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Unformatted text preview: Item 0940 OTHER: Flight Dynamics- General- Precession - Gyroscopic and Aerodynamic Positional Statement; 1. Gyroscopic precession (torque-induced precession) explains the activity inside the flapping/teetering hinges. 2. Aerodynamic precession, including non-gyroscopic dynamics, explains the activity outside the flapping/teetering hinges. 3. An exception to 2. is the rigid rotor where, because of the flapping stiffness, some of the blade that is out is of the virtual hinge will partially contribute to the gyroscopic precession. Therefor; & Gyroscopic precession has nothing to do with teetering rotors. & Gyroscopic precession has very little to do with all other rotors. Elaboration: The reference to gyroscopic precession was an easy-to-understand method of explaining aerodynamic precession to those who did not need to know the intricacies of the rotor. This was particularly true in the past, when gyrocopters and most helicopters had teetering or gimbaled rotors; and the phase angle was 90. All modern helicopter rotors have phase angles that are less than 90. In addition: The rotor of a helicopter has very little inertia for its size. In fact, most pilots would like it to have more inertia during autorotation. I.e. The rotational speed of a helicopter rotor is very slow compared to that of a gyroscope. The mass at the circumference of a helicopter rotor is very small compared to that of a gyroscope. The following excerpt is the conclusion of a mathematical description about gyroscopic precession; "Now is the time to confess that there is an implicit assumption buried in our reasoning. We have assumed that the angular momentum was all due to the rotation of the rotor. In fact, the precessional motion also contributes to the total angular momentum. Our analysis is valid only as long as [the precessional frequency] is much smaller than [the angular velocity]. This condition is met when [the angular momentum] is large compared to [the applied torque]. & Otherwise, the motion of the gyroscope is much more complicated, as you might observe in an actual experiment where the rotation of the rotor slows down over time. We can see that as the rotor slows, the precessional frequency increases. At some point when the precessional frequency exceeds a critical value, the gyroscope will begin to wobble and eventually tumble in its gimbals." In other words; the rotational inertia of the helicopter's rotor visa vie, the to the aerodynamic tipping force of the cyclic, is too small for equations of gyroscopic precession to apply. Overview: The realignment the rotor disk, due to control input or perturbations can be explained; 1. solely in aerodynamic terms, or 2. in terms of an integrated gyroscopic precession and aerodynamic precession , but 3. not solely in gyroscopic terms....
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This note was uploaded on 01/05/2011 for the course DU 3 taught by Professor Frando during the Spring '10 term at University of Dundee.
- Spring '10