Wing Ballast Distribution

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

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