Mass Balancing explanation request.

Could someone explain the idea, in semi layman's terms, why it's important, how it's done, what could be experienced if the balancing were out of whack, and if it is something that could be checked yourself with a procedural guide...
I have a full flying elevator. I'm not experiencing any problems, just a little fuzzy on this....

Thanks....

OK - I will bite.

Any plate shaped thing that is disturbed - moved by some external force
- will tend to rotate. This tendency to rotate is caused by the distance
between the point of action of whatever force disturbed your plate, and
the centre of gravity.

This is very elementary physics.

So - what happens when you attach a plank shaped thing to a structure by
a hinge on the extreme edge?
When the structure moves in one direction, the plank will trail because
of inertia - the CG is some distance from the hinge.

But on control surfaces this is undesirable. Think of ailerons:
1- wing is displaced upwards by a gust.
2- aileron hinge is attached at forward edge, so the CG of the control
tends to "stay behind".
3- Aileron moves into a down orientation relative to the wing.
4- aerodynamics does what you would expect and lifts the tip
5- oops we now have a "divergent" control behaviour.

If - to make things worse the structure it is attached to is springy
(Like a wing) then when the structure reaches the end of its elastic
range it will stop suddenly.
Inertia will keep the CG of the control moving in the previous
direction. So:
6 - Inertia is in charge - now the aileron overshoots the neutral
position and moves into an "up" displaced position on the wing.
7- Aerodynamics takes over and generates a downward force on the wing
where the aileron is attached.
8 - the force added to the elastic reaction of the wing starts the
wingtip moving downward - quite energetically...
9 - at some point the elastic force reverses and starts slowing the
movement.
10 - inertia takes over and the aileron CG starts to move downward
relative to the decelerating wing.
11 - as the wing reaches it's elastic limit , this time in the down
displacement the aileron is in the down displaced position because of
inertia. and the cycle repeats.

For obvious reasons the opposite wingtip will be doing exactly the
opposite movement. (180 degrees out of phase)

Welcome to flutter.

So the idea is to ensure that the control surface has the smallest
possible distance between the CG and the hinge line.
This is why hinges are mounted offset into the control, with weights on
the leading edge. If you get that part perfectly balanced, then the
surface will not rotate at all when displaced at 90degrees along the
hinge line.

An all moving control surface is similar - it has the hinge close to the
CG , and unfortunately, unless there has been careful selection of
aerofoils - will have very little force change with angle of attack
change. So no feedback on the stick. In gliders, the reflexed aerofoil
shape required for good control force is also relatively speaking quite
a significant drag source. So the designers don't like it. This is one
of the reasons all flying elevators tend to feel "light" - and went out
of fashion in the 70s...

I like it that way, but safety indicates that you want more control
force and that it should damp itself out.

Too much weight in front of the hinge and you have a different problem
in that the control inertia will tend to rotate around the hinge so as
to resist movement. LEading to strange control forces and stability you
may not want...

There is an excellent video (on youtube) from the akafliegs showing low
speed flutter on a DG where the mass balance had been reduced on the
ailerons for experimental demonstration. Not pretty to watch- it is
similar to a dog shaking the water off its back.

Helpful?

Bruce

On 2012/07/16 4:33 AM, POPS wrote:
Could someone explain the idea, in semi layman's terms, why it's
important, how it's done, what could be experienced if the balancing
were out of whack, and if it is something that could be checked yourself
with a procedural guide...
I have a full flying elevator. I'm not experiencing any problems, just a
little fuzzy on this....

Thanks....





--
Bruce Greeff
T59D #1771

Fantastic explanation that even regular non engineers can understand. Thanks for taking the time. I have also wondered about this same thing.

Bruno - B4

OK, a little less foggy, thanks for that...

Originally Posted by BruceGreeff

OK - I will bite.

Any plate shaped thing that is disturbed - moved by some external force
- will tend to rotate. This tendency to rotate is caused by the distance
between the point of action of whatever force disturbed your plate, and
the centre of gravity.

This is very elementary physics.

So - what happens when you attach a plank shaped thing to a structure by
a hinge on the extreme edge?
When the structure moves in one direction, the plank will trail because
of inertia - the CG is some distance from the hinge.

But on control surfaces this is undesirable. Think of ailerons:
1- wing is displaced upwards by a gust.
2- aileron hinge is attached at forward edge, so the CG of the control
tends to "stay behind".
3- Aileron moves into a down orientation relative to the wing.
4- aerodynamics does what you would expect and lifts the tip
5- oops we now have a "divergent" control behaviour.

If - to make things worse the structure it is attached to is springy
(Like a wing) then when the structure reaches the end of its elastic
range it will stop suddenly.
Inertia will keep the CG of the control moving in the previous
direction. So:
6 - Inertia is in charge - now the aileron overshoots the neutral
position and moves into an "up" displaced position on the wing.
7- Aerodynamics takes over and generates a downward force on the wing
where the aileron is attached.
8 - the force added to the elastic reaction of the wing starts the
wingtip moving downward - quite energetically...
9 - at some point the elastic force reverses and starts slowing the
movement.
10 - inertia takes over and the aileron CG starts to move downward
relative to the decelerating wing.
11 - as the wing reaches it's elastic limit , this time in the down
displacement the aileron is in the down displaced position because of
inertia. and the cycle repeats.

For obvious reasons the opposite wingtip will be doing exactly the
opposite movement. (180 degrees out of phase)

Welcome to flutter.

So the idea is to ensure that the control surface has the smallest
possible distance between the CG and the hinge line.
This is why hinges are mounted offset into the control, with weights on
the leading edge. If you get that part perfectly balanced, then the
surface will not rotate at all when displaced at 90degrees along the
hinge line.

An all moving control surface is similar - it has the hinge close to the
CG , and unfortunately, unless there has been careful selection of
aerofoils - will have very little force change with angle of attack
change. So no feedback on the stick. In gliders, the reflexed aerofoil
shape required for good control force is also relatively speaking quite
a significant drag source. So the designers don't like it. This is one
of the reasons all flying elevators tend to feel "light" - and went out
of fashion in the 70s...

I like it that way, but safety indicates that you want more control
force and that it should damp itself out.

Too much weight in front of the hinge and you have a different problem
in that the control inertia will tend to rotate around the hinge so as
to resist movement. LEading to strange control forces and stability you
may not want...

There is an excellent video (on youtube) from the akafliegs showing low
speed flutter on a DG where the mass balance had been reduced on the
ailerons for experimental demonstration. Not pretty to watch- it is
similar to a dog shaking the water off its back.

Helpful?

Bruce

On 2012/07/16 4:33 AM, POPS wrote:
Could someone explain the idea, in semi layman's terms, why it's
important, how it's done, what could be experienced if the balancing
were out of whack, and if it is something that could be checked yourself
with a procedural guide...
I have a full flying elevator. I'm not experiencing any problems, just a
little fuzzy on this....

Thanks....





--
Bruce Greeff
T59D #1771

On Monday, July 16, 2012 6:22:14 AM UTC-4, BruceGreeff wrote:
OK - I will bite.

Any plate shaped thing that is disturbed - moved by some external force
- will tend to rotate. This tendency to rotate is caused by the distance
between the point of action of whatever force disturbed your plate, and
the centre of gravity.

This is very elementary physics.

So - what happens when you attach a plank shaped thing to a structure by
a hinge on the extreme edge?
When the structure moves in one direction, the plank will trail because
of inertia - the CG is some distance from the hinge.

But on control surfaces this is undesirable. Think of ailerons:
1- wing is displaced upwards by a gust.
2- aileron hinge is attached at forward edge, so the CG of the control
tends to "stay behind".
3- Aileron moves into a down orientation relative to the wing.
4- aerodynamics does what you would expect and lifts the tip
5- oops we now have a "divergent" control behaviour.

If - to make things worse the structure it is attached to is springy
(Like a wing) then when the structure reaches the end of its elastic
range it will stop suddenly.
Inertia will keep the CG of the control moving in the previous
direction. So:
6 - Inertia is in charge - now the aileron overshoots the neutral
position and moves into an "up" displaced position on the wing.
7- Aerodynamics takes over and generates a downward force on the wing
where the aileron is attached.
8 - the force added to the elastic reaction of the wing starts the
wingtip moving downward - quite energetically...
9 - at some point the elastic force reverses and starts slowing the
movement.
10 - inertia takes over and the aileron CG starts to move downward
relative to the decelerating wing.
11 - as the wing reaches it's elastic limit , this time in the down
displacement the aileron is in the down displaced position because of
inertia. and the cycle repeats.

For obvious reasons the opposite wingtip will be doing exactly the
opposite movement. (180 degrees out of phase)

Welcome to flutter.

So the idea is to ensure that the control surface has the smallest
possible distance between the CG and the hinge line.
This is why hinges are mounted offset into the control, with weights on
the leading edge. If you get that part perfectly balanced, then the
surface will not rotate at all when displaced at 90degrees along the
hinge line.

An all moving control surface is similar - it has the hinge close to the
CG , and unfortunately, unless there has been careful selection of
aerofoils - will have very little force change with angle of attack
change. So no feedback on the stick. In gliders, the reflexed aerofoil
shape required for good control force is also relatively speaking quite
a significant drag source. So the designers don't like it. This is one
of the reasons all flying elevators tend to feel "light" - and went out
of fashion in the 70s...

I like it that way, but safety indicates that you want more control
force and that it should damp itself out.

Too much weight in front of the hinge and you have a different problem
in that the control inertia will tend to rotate around the hinge so as
to resist movement. LEading to strange control forces and stability you
may not want...

There is an excellent video (on youtube) from the akafliegs showing low
speed flutter on a DG where the mass balance had been reduced on the
ailerons for experimental demonstration. Not pretty to watch- it is
similar to a dog shaking the water off its back.

Helpful?

Bruce

On 2012/07/16 4:33 AM, POPS wrote:
> Could someone explain the idea, in semi layman's terms, why it's
> important, how it's done, what could be experienced if the balancing
> were out of whack, and if it is something that could be checked yourself
> with a procedural guide...
> I have a full flying elevator. I'm not experiencing any problems, just a
> little fuzzy on this....
>
> Thanks....
>
>
>
>

--
Bruce Greeff
T59D #1771

Excellent explanation, Bruce. The video you were referring to is most likely the one showing the SB9 flutter trials:

http://www.youtube.com/watch?v=jiN1dAdqQv4

GM

Why thank you sir.



On 2012/07/16 3:20 PM, wrote:
Fantastic explanation that even regular non engineers can understand. Thanks for taking the time. I have also wondered about this same thing.

Bruno - B4


--
Bruce Greeff
T59D #1771