Wing Ballast Distribution

[jcarlyle]

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

[n7ly]

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

[SoaringXCellence]

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.

[sisu1a]

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

I think increased outboard mass increases the calculated flutter
speed.

-Paul

[Craig[_2_]]

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

[Andy[_10_]]

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

[Dave Nadler]

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.

[JJ Sinclair]

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.

[Frank Whiteley]

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

[JJ Sinclair]

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

[tstock]

>
> Also helps stabilize the glider in a spin.

Could you explain this statement for us newbies... this is opposite of
what I would have expected so I would like to understand why it is so.

Thanks!
-tom

[John Smith]

tstock wrote:
>> Also helps stabilize the glider in a spin.
>
> Could you explain this statement for us newbies...

Just don't worry. All moderately current regulations ask that a glider
is recoverable from a fully developed spin with any possible ballast
distribution.

[Craig[_2_]]

On Dec 10, 4:37*pm, tstock > wrote:
> > Also helps stabilize the glider in a spin.
>
> Could you explain this statement for us newbies... this is opposite of
> what I would have expected so I would like to understand why it is so.
>
> Thanks!
> -tom

Dave's right, but I think he was just poking fun. The higher
rotational inertia tends to flatten the spin and make it harder to
recover from. Most of the long wing ships aren't certified for
spinning even without ballast.

Craig

[Greg Arnold]

Craig wrote:
> On Dec 10, 4:37 pm, tstock > wrote:
>>> Also helps stabilize the glider in a spin.
>> Could you explain this statement for us newbies... this is opposite of
>> what I would have expected so I would like to understand why it is so.
>>
>> Thanks!
>> -tom
>
> Dave's right, but I think he was just poking fun. The higher
> rotational inertia tends to flatten the spin and make it harder to
> recover from. Most of the long wing ships aren't certified for
> spinning even without ballast.
>
> Craig


Presumably, the higher rotational inertia also makes it harder to get
into a spin.

[Brian Whatcott]

jcarlyle wrote:

> 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

Loading the wings mitigates wing bending. Wings want to fold up from
fuselage loads, so spreading a wing loading can reduce stress at the
wing/fuselage junction.

Brian W

[Brian Whatcott]

Andy wrote:

> 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. /snip/
> 9B

For the usual wing that has some sweep back, wing bending provides
reduced AofA which is favorable: swept forward wings do the opposite,
unless they are very stiff. Lateral weight distribution plays into this
effect too.

Brian W

[Brian Whatcott]

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.

Hmmm... I don't think so. Twin jets are noted for more severe spin
characteristics.

Brian W

[Brian Whatcott]

Spin loading and sharp edge gust loading are different wing cases.

Brian W

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
>
> 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.

[Brian Whatcott]

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

Front loading of aeroelastic surfaces has a comparable effect to forward
(dynamic) balancing of control surfaces like flaps, elevator, ailerons:

It pushes away the flutter envelope.

Brian W

[Andreas Maurer]

On Thu, 10 Dec 2009 10:02:55 -0800 (PST), 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.

I beg to differ - at least for certain gliders it's vice-versa. ;)

In the AS 22-2 (the mother of all ASH-25s) it is strictly required to
fill the outer wing tanks first (and empty them last) - the limiting
factor is the bending momentum of the wing of course.

The interesting part:
Even with completely filled outer wing bags (about 120 lbs per wing)
the roll rate doesn't suffer at all.
But filling the inner wing tanks (about 100 lbs per wing) instead of
having a co-pilot on board reduces the roll rate significantly.

Don't ask me why.

[Eric Greenwell]

John Smith wrote:
> tstock wrote:
>>> Also helps stabilize the glider in a spin.
>>
>> Could you explain this statement for us newbies...
>
> Just don't worry. All moderately current regulations ask that a glider
> is recoverable from a fully developed spin with any possible ballast
> distribution.
How old is "currently"? Are they really certified to recover with one
wing full of ballast and the other wing empty?

Best to check the manual!

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

[Eric Greenwell]

Andy wrote:
>>
>> 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.
This sounds backwards to me. Won't that move the flutter speed to a
lower speed? I seem to recall one concern with adding winglets to older
gliders is the potential lowering of the speed for flutter onset.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* Sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

[John Smith]

Eric Greenwell wrote:
> Are they really certified to recover with one
> wing full of ballast and the other wing empty?

JAR 22.221 General

(a) [...] the demonstrations of sub-paragraphs (b)
through (g) must also be made for the most critical
water-ballast loadings.

(b) The sailplane must be able to recover from spins
of at least five turns [...] by applying the controls
in a manner normal for recovery [...]

[jcarlyle]

Wow - thanks to all who answered, especially Craig and Andy. I
understand now why weighted wing tips are good.

I’m puzzled about Andreas’ comments on the ASW-22, though:

> Even with completely filled outer wing bags (about 120 lbs per wing)
> the roll rate doesn't suffer at all.
> But filling the inner wing tanks (about 100 lbs per wing) instead of
> having a co-pilot on board reduces the roll rate significantly.

This behavior seems to defy physics!

-John

[Andreas Maurer]

On Fri, 11 Dec 2009 06:00:23 -0800 (PST), jcarlyle
> wrote:


>This behavior seems to defy physics!

You got it. :)

I have the feeling that the cause might be some minor dihedral change
due to the water ballast, but I can't nail it down.



Bye
Andreas

[Andy[_10_]]

On Dec 10, 7:08*pm, Eric Greenwell > wrote:

> > 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.
>
> This sounds backwards to me. Won't that move the flutter speed to a
> lower speed? I seem to recall one concern with adding winglets to older
> gliders is the potential lowering of the speed for flutter onset.

Hmmm...maybe you're right. I'm not an expert on aeroelastics. My
thinking was that flutter at its core is like a mass-spring-damper
system. Given that the input aerodynamic forces don't change with mass
loading, adding mass to the wing ought to make it respond less in
bending to the input force because it has more inertia. I would expect
most of the bending resistance would be structural (spring) resistance
rather than inertial (mass) resistance, so the effect could be small.
The lower natural frequency of the wing would correspond to a lower
speed for flutter onset, but the aerodynamic forces would be lower as
a function of the square of the speed, so what's the net effect? With
enough mass might you not get any flutter at all? May there also be
harmonic effects related to how many bending "waves" you get along the
span?

In any event, I think the main effect is the increase in g-limit due
to the reduced bending loads from the change in spanwise weight
distribution.

9B

[Eric Greenwell]

John Smith wrote:
> Eric Greenwell wrote:
>> Are they really certified to recover with one wing full of ballast
>> and the other wing empty?
>
> JAR 22.221 General
>
> (a) [...] the demonstrations of sub-paragraphs (b)
> through (g) must also be made for the most critical
> water-ballast loadings.
>
> (b) The sailplane must be able to recover from spins
> of at least five turns [...] by applying the controls
> in a manner normal for recovery [...]
The paragraph (a) for the Aug 2001 Jar copy I found includes:

(a) ...
*Unless it can be shown that asymmetric
water-ballast is unlikely to occur by malfunction
or with lateral accelerations during a spin,* the
demonstrations of sub-paragraphs (b) through
(g) must also be made for the most critical
water-ballast loadings.

Are exemptions for asymmetric water-ballast used for any of our gliders,
and how would a pilot know? Is it something in the flight manual, perhaps?

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

[Brian Whatcott]

Andreas Maurer wrote:

> Even with completely filled outer wing bags (about 120 lbs per wing)
> the roll rate doesn't suffer at all.
> But filling the inner wing tanks (about 100 lbs per wing) instead of
> having a co-pilot on board reduces the roll rate significantly.
>
> Don't ask me why.
>
>
Adverse aeroelastic wing twist due to aileron action anti-servoing the
wing??

Brian W

[Martin Gregorie[_5_]]

On Fri, 11 Dec 2009 07:07:43 -0800, Andy wrote:

> Hmmm...maybe you're right. I'm not an expert on aeroelastics. My
> thinking was that flutter at its core is like a mass-spring-damper
> system. Given that the input aerodynamic forces don't change with mass
> loading, adding mass to the wing ought to make it respond less in
> bending to the input force because it has more inertia. I would expect
> most of the bending resistance would be structural (spring) resistance
> rather than inertial (mass) resistance, so the effect could be small.
> The lower natural frequency of the wing would correspond to a lower
> speed for flutter onset, but the aerodynamic forces would be lower as a
> function of the square of the speed, so what's the net effect? With
> enough mass might you not get any flutter at all? May there also be
> harmonic effects related to how many bending "waves" you get along the
> span?
>
I remember hearing a talk about this a few years ago. Anything that moves
the wing section CG forward relative to the effective CP will reduce the
tendency of the surface to flutter - hence lead in control surface
leading edges and the recommended lead in the tip LE of the modified
ASW-22.

All modern gliders carry water in front of the spar. As a result the CG
moves forward and you'd expect some increase on the speed at which
flutter starts.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

[Andy[_10_]]

On Dec 11, 12:57*pm, Martin Gregorie >
wrote:
> On Fri, 11 Dec 2009 07:07:43 -0800, Andy wrote:
> > Hmmm...maybe you're right. *I'm not an expert on aeroelastics. My
> > thinking was that flutter at its core is like a mass-spring-damper
> > system. Given that the input aerodynamic forces don't change with mass
> > loading, adding mass to the wing ought to make it respond less in
> > bending to the input force because it has more inertia. I would expect
> > most of the bending resistance would be structural (spring) resistance
> > rather than inertial (mass) resistance, so the effect could be small.
> > The lower natural frequency of the wing would correspond to a lower
> > speed for flutter onset, but the aerodynamic forces would be lower as a
> > function of the square of the speed, so what's the net effect? With
> > enough mass might you not get any flutter at all? May there also be
> > harmonic effects related to how many bending "waves" you get along the
> > span?
>
> I remember hearing a talk about this a few years ago. Anything that moves
> the wing section CG forward relative to the effective CP will reduce the
> tendency of the surface to flutter - hence lead in control surface
> leading edges and the recommended lead in the tip LE of the modified
> ASW-22.
>
> All modern gliders carry water in front of the spar. As a result the CG
> moves forward and you'd expect some increase on the speed at which
> flutter starts.
>
> --
> martin@ * | Martin Gregorie
> gregorie. | Essex, UK
> org * * * |

The only glider wing flutter I've seen is that DG-100 video that shows
asymmetric flutter with a fair amount of aileron involvement. Aileron
flutter is driven by the lack of mass balance because the aileron is
hinged at the leading edge. The wing itself is "hinged" more about the
spar I think (not really a hinge I know). This would seem to be more
"mass balanced" so adding weight in the D-tube may or may not produce
the same effect.

9B

[Morgans[_2_]]

"tstock" > wrote

> Could you explain this statement for us newbies... this is opposite of
> what I would have expected so I would like to understand why it is so.

Go back to the ice skater analogy. Weight out at tips (like arms
outstretched while spinning) provides for a slower spin, and make it a
little easier to stop a spin.
--
Jim in NC