MOD 8.4 PART 2.pdf

British Airways Global Learning Academy – Basic Aerodynamics Dutch Roll And Spiral Instability Oscillating instability is not easily corrected by the pilot and if the aircraft is prone to this form of dynamic instability, an automatic yaw damper will be installed in the aircraft. The yaw damper wil...

British Airways Global Learning Academy – Basic Aerodynamics Dutch Roll And Spiral Instability Oscillating instability is not easily corrected by the pilot and if the aircraft is prone to this form of dynamic instability, an automatic yaw damper will be installed in the aircraft. The yaw damper will sense a roll or yaw motion and activate the correct flight controls to overcome them. The effect of dihedral is to roll the aircraft in the direction opposite that in which it is slipping, and the vertical fin will try to yaw the aircraft in the direction of the slip. These two forces both affect the lateral and directional stability of an aircraft. If the dihedral effect is greater than that of the fin, the aircraft will have a tendency to Dutch roll in flight (Figure 13). A Dutch roll is an oscillation of very low magnitude about both the longitudinal and vertical axes. It is objectionable as far as flight comfort, but it is generally not a serious flight condition. If the aircraft is yawed to the right, the left wing advances (sideslip) and generates more lift, whilst the right wing slows down and produces less lift. The result of the imbalance in lift is to roll the aircraft in the direction of the initial yaw. The left wing also produces greater drag due to the larger areas exposed to the airflow, which causes the aircraft to yaw in the opposite direction. Figure 13 – Dutch Roll This results in the right wing producing more lift than the left wing, reversing the direction of the roll. The final result is a snaking motion, where the rolling and yawing oscillations have the same frequency, but are out of phase with each other. Dutch Roll is characterised by a coupling of directional and lateral oscillation that produces the tendency of the aircraft to "wander" about the roll and yaw axes. Dutch Roll generally occurs when an aircraft's dihedral effect is larger than its static directional stability. Module 08B ETBN 0492 October 2023 Edition 28 Basic Aerodynamics – Flight Stability British Airways Global Learning Academy – Basic Aerodynamics Manoeuvrability Manoeuvrability is also an important characteristic of an aircraft design process. This is the ability of an aircraft to be directed along a selected flight path. The smooth and easy response of the aircraft to its controls is very important. Aircraft are designed to have varying degrees of stability depending on their role. If the aircraft lacks manoeuvrability, it will be easy to fly straight and level but difficult and tiring through manoeuvres. It is important to achieve a good balance between stability and manoeuvrability in any aircraft design process. Commercial transport aircraft need to be positively stable because they spend many hours in straight and level cruising flight but machines required to be highly manoeuvrable such as fighters or aerobatic types are only marginally stable and require a lot of attention from the pilot. Module 08B ETBN 0492 October 2023 Edition 29 Basic Aerodynamics – Flight Stability