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pythonrock

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What software do you use to evaluate fin construction? I looked for Aero finsim and they are no longer selling software. Is there any other? Also, is there a good resource for the shear modulus for different materials?
Thanks
 
Ummmmm,

I'm silly enough to confess I bought into it several years ago and it's sitting on a hard drive on a shelf somewhere. I don't know how accurate the materials analysis was in the list of materials that could be used for the calculations. I don't know if new materials could be added to the program. I got bored, found it pretty hard to understand and decided I probably wouldn't be going into that realm. If you are an engineering type would probably be easier to understand. Why don't you email the fellow and see if he'll "donate" a copy to you. I don't know why
he doesn't simply closeout or close down the site. It's still up but can't buy. Kurt
 
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I am a long-time proponent of FinSim and use it regularly. It helps tremendously in guarding against flutter conditions. Saying that, it really is only math; surely there are other stand-alone packages or plugins to Excel and the like that do the same thing.
 
dixontj93060 said:
... surely there are other stand-alone packages or plugins to Excel and the like that do the same thing.
Click to expand...

I have used a spreadsheet I designed, with math/method taken from Apogee newsletter 291. It does all the math, you just fill in some details. The (very) hard part is determining the shear modulus for various materials. Anyone can use it if they think it would help.

View attachment FinFlutter (291).xls

https://www.apogeerockets.com/education/downloads/Newsletter291.pdf

Note: Edit the tan colored boxes only.
 
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Many of the physical properties of common materials we use can be found online. One source is: MatWeb.

The bigger hurdle for me over the years in examining flutter was that laminate composite struture was the common means to combat flutter and thus in high performance rockets we seldom have fins that are homogeneous with defined physical properties that can be referenced in a database. That is where The Laminator came in in solving this issue; without it, you can not validate the behavior and performance of a composite structure.
 
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+1. Thanks Tim.
I downloaded the trial Laminator and the list basically only has carbon or glass - epoxy. If purchased, does it include foams, honeycomb, plywood etc? That software may have a learning curve.
MatWeb is great source.
Just what I was looking for. With those and finflutter(291).xls, It covers the bases.
 
clreynolds said:
I have used a spreadsheet I designed, with math/method taken from Apogee newsletter 291. It does all the math, you just fill in some details. The (very) hard part is determining the shear modulus for various materials. Anyone can use it if they think it would help.

View attachment 181954

https://www.apogeerockets.com/education/downloads/Newsletter291.pdf

Note: Edit the tan colored boxes only.
Click to expand...

I found that spreadsheet to yield very questionable values. A 1/16" carbon fiber fin with 12" roots and roughly 4" span was predicted to flutter at Mach 0.3 and it flew to 1.6 without any flutter at all.

The longer the root, the worse the flutter was predicted to be. I think it was designed more for the slender wing regime than the stubby delta shaped fins common on rockets.
 
pythonrock said:
+1. Thanks Tim.
I downloaded the trial Laminator and the list basically only has carbon or glass - epoxy. If purchased, does it include foams, honeycomb, plywood etc? That software may have a learning curve.
MatWeb is great source.
Just what I was looking for. With those and finflutter(291).xls, It covers the bases.
Click to expand...

No, it doesn't. You have to get this info from online, which is fine for common materials such as birch plywood, balsa, etc. Other materials such as foams and honeycomb may not be online, often for those materials you need to go direct to the manufacturer to get a spec sheet. Of course, the software allows you to load the 4 or 5 key parameters that you need for new materials and save them in your database.
 
Thanks for the input CarVac.

Unfortunately, I can't vouch for all of the math/science to backup these calculations. I can only refer to the following info.
The equations used in this spreadsheet are based on NACA Technical Note 4197 (see NACA TN-4197 which doesn't seem to be publicly available anymore) which presents a simplified method for calculating fin flutter velocity based on the method of Theoderson. NACA TN-4197 has confirming data up to "at least mach 1.3", so this may analysis may not hold up for higher mach numbers. NACA TN-4197 covers three different types of flutter: pitch-bending flutter, stall flutter, and torsion-bending flutter (or as the British say, "flexure-torsion flutter"). The flutter that we experience in amateur rocketry is torsion-bending flutter, that is, flutter due to the fins twisting and bending because they are not stiff enough.
A couple of difficulties. First, you have to know the fins aspect ratio which can be tough to calculate except for simple geometric cases. The solution here was to just calculate the simple geometric cases of tapered, clipped delta, swept delta, and triangular. You need numerical integration such as RockSim uses to calculate the aspect ratio of arbitrary shapes.
The second more difficult problem is finding the shear modulus for composite materials. Composite materials include carbon fiber, fiberglass, kevlar, and anything else that is epoxied together or to some sort of substrate. Plywood is also a composite as is G10.
Click to expand...
The results do compare with work done by others including another similar set of spreadsheets and research into materials done by Duncan McDonald some 14-15 years ago.

You are correct that some assumptions are made about basic shapes of fins that we use. But also note that "flutter" does not necessarily mean shred. We have all seen the (cool) videos of fin flutter during flight.

Last, a 1/16" flat sheet with 12" root and 4" span seems at the extreme, but a quick search produced a shear modulus for a carbon fiber composite of all most 7.8 million. You didn't mention the tip cord, but when I use a sample with an 8" (clipped delta) cord, I get a flutter boundy of 1841 fps, easily surviving a mach+ flight. (and one I would very much like to see :) )

Again, I am not trying justify someone elses work (NACA) but it seems to be a valuable model for most of what we do.
 
clreynolds said:
Thanks for the input CarVac.

Unfortunately, I can't vouch for all of the math/science to backup these calculations. I can only refer to the following info.

The results do compare with work done by others including another similar set of spreadsheets and research into materials done by Duncan McDonald some 14-15 years ago.

You are correct that some assumptions are made about basic shapes of fins that we use. But also note that "flutter" does not necessarily mean shred. We have all seen the (cool) videos of fin flutter during flight.

Last, a 1/16" flat sheet with 12" root and 4" span seems at the extreme, but a quick search produced a shear modulus for a carbon fiber composite of all most 7.8 million. You didn't mention the tip cord, but when I use a sample with an 8" (clipped delta) cord, I get a flutter boundy of 1841 fps, easily surviving a mach+ flight. (and one I would very much like to see :) )

Again, I am not trying justify someone elses work (NACA) but it seems to be a valuable model for most of what we do.
Click to expand...

It might have been 5 inch span and 2 inch tip. If you search for tailcone research rockets that is the one in question.

They flew so perfectly straight, and they reached the predicted altitudes, so I wouldn't expect the fins to have fluttered. I tried two different values for the shear modulus, but both directions gave subsonic flutter speeds. I don't exactly remember what the values I used were, though.
 
That's ok. It is always interesting to compare the model to the real world.:)
I think I have acuratley recreated the math formulas, but I have been wrong before :facepalm:, if anyone has a calc done by another program/method to compare that would be nice.
 
If there is a fillet then the confined root area is not capable of the same bending as the thinner remaining part of the fin. That could affect the results in the flutter computation quite a bit. What results are achieved when the fillet is subtracted from the fin span and chord?

Gerald
 
G_T said:
If there is a fillet then the confined root area is not capable of the same bending as the thinner remaining part of the fin. That could affect the results in the flutter computation quite a bit. What results are achieved when the fillet is subtracted from the fin span and chord?

Gerald
Click to expand...

The fillets were very small.
 
Subtracting the fillet area from the span would increase the limit, but I was thinking the thickness of only 1/16" has a much bigger effect.
 
clreynolds said:
Subtracting the fillet area from the span would increase the limit, but I was thinking the thickness of only 1/16" has a much bigger effect.
Click to expand...

I remember playing around with the spreadsheet and it had some counterintuitive results, such as a decreasing flutter speed with increasing root chord. That just seems unreasonable. I will have to check it out again when I get home.

I did end up adding a little reinforcement to the first one, and full tip-to-tip on the second one, but that was only a single layer of carbon on both sides.
 
I have had excellent results using this excel workbook authored by David Harris. Peering into the formulas in the cells, the math looks right - and the predictions have been good.

All the best, James

View attachment 182018
 
Last edited by a moderator:
G_T said:
If there is a fillet then the confined root area is not capable of the same bending as the thinner remaining part of the fin. That could affect the results in the flutter computation quite a bit. What results are achieved when the fillet is subtracted from the fin span and chord?

Gerald
Click to expand...

Yes,

Plus, how does one take into account tip to tip lamination in the 1/3rd, 2/3rds and full layered glass lamination? That certainly can add to flutter resistance.

It's like the response, "Burnsim told me this mix would be allright at this given Kn. Why did it blow?" A sim can't take into account every little variable and modification that a person might make to improve the situation. The materials in the simulation might not match those that were actually used in the build so will not explain the result seen. Kurt
 
clreynolds said:
Thanks for the input CarVac.

Unfortunately, I can't vouch for all of the math/science to backup these calculations. I can only refer to the following info.

The results do compare with work done by others including another similar set of spreadsheets and research into materials done by Duncan McDonald some 14-15 years ago.

You are correct that some assumptions are made about basic shapes of fins that we use. But also note that "flutter" does not necessarily mean shred. We have all seen the (cool) videos of fin flutter during flight.

Last, a 1/16" flat sheet with 12" root and 4" span seems at the extreme, but a quick search produced a shear modulus for a carbon fiber composite of all most 7.8 million. You didn't mention the tip cord, but when I use a sample with an 8" (clipped delta) cord, I get a flutter boundy of 1841 fps, easily surviving a mach+ flight. (and one I would very much like to see :) )

Again, I am not trying justify someone elses work (NACA) but it seems to be a valuable model for most of what we do.
Click to expand...

It is there, just under a different link https://ntrs.nasa.gov/search.jsp?R=19930085030&hterms=4197&qs=N%3D0%26Ntk%3DAll%26Ntt%3D4197%26Ntx%3Dmode%2520matchallpartial%26Nm%3D123%7CCollection%7CNASA%2520STI%7C%7C17%7CCollection%7CNACA
 
clreynolds said:
Thanks for the input CarVac.

(Snip for Brevity's Sake)

You are correct that some assumptions are made about basic shapes of fins that we use. But also note that "flutter" does not necessarily mean shred. We have all seen the (cool) videos of fin flutter during flight.
Click to expand...

Ahhhhhhh, The only problem there is the material fatigue and micro cracks that could occur with the material. Get away with it once, O.K.
Try to send the rocket at the same velocity a second or third time and that's when it'll shred on you. Heck, with loss of strength, the flutter
might occur at lower velocity too! If the rocket isn't carrying a downward facing camera, might not even know if flutter is occurring. Kurt
 

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