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Since a propeller has gyroscopic characteristics it will tend to stay in its plane of rotation until it is diSplaced by some strong external force. when such a force or moment is applied, the propeller reacts in a direction 90 degrees to the force. For example, if the propeller is displaced upward the reSistance of the structure applies a nosedown pitching moment causing the propeller disc to swing to the left due to precession. The yaw stiffness resists this motion causing preceSSion downward, reSisted by pitching stiffness which produces a precessional swing to the right. This, in turn, is resisted to cauSe an upward pTBCBSSlon to complete the cycle. This effect is termed "whirl mode," and its direction of rotation is counter to that of the propeller.
Normally, whirl mode can operate only within the fleXibility limits of the engine mounting structure and is quickly damped. If, however, the stiffness of the supporting system is reduced through improperly installed, failed, or damaged powerplant structure, mounts, or nacelle structure, the damping of whirl mode is reduced to a degree depending on the amount of stiffness reduction.
Structural weakness or damage does not change the conditions under which whirl mode may be initiated, but in three ways it makes the phenomenon a potential
danger:
l. The greater flexibility of a weakened system can allow whirl mode more freedom, hence it can become more violent. In an undamaged system the stiffness increases with increasing deflections but this is not necessarily true if the
structure is damaged.
2. In a weakened installation, the increasing Violence of whirl mode can further,damage the supporting structure, in turn leading progressively to more violence and even further damage.
3% As the structural system.is damaged, reducing the spring~constant, the amplitude of whirl mode increasss and the frequency decreaSes from its natural value to lower values which, in the case of the Electra, approach the wing
fundamental frequencies.
The natural frequency of whirl mode in an undamaged installation is approxi- mately five cycles per second. The wing torsional frequency is about 3.5, and wing bending about two cycles per second, with some slight variation with fuel
loading.
As whirl mode progresses in an overly flexible or damaged powerplent instal— lation, its frequency can reduce from five to three c.p.s. where it will drive the wing in three c.p.s. torsional and bending oscillations. These wing oscils lations will re—enforce and perpetuate the whirl mode. The three oscillations are then coupled at the same frequency of about three c.p.s., thus becoming a form of induced flutter forced by a powerful harmonic oscillation. This phenome— non can exist, as demonstrated in wind tunnel tests and in analytical methods, at an airspeed far below that at which classical flutter can develop.
The design stiffness factor for an Electra powerplant installation is 15.9 x 10_ inch pounds per radian (root—mean-square). The tests indicated that at this stiffness level whirl mode cannot force wing oscillation at an airspeed lower
