ParagonRocketry
Member
Hi all!
I was wondering if I could get some help considering shear modulus G for my Level 2 HPR certification. I'm planning on using the same rocket that I used to get my Level 1 certification (3.1" LOC Sandhawk), and currently with my OpenRocket sim and possible motor configuration I'm looking at running at about Mach 0.9. Because of this, I'm a bit worried about fin flutter during my flight. I've found the equation regarding calculating fin flutter velocity online, but I'm running into some problems with the value of G.
After consulting Matweb, the G value for plywood is 20,000-30,000 psi (0.138-0.207 GPa), and by my calculations it needs to be over 160,000 psi (1.103 GPa). I know that going tip-to-tip fiberglassing would work, but I don't have enough time to do that for my best-case timeframe (and I would absolutely hate re-stripping the bottom third of my rocket). If I stay subsonic with my motor, and my fins can handle it, I think it would work without needing to go tip-to-tip.
So, my question for you all is: what's an accurate shear modulus value for aircraft-grade plywood?
EDIT: Thank you guys so much for the information, I really appreciate all the good advice here and in my messages. I think the primary problem I had was that my fin thickness was off; for some reason, it was in my OpenRocket sim as 0.118 inches, while LOC and people who had experience with those fins said that it was 0.25 inches. I checked and they were right- which lead to a much more reasonable necessary G of 16,422 psi. I also thought I should post the article where I'm getting my calculations from, so here that is: https://apogeerockets.com/education/downloads/Newsletter291.pdf
Thanks again so much!
I was wondering if I could get some help considering shear modulus G for my Level 2 HPR certification. I'm planning on using the same rocket that I used to get my Level 1 certification (3.1" LOC Sandhawk), and currently with my OpenRocket sim and possible motor configuration I'm looking at running at about Mach 0.9. Because of this, I'm a bit worried about fin flutter during my flight. I've found the equation regarding calculating fin flutter velocity online, but I'm running into some problems with the value of G.
After consulting Matweb, the G value for plywood is 20,000-30,000 psi (0.138-0.207 GPa), and by my calculations it needs to be over 160,000 psi (1.103 GPa). I know that going tip-to-tip fiberglassing would work, but I don't have enough time to do that for my best-case timeframe (and I would absolutely hate re-stripping the bottom third of my rocket). If I stay subsonic with my motor, and my fins can handle it, I think it would work without needing to go tip-to-tip.
So, my question for you all is: what's an accurate shear modulus value for aircraft-grade plywood?
EDIT: Thank you guys so much for the information, I really appreciate all the good advice here and in my messages. I think the primary problem I had was that my fin thickness was off; for some reason, it was in my OpenRocket sim as 0.118 inches, while LOC and people who had experience with those fins said that it was 0.25 inches. I checked and they were right- which lead to a much more reasonable necessary G of 16,422 psi. I also thought I should post the article where I'm getting my calculations from, so here that is: https://apogeerockets.com/education/downloads/Newsletter291.pdf
Thanks again so much!
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