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....

On Jul 15, 2:38*pm, 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 thiSs....

Thanks....

--
POPS

the not so simple answer is "it depends"... You will notice some
classic aircraft (cubs, Taylorcraft etc) do not have either mass or
aerodynamic balances. On my own design however, I was very particular
about mass bala Elevators, as out of all the controe one that will
kill you the quickest! The question whether you need it or not depends
onlevators? how are the elevators actuated? are you likely to be
putting large elevator inputs in at high energy levels? Lower
performance aircraft with a narrow speed range are unlikely to hit the
flutter range for any length of time, so mass balancing is of less
importance. Larger elevators increase the effects of flutter, a




d tend to be more prone to it and, especially if the chord is quite
wide, will need quite a lot of weight or a long moment arm to balance
correctly. If the balance is not accurate and still behind the the
hinge, it will reduce the flutter effect, but not eliminate it -
essentially it makes the elevator behave as if the chord is smaller.
the actuation method is quite important - I use a heavy push - pull
cable on my elevators, and this has a certain amount of inherant
damping effect from the cable friction. Conventional cables may in
some circumstances allow a bit of movement and even resonate to
amplify flutter tendencies. Pushrods are more solid, but also can have
a lot less system friction, so it depends on the installation. Powered
flight controls are non - reversible (the movement of the surface
doesnt get fed back into the system) so are generally resistant to
flutter, but I'm guessing not many homebuilts have fully powered
elevators!. I'd say, If in doubt, balance them as close to neutral as
you can - it gives piece of mind if nothing else.

http://eaaforums.org/showthread.php?...e-for-Elevator

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.

http://www.youtube.com/watch?v=kQI3AWpTWhM

Not quite. The entire wing was re-engineered for reduced stiffness (stretched to 17m), then overloaded with water to increase mass, thus allowing the entire wings to flutter at lower speeds for controlled study of aero-elastic flutter (bending of structure, not control surface flutter). They did it with the ailerons locked, to achieve a limited oscillation to keep the ship in one piece, and to my knowledge the ailerons were normally balanced. Below is a copypasta of a synopsis:

-p

--------------------------------------------------------------------------------
A flutter test with the DG-300/17 of the DLR Braunschweig:
FLUTTER "Der heiligen DG"
Here you can download a spectacular video showing a Flutter Test, something you normally do not see.
We are lucky to have pilots who will risk such potentially dangerous tests to give us the opportunity the better understand the Phenomena of Flutter.
The following information is important to be read to understand this video with the fluttering wing on a DG-300/17:
----------------------------------------------------
Dear Mr. Weber,

this DG-300/17 is a Research Plane, in comparison to the factory DG-300 is it a plane on which the wingspan was increased from 15m to 17m. The additional wingspan was added towards the wing root.

The filmed flutter with limited amplitude only occurred with a high water ballast. The too large water tanks contained more water than the allowed amount. The flutter tendency with increasing amount of water was known based on flatter calculation and static flutter tests by the DLR Institut for Aero-elastic. During these tests a small reduction within a limited speed range was discovered.

The observed flutter oscillation during this experiment of the glider in actual flight gave us the opportunity, to prove the results of the theoretical methods used. Of course it goes without saying that such high risk flight tests could only be planned and carried out by highly experienced specialists of the DLR Braunschweig.

To obtain the airworthiness certificate the water ballast was reduced to such an amount that for the normal use of this plane, for research purposes within the DLR and at the yearly IDAflieg comparison glider performance program, such Flutter cannot occur.

The film shows the flutter occurring at an airspeed between 140 and 150 km/hr during which the anti-symmetric wing bending and rotation momentum of the aileron are involved. By fastening the controls a non-linear reaction occurs , which develops a flutter with limited amplitude. Because of this, an overload and breakup of the plane does not occur.

When the airspeed by pulling on the stick, without hindering the sidewards movements of the stick, is reduced, the flutter oscillations will stop, but only when an appreciably slower airspeed is reached. This speed is the actual flutter airspeed of the plane. Flutter will start, when this airspeed is exceeded and the right disturbance influences the plane at this point.

Please Note:
All planes having an airworthiness certificate have gone through extensive static oscillation tests and flutter calculations in addition to a flight test program during which they have been checked for critical flutter behavior. Within approved operation field and its limitations as shown in the flight handbook one can be assured that no aero-elastic instabilities/flutter will occur.

Jan Schwochow
Flugabteilung des DLR (Deutsches Zentrum fuer Luft und Raumfahrt)
Braunschweig mit technisch/wissenschaftlicher Unterstützung
des Instituts für Aeroelastik des DLR in Göttingen
--------------------------------------------------------------------------------

On Sunday, July 15, 2012 2:38:47 PM UTC-7, 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....




--
POPS



On Sunday, July 15, 2012 2:38:47 PM UTC-7, 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....




--
POPS



On Sunday, July 15, 2012 2:38:47 PM UTC-7, 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....




--
POPS



On Sunday, July 15, 2012 2:38:47 PM UTC-7, 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....


Pops,
Without the physics, mass balance is used to prevent control surface flutter. Flutter was a problem with the Janus C elevator, and the solution was a technical bulletin requiring installation of a mass balance. Really good tight linkages can prevent the onset of flutter, as well as keeping the surface 'loaded'.
Jim Wynhoff

I think you are largely right.

Yes - this was a fixed control test. But, as I understand it - The
ailerons were not fixed - the stick was prevented from moving side to
side during the test. The flaps & Ailerons still fluttered - you can see
the port surfaces lift above the neutral if you watch in slow motion.

And yes - this is harmonic aeroelastic wing flutter not control surface
induced. However it is the same principle really, harmonic motion
induced by the interaction of airflow and inertia. Here - the wing can
be thought of as a plate hinged around the main spar.

In this case adding the additional water ballast caused the inertial
energy to be high enough to overcome the torsional stiffness of the wing
- which was extended to reduce the stiffness and presumably lower the
frequency.

Interestingly the test resulted in some painful and presumably
superfluous regulation - all EAS22 compliant gliders have stiffer
controls with greater mass balance. There was a discussion on the DG1000
design where they were forced to redesign controls to comply.

Interesting aside - I understand one of the mechanisms the designers
have used to increase aeroelastic flutter speed on those high aspect
ratio wings is to introduce the multi trapezoidal leading edges. I
believe Schempp-hirth started this.

Basically - look at an older design like an ASW20 in a high load
situation the wingtip has substantial vertical displacement. It appears
the wing is flexing to high Angle of attack on the outboard panels.
Possibly this is caused by a combination of the rotational drag force
from winglets as well as the aerodynamic load induced bending.

The straight leading edge means that the centre of pressure on the wing
remains ahead of the main spar all the way to the wingtip - at high load
this tends to rotate the weakest (torsionally) part of the wing to
higher AoA than desired.

On the polyhedral designs the aerofoil and structure is effectively
swept back - neutralising this rotational force, by putting the centre
of pressure behind the (projected) spar. In this case high load tends to
reduce the AoA at the tip relative to the rest of the structure,
transferring load inboard. (positive and negative load would have the
same effect)

Result is less bending of tips at high speed - up and down, So less
probability of flutter.

Sensible?

On 2012/07/16 6:05 PM, sisu1a wrote:
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.

http://www.youtube.com/watch?v=kQI3AWpTWhM

Not quite. The entire wing was re-engineered for reduced stiffness (stretched to 17m), then overloaded with water to increase mass, thus allowing the entire wings to flutter at lower speeds for controlled study of aero-elastic flutter (bending of structure, not control surface flutter). They did it with the ailerons locked, to achieve a limited oscillation to keep the ship in one piece, and to my knowledge the ailerons were normally balanced. Below is a copypasta of a synopsis:

-p

--------------------------------------------------------------------------------
A flutter test with the DG-300/17 of the DLR Braunschweig:
FLUTTER "Der heiligen DG"
Here you can download a spectacular video showing a Flutter Test, something you normally do not see.
We are lucky to have pilots who will risk such potentially dangerous tests to give us the opportunity the better understand the Phenomena of Flutter.
The following information is important to be read to understand this video with the fluttering wing on a DG-300/17:
----------------------------------------------------
Dear Mr. Weber,

this DG-300/17 is a Research Plane, in comparison to the factory DG-300 is it a plane on which the wingspan was increased from 15m to 17m. The additional wingspan was added towards the wing root.

The filmed flutter with limited amplitude only occurred with a high water ballast. The too large water tanks contained more water than the allowed amount. The flutter tendency with increasing amount of water was known based on flatter calculation and static flutter tests by the DLR Institut for Aero-elastic. During these tests a small reduction within a limited speed range was discovered.

The observed flutter oscillation during this experiment of the glider in actual flight gave us the opportunity, to prove the results of the theoretical methods used. Of course it goes without saying that such high risk flight tests could only be planned and carried out by highly experienced specialists of the DLR Braunschweig.

To obtain the airworthiness certificate the water ballast was reduced to such an amount that for the normal use of this plane, for research purposes within the DLR and at the yearly IDAflieg comparison glider performance program, such Flutter cannot occur.

The film shows the flutter occurring at an airspeed between 140 and 150 km/hr during which the anti-symmetric wing bending and rotation momentum of the aileron are involved. By fastening the controls a non-linear reaction occurs , which develops a flutter with limited amplitude. Because of this, an overload and breakup of the plane does not occur.

When the airspeed by pulling on the stick, without hindering the sidewards movements of the stick, is reduced, the flutter oscillations will stop, but only when an appreciably slower airspeed is reached. This speed is the actual flutter airspeed of the plane. Flutter will start, when this airspeed is exceeded and the right disturbance influences the plane at this point.

Please Note:
All planes having an airworthiness certificate have gone through extensive static oscillation tests and flutter calculations in addition to a flight test program during which they have been checked for critical flutter behavior. Within approved operation field and its limitations as shown in the flight handbook one can be assured that no aero-elastic instabilities/flutter will occur.

Jan Schwochow
Flugabteilung des DLR (Deutsches Zentrum fuer Luft und Raumfahrt)
Braunschweig mit technisch/wissenschaftlicher Unterstützung
des Instituts für Aeroelastik des DLR in Göttingen
--------------------------------------------------------------------------------


--
Bruce Greeff
T59D #1771

Ok, I think I have it now.... M-B normally good.. flutter definitely bad...nice flutter video too...

Thanks you ya-all!

Originally Posted by

On Sunday, July 15, 2012 2:38:47 PM UTC-7, 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....




--
POPS



On Sunday, July 15, 2012 2:38:47 PM UTC-7, 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....




--
POPS



On Sunday, July 15, 2012 2:38:47 PM UTC-7, 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....




--
POPS



On Sunday, July 15, 2012 2:38:47 PM UTC-7, 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....


Pops,
Without the physics, mass balance is used to prevent control surface flutter. Flutter was a problem with the Janus C elevator, and the solution was a technical bulletin requiring installation of a mass balance. Really good tight linkages can prevent the onset of flutter, as well as keeping the surface 'loaded'.
Jim Wynhoff

On 7/16/2012 11:11 PM, BruceGreeff wrote:

Basically - look at an older design like an ASW20 in a high load
situation the wingtip has substantial vertical displacement. It appears
the wing is flexing to high Angle of attack on the outboard panels.
Possibly this is caused by a combination of the rotational drag force
from winglets as well as the aerodynamic load induced bending.

The straight leading edge means that the centre of pressure on the wing
remains ahead of the main spar all the way to the wingtip - at high load
this tends to rotate the weakest (torsionally) part of the wing to
higher AoA than desired.

I think you are making unwarranted assumptions:

* that Schleicher did not make the ASW 20 wing torsionally stiff enough
to avoid twisting; in fact, it had plenty of glass fiber in the skins
(the part of the wing that gives it torsional stiffness - the spar is
mostly for bending loads) to do just that.
* That the ASW 20 had winglets - it did not
* That the outer part of the wing is is the weakest torsionally; even if
it is, it is also the portion with the least torsional load on it

Modern two seaters often have the wing swept forward until about
midspan, which contradicts your claim.

The basic claim that the trapezoidal wings are a way to deal with
flutter might be right (I have not seen this claim before -
references?), but to claim wings without it are inadequate to meet their
design requirements is unsupported. There are several ways to increase
flutter speeds, even with straight leading edges, and designers used
them as needed.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl
- "A Guide to Self-launching Sailplane Operation Mar/2004" Much of what
you need to know tinyurl.com/yfs7tnz

On Jul 16, 11:11*pm, BruceGreeff wrote:

Basically - look at an older design like an ASW20 in a high load
situation the wingtip has substantial vertical displacement. It appears
the wing is flexing to high Angle of attack on the outboard panels.
Possibly this is caused by a combination of the rotational drag force
from winglets as well as the aerodynamic load induced bending.

Evaluating sectional angle of incidence from a photo or video is a
very difficult proposition with many confounding factors. I'd want to
see reference posts secured to the wing before I would try to draw any
conclusions.

The straight leading edge means that the centre of pressure on the wing
remains ahead of the main spar all the way to the wingtip - at high load
this tends to rotate the weakest (torsionally) part of the wing to
higher AoA than desired.

"Center of Pressure" is a rather arbitrary concept mosty used in
elementary aerodynamics and then abandoned. It is more useful to
discuss the forces on a wing section with coefficients of lift and
drag and pitching moment.

On the polyhedral designs the aerofoil and structure is effectively
swept back...

Except, of course, on those polyhedral designs where the wing is swept
forward.

Thanks, Bob K.

It's all very complicated, but I am sure of three things.

1- there are others who know more about this than I do.
2 - Over simplification leads to problems.
3 - Angle of Attack can be and has been accurately measured in flight,
in the design of winglets for a couple of gliders. In one case there was
indication of rotation under load - not conjecture.

Again - since they were only measuring actual angle of attack at
specific span points, it is impossible to say what the exact cause was.

Would it be better to say that the end result of forces acting on any
section of a wing resolve into two elements.
One bending moment/force that will have a chordwise displacement and an
angle (probably not exactly 90 degrees) to the chord line, or zero lift
chord line?
Secondly there will be a rotational moment, similarly at some, probably
different displacement along the same chosen line. And further that
these two origin displacements will move relative to the structure and
relative to eachother - depending on free stream velocity, density,
angle of attack, airfoil shape, contamination, turbulence (how many
parameters do we want to consider). Effectively, how hard the wing is
working.

The resultant force will be a single, constantly varying vector which
due to it's sense, origin and magnitude will constantly induce a varying
tendency to deform the section of wing. Which it will achieve to a
degree that depends on the origin, magnitude and direction of the
vector, and 3D stiffness in the section under consideration.

Pictures are easier - but is that a reasonable attempt?

Cheers
Bruce

On 2012/07/18 5:07 PM, Bob Kuykendall wrote:
On Jul 16, 11:11 pm, BruceGreeff wrote:

Basically - look at an older design like an ASW20 in a high load
situation the wingtip has substantial vertical displacement. It appears
the wing is flexing to high Angle of attack on the outboard panels.
Possibly this is caused by a combination of the rotational drag force
from winglets as well as the aerodynamic load induced bending.

Evaluating sectional angle of incidence from a photo or video is a
very difficult proposition with many confounding factors. I'd want to
see reference posts secured to the wing before I would try to draw any
conclusions.

The straight leading edge means that the centre of pressure on the wing
remains ahead of the main spar all the way to the wingtip - at high load
this tends to rotate the weakest (torsionally) part of the wing to
higher AoA than desired.

"Center of Pressure" is a rather arbitrary concept mosty used in
elementary aerodynamics and then abandoned. It is more useful to
discuss the forces on a wing section with coefficients of lift and
drag and pitching moment.

On the polyhedral designs the aerofoil and structure is effectively
swept back...

Except, of course, on those polyhedral designs where the wing is swept
forward.

Thanks, Bob K.


--
Bruce Greeff
T59D #1771

Concerning how controls surfaces are hinged to allow for mass balancing:
How are the ailerons balanced on the Grob Speed (dis)Astir where the
aileron hinge is a flexible section of the upper wing skin? Balance
weight somewhere down the line in the control system?


WB