AeroFinSim 4.5 CN-Alpha Menu Question

HyperSonic

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I was wondering about what options I should use when I run my fins in FinSim. Under the menu CN-Alpha there are 3 or 4 options you can choose. There is Classical 2D Lift Slope, Barrowman 3D Lift Slope, NACA TN 4197 Method. I have tried all of them and the smallest numbers for divergence and flutter came from NACA TN 4197. A very close 2nd was Classical 2D. When I used Barrowman 3D the numbers got much bigger than the first two. Does anybody know which option I should be using and Why? Also CN reference to Body Tube or Fin Area?
 

AeroAggie

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There's not really one answer to your questions, as it depends on the configuration and velocities you expect to see.

The NACA TN 4197 equations are intended for high aspect ratio wings for subsonic flight. (see first paragraph here: https://aerorocket.com/FinSim_Supersonic_Flutter.html). If you're using Pine's Method (the default method when you first open FinSim), the developer says to use both the Classical 2D and Barrowman 3D methods to bound the problem, as your critical velocities will be somewhere between those two answers.

If you're scratch building a sport flyer and not optimizing for max velocity or altitude, then just use the Classical 2D method and be done with it. If you're building a high performance kit, you may find that the previously demonstrated rocket performance greatly exceeds the Classical 2D predictions but presumably still bounded by the Barrowman 3D predictions.

Can you post some more details on your rocket? You'll get better feedback if we know what your goals are.
 
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HyperSonic

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Can you post some more details on your rocket? You'll get better feedback if we know what your goals are.[/QUOTE said:
This will be a scratch built 4 inch MD carbon fiber airframe with 4 solid CF fins that are about .225 thick. Nose cone is VK fiberglass and the total length is around 108 inches. I'm going to use the length as a variable to adjust my CG where I want it and maybe not have to add dead weight to this rocket. Motor will be the CTI N5800 and the total weight will be around 47 pounds. (optimum mass according to RASAero 2) When I sim this rocket in RASAero 2, it will touch Mach 3 at around 6000 feet 3.3 seconds into flight. Its all down hill from there.

My goal is to keep the fins as thin as possible but still be able to handle 0 to Mach 3 in 3.3 seconds at 6000 feet. These fins will be solid IM7 6K CF. I used the IM6 CFRP option when using finsim.

Fins are as follows: root chord = 15 inches, Leading edge sweep = 12.75 inches, tip chord = 1.25 inches, span is 4.5 inches, thickness = .225 inches. Pic of shape below.

20180111_104527.jpg
 
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