Answer:
The possible modes of lateral-directional motion of an aircraft when disturbed slightly from steady flight are divergence and subsidence.
When an aircraft is disturbed slightly in the lateral plane, its motion can either diverge or subside depending on the sign of (LvNr−LrNv), where:
- Lv is the rolling moment due to sideslip (yaw) velocity
- Nr is the yawing moment due to rolling velocity
- Lr is the rolling moment due to yawing velocity
- Nv is the yawing moment due to sideslip velocity
To understand this, let's consider two scenarios:
1. Divergence: If (LvNr−LrNv) is positive, the aircraft will experience divergence motion. This means that any slight disturbance will cause the aircraft to roll or yaw more and more, leading to an unstable condition.
2. Subsidence: If (LvNr−LrNv) is negative, the aircraft will experience subsidence motion. In this case, any slight disturbance will cause the aircraft to gradually return to its steady flight condition, indicating a stable behavior.
Now, let's discuss how the stability of an aircraft in this mode changes with an increase in fin size. The fin of an aircraft, also known as the vertical stabilizer or the vertical tail, plays a crucial role in maintaining lateral-directional stability.
With an increase in fin size, the stability of the aircraft in the lateral-directional mode also improves. A larger fin provides more surface area and leverage, allowing it to generate more corrective forces to counteract any disturbances in the lateral plane. As a result, the aircraft becomes more stable and less prone to divergence or subsidence.
In summary, the possible modes of lateral-directional motion when an aircraft is disturbed slightly from steady flight are divergence and subsidence. The sign of (LvNr−LrNv) determines whether the motion will be divergent or subsiding. Increasing the fin size improves the stability of the aircraft in this mode by providing more corrective forces to counteract disturbances.