FinSim Validation

[CoyoteNumber2]

From what I've read, the Classical 2-D and Barrowman 3-D methods produce lower and upper limits to a range in which fin flutter is possible. FinSim produced these numbers (feet/sec) for my rocket:

Method	Divergence	Flutter
Classical 2-D	1169.02	1590.27
Barrowman 3-D	3015.14	4101.64

OpenRocket shows the maximum velocity at 2633 feet/sec - well above the 2-D numbers but okay on the 3-D numbers.

Does anyone have empirical data on one of their flights where the rocket's velocity fell between the 2-D and 3-D numbers? What were the results?

To make things even more interesting, my fins are airfoiled. I'm using an average thickness for calculations purposes, but I think I read somewhere that fins with non-uniform thickness (i.e. airfoiled) are less prone to flutter. (Please correct me if I'm off-base.)

.FIN file attached (renamed to .txt)

View attachment HS Mk II (2).txt

 

[markkoelsch]

Tapered fins are generally less prone to flutter. You calculations will likely be incorrect by using an average thickness.


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[CoyoteNumber2]

You calculations will likely be incorrect by using an average thickness.

No doubt. But in lieu of input fields for varying fin thickness in the software, I used the average thickness per the FinSim documentation. Can you suggest a better way of doing it?

 

[AeroAggie]

Can you suggest a better way of doing it?

A better way would be to run your own detailed finite element model and your own supersonic wind tunnel test, and correlate the two. But I'm guessing that since you're asking, you don't have access to those kinds of tools.

I think you're correct to use an average thickness, if it maintains the same cross sectional area and mass as the original fin. The FinSim documentation describes the equations and correlation to NACA methods and test data fairly well. This tool really is about as good as you're going to get outside of academic research or launch vehicle industry, but it is just that - a tool - and it will give you numbers that are close enough to help you make good design decisions at a hobby level.

What rocket and what motor are you using? Are your fins through the wall or surface mounted?

You can do things to improve your flutter margins like add fiberglass or carbon plies to increase thickness, cut new fins from thicker material, or reduce the size of the fin as much as your stability calculations will allow. If your rocket is already built, then either use motors that keep your speed below the critical speeds, or just know that every flight might be your last and let 'er rip! A savvy RSO might disallow this approach though. :wink:

One thing I don't see in your numbers is the altitude at which you reach your max speed. You'll want to run FinSim for that altitude, not sea level.