Re: Boeing 777 Wriggle

Date:         30 Mar 98 04:31:25 
From:         domk1031@cetus.zrz.TU-Berlin.DE (Burkhard Domke)
Organization: Technical University of Berlin, Germany
References:   1
View raw article
  or MIME structure

On 24 Mar 98 11:38:39 , wrote:

>I remember reading on this group at least a year or so ago about a problem
>Boeings 777s were having: it seems that when seated in the back of the plane
>that there was a perceptable side-to-side wiggle, of sorts.
>If my memory serves me correctly, several posters said this problem was fixed.
>I don't think so. I've taken a dozen or so transatlantic legs on UA's 777 in
>the last few years and have never noticed the wiggle. However, I've never sat
>further back than the trailing edge of the wing. On a recent trip, I was in
>the last row and, let me tell you, they haven't fixed the wiggle problem.
>It lasted the entire flight. It basically felt like you were being shaken side
>to side every second for five hours.
>So what's the deal?

The tail-wag is related to aeroelasticity, i.e. aerodynamic loads
inducing a structural response and vice versa. In this case it's an
interaction bewteen the first mode of structural bending of the
fuselage and aerodynamic forces acting on the tail/rudder assembly (or
the empennage altogether). The usual approach to get rid of the
tail-wag is to task the yaw-damper to counter-act the wag with proper
rudder inputs or to rebalance the rudder.

Gets a tad more complex with sensible side-stick controls, as the
tail-wag implies a nose-wag, which in turn forces the side-stick into
the pilot's hand/arm (body inertia !), thus inducing unwanted
(disturbing) control inputs...