Is there a simple (or complex) guideline for the relationships between
CG, pivot axis and aerodynamic center for a control surface to prevent
flutter?

Is it as simple as keeping the CG at or ahead of the aerodynamic
center, similar to designing for stability of the aircraft itself?

--

FF

The following is an article that you may find interesting, even though it
doesn't directly answer your question.
http://www.soaridaho.com/Schreder/Construction/HBldr_Hall.html#flutter

Wayne
http://www.soaridaho.com/Schreder

> wrote in message
oups.com...
> Is there a simple (or complex) guideline for the relationships between
> CG, pivot axis and aerodynamic center for a control surface to prevent
> flutter?
>
> Is it as simple as keeping the CG at or ahead of the aerodynamic
> center, similar to designing for stability of the aircraft itself?
>
> --
>
> FF
>

I had to rebuild my ailerons which started fluttering when approaching
175kts IAS. The ailerons should essentially balance horizontally when
suspended from the hinges. If they are tail heavy and the wing starts
shaking the aileron tail "shakes" in opposite direction to the wing.
But that is just the orientation that will help the wing shake even
more which in turn will shake (flutter) the aileron with an increasing
amplitude - with possible destructive effects. This is a forced
resonance effect. If your plane is slow this scenario may not effect
things much. But it all depends on wing mass, stiffness of wing
support, etc. In my case I had to include more weight on the front part
of the aileron to balance.
-----------------------------------------
SQ2000 canard: http://www.abri.com/sq2000

wrote:
> Is there a simple (or complex) guideline for the relationships between
> CG, pivot axis and aerodynamic center for a control surface to prevent
> flutter?
>
> Is it as simple as keeping the CG at or ahead of the aerodynamic
> center, similar to designing for stability of the aircraft itself?
>
> --
>
> FF

On 6 Jun 2005 09:39:45 -0700, wrote:

>Is there a simple (or complex) guideline for the relationships between
>CG, pivot axis and aerodynamic center for a control surface to prevent
>flutter?
>
>Is it as simple as keeping the CG at or ahead of the aerodynamic
>center, similar to designing for stability of the aircraft itself?

The Bearhawk and Christavia both have ailerons that have a pivot point
aft of the nose of the aileron. The Bearhawk requires that lead (or
some heavy substance) be placed in the nose of the aileron such that
the aileron is neutral while balancing on the hinge points. I
understand that the nose could be a bit heavy (such that the nose
pivots down when balanced on the hinges) and not compromise flutter
avoidence. But the Christavia does not require counterweighting,
although I've heard of one or two doing it anyway. The Christavia is
a lot slower than the Bearhawk, which makes a difference.

The other parts to flutter avoidence involve making sure there is no
slack in the control cables or rods, and that the controls themselves
are stiff enough such that their flexibility does not contribute to
the problem.

I'm not an aerodynamicist and I do not know or offer mathmatical
figures to support what I just said. When I built the ailerons and
other control surfaces for my Christavia, I did not change anything,
they are built according to the plans. No Christavia has reported a
problem with flutter and I'm not going to risk changing that.

Well, to be honest, I did drill lightening holes in the aileron spars,
but my rational there was that the spars were way over designed and
hang off three hinges. The control cable attaches right in the middle
of the aileron so there's no way for the aileron to twist when a
control input is applied. The holes aren't real close together and
not very big either.

It's a drop in the bucket anyway because the wings are massive,
overdesigned and very heavy (talking about a Mk 4 version of the
Christavia). Besides, max speed is 130 or so.

Corky Scott

On 6 Jun 2005 09:39:45 -0700, wrote:

>Is there a simple (or complex) guideline for the relationships between
>CG, pivot axis and aerodynamic center for a control surface to prevent
>flutter?
>
>Is it as simple as keeping the CG at or ahead of the aerodynamic
>center, similar to designing for stability of the aircraft itself?

basically that is it.
ailerons have an added complexity that while they need to be balanced,
the overall wing also needs balance in the correct position.(typically
ahead of the main spar)

not all surfaces on all aircraft are actually fully balanced. the
ailerons on a cessna 150 are balanced on the hinge line with a bar of
lead rivetted onto the frise lip. the rudder and stabiliser have a
specification for the maximum amount of allowable unbalance.

Vne is the normal never exceed speed. Vd is the dive speed which is
established by test flying. it is the maximum speed that can be flown
before the onset of flutter (or by other structural limits) and is
determined by very careful test flying. Vne is set at 90% of Vd for
safety.

so you can build in flutter proofing by balance, absence of hinge slop
and control tensions but in the end it is up to some careful test
flying to determine whether there is an onset of flutter.

hope this helps
Stealth Pilot

"Stealth Pilot" > wrote in message
...
> On 6 Jun 2005 09:39:45 -0700, wrote:
>
> >Is there a simple (or complex) guideline for the relationships between
> >CG, pivot axis and aerodynamic center for a control surface to prevent
> >flutter?
> >
> >Is it as simple as keeping the CG at or ahead of the aerodynamic
> >center, similar to designing for stability of the aircraft itself?
>
> basically that is it.
> ailerons have an added complexity that while they need to be balanced,
> the overall wing also needs balance in the correct position.(typically
> ahead of the main spar)
>
> not all surfaces on all aircraft are actually fully balanced. the
> ailerons on a cessna 150 are balanced on the hinge line with a bar of
> lead rivetted onto the frise lip. the rudder and stabiliser have a
> specification for the maximum amount of allowable unbalance.
>
> Vne is the normal never exceed speed. Vd is the dive speed which is
> established by test flying. it is the maximum speed that can be flown
> before the onset of flutter (or by other structural limits) and is
> determined by very careful test flying. Vne is set at 90% of Vd for
> safety.
>
> so you can build in flutter proofing by balance, absence of hinge slop
> and control tensions but in the end it is up to some careful test
> flying to determine whether there is an onset of flutter.
>
> hope this helps
> Stealth Pilot

In some cases, Vd flight tests have used small air/oil damper cylinders
(basically tiny shock absorbers) fitted to the control surface control
horns. These allow normal control motion but strongly damp motion at faster
rates. This is the same idea as steering dampers on trucks.

I've often thought that any homebuilt where there is a concern for control
flutter could use these as a permanent safety feature since they are light
and cheap.

Bill Daniels

"Stealth Pilot" > wrote

> so you can build in flutter proofing by balance, absence of hinge slop
> and control tensions but in the end it is up to some careful test
> flying to determine whether there is an onset of flutter.

The flutter test for the brave, goes something like this. Get plenty of
altitude, say 8 thousand feet AGL, put the plane into a pretty steep dive,
WOT, and give the stick a sharp rap. If the plane keeps flying, and you are
satisfied with this speed as Vne, OK, you are done. If the plane starts to
flutter, and the wing falls off, use you parachute, now.

Oh, you did have a parachute on, didn't you? <g>
--
Jim in NC

On Tue, 7 Jun 2005 20:03:14 -0400, "Morgans"
> wrote:

>
>"Stealth Pilot" > wrote
>
>> so you can build in flutter proofing by balance, absence of hinge slop
>> and control tensions but in the end it is up to some careful test
>> flying to determine whether there is an onset of flutter.
>
>The flutter test for the brave, goes something like this. Get plenty of
>altitude, say 8 thousand feet AGL, put the plane into a pretty steep dive,
>WOT, and give the stick a sharp rap. If the plane keeps flying, and you are
>satisfied with this speed as Vne, OK, you are done. If the plane starts to
>flutter, and the wing falls off, use you parachute, now.
>
>Oh, you did have a parachute on, didn't you? <g>


Uh, you might have made a slight error here... the correct
method is to get into a dive, pull up and at the correct
test velocity, give the stick a sharp rap. The difference
is that you're slowing down NOT speeding up as you enter the
"test".... If you do encounter flutter, it's hopeful it
will stop as you slow down.

John

"John Ammeter" > wrote

> Uh, you might have made a slight error here... the correct
> method is to get into a dive, pull up and at the correct
> test velocity, give the stick a sharp rap. The difference
> is that you're slowing down NOT speeding up as you enter the
> "test".... If you do encounter flutter, it's hopeful it
> will stop as you slow down.

Good enough. It's been a while since I read about that. I guess not being
able to slow down would be a "bad thing", but it *would* give my parachute
scenario a better shot. <g>
--
Jim in NC

wrote:
> Is there a simple (or complex) guideline for the relationships between
> CG, pivot axis and aerodynamic center for a control surface to prevent
> flutter?
>
> Is it as simple as keeping the CG at or ahead of the aerodynamic
> center, similar to designing for stability of the aircraft itself?
>
> --
>
> FF

On Tue, 07 Jun 2005 18:23:12 -0700, John Ammeter
> wrote:

>On Tue, 7 Jun 2005 20:03:14 -0400, "Morgans"
> wrote:
>
>>
>>"Stealth Pilot" > wrote
>>
>>> so you can build in flutter proofing by balance, absence of hinge slop
>>> and control tensions but in the end it is up to some careful test
>>> flying to determine whether there is an onset of flutter.
>>
>>The flutter test for the brave, goes something like this. Get plenty of
>>altitude, say 8 thousand feet AGL, put the plane into a pretty steep dive,
>>WOT, and give the stick a sharp rap. If the plane keeps flying, and you are
>>satisfied with this speed as Vne, OK, you are done. If the plane starts to
>>flutter, and the wing falls off, use you parachute, now.
>>
>>Oh, you did have a parachute on, didn't you? <g>
>
>
>Uh, you might have made a slight error here... the correct
>method is to get into a dive, pull up and at the correct
>test velocity, give the stick a sharp rap. The difference
>is that you're slowing down NOT speeding up as you enter the
>"test".... If you do encounter flutter, it's hopeful it
>will stop as you slow down.
>
>John

thank you john. that's exactly how it is done. if you lose control
because of flutter the aircraft is moving back into a more inherently
safe speed range. ...if it lasts that long. :-)

Stealth Pilot

wrote:
>
> wrote:
>
>>Is there a simple (or complex) guideline for the relationships between
>>CG, pivot axis and aerodynamic center for a control surface to prevent
>>flutter?
>>
>>Is it as simple as keeping the CG at or ahead of the aerodynamic
>>center, similar to designing for stability of the aircraft itself?
>>
>>--
>>
>>FF
>
>

No. This is aviation, dealing with real physics. Nothing is as ever as
simple as it seems.

A big component in flutter is the stiffness of the structure. The
stiffer it is, the closer you get to the idealized world where it is all
about CG and aerodynamic centers.

The biggest problem with flutter is that you have several complex and
unquantified forces creating stresses in complex structures, resulting
in unquantified deflections which create another set of compex and
unquantified forces. The reason it is all unquantified is that the
forces are constantly moving in an almost random manner, and only at
just the wrong time they'll settle into a pattern that'll rip the wings off.

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."