Fin Sim help needed

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SpaceManMat
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Fin Sim help needed

Postby SpaceManMat » Thu Nov 12, 2015 11:23 pm

I'm looking at building a Purple Parrot as a 38mm MD.
I've done a whole bunch of sims but am not liking what FinSim is telling me. I've not decided on glue or fillet size at this stage, what concerns me is fin flutter and divergence. For those of you who don't have FinSim it is now available for free download, but be warned its not for the faint hearted.
Anyway here is what I am inputting. The flight should hit about Mach 1.8 at just over 3000 feet if flown at Westmar.
FinSim Design.jpg
FinSim Design.jpg (84.19 KiB) Viewed 1129 times

FinSim Main.jpg
FinSim Main.jpg (116.08 KiB) Viewed 1129 times

FinSim Results1.jpg
FinSim Results1.jpg (134.05 KiB) Viewed 1129 times

FinSim Results2.jpg
FinSim Results2.jpg (131.29 KiB) Viewed 1129 times


Attached is the FinSim file and my conversion spread sheet
Attachments
FinSim Purple Parrot.zip
(9.67 KiB) Downloaded 45 times
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

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OverTheTop
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Re: Fin Sim help needed

Postby OverTheTop » Fri Nov 13, 2015 7:00 am

Fin thickness does seem quite thin, given the amount of sweep you have.

Since the stiffness of the fins is proportional to the cube of the thickness, you will get good bang-for-buck if you thicken them somewhat.

You could think of stiffening just part of the fin closer to the root. Add 0.5mm G10 each side, and fair it onto the thinner piece?

Other thoughts are to increase chord lengths and shorten span a little.

Just slightly random thoughts...
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SpaceManMat
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Re: Fin Sim help needed

Postby SpaceManMat » Fri Nov 13, 2015 7:47 am

I know how to increase the fins strength OTT. This is the kits standard fins and I am pretty surprised at what I'm seeing here which leads me to think I either have not used the program properly or do not know how to properly interpret the results. One of the other things that concerns me is that I could not find any details about the properties of the G10 FG that are used by the program, this leads me to ask questions about are the properties used typical or are they on the safe (softer) side?

After handling the actual fin and testing its firmness by hand, my gut feel is that it would at the very least go Mach. The fin may be thin but it is also not that large so it is proportionally thick. By comparison my Flying Dutchman fins are 2.4mm but are sigificantly larger.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

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OverTheTop
It's only money...
It's only money...
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Location: Melbourne, Australia

Re: Fin Sim help needed

Postby OverTheTop » Fri Nov 13, 2015 9:06 am

You are right about the different stiffness of "G10". We had some circuit boards that changed manufacturer. The newer boards felt about half as stiff as the old ones. We were relying on the boards to seat o-rings on pressure transducers. Ended up having to change o-ring material to something softer.
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"Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

martymonsta
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Re: Fin Sim help needed

Postby martymonsta » Sat Nov 14, 2015 7:12 am

Hey Matt I had a look at the file and i'm no expert on fin sim but from what I understand you need to provide the method that the fins are going to be attached to give a flutter and more critically divergence speed. you had that set to the default "Elmers wood glue" with a 1mm fillet so I changed it to WS 105/205 and 5mm fillet and added a couple more thickness of the same shape fin as a comparison.

Image

Image

I can't compress files so I sent it back to you on FB. have a play with the AOA and the different fin thicknesses under the "Fin bending stress analysis" Button. Hope that help.
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SpaceManMat
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Re: Fin Sim help needed

Postby SpaceManMat » Sat Nov 14, 2015 2:10 pm

Details of the Fin Stress Bending Analysis screen are not saved into the file and these are not used to determine Fin Flutter and Divergence.

I think that the answer lies in understanding the materials and possibly interpreting the results.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

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SpaceManMat
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Re: Fin Sim help needed

Postby SpaceManMat » Mon Nov 16, 2015 9:22 am

Tracked down this spreadsheet from the FinSim secure download page

http://aerorocket.com/download/NACA_TN_ ... er-v1.xlsx

I won't post online as this I suspect John would consider this a breach of copyright, however the figures are in the spreadsheet. You will need to register to obtain a copy.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing


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