On Mar 14, 3:59 pm, Dave wrote:
Soooo..

How come he (MX) was able to give a rather complete and correct
answer to the poster's question?

Care to give it a try?

Dave

On Tue, 13 Mar 2007 22:44:15 -0700, C J Campbell

wrote:
On 2007-03-13 17:43:50 -0700, megaMAX
said:

On Wed, 14 Mar 2007 01:18:33 +0100, Mxsmanic
wrote:

Most commercial flights are flown on autopilot for the vast majority of the
trip. Thus you would have seen autopilot corrections, not pilot corrections.

Just so you know, Msxmanic does not know any more about flight than you
do. He is not a pilot. He is just some nut who hangs out here and
pretends to know what he is talking about.

MX's knowledge is very sophomoric, and heavily stilted towards biases
introduced by inaccuracies in his simulations.

As in this case.

Minor Harmonic oscillations in flights are a natural byproduct of the
dynamic stability of modern aircraft. The Autopilot has no part in
it.

When the oscillations are on the Longitudinal axis, they are called
phugoid oscillations, I'm not sure if there is a similar term for the
roll equivolents, but it is the same deal- Essentially what is
happening is a battle between the aircraft's substantial momentum, and
the aerodynamic forces which keep it pointed into the wind... Large
out-of-trim deflections (both roll and pitch) are damped quickly, but
there is a threshold at which the amplitude of the oscillations falls
below the aerodynamics ability to produce large enough forces to damp
the behavior...

The period and amplitude of these harmonic functions are directly
related to the configuration and chord of the aerodynamic surfaces vs.
The aircraft's distribution of mass. In general, thinner aerodynamic
surfaces create stronger off-center aerodynamic forces and hence
better damping, and lighter aircraft produce weaker momentum forces
and again, fewer oscillations. Granted, 'fewer' in this case is a
matter of perception... the oscillations you were seeing were really
very minor (less than a degree), its just that the solid horizon
combined with the long arm of the wing made them more obvious than the
same fraction of a degree roll oscillation would be in a smaller
aircraft.

No MX... it has nothing to do with the autopilot...

FWIW, I remember reading somewhere that the 747 prototype actually had
a problem on its first few flights - the engineers had designed the
aerodynamic surfaces to damp the phugoid (longitudinal) harmonics
based on a theoretical perfectly rigid aircraft, however, in reality,
it turned out that the fuselage had a natural bending resonance
frequency which matched the aerodynamic harmonic frequency, and they
amplified eachother to produce an in flight longitudinal harmonic that
was actually quite substantial... the result, IIRC, was a very sick
planeload of journalists on the aircraft's first publicity flight.

For more information, read up on both Phugoid oscillations and Roll
stability via using wing dihedral.