"flutter" is bad, and why exactly?
 

My question centers here on the "why".
This topic was touched on in the recent Bonneville thread, and I felt best not to clutter that thread up, and sorry if this has been addressed elsewhere, unknown to me.

It seems bad, it looks bad, and "common sense" seems to indicate its absorbing energy, bending the panel that is fluttering, converting some portion of the energy to heat, I suspect.

But the question here is, is there ANY aero benefit (less overall drag?)by allowing the air to seek a path of lower resistance as the fluttering panel reacts to the varying(/) pressure waves, or is just increasing drag, turbulence, etc?

On a slightly related topic, it has always been the goal in high powered speaker cabinet design, to build cabinets as vibration free as practically possible for the situation, because any cabinet vibration has tonal artifacts, and more pertinent to this thread, wastes acoustic energy converting into mechanical energy and the resultant heating.

So is it a fair conclusion that when ever one sees random panel fluctuations in vehicles due to aero loads, the design is lacking in some degree?

How is the heat being generated? Pressure fluctuations. The heat produced is likely minimal compared to the clutching fingers of turbulent air.

First thing that comes to mind is a flag. How much drag is created by those ripples in a flag. Maybe not apples to apples, but it seems a flag being towed by a car at 30mph is similar to a weak body panel at 300mph, no?

Flutter will cause a plane to crash out of control. It is an oscillation resonance excited by the energy in the air flow, If the cop ( center of pressure) and CG center of gravity do not coincide, if you look closely at the planes control surfaces, you will see weights ahead of the hinge. A flag waving id the wind is flutter.

https://en.wikipedia.org/wiki/Aeroelasticity

It is my understanding that you generally want CG in front of COP for a stabilizing effect. If the converse were true, the surface mass weights would be attached to the rear not the front.

My thinking is bending any material, requires energy, and that generates internal heating of the material. Is it likely minor, yes, and I thought obvious, which is why my original question has more of an aero leaning, so does the "flutter" induce additional drag/turbulence/flow separation/etc or is the observed panel flutter simply the result of the above?

This question was first considered by me decades back when Nascar had roof mounted live action cameras and it was apparent the severely lightened hoods would deflect/flutter grossly at speed or when it the wake of another car, and wasn't sure if it mattered little, or they felt it was of no real consequence. It seemed the Bonneville car thread had similar issues.

But what is causing the material heating? Viscosity transferring energy from the turbulence in the unseparated flow. The turbulence itself robs energy that flows away in the [slightly] heated air.
en.wikipedia.org/wiki/Parasitic_drag

why bad
 

* flutter is cyclic deformation
* cyclic deformation can lead to 'work-hardening'
* work-hardening can lead to embrittlement of the panel
* embrittlement can lead to fracture
* fracture leads to panel failure
* Panel failure alters pressure distribution in the vicinity of the failed panel
* the altered pressure distribution can affect aerodynamic stability.
* Flutter-induced aerodynamic instability has killed a number of people. Aircraft and record cars.

What could possibly go wrong?

https://2.bp.blogspot.com/-KauPX-RQ1...27714304_n.jpg
On the return pass, at 427 mph, the lack of good body mounting, and not enough clearance at speed with air pressure, centrifigal force, and kinematics, caused tire deflection which allowed a panel to rub the tire. Which killed the left rear tire.

In stereo-speak "heat" is the mysterious energy created by vibration and resonance when absorbed into a structure (rack/stand/enclosure).

Interesting incident on the Flashpoint car, thanks for posting.

I think buffeting and flutter is energy at random vectors, and we want all those energies and vectors parallel to the direction our body is moving for maximum efficiency.

Analogy, if I may:

Walking in a crowd with people constantly knocking into you is harder and takes more energy than walking a path unencumbered by rude idiots.

Why Bad

Granted, and you are stating the obvious IMO, my question implied, was from mainly an aero perspective rather than structural.

Ok then simple: calculate the square footage of the surface movement or even guesstimate it, figure out how much your CoD is in square feet, and add this to the flutter drag. Shouldn't be a huge number unless the panel has a loose end.

It is curious that both the Wikipedia entry on "Aeroelasticity" and Aerohead's reply focus on the structural effects of flutter.

Is there a way to exclude the structural effects and just focus on the aerodynamic energy it takes to create and dissipate flutter?

Are these two elements so intertwined that one cannot talk about one without talking about the other?

I suppose it would help if we define an "A" and a "B" situation.

A: Flutter caused by structural deficiency, and why is it bad.

B: Flutter caused by shape/body/form imperfection, and why is it bad.

It seems to be we are talking about "A" when the original poster wants to talk about "B".

EDIT:

The PDF in this link may help explore the different aspects of this topic, I like the pyramid diagram it begins with.

https://www.google.com/url?sa=i&url=...AAAAAdAAAAABAD

Finding the right question comes before finding the right answer.

Finding the right terms, the correct language is needed to communicate intended thought(s).

Cut out the middle man: http://www.ltas-cm3.ulg.ac.be/AERO00...elasticite.pdf

The triangle is a good frame. :)

I suspect the effect is more acoustic than thermal.

Most certainly.:thumbup:

Thanks for the link, might take few reads for all that to sink in, but this stood out:

"There are no empirical or statistical design
methods for aeroelastic design; flutter is a very
complex phenomenon"

aero perspective
 

Then we'd want to look at 'where' the flutter occurs.
Side body deformation could affect yaw moments and directional stability, depending on velocity, winds, gusts, etc..

If it's anywhere in the aft-body, it could trigger separation and induced-lift, laterally, or vertically.
Cyclic deformation could induce Karman vortex street-esque phenomena, tail wagging the dog.
Aeroelasticity implies unstable drag coefficient. I don't know how that could ever be beneficial.