11 WWW.GOFLY.ONLINE Y ONLI NE FLI GHT SCHOOL PRINCIPLES OF FLIGHT motions. A forward CG position may decrease elevator effectiveness, requiring more force or deflection to control pitch. Conversely, moving the CG aft can decrease longitudinal stability but may increase elevator effectiveness. Changes in the centre of pressure If the centre of pressure (CP) shifts forward, it can destabilise the aircraft longitudinally, making it more prone to pitch-up tendencies. A forward shift in CP may require increased elevator deflection to maintain pitch control, while a rearward shift may reduce elevator effectiveness. Changes in thrust An increase or decrease in power/thrust in most light aircraft will affect its longitudinal stability. In a single engine aircraft, when power is increased the nose will pitch up, and when power is reduced the nose will pitch down. Tail down force The tail plane of an aircraft is generally set at an angle of attack that will provide a down force at the rear of the aircraft in level flight, ensuring longitudinal stability. It helps counteract pitching moments, enhances the aircraft’s static margin, and affects elevator effectiveness, all of which contribute to the aircraft’s ability to maintain a steady pitch attitude. Lateral Stability Lateral stability refers to an aircraft’s stability when rolling, which involves the rotation of the aircraft around its longitudinal axis (nose to tail). An aircraft with lateral stability will have the tendency to return to wings level after a roll. High wing versus low wing A high wing aircraft has the wing located above the aircraft fuselage. Due to the mass being located below the lift, the aircraft tends to be very stable. Being very stable has its benefits, however this usually translates into being not as responsive as a low wing aircraft. A low wing aircraft has the mass located above the lift and is therefore less stable than a high wing aircraft. This also makes a low wing aircraft more manoeuvrable in roll than a high wing aircraft. To make a low wing aircraft more stable, they often have more dihedral than a high wing aircraft. High wing aircraft tend to be more laterally stable than a low wing aircraft. Dihedral versus anhedral The upward angle of the wings in relation to a straight line reference point, running from the front of the aircraft to the back, is called the aircraft dihedral. A larger dihedral angle generally means the aircraft is more stable in the longitudinal axis, that is, dihedral increases lateral stability. A downward angle of the wings in relation to a straight line reference point, running from the front to the back of the aircraft, is called anhedral. Anhedral decreases lateral stability. This can be an advantage for large freight aircraft, such as the Antonov, which carries heavy cargo weights which would otherwise make the aircraft hard to roll. Sweepback Most low speed light aircraft do not have swept back wings. The swept back wing is normally only used in high speed aircraft to lower drag and compressibility issues experienced in high speed flight. Sweepback enhances lateral stability, similar to dihedral, but not as pronounced. Sweepback and dihedral are often used together. Directional Stability Directional stability refers to an aircraft’s stability in yaw, where the aircraft rotates around its vertical axis. Changes in centre of gravity Moving the centre of gravity (CG) forward tends to decrease directional stability. A forward CG shifts the aerodynamic forces acting on the aircraft, reducing the effectiveness of the vertical stabiliser and rudder in counteracting yawing moments. Moving the CG aft tends to increase directional stability, making the aircraft more resistant to yaw disturbances. Differences in fin and rudder size Larger fin size and longer rudder moment arms generally contribute to greater directional stability. A larger fin size
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