Phugoid
Originally Posted by CONF iture
Such oscillations were taking place in Bilbao due to atmospheric disturbance.
The phugoid oscillation as such has no relation with atmospheric disturbances. The initiating disturbance can be anything, a gust, longitudinal control input, thrust increase ...
QUOTE
Phugoid
The long-perioid of phugoid mode involves a trade between kinetic and potential energy. In this mode, the aircraft, at nearly constant angle of attack, climbs and slows, then dives, losing altitude while picking up speed. The motion is usually of such a long period (about 93 seconds for a 747) that it need not be highly damped for piloted aircraft. This mode was studied (and named) by Lanchester in 1908. He showed that if one assumed constant angle of attack and thrust=drag, the period of the phugoid could be written as: T = p V2 U/g = 0.138 U. That is, the period is independent of the airplane characteristics and altitude, and depends only on the trimmed airspeed.
P.S.
The formula for the period should read: T = π*√2*U/g, where U is the trimmed airspeed (TAS) and g the acceleration of gravity. With U in ft/s the period in seconds is T=0.138*U .
Last edited by HazelNuts39; 23rd Feb 2014 at 09:19.
Reason: P.S., grammar