Wing Ballast Distribution

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

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

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 [...]

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

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

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

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

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

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 |

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