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