On Dec 10, 11:01*am, Craig wrote:
On Dec 10, 10:44*am, sisu1a wrote:

Why would aircraft designers want to keep weight towards the wing
tips?

I think increased outboard mass increases the calculated flutter
speed.

-Paul

N7LW has it right. *It has to do with bending moments developed in the
spar. *At any given flight condition the wing supports a given amount
of lift that can be expressed in lbs/ft of span (or N/m if you
prefer). *The fuselage doesn't produce any lift and has to be
supported by the lift produced on these long cantilever beams on each
side. *Weight toward the middle of the aircraft increases the wing
bending. If the weight can be moved toward the outer portions of the
wing the bending loads are decreased. *The paper airplane is a good
way to visualize this.

Craig

Yup.

Ballast towards the wings tips increases the g-limit at any given
weight versus ballast towards the wing root. It may or may not affect
the flutter limits - but my initial hypothesis would be that it
reduces the natural frequency of the wing in bending which would
probably help on flutter.

The tradeoff may have been that on the LS-8a they had enough g-margin
at Vne to allow the outer tanks to drain first, allowing the glider
better roll rate with half ballast. The longer wings on the 18 meter
variant may have required the outboard weight to keep everything
within the wing's structural limits. If they raised MTOW on the -18
then this would also require bending load relief in the form of
outboard ballast to stay within limits.

Even if it didn't affect actual certification, it's not a bad way to
go with the longer wings.

9B