I've just read the latest Airbus flight safety magazine - 'Safety First' - and I see that it has a fascinating article on just thsi topic, written in a very readable style:
(sorry about the formatting - the two column copy from a pdf didn't transfer well!)
The Lateral Flight Control Laws
On July 27th 2005, in Toulouse we had a strong wind from the south, called “vent d’Autan”, giving rise to a lot of turbulence. It was flight 51 of the f irst A380. We performed several landings and it became apparent that the lateral flight control laws would have to be tuned again: the pilots
were very active on the stick, the ailerons were moving a lot and created unpleasant lateral accelerations, mainly at the back of the aircraft. The flight
test engineers had several possibilities to adjust the control laws: gains, damping…, but none of them could solve the issue. This typical development
flaw had to be corrected, but it was not an easy task. Mid October, new PRIM
flight controls computers were delivered with a new control law for the ailerons that the engineers of the design office called “VDA” or “Valse Des Ailerons” (ailerons waltz). As an example, when moving the stick to the
left, on the left wing, the internal aileron started to move up immediately. The outer aileron was doing the same, but with a different deflection. Finally,
the centre aileron was either initially going down, in opposition to the two others, then taking an upward position, or going up after a very short delay in a neutral position. Several adjustments were available for the flight engineers, for example, the ratio between the deflection of inner and outer ailerons and the logic of the centre aileron. The target of this strange
kinematic was to “break” some wing oscillations as two of them had very close frequencies and, in certain circumstances, they had the possibility to couple together. Looking at the page dedicated to the flight controls
on the screen at the disposal of the crew, it was easy to understand
why this strange motion of the ailerons received this nickname of “VDA”. A similar differential deflection was also implemented on the two rudders
and was called “VDR” or “Valse Des Rudders” (rudders waltz), a typical Airbus “British – French” acronym, as rudder is not a French word! The
improvement on comfort was spectacular. However, some tuning was still needed. In January 2006, we installed a new standard of the computers,
with some improvements on the VDA laws. The main one was a reduction in the activity of the ailerons. The adjustments were again performed in flight. The final tuning is such that, for speeds below 300 kts, the deflection
of the inner aileron is 2.5 times the value of the outer one. The centre aileron follows the inner, but with a time delay of 350 milliseconds. Some more
modifications were needed at high altitude due to the Mach effect.
But we had another issue: the tuning of the spoilers. At the beginning,
they were deflected as soon as there was a command in roll and this created some buffet. Mid February 2006, new settings were proposed by the
design office in order to reduce these vibrations, with a limitation of the deflection to 3° as long as there was not a strong demand from the pilot. Without this trick, one of our British test pilots told us that he had the
impression of being “punished” by this buffet when entering a standard turn! On top, in the final tuning, when more manoeuvrability was needed, there was a higher deflection of the outer spoilers than of the inner ones, because they were creating less buffet. For all these flights where it was
important to get an idea on the comfort, a qualitative judgement at various locations in the plane was needed. In the cockpit, the pilots gave their impressions, both on the ease of flying and on the comfort in the forward
part of the aircraft. The flight test engineers, seating close to the centre of gravity, gave their sensations based on their feelings and the available traces. At the back, close to the most rear door of the main deck, we
installed a seat equipped with an intercom connected to the other crew members. A young flight test engineer was sat there, to give his opinion about his perception of comfort. Taking into account the number of roll manoeuvres we were performing on each flight, we had to hope that he would not become sick! It is true that a choice could have been made based on an analysis of the traces of several parameters of the motion at the
various positions in the plane, but we considered that the opinion of a potential “passenger” was fundamental in order to make the final decision. Obviously, all the records of these parameters were used by the design office to make progress in the tuning of the flight controls laws. It is to be noted that, at the beginning of the program, we were concerned by a possible
difference of comfort between the two decks. The f irst flights demonstrated that this was not an issue. At the opportunity of your next flight on an A380, if you travel in business or economy class, I recommend that you book a “window seat”, close to the wing or at the back of the plane in order to see how the ailerons are working (in first class you will not have this chance as you will be too far forward!). The effect is best observed just after take-off and during the early climb manoeuvres with the ailerons moving around their neutral position. You will see that when entering into a simple turn
or for a unique roll correction, taking as a base the inner aileron, the one closest to you, the outer aileron will move simultaneously but with a smaller deflection. Then with a small time delay, the centre will join the inner. If several corrections are made by the pilot, in one direction then in another, taking into account the different deflections and the time delay, you will see
the ailerons in totally different positions, up and down. The nickname “Valse Des Ailerons” is really well chosen and it is effective