Lateralldirectional oscillation

The Lateral/Directional Oscillation (LDO) of a helicopter can be regarded as the lateral/directional equivalent of the longitudinal long-term mode. In most rotorcraft the LDO is very similar to the Dutch roll mode found in conventional fixed wing aircraft. Depending on the relative magnitudes of lateral static stability and directional static stability the Dutch roll will be either convergent or divergent, and highly oscillatory or deadbeat. The characteristics of the mode may also change markedly with flight condition as fin effectiveness can be strongly influenced by the skew angle of the main rotor wake. The ease of excitation, ease of suppression, the frequency, the roll-to-yaw ratio and the intended role of the helicopter will determine if the LDO can be tolerated without augmentation.

6.5.1.3 Spiral stability

For rotorcraft, the aerodynamic factors contributing to good LDO characteristics will often degrade the spiral stability. At high angles of bank the weight of the vehicle will produce a large into-turn moment which may overcome the existing aerodynamic forces thus degrading stability still further. It may be impossible to endow the helicopter with acceptable LDO and spiral mode characteristics throughout its flight envelope without resorting to artificial aids.

6.5.1.4 Directional static stability

The directional static stability characteristics of a helicopter can be poor, especially at low values of forward speed and low angles of sideslip. The main cause of this phenomenon is the lack of adequate fin effectiveness at low values of lateral velocity or total velocity. Blanking of the fin by the fuselage or inadequate tailoring of the main rotor wake are typical reasons. The resulting low values of Nv may lead to poor natural sideslip control that may be unacceptable in certain missions, such as ground attack. Directional stability may be further degraded if there is a large amount of side area towards the front of the fuselage, such as when external stores are carried or floats fitted. Aerodynamic fixes may be difficult to engineer and are usually expensive to install. Often the most suitable solution is stability augmentation in the form of a yaw damper or the provision of a heading hold.

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