Re: An Accelerometer in Paradise

From: (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         21 Mar 96 02:38:07 
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In article <airliners.1996.340@ohare.Chicago.COM> Ian Judge <> writes:
>These sound particularly high when you consider that the maximum g-loading
>of a B757 is 3g. Approx 3 yrs ago Caledonian severely bent a B757 with a
>heavy landing of 2.76g.

I would rate a hard landing as a "collision" with a large immobile object,
and wouldn't use it to generalize the aircraft's ability to handle high loads.
If one wanted to push this implausible premise, one could also argue that
a hard landing loading indication could be viewed as negative G's.  I.e.,
the airplane going down and the gear pushing the airplane up.  Negative G
ratings, in any event, are normally quite low, usually about -1 G's.

In practice, Boeing aircraft have been known to be incredibly overdesigned,
and can take significant beatings.  747s have been clocked in excess of 7 g's,
which is usually when the FDR or DFDR stops recording reliable data.

Modern design practices can throw all this in question: fly-by-wire, for
example, may be used by overoptimistic engineers to save money by trimming
down the structural requirements.  Fly-by-wire can also be used to save
structural costs by (for example) allowing smaller control surfaces, and
increasing aircraft instability, which is artificially controlled.  This
has operational benefits as well.  Load alleviation functions can pave the
way to a lighter, weaker wing.  Computer-aided design techniques may
incline engineers to think that they have a "precise" understanding of real-
world requirements.  And Simulation is also being increasingly used to
justify and validate design decisions.

It's an evolving area.  FBW's applicability to these areas is thus far
limited, but the 777 was completely computer-designed.  In the future, at
least, I suspect we won't read about 7 G recovery maneuvers much.

Robert Dorsett                         Moderator, sci.aeronautics.simulation