Mass Balancing explanation request.

POPS

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Jul 15, 2012, 5:38:47 PM7/15/12

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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

Karen

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Jul 15, 2012, 10:58:29 PM7/15/12

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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?2188-Mass-Balance-for-Elevator

POPS

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Jul 15, 2012, 10:33:16 PM7/15/12

to

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

BruceGreeff

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Jul 16, 2012, 6:22:14 AM7/16/12

to

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

Bruce Greeff
T59D #1771

bruno...@gmail.com

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Jul 16, 2012, 9:20:28 AM7/16/12

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

Bruno - B4

sisu1a

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Jul 16, 2012, 12:05:31 PM7/16/12

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>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
--------------------------------------------------------------------------------

POPS

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Jul 16, 2012, 2:36:21 PM7/16/12

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

> 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

--
POPS

plodd...@gmail.com

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Jul 16, 2012, 5:21:20 PM7/16/12

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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