Fly by Wire questio0n

[Chris Scott]

Artificial Stability and Airliners

Quote from Milt:
One big advantage of FBW is that it enables the use of artificial stability with the computers, usually triplicated, working like mad things to give us a smooth progress. Instead of the usual down load on the tail to produce natural stability in pitch the designer can now program the computers to provide a significant up load from the tail and at the same time reduce its size and weight. This also leads to lower weight for the wing's structure as they have less of a lifting task. Result - a big increase in efficiency. Stability in roll and yaw is simplified and smoothed.
[Unquote]

My knowlwedge on FBW is very limited, but what Milt has said is in need of some qualification in relation to current airliners. As no one else has yet done so, I'll do my best. The design of airliners has not yet been allowed to advance as far as he is suggesting.

My understanding is that well-established military FBW aircraft, the F-16 being an old example, have either neutral static stability or even static instability - whether it is one or the other doesn't affect what I am here for.

One of the advantages of a canard layout - where the horizontal stabiliser is forward of the main-plane, is that a canard is inherently stable in pitch but, nevertheless, produces positive lift from its stabiliser, thus reducing the lift requirement of the main-plane. In a 'conventional' layout, however - with the horizontal stabiliser aft of the main-plane, static stability in pitch is only achieved by the stabiliser generating negative lift, which has to be offset by extra lift from the mailnplane (as Milt has said).

So an aeroplane with a rear-mounted horizontal stabiliser (let's just call it a tailplane, so much less of a mouthful) - like all current airliners since the grounding of Concorde - can only be stable if its tailplane produces negative lift throughout the flight envelope. And they all do.

That all FBW airliners still have negative-incidence tailplanes and positive static stability is best illustrated, I think, by the fact that it is still possible for the pilot to fly them with a complete failure of FBW. [This is usually done using the stabiliser trim for pitch and the rudder for yaw/roll.] But they do tend to be trickier to handle than the pre-FBW airliners were.

Returning to punkalouver's original question:
"In modern airliners, the primary advantage of FBW is that it allows engineers to use lighter wing and tail structures." Why?

Milt is right, I think, as to why FBW has produced weight and efficiency savings already.
1) The tailplanes can be and are smaller, because FBW reduces the pitch stability required (hence the trickier handling with total FBW failure).
2) On Airbus, FBW limits vertical acceleration to +2.5g, the certification limit for all airliners.
3) Certain gust-alleviation features are sometimes available, like the LAF system on the A320, which alleviates the wing loads in turbulence by rapid movements of the spoilers and ailerons.

In summary, airliners are well behind the military in exploiting the possible (cost) benefits of FBW. This is because the regulatory authorities, thank goodness, take a conservative approach to airliner stability. But it's exactly 20 years since digital full FBW first flew passengers (A320), and a lot longer since analogue FBW was pioneered by "the pointy one". Civil digital FBW has largely confounded the doom-merchants, and policies are likely to move on.

Well, that's my tuppence-worth. I now await a rash of corrections from the cognoscenti...