Re: An-124 and wing dihedral

Date:         06 Jun 98 15:39:04 
From:         johnmhunt@ipa.net (John M. Hunt)
Organization: Internet Partners of America
References:   1
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Pete Mellor <pm@csr.city.ac.uk> wrote:
>In general connection with this thread, I am aware that
>yaw-damping is one of the functions of the digital flight
>control system on fly-by-wire aircraft such as the A320
>and its successors and the B777. However, Dutch roll is
>a vice of any delta-winged aircraft. Presumably, automatic
>yaw-damping was achieved in pre-FBW days by a simple
>analogue system (possibly part of the autopilot)?
>
>I'm guessing. Does anyone have any definite information?

Early yaw dampers employed a simple rate gyro to determine angular
velocity about the yaw axis of the aircraft.  Rudder deflection
proportional to the measured yaw rate effectively increased the
damping in yaw of the aircraft, ;which is very effective in damping
the combined yaw/roll motion commonly called Dutch Roll.

Pilots can accomplish the same by applying manual rudder deflection
proportional to yaw velocity, providing that they can sense angular
velocity and not be overly impressed by the perceived side slip (yaw
angle).  Presumably this is very tiring over a long period of time.

I presume that the sophisticated flight control computers of today
take advantage of the information from the inertial platforms, using
both roll and yaw velocity and perhaps yaw angle to do a more elegant
job.

Incidentally, does anyone know if their is any relationship to this
subject and the reported continuous minor yawing motion reported to be
noticeable in the extreme aft portion of the 777, at least in its
earlier days.  If such continuous yawing exists, is it caused by pure
aerodynamics or is it a manifestation of some sort of limit cycle
oscillation resulting from non-linearities in the flight control
servos?

-

John M. Hunt
johnmhunt@ipa.net