Unconditionally Flutterless Fins

[Feckless Counsel]

TRF,

Theory apparently says a fin will never flutter when the fin's center of gravity is forward of the fin's elastic axis. Could anyone give a practical planform for such a fin?

Feckless

[Handeman]

Interesting theory, where does it come from? Do you have a link to the paper/report that explains it?

Sounds like something that would be purely theortical to me. I don't see were CG of the fin really comes into it because it's attached to the BT and the flutter is a function of air flow and physical properties.

Certainly wouldn't be the first time I was way off.

[rstaff3]

I'm with Handeman on this one. Everything I've read about flutter (ie a recent Apogee newsletter) says the subject is a lot more complicated than that. However, the stated condition could be a general rule of thumb that would reduce flutter. (?)

[Feckless Counsel]

Gents,

That is from the technical notes included with AeroFinSim with reference to the text "Principle of Aeroelasticity" by Bisplinghoff. I do not fully understand the statement but speculate something in that condition creates provides heavy, unconditional fin damping as in a spring / mass / dashpot system.

Feckless

[rstaff3]

Too bad they did not have a design example.

[MClark]

Control surfaces on airplanes are balanced in this way so they do not flutter. It is done by weight ahead of the hinge line.
It could be done by making the front 25% or so of the fin of a heavier material and the back part out of lighter. Example, front part of oak and back of balsa.

Mark

[Peter Olivola]

In Stine's illustration of an "ideal" rocket there is one dimension not discussed in the text: A 2:1 root to tip taper of the fins. Such a taper would minimize the effect of harmonics within the structure. That is, adjacent segments would resonate at different frequencies and isolate each other. It was shown as part of a clipped delta fin design, but it's certainly possible that other designs could negate the dampening.

Harmonic resonance is the thing to be avoided. Once initiated, it's difficult to stop and the typical flight profile of a rocket produces exactly the wrong effect, i.e., it will exacerbate harmonic resonance rather than dampen it.

In many cases, the choice of fin material can make things worse. For instance, birch ply fins may have a harmonic resonance frequency achieved at a velocity that a given rocket transitions through too quickly to initiate flutter while G10 achieves resonance at a higher velocity that the rocket sustains long enough to initiate the phenomenon.

Rocket science is an ideal example of complexity.

[Handeman]

Control surfaces on airplanes are balanced in this way so they do not flutter. It is done by weight ahead of the hinge line.
It could be done by making the front 25% or so of the fin of a heavier material and the back part out of lighter. Example, front part of oak and back of balsa.

Mark

I understand what you're saying, but does that really apply to rocket fins that don't have hinge lines? They are solidly attached to the BT along a line perpendicular to what a hinge line would be. If there is no hinge line that allows movement, is there any need to make adjustments to the CG like an airplane?

[jderimig]

TRF,

Theory apparently says a fin will never flutter when the fin's center of gravity is forward of the fin's elastic axis. Could anyone give a practical planform for such a fin?

Feckless

Forward with respect to what? Airflow?

[Feckless Counsel]

John,

My understanding is with respect to airflow.

Handeman,

I'm a novice explorer in this subject. That being said I believe it does apply to fixed panels because those are elastic and are able to bend as if there were ailerons.

[bandman444]

I found this interesting so I looked it up and thought I would share one source that
supports this claim

I know nothing on the topic other than that reading, but it would be interesting to hear what others have to say?

Bryce