Question - rudder flutter ?

What is rudder flutter ?

"Bravo Two Zero" wrote in news:EyZbj.1217$Gr4.669
@newsfe08.phx:

What is rudder flutter ?




A bit like the way a flag whips in a high wind. Any control surface can
flutter if the supporting surface is flexible enough to allow a
complimentary cyclic oscillation of the two surfaces (control and
stabilising surfaces).
Here's how it works. The control surface is displaced, loading up the
adjacent stabilising surface and bending it over to one side (the fin
in this case, but it can be the stab or the wing just as easily) The fin
has now suddenly been pushed to one side under tension and may now
spring back in the other direction carrying the hinged edge of the
rudder with it. the trailing edge of the rudder, however, will be left
behind in this move, deflecting and thereby making a control input that
contributes considerable force in the direction the rudder is now
moving. ( I know, I know, a pic here would help, but bear with me) the
fin is now stretched out in the other direction and ready to spring back
the other way repeating the cycle.
If the rudder is balanced on the hinge line, it will be far less likely
to support this cycle. The other way you can stem it is to make the stab
fin or wing very rigid and this is why biplanes can have as high a
redline as , say a Mooney might though the bipe might have no balance
area at all and the Mooney would. All the surfaces are too rigid to
allow the cycle to start.
You would think that a greater load on the surface, i.e, "G". would tend
to dampen the action, but the opposite is the case.
Flutter will appear at a lower speed if the surface has an aerodynamic
load on it.
Aerobatics are the most common arena for flutter, but poor maintenance
can also bring it on. A lot of high performance light planes have been
lost simply because they have been repainted without rebalancing the
surface, or the balance material has come loose or lost in a rebuild.
Mass balance is typically a bit of lead buried in a balance horn or some
other bit of the control surface ahead of the hinge line.
Aerodynamic balance can also be employed to help in this and that's
often where the mass balance is housed as well.
Large aircraft often use depleted uranium for the mass balance, BTW!




Bertie

On Dec 25, 4:18 pm, Bertie the Bunyip wrote:
"Bravo Two Zero" wrote in news:EyZbj.1217$Gr4.669
@newsfe08.phx:

What is rudder flutter ?

A bit like the way a flag whips in a high wind. Any control surface can
flutter if the supporting surface is flexible enough to allow a
complimentary cyclic oscillation of the two surfaces (control and
stabilising surfaces).
Here's how it works. The control surface is displaced, loading up the
adjacent stabilising surface and bending it over to one side (the fin
in this case, but it can be the stab or the wing just as easily) The fin
has now suddenly been pushed to one side under tension and may now
spring back in the other direction carrying the hinged edge of the
rudder with it. the trailing edge of the rudder, however, will be left
behind in this move, deflecting and thereby making a control input that
contributes considerable force in the direction the rudder is now
moving. ( I know, I know, a pic here would help, but bear with me) the
fin is now stretched out in the other direction and ready to spring back
the other way repeating the cycle.
If the rudder is balanced on the hinge line, it will be far less likely
to support this cycle. The other way you can stem it is to make the stab
fin or wing very rigid and this is why biplanes can have as high a
redline as , say a Mooney might though the bipe might have no balance
area at all and the Mooney would. All the surfaces are too rigid to
allow the cycle to start.
You would think that a greater load on the surface, i.e, "G". would tend
to dampen the action, but the opposite is the case.
Flutter will appear at a lower speed if the surface has an aerodynamic
load on it.
Aerobatics are the most common arena for flutter, but poor maintenance
can also bring it on. A lot of high performance light planes have been
lost simply because they have been repainted without rebalancing the
surface, or the balance material has come loose or lost in a rebuild.
Mass balance is typically a bit of lead buried in a balance horn or some
other bit of the control surface ahead of the hinge line.
Aerodynamic balance can also be employed to help in this and that's
often where the mass balance is housed as well.
Large aircraft often use depleted uranium for the mass balance, BTW!

Bertie

To add to Bertie

The bottom line is that If the COG of the surface is behind it's
center of lift flutter is promoted. That's why most control surfaces
add weights (depleted uranium!?) to bring the COG forward. On the
preflight you check that that balance weights are there and fixed.
Flutter will destroy the surface, structure and it's hinges pretty
quickly.

Cheers

WingFlaps wrote in news:797747ee-e7ec-4c72-b61b-
:


The bottom line is that If the COG of the surface is behind it's
center of lift flutter is promoted. That's why most control surfaces
add weights (depleted uranium!?) to bring the COG forward. On the
preflight you check that that balance weights are there and fixed.
Flutter will destroy the surface, structure and it's hinges pretty
quickly.
Actually, that's not quite correct. the CG of the surface has to be at the
hinge line, not it's Cl. to eliminate this tendency. It's down to the
rididity of the hinge in space, though. The less rigid the greater the need
for balancing. Most wings and stab surfaces are quite flexible, though, so
at least some balance, if not 100% (which would put it at the hinge line)
is reguired for most airplanes.


I saw a video of a twin Comanche on You tuvbe flutterign badly during
tests, but I can't find it now. The other ones there don't illustrate it
very well excepet maybe this one. http://www.youtube.com/watch?v=
8D7YCCLGu5Y
No control surface, but it's the flexibility of the wing that's causing the
problem. It's increasing in amplitude because each subsequent oscillation
is further and when it moves further it increases alpha which in turn
imparts more energy to the next oscillation..


Bertie

On Dec 26, 12:15 am, Bertie the Bunyip wrote:
WingFlaps wrote in news:797747ee-e7ec-4c72-b61b-
:



The bottom line is that If the COG of the surface is behind it's
center of lift flutter is promoted. That's why most control surfaces
add weights (depleted uranium!?) to bring the COG forward. On the
preflight you check that that balance weights are there and fixed.
Flutter will destroy the surface, structure and it's hinges pretty
quickly.

Actually, that's not quite correct. the CG of the surface has to be at the
hinge line, not it's Cl. to eliminate this tendency. It's down to the
rididity of the hinge in space, though. The less rigid the greater the need
for balancing. Most wings and stab surfaces are quite flexible, though, so
at least some balance, if not 100% (which would put it at the hinge line)
is reguired for most airplanes.


Err, I didn's say the Cl was at the hinge line (rather that COG must
be forward of control surface lift). But I agree, if the COG _can_ be
put at the the hinge line then not just control surface flutter is
reduced but also flying surface flutter (in this case fin + rudder).
The trouble with the lightt structures in a plane is that balancing
one end (e.g. with a leaded horn) may lead to tortional problems...

Cheers

On Dec 26, 12:15 am, Bertie the Bunyip wrote:


I saw a video of a twin Comanche on You tuvbe flutterign badly during
tests, but I can't find it now. The other ones there don't illustrate it
very well excepet maybe this one.http://www.youtube.com/watch?v=
8D7YCCLGu5Y
No control surface, but it's the flexibility of the wing that's causing the
problem. It's increasing in amplitude because each subsequent oscillation
is further and when it moves further it increases alpha which in turn
imparts more energy to the next oscillation..

That sounds like divergence (not flutter)? Divergence is due to the
flexibility of the structure and a great example is Tacoma narrows?

Cheers

That

WingFlaps wrote in
:

On Dec 26, 12:15 am, Bertie the Bunyip wrote:
WingFlaps wrote in
news:797747ee-e7ec-4c72-b61b-
:



The bottom line is that If the COG of the surface is behind it's
center of lift flutter is promoted. That's why most control
surfaces add weights (depleted uranium!?) to bring the COG forward.
On the preflight you check that that balance weights are there and
fixed. Flutter will destroy the surface, structure and it's hinges
pretty quickly.

Actually, that's not quite correct. the CG of the surface has to be
at the hinge line, not it's Cl. to eliminate this tendency. It's down
to the rididity of the hinge in space, though. The less rigid the
greater the need for balancing. Most wings and stab surfaces are
quite flexible, though, so at least some balance, if not 100% (which
would put it at the hinge line) is reguired for most airplanes.


Err, I didn's say the Cl was at the hinge line (rather that COG must
be forward of control surface lift).


I know. I understood what you said, but the Cl of the surface is
irrelevant to flutter.


But I agree, if the COG _can_ be
put at the the hinge line then not just control surface flutter is
reduced but also flying surface flutter (in this case fin + rudder).

They're th esame thing, really. Although you can have eiter, they're
caused by the same situation, the surface as a whole, whether or not it
has a seperate flying surface or not, is producing and flexing the
surface.
The control surface element is caused by the rudder or whatever, lagging
behind the moving fin due to inertia. This causes displacement of the
rudder and creates a rudder input which provokes the movement of the fin
even further. the combined inertia stretches the elastic fin and when it
gets as far as it;'s going to go in that direction, the elastic property
of the construction material flips it back the other way, leaving the
rudder behind and causing another rudder input in the opposite direction
and so on, increasing in amplitude until something fails, be it the
hinge, the spar, or whatever reahes it's limit first. Usually it's the
spar.

The trouble with the lightt structures in a plane is that balancing
one end (e.g. with a leaded horn) may lead to tortional problems..

Huh?
Torsional flutter is kinda outside the scope of this explanation, but
the amount required if you put it at the tip would be lower anyway if
you're not shooting for 100%, so that's why it is placed at the end if
that's the solution required. There are a few types where massive lumps
are mounted at the outboard ends of the ailerons. Others, like Cessnas,
just have loooong strips inside the hinge gap.

It's done all the time, but it doesn;t have to be at one end and it
doesn't have to be a horn. It also doesn't have to be a 100% balance if
the performance doesn't neccesitate it or if the stab or wing is rigid
enough for flutter not to be an issue within the speed envelope.
But for anythng that goes very fast, you have to have 100% balancing,
which means it's statically balanced at the hinge line.


Bertie

The other way you can stem it is to make the stab
fin or wing very rigid and this is why biplanes can have as high a
redline as , say a Mooney might though the bipe might have no balance
area at all and the Mooney would.

Is this because of the welded 4130 tube structure?

WingFlaps wrote in
:

On Dec 26, 12:15 am, Bertie the Bunyip wrote:


I saw a video of a twin Comanche on You tuvbe flutterign badly during
tests, but I can't find it now. The other ones there don't illustrate
it very well excepet maybe this one.http://www.youtube.com/watch?v=
8D7YCCLGu5Y
No control surface, but it's the flexibility of the wing that's
causing the problem. It's increasing in amplitude because each
subsequent oscillation is further and when it moves further it
increases alpha which in turn imparts more energy to the next
oscillation..

That sounds like divergence (not flutter)? Divergence is due to the
flexibility of the structure and a great example is Tacoma narrows?



hmm, true.

Well, flutter is also down to flexibility. All kinds of flutter in all it's
incarnations.
So I suppose it could be said that flutter is a species of divergence.
Dunno, I'm not an engineer! What I've given is the dumbed down pilot
version of flutter.


Bertie

wrote in news:d6ec528a-01a1-4a71-a603-
:

The other way you can stem it is to make the stab
fin or wing very rigid and this is why biplanes can have as high a
redline as , say a Mooney might though the bipe might have no balance
area at all and the Mooney would.

Is this because of the welded 4130 tube structure?


No, the rigging. The wings on a bipe won't budge at all because of the
flying wires. Same for the stab. 4130 will flex really easily by itself.
You can bend it by hand!
But even the tail surfaces on those airplanes are braced at about half span
so they don't flex significantly. A cantilever aluminum or even a wood
structure will flex quite a lot so that must be taken into consideration if
you want any kind of reasonable redline.
That kind of exhausts my knowledge of the subject, but there are some guys
over in rec.aviation.homebuilt, amongst other places, that know this stuff
backwards and forwards.
Anthony also probably knows it just backwards



Bertie