REFLECTOR: More on Airframe Flutter
Ron Brown
romott at roadrunner.com
Tue Mar 27 08:21:08 CDT 2007
The Flutter issue is hot over at the RV list as well. (I would be very careful in allowing vibration - buzz to continue - either fix the problem or avoid the flight condition that is causing it. As Brendon found, this ain't nothing to play around with!!!!)
Time: 07:39:23 AM PST US
Subject: RV-List: Re: TAS and VNE
From: "Glaeser, Dennis A" <dennis.glaeser at eds.com>
I must respectfully disagree. Flutter and dynamic stability are vastly
different things. Flutter is NOT the trailing edge of a control surface
flapping in the breeze like a flag in high wind. Flutter is the some
portion of an airframe (wing, tail, fuselage) acting like a rubber band
due to forces in resonance with one or more of it's natural frequencies.
Since it is a frequency, not force, related phenomenon, that is why TAS
is key instead of IAS. At the right frequency, it only takes a small
force to start the process (think tuning fork).
The force that reacts to bring the airframe back after a disturbance is
the stiffness of the structure (again, like a tuning fork) not the
control surface. The reason for balancing controls is to minimize their
impact in increasing and/or maintaining the disturbing forces. Even a
wing without ailerons or flaps (a helicopter rotor blade for example)
can flutter. My first job was rotor blade design - I've seen wind
tunnel videos of it happening.
If you've never seen the bridge video mentioned in Ken's article - find
it and take a look. The Kruger article is accurate.
Dennis Glaeser
BSAE - building an RV-7A
------------------------------------------------------------------------
From: Wheeler North
I also don't entirely agree with the Kruger article.
Flutter is not a function of velocity directly, it is a function of
force acting upon the components which may flutter. While the air
molecules are going by at a faster rate there are far less of them so
the resultant force doesn't nearly increase at the rate TAS does with
gains in altitude.
Flutter is properly called negative dynamic stability and it requires
positive static stability to occur. Positive static force pushes the
control back to its neutral condition. When it's coupled to inertia and
the resultant forces are able to move the control beyond that to a
position greater then the original upset than that is negative dynamic
stability AKA flutter. Mass aft of the pivot line adds to the inertia,
mass at or just fwd of it takes away from the inertia. However it is
force that causes the initial positive static desire to return to
neutral and beyond. And it doesn't change too much with changes in
altitude. If it did you would be able to notice it in the feel of the
stick as altitude changed.
This is one of the primary reasons we use VNE and all the other numbers
in indicated units.
W
Time: 12:30:52 PM PST US
From: "Ed Anderson" <eanderson at carolina.rr.com>
Subject: RV-List: A internet view of TAS and VNE
Here is an explanation that appears to make it fairly clear as to why
TAS rather than IAS is the critical factor in flutter.
Ed
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
http://www.auf.asn.au/groundschool/flutter.html#flutter
Indicated airspeed really reflects dynamic pressure rather than airflow
velocity so if the structural limitations which define Vne for an
aircraft type are particularly associated with the distribution of
forces associated with velocity then the specified indicated Vne has to
be decreased as altitude is increased - to adjust for the increase in
true airspeed which latter is about 1.5% greater than IAS/CAS for every
1000 feet of altitude, see rule of thumb #2.
13.3 Aerodynamic reactions to flight at excessive speed
Flutter
Wing structures are akin to a 'tuning fork' extending from the fuselage.
When a tuning fork is tapped the fork vibrates at a particular
frequency, the stiffer the structure the higher its 'natural' frequency.
The natural frequency of a wing or tailplane structure may apply another
limiting airspeed to flight operations - related to structural
instabilities: flutter and wing divergence.
When the airflow around a wing or control surface is disturbed by
aerodynamic reactions or pilot inputs, the structure's elastic reactions
may combine as an oscillation or vibration of the structure (possibly
evident as a buzz in the airframe) which will quickly damp itself out at
normal cruise speeds. At some higher speed - the critical flutter speed
- where the oscillations are in phase with the natural frequency of the
structure the oscillations will not damp out but will resonate, rapidly
increasing in amplitude. (Pushing a child on a swing is an example of
phase relationships and amplification). This condition is flutter and,
unless airspeed is very quickly reduced, the severe vibrations will
cause control surface [or other] separation within a very few seconds.
The following paragraph is an extract from an article by William P.
Rodden appearing in the McGraw-Hill Dictionary of Science and
Technology; it provides a succinct description of flutter:
"Flutter (aeronautics) - An aeroelastic self-excited vibration with a
sustained or divergent amplitude, which occurs when a structure is
placed in a flow of sufficiently high velocity. Flutter is an
instability that can be extremely violent. At low speeds, in the
presence of an airstream, the vibration modes of an aircraft are stable;
that is, if the aircraft is disturbed, the ensuing motion will be
damped. At higher speeds, the effect of the airstream is to couple two
or more vibration modes such that the vibrating structure will extract
energy from the airstream. The coupled vibration modes will remain
stable as long as the extracted energy is dissipated by the internal
damping or friction of the structure. However a critical speed is
reached when the extracted energy equals the amount of energy that the
structure is capable of dissipating, and a neutrally stable vibration
will persist. This is called the flutter speed. At a higher speed, the
vibration amplitude will diverge, and a structural failure will result."
Inertia has a role in flutter development requiring that control
surfaces - ailerons, elevators, rudder - be mass balanced (i.e. the
centre of gravity of the control surface coincides with the hinge line)
to limit the mass moment of inertia. It may be acceptable for the
control surface to be over-balanced, i.e. the cg is slightly forward of
the hinge line. Mass balancing of the control surfaces will prevent them
fluttering but the possibility for wing [for example] flexing/twisting
flutter may still exist.
The critical flutter airspeed [or something akin to it] may eventuate
well below Vne if wear in control surface hinges, slop in actuating
rods/cables/cranks/torque tubes, water or ice inside control surfaces or
absorbed within a foam core, mud outside, faulty trim tabs, additional
surface coatings applied after balancing or other system disturbances
exist which alter the structure's reactions.
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