Phugoid

The phugoid is a technical term from the flight mechanics and means a web vibration of longitudinal movement.

Appearance

The phugoid is an oscillatory change of potential energy ( altitude ) with kinetic energy ( airspeed ) that at very low frequencies (typically 30 seconds to several minutes per cycle) and with very low attenuation expires. A mitfliegenden this observer looks as if the vibrating plane describes a standing ellipse around a mean position. The exact frequency of the phugoid primarily depends on the airspeed and longitudinal stability.

Calculation

For the calculation of eigenmodes the equations of motion are established and written into a matrix whose coefficients. The eigenvalues ​​of the matrix are two complex conjugate pairs of values ​​; they correspond to the frequencies of the natural modes ( in the longitudinal movement of these are the Anstellwinkelschwingung and phugoid ) and the eigenvectors describe the appearance of the natural form.

Driving force of the phugoid is the longitudinal static stability of the aircraft, so his desire to return to a deviation from the trimmed flight condition. With decreasing longitudinal stability reduces the frequency at neutral stability, the mode shape is aperiodic, and for a negative stability of the complex conjugate pair of values ​​two aperiodic mode shapes, one of which passes fanned. This represents the divergence movement of the unstable aircraft.

The damping is dominated by the drag of the aircraft, which is why the phugoid can be easily flown especially for gliders. In some types it is even slightly fanned, but due to the low frequency simply auszusteuern.