Counterweighting to prevent flutter

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

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