominating pro-spin moment. The relatively large changes in drag coefficient with rolling motion imply a predominance of yaw for the spin of the high speed airplane configuration.

Actually, various other factors contribute to the predominating yaw tendency for the spin of the modern airplane configuration. The static directional stability deteriorates at high angles of attack and may be so weak that extemely large yaw displacements result. In certain instances, very high angles of attack may bring such a decay in directional stability that a “slice” or extreme yaw displacement takes place before a true spin is apparent. At these high angles of attack, the adverse yaw due to roll and aileron deflection can be very strong and create large yaw displacements of the airplane prior to realizing a stall.

The aircraft with the relatively large, long fuselage can exhibit a significant moment con­tribution from the fuselage alone. The cross flow pattern on the fuselage at high angles of attack is capable of producing pro-spin mo­ments of considerable magnitude which con­tribute to the self-sustaining nature of the spin. Also, the large distributed mass of the fuselage in rolling-yawing rotation contributes to inertia moments which flatten the spin and place the aircraft at extreme angles of attack.

The spin recovery of the modern high speed airplane involves principles which are similar to those of the spin recovery of the conven­tional airplane. However, the nature of the spin for the modern configuration may involve specific differences in technique necessary to reduce the sideslip and angle of attack. The use of opposite rudder to control the sideslip and effect recovery will depend on the effective­ness of the rudder when the airplane is in the spin. At high positive angles of attack and high sideslip the rudder effectiveness may be reduced and additional anti-spin moments must be provided for rapid recovery. The deflection of ailerons into the spin reduces the autorota­tion rolling moment and can produce adverse yaw to aid the rudder yawing moment in effecting recovery.

There may be many other specific differences in the technique necessary to effect spin re­covery. The effectiveness of the rudder during recovery may be altered by the position of elevators or horizontal tail. Generally, full aft stick may be necessary during the initial phase of recovery to increase the effectiveness of the rudder. The use of power during the spin recovery of a propeller powered airplane may or may not aid recovery depending on the specific airplane and the particular nature of the slipstream effects. The use of power during the spin recovery of a jet powered airplane induces no significant or helpful flow but does offer the possibility of a severe compressor stall and adverse gyroscopic moments. Since the airplane is at high angle of attack and sideslip, the flow at the inlet may be very poor and the stall limits considerably reduced. These items serve to point out possible dif­ferences in technique required for various con­figurations. The spin recovery specific for

NAVWEPS 00-80T-80

stability and control

each airplane is outlined in the pilot’s hand­book and it is imperative that the specific tech­nique be followed for successful recovery.