Wing Ballast Distribution

When you water ballast the four wing tanks of an LS8a you fill the
inner tanks first, and when you dump the water there’s provision for
the inner tanks to drain last. With a RS LS8-18 (and the newer DG LS8-
s) this procedure is reversed - the outer tanks are filled first and
there’s provision for the outer tanks to drain last. This was a
deliberate decision, because mechanical changes were made to the two-
handled dump system levers.

Why would aircraft designers want to keep weight towards the wing
tips? Does it have something to do with going from a 15m to an 18m
span? Wouldn’t the increased moment of inertia and wing spar bending
negatively affect performance?

-John

On Dec 10, 8:31*am, jcarlyle wrote:
Weight in the wing is load relieving. Try it with a paper airplane.
Tape coins around.


When you water ballast the four wing tanks of an LS8a you fill the
inner tanks first, and when you dump the water there’s provision for
the inner tanks to drain last. With a RS LS8-18 (and the newer DG LS8-
s) this procedure is reversed - the outer tanks are filled first and
there’s provision for the outer tanks to drain last. This was a
deliberate decision, because mechanical changes were made to the two-
handled dump system levers.

Why would aircraft designers want to keep weight towards the wing
tips? Does it have something to do with going from a 15m to an 18m
span? Wouldn’t the increased moment of inertia and wing spar bending
negatively affect performance?

-John

On Dec 10, 9:23*am, n7ly wrote:
On Dec 10, 8:31*am, jcarlyle wrote:
Weight in the wing is load relieving. Try it with a paper airplane.
Tape coins around.



When you water ballast the four wing tanks of an LS8a you fill the
inner tanks first, and when you dump the water there’s provision for
the inner tanks to drain last. With a RS LS8-18 (and the newer DG LS8-
s) this procedure is reversed - the outer tanks are filled first and
there’s provision for the outer tanks to drain last. This was a
deliberate decision, because mechanical changes were made to the two-
handled dump system levers.

Why would aircraft desgigners want to keep weight towards the wing
tips? Does it have something to do with going from a 15m to an 18m
span? Wouldn’t the increased moment of inertia and wing spar bending
negatively affect performance?

-John- Hide quoted text -

- Show quoted text -

The weight toward the tips provides an interial resistance to roll.
That means the ride is a little smoother as the wing has greater
resistance to differential vertical gusting across the wingspan. It
is the same principle that tightrope walkers use by carrying a long
heavy pole.

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

I think increased outboard mass increases the calculated flutter
speed.

-Paul

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

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

On Dec 10, 1:02*pm, SoaringXCellence wrote:
The weight toward the tips provides an interial resistance to roll.
That means the ride is a little smoother as the wing has greater
resistance to differential vertical gusting across the wingspan. *It
is the same principle that tightrope walkers use by carrying a long
heavy pole.

Also helps stabilize the glider in a spin.

I believe it was the Constellation that was bpopping rivets in
turbulence, the fix was 300# of lead in each wing tip.
JJ

Dave Nadler wrote:
On Dec 10, 1:02*pm, SoaringXCellence wrote:
The weight toward the tips provides an interial resistance to roll.
That means the ride is a little smoother as the wing has greater
resistance to differential vertical gusting across the wingspan. *It
is the same principle that tightrope walkers use by carrying a long
heavy pole.

Also helps stabilize the glider in a spin.

On Dec 10, 3:25*pm, JJ Sinclair wrote:
I believe it was the Constellation that was bpopping rivets in
turbulence, the fix was 300# of lead in each wing tip.
JJ

LAK-12 has 20lbs in the leading edge of each tip.

Frank Whiteley

Schleicher made us put 3kg of lead down the inside of the outboard
wing leading edge when we installed Heinzies 26.5 meter 'bent wing
tips' on the ASH-25. They wanted it spread out for a good 10 feet.
JJ

Frank Whiteley wrote:
On Dec 10, 3:25*pm, JJ Sinclair wrote:
I believe it was the Constellation that was bpopping rivets in
turbulence, the fix was 300# of lead in each wing tip.
JJ

LAK-12 has 20lbs in the leading edge of each tip.

Frank Whiteley