Fin Flutter

In another thread I asked about Fin Flutter. There was some good info in there but because of the thread I wonder if a lot of input was missed.

The questions:

Is there a good practical rule of thumb to guide us as to when to worry about fin flutter?

Is there a reasonably easy to use method to calculate fin flutter?

Are there rules of thumb to help tell you when you don't need to worry at all?


The answers we have:

It was stated the fin flutter is a resonance issue and can occur at any speed. This is not a mach only issue.

It was also stated that simply having different materials does not mean that flutter will not occur - resonances can be dampened by this method but they can also be amplified if you are unlucky or have a poor design.

I think this is a really tough question.....there is literature out there that can get into the physics of the forces on the fins and determining the resonant frequency of a system is not for the faint of heart.

I've seen some (well, one anyway) fin flutter calculators out there but I don't put much stock in them....they seem pretty simplistic and some of the results are just not realistic in my opinion.

I don't know of any rules of thumb except that if the fin looks like a rocket fin (i.e. you aren't building a Klingon Bird of Prey) and is reasonably proportioned for it's size (span, chord and thickness) it will probably be ok up to transonic. If the fin leaves the realm of rocketness and you have a high aspect ratio or it's really flexible then it's anybodies guess.

As you press to test on the velocity the best answer I can give you is to make it as stiff as you can make it given your goals and materials......low aspect ratio, planform that is inherently flutter resistant, stiffening techniques such as correctly oriented carbon fiber and good building techniques.

As far as the whole different materials having vibratory properties that "cancel" each other out that is just not so in a bonded system. Each layer of the composite structure is bonded to it's neighbor and therefor they vibrate as a single system so there is no cancellation possible unless your layers of composite debond and they can move independently.

That being said, each layer or component WILL change the frequency at which the part will vibrate and therefor change the sympathetic frequency which will cause flutter. The reason is each "layer" changes the mass distribution and stiffness in any given direction (with uni it's in only one axis, with woven cloth it can be in two or three axis' in a plane etc.)

I can imagine if you are using unidirectional cloth for stiffness, there is a point when pushing the edge of it's performance that just the orientation of the carbon fibers could mean the difference between success and disaster.

Oh well, that's my .02 worth....it's time to get back to work. Didn't contain any pat answers for you...maybe someone out there has some better information.

Thanx, i suggested he go there first.

And smurf I also agree wholeheartedly, once a bunch of stuff gets bonded together chemically, it loses cancellation properties, it becomes a glob that has its own stiffness and weight--maybe a lower Q but still one material.

Now over on Composite rocketry forum it was suggested that by using visco-elastic polymers as a core, the problem might be mitigated to some extent. I'm open minded about this as anyway to turn motion into heat seems good, and in fact have been looking at building speakers this way.

Good thread.
John S

Originally posted by rocketguy101
fin flutter

Click to expand...

Denverdoc sent me there - but I have had difficulty making heads or tails of it. The instructions seem cryptic and I'm not having a lot of luck with it.

Originally posted by uncle_vanya
Denverdoc sent me there - but I have had difficulty making heads or tails of it. The instructions seem cryptic and I'm not having a lot of luck with it.

Click to expand...

I'm assuming you got to the excel spread sheet, maybe to start try opening the tabs from GLR or PML. The flutter velocity as a function of material is shown in the right hand cells. Different planforms form the columns.

Then you can play with width, watch what happens as you override the given thickness, etc
JS

Not really a post about why/when it happens, but if you want to SEE it... pick up the latest LDRS video from Rockets Magazine... Art Upton's on board video shows the fins on his bird as they start to ever so slightly flex...they go back and forth a few times, farther and farther, then *POOF*... the just seemed to vaporize... I've been flying about 9 years, and knew a bit about flutter, but to actually see it was incredible!

Ron

Originally posted by rocketguy101
from you tube

Click to expand...

Flutter, plus (mostly) an artifact of the camera's rolling shutter.

GC

"As far as the whole different materials having vibratory properties that "cancel" each other out that is just not so in a bonded system. Each layer of the composite structure is bonded to it's neighbor and therefor they vibrate as a single system so there is no cancellation possible unless your layers of composite debond and they can move independently."

Thank God, someone said it, I was getting so tired of people pulling this "cancellation" crap out their arses.

Originally posted by aerospike
"As far as the whole different materials having vibratory properties that "cancel" each other out that is just not so in a bonded system. Each layer of the composite structure is bonded to it's neighbor and therefor they vibrate as a single system so there is no cancellation possible unless your layers of composite debond and they can move independently."

Thank God, someone said it, I was getting so tired of people pulling this "cancellation" crap out their arses.

Click to expand...

Mea Culpa - I misunderstood this like many others.

Originally posted by AZ_Ron
Not really a post about why/when it happens, but if you want to SEE it... pick up the latest LDRS video from Rockets Magazine... Art Upton's on board video shows the fins on his bird as they start to ever so slightly flex...they go back and forth a few times, farther and farther, then *POOF*... the just seemed to vaporize... I've been flying about 9 years, and knew a bit about flutter, but to actually see it was incredible!

Ron

Click to expand...

Hi Ron,

I also recomend folks pick up the LDRS video DVD. Lots of great stuff on that, and the photos on the other disk, what a value!

here is a link to the still frames and my web video.

https://www.boostervision.com/bbn1100/

Originally posted by aerospike
"As far as the whole different materials having vibratory properties that "cancel" each other out that is just not so in a bonded system. Each layer of the composite structure is bonded to it's neighbor and therefor they vibrate as a single system so there is no cancellation possible unless your layers of composite debond and they can move independently."

Thank God, someone said it, I was getting so tired of people pulling this "cancellation" crap out their arses.

Click to expand...

Aerospike,

Might not it be possible to lower the q of the system by using a heterogeneous composition with careful attn to ply bias--in other words I'm in full agreement about self-cancelling crap, but wonder if that wasn't tell a secret round the room kind of distortion about potential advantages of using different materials.

Just curious as to your opinion on the issue.

It really depends, John. If you have something that actually dampens as it flexes, and use it as a core, it might help. On the other hand, the idea that if you have, say, a layer of glass, then a layer of kevlar, then a layer of carbon, that they will somehow cancel and never flutter, that is the "cancellation" crap that is being referred to. As soon as the cloth materials are bonded, they are one plate, and although the addition of different materials will modify the frequency at which the plate will flutter, it will absolutely still flutter, and this will not by any means completely or even significantly cancel out. As Aerospike said, this is because they are fully bonded together and act as one sheet. The Tacoma Narrows bridge was made of more than one material, but it was solidly attached together - nobody will argue that it didn't "flutter."

Originally posted by cjl
It really depends, John. If you have something that actually dampens as it flexes, and use it as a core, it might help. On the other hand, the idea that if you have, say, a layer of glass, then a layer of kevlar, then a layer of carbon, that they will somehow cancel and never flutter, that is the "cancellation" crap that is being referred to. As soon as the cloth materials are bonded, they are one plate, and although the addition of different materials will modify the frequency at which the plate will flutter, it will absolutely still flutter, and this will not by any means completely or even significantly cancel out. As Aerospike said, this is because they are fully bonded together and act as one sheet. The Tacoma Narrows bridge was made of more than one material, but it was solidly attached together - nobody will argue that it didn't "flutter."

Click to expand...

Forget the bridge. Thats too far out there. And forget VE's for the moment. Forget even ply bias. Lets just assume we have a number of materials, each with a different k and rho. Neglecting many real world issues for the moment, since each has inherent damping, they all have specific frequencies of resonance proportional to sqrt(k/m). Now I also understand that flutter is not simply a resonance issue, but by having materials with different resonant frequencies, could that possibly contribute to a material with a less peaky frequency response, and therfore maybe less vulnerable to flutter phenomenon. I dont believe I'm saying that they wont flutter, but that the flutter onset and decay may be less abrupt, along with amplitude reduction.

The problem with that is that there is not really any reason to believe that a sheet of carbon/kevlar/glass will behave like 3 separate sheets of those materials that are then subsequently placed together. I maintain that it will still have very significant (and catastrophic) resonance points, they will just be moved or altered. The ideal goal IMO is stiffness, as that vastly increases the resonant frequency and brings it out of range of any potential flight profile. Of course, the stiffer an item is, the more shock load it will create on landing, so you have to take that into account too...

There's a reason they call this "rocket science"

Originally posted by denverdoc
by having materials with different resonant frequencies, could that possibly contribute to a material with a less peaky frequency response, and therfore maybe less vulnerable to flutter phenomenon

Click to expand...

That's a very good qstn. I don't have an analytical answer for you, but my intuition tells me that a compound material system would lose overall stiffness faster than it would exhibit some kind of muted or attenuated response.

My worry would be more along the lines of: a compound material system, pushed toward its limits of stiffness and strength (whatever they are), is also going to undergo some significant strains (deflections from the 'nominal' static position) as you approach flutter onset. Will the dis-similar chemistry of the various fibers, at extreme strain levels, all behave consistently or will you begin to get fiber/matrix separation, rapidly leading to ply delamination?

Remember also that when you stack up relatively weak materials (glass fibers) with significantly more stiff materials (carbon fibers) the loads tend to dump into the stiffer structural paths. The weak materials end up mostly just taking up space, going along for the ride, adding weight and frontal area, rather than contributing any significant structural benefit.

PB,

As I stated, just playing the devils advocate so we could all develop a greater understanding of the engineering principles, and maybe understand where this notion got started.

I too have my doubts about gaining any advantages purely from a resistance to flutter perspective and said so on the mother thread from whence this migrated. But I can see right away where one might get cheaper and in some ways, all around better fins by combining materials, eg using kevlar to take advantage of its incredible resistance to shear, FG for its cost, relatively high strength, etc...

Since we can control how the fibers are oriented on the fin, how do you determine which way the fin will flutter??

I imagine there is an optimal "antiflutter" bias for any given fin design and the secret to not "overbuilding" it is to concentrate the applied stiffening in that direction. I guess what I'm trying to say is we seem to try to stiffen our fins in every direction with the multiple ply layups but is that necessary?? (I would think not intuitively but am not sure.) If not, how do you determine the best orientation for your fibers for a given fin planform?

I think the main advantage with composite construction vis a vis flutter, is that there is more control over mass distribution of the structural component. Flutter is not only dependant on the shape of the structure, stiffness, vehicle attitude, Q, etc but also how the mass of the structure in question is distributed.

Powderburner brings up another good point: interlaminar shear. This is a particular problem when there is a great jump in shear strength of adjacent plys.

Digression: I watched some cable show on tanks last night, the brits have developed a FG prototype that weighed a 1/3 less than its metal counerpart--looked like 400 plies or so, and are working on a CF version so light it it might actually be airdroppable. Composites rule!
JS

Sounds interesting, though I would question a CF tank's ability to take a hit as well as a standard one with reactive armor.

CJL,

Might be right, I say stick a 200 plies of kev in the middle
But one one advantage cited--field repairs. Something akin to a chopped fiber, 'poxy goo you just shove into the wound. And off you go....
JS

Originally posted by denverdoc
CJL,

Might be right, I say stick a 200 plies of kev in the middle
But one one advantage cited--field repairs. Something akin to a chopped fiber, 'poxy goo you just shove into the wound. And off you go....
JS

Click to expand...

Anti-Armor rounds spew molten metal in a shaped charge. How would a kev/CF composite hold up to the spall?

Art, These are not meant to be the ultimate tank. The advantage cuz of their weight is they could be deployed quickly by airdrop and function very well. There are many approaches to building tanks, but in many sims you might have plenty of strength for the weapons encountered..The kev was a joke related to the thread.

Get something on the field fast, blow the crap out of enemy targets makes some sense to me vs wait two weeks while things get out of hand, to bring in heavier armor, meanwhile they have heavier armor,etc.

IIRC the Brits invented the tank idea. Military science is a bore to me and as a physician somewhat antithetical to my profession, lets move on please.
JS

Originally posted by denverdoc
Art, These are not meant to be the ultimate tank. The advantage cuz of their weight is they could be deployed quickly by airdrop and function very well. ....
Get something on the field fast, blow the crap out of enemy targets makes some sense to me vs wait two weeks while things get out of hand, to bring in heavier armor, meanwhile they have heavier armor,etc....

Click to expand...

Well, that sounds like a new Sherman Tank then

the American Sherman tanks could be blown up easy. But we built so many of them, huge numbers that they did have an impact.

It took like 4-6 of them to gang up on a Panzer to get a kill

But it seems like we were able to build them 10 - 1

and who won the war?

You'd be hard pressed to say we won the tank war though. We won for other reasons than that (I would say primarily economic). With the same economic resources, the germans would have been able to crank out far more tanks, and would have absolutely annihilated the shermans.

While the sheer number of tanks that we produced helped significantly, I would say the fact that we could fuel our tanks is what won the war. The tiger and the leopard were better tanks, but without fuel to move them, they were worthless. One of the major reasons the battle of the bulge didn't work for the Germans.

-Aaron