Dihedral with FBW - spiral stability /wingtip strike angle vs divergent Dutch Roll

[Mach Stall]

I'm curious what design priority pilots would prefer with a FBW system, with respect to tuning the wing dihedral of the aircraft. Of course, with more dihedral comes worse Dutch Roll but better spiral stability; also, the roll angle at which a wingtip will strike the runway increases. But with FBW, it would only be with FBW failure resulting in direct mode that natural dynamic stability modes would matter (and even then, it is not truly direct but still has stability augmentation as I understand it).

But let's say you could get a few extra degrees of wingtip strike angle at the expense of a much higher workload situation with more divergent Dutch Roll at altitude -- that is, more divergent only in the incredibly unlikely event of FBW failure (but with the advantage of less divergent spiral stability in such FBW failure). What would you prefer/prioritize?

[Clandestino]

You don't understand it.

Airbus FBW is absolutely no SAS and FBW Airbi can be neatly flown in direct law i.e. no FBW intervention between stick and control surface displacement. Rudder and yaw damping on minibus are not FBW anyway. I would be very surprised if Dassault, Boeing, Embraer or Tupolev FBWs turn out to be different.

Low wing airliners are landed in wings level, or at the very worst, very slight bank into wind. FBW or not, when properly flown, wingtip strike angle does not come into play. "Would you like to have extra couple degrees of bank angle at touchdown?" is quite nonsensical.

I would now like to see couple of opinions from resident Airbus experten, CONFiture, Machinbird, roullisholandais, Retired F4 etc. before we let this thread die.

[No Fly Zone]

@Mach: Have you checked your TWF connections today? How much EtOH within the last 12-24 hours? A stimulating question, buy it may fly better in 24 hours.:eek:

[Mach Stall]

Clandestino, I'm certainly open to being wrong, but as I understood it, the B777 and B787 have 3 basic modes for manual control- Normal, Secondary, and Direct, and that all three involve pitch stability augmentation. I take it this is incorrect?

Also, with respect to wingtip strike, when you say "when flown correctly," a human factors red light goes off in my mind. Aircraft designers can't only design for "when flown correctly," as the human is guaranteed to make mistakes. There are a number of videos on youtube that show such mistakes with respect to close calls and wingtip strikes. And then there are emergency situations of unknowable circumstances.

I'm not suggesting a plane be designed for dynamic stability such that a pilot couldn't handle flying it in a truly direct mode. I was simply asking whether a pilot would prefer better: (i) natural Dutch Roll damping (ie, less divergent) or (ii) better natural spiral stability and wingtip strike clearance, in a FBW plane.

Though I'm open to being enlightened on many things, to say the question is entirely stupid is to say it doesn't really matter what wing dihedral is chosen.

[Mach Stall]

No Fly, probably not enough EtOH, maybe that's my problem -- I'll go work on that...

[Amadis of Gaul]

Do we really need to explore this kind of rocket surgery during the weekend? Can't it wait until Tuesday? Words like divergent, convergent etc just make me sad...