Phygoids
The phygoid is a technical term from flight mechanics and describes a path oscillation of the longitudinal movement.
Appearance
The phygoid is an oscillatory change of potential energy (flight altitude) with kinetic energy (flight speed), which takes place at very low frequencies (typically 30 seconds to several minutes per cycle) and with very little damping . For a flying observer, this looks as if the swinging aircraft describes a standing ellipse around a mean position.
The exact frequency of the phygoids depends primarily on the flight speed and the longitudinal stability .
calculation
To calculate the eigenmodes , 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 (in the longitudinal motion these are the angle of attack oscillation and the phygoids), and the eigenvectors describe the appearance of the eigenmode.
The driving force of the phygoids is the static longitudinal stability of the aircraft, i.e. its endeavor to return to the trimmed flight condition after a deviation. With decreasing longitudinal stability, the frequency decreases, with neutral stability the eigenform becomes aperiodic , and with negative stability the conjugate complex value pair becomes two aperiodic eigenforms, one of which is fanned. This represents the divergence movement of the unstable aircraft.
The damping is dominated by the resistance of the aircraft, which is why the phygoids can be flown easily, especially with gliders . With some types it is even slightly fanned, but is easy to control due to the low frequency.
See also
literature
- Xaver Hafer, Gottfried Sachs: Flight Mechanics. Springer-Verlag, Berlin 2002.