Angle of attack oscillation

Oscillation (pitching) around the transverse axis.

The angle of attack oscillation , also known as alpha oscillation ( short period ), describes an oscillation around the transverse axis in flight mechanics .

Appearance

The angle of attack oscillation is an oscillatory movement around the transverse axis , whereby the angle of attack changes, but the flight speed and orbit angle remain almost constant. Depending on the type of aircraft and flight speed, this oscillation occurs with frequencies of a few Hertz . In configurations with a separate tail unit , it is very strongly damped and decays before there is an oscillatory movement, but in flying wing aircraft it can appear as a vibration due to the lower damping .

calculation

To calculate the angle of attack oscillation, the equations of motion are set up and their coefficients are written into a matrix . The eigenvalues ​​of the matrix are two conjugate complex value pairs; they correspond to the frequencies of the eigenmodes of the angle of attack oscillation and the phygoids . The eigenvectors describe the appearance of the eigenmode.

Occur

The angle of attack oscillation occurs as a response of the aircraft to elevator inputs and can lead to flutter in unfavorable circumstances . In the case of a raised flying wing with a swept wing, it can couple with the bending vibration of the wing, and both vibrations stimulate each other when their natural frequencies approach.

In aircraft with very sensitive altitude controls, the pilot can unconsciously fan this oscillation if he oversteers and tries to compensate for this control input. If its response delay is close to half the period of the angle of attack oscillation , its control signal will only be issued when the aircraft itself has already returned to the trimmed angle of attack and amplify the oscillation. This is induced by the pilot oscillation ( pilot-induced oscillation ).

annotation

1. ↑ The orbit angle is calculated from the pitch angle (angle of the aircraft to the horizon) minus the angle of attack.

literature

• Xaver Hafer, Gottfried Sachs: Flight Mechanics . Springer-Verlag, Berlin 2002.