Fin materials When is it 'enough'? G11? Uni Carbon plate?

I am trying to better understand fin flutter, and how to know when a chosen fin material, and thickness, is sufficient for a given project without overbuilding.

The specific project is a 4" diameter rocket, about 75" long, weighing about 29lbs with the motor. The maximum velocity expected is mach 2.2, and a projected altitude of 33k' ish, possibly higher with some adjustments. The current fin design that I am looking at has a root chord of 11", a height of 3.75", a sweep length of 10.5", a tip chord of 1.5", and a sweep angle of just over 70 degrees. There will be a simple bevel on the edges, of 7-10 degrees. There are 4 fins. The stability is sitting at 1.58 cal. This is a through the wall construction, with a 75mm motor mount. This is not a tip to tip cloth reinforced design. There will be internal and external fillets.

The material I have been thinking of is 3/16" G11, and I am basing this on what I can gleam from other peoples successful projects and what is sold with kits.

In the OR sims, the difference between the 3/16" G11 fins, and some .125" pre-preg 0-90 uni Dragonplate CF, is around 1000' and slightly improved stability.

I have looked for a fin flutter calculator, but the one I found looks to be unavailable to common folk. I would rather have the actual numbers to back up the 'how much is enough', rather than just trying to 'guess on the safe side'.

With the above fin profile, is it likely that the CF plate I mentioned would be stiff enough?

If it is easy for someone to run a fin flutter analysis, or point me in a direction to where I can do this myself, that is the info I would prefer.


Thank you all, in advance.

Have you seen the program FinSim? I haven't used it, but I understand it's good for flutter, etc. analysis

https://www.aerorocket.com/products/products.html#ANCHOR10


https://www.aerorocket.com/finsim.html

Yes, FinSim is the one I was looking at. It looks to be available as a free (protected) download for students, only. I am a US citizen, which is one of the other requirements.

Based on the lower section, I think you could ask for a download code assuming this is for non commercial use? Worth a try, I'd think.

Chris_H said:

Yes, FinSim is the one I was looking at. It looks to be available as a free (protected) download for students, only. I am a US citizen, which is one of the other requirements.

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I believe you have to: State you are a US citizen, Tell him you are in the US, and Give him a clear description of how you will use FINSIM. I used it on my L3 rocket last year.

djkingsley said:

I believe you have to: State you are a US citizen, Tell him you are in the US, and Give him a clear description of how you will use FINSIM. I used it on my L3 rocket last year.

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Cool.

I will contact him.

Of course, I've always been skeptical that Finsim's results had anything to do with reality, but that seems to be a minority view around here.

Even if the treatment is correct, the material properties of composite materials are notoriously variable and hard to quantify.

YMMV.

mikec said:

Of course, I've always been skeptical that Finsim's results had anything to do with reality, but that seems to be a minority view around here.

Even if the treatment is correct, the material properties of composite materials are notoriously variable and hard to quantify.

YMMV.

Click to expand...

Agreed that there is going to be variation between different carbon plates of a given thickness, absolutely. But, hopefully this Dragonplate product is somewhat consistent in it's specs, and not far off from whatever valued are used by FinSim. I am still not sure I want to use it, but maybe, if it seems strong enough.