Building a 4.1x upscale EAC Viper for my level 2 rocket. Wanted to run I-K motors. The fins are a high aspect ratio design. Understanding that Flutter may be an issue, I searched through the TRF archives for solutions.
As I tried to understand the math in the flutter spreadsheets that were posted here in the early 2010s, most started with NACA TN 4197. https://ntrs.nasa.gov/api/citations/19930085030/downloads/19930085030.pdf
One from Duncan McDonald, was linked and posted a bunch of times in various revisions (Duncan McDonald FinFlutter), and is relatively accurate and true to 4197 (depending on the version, there were multiple posted). He also has anecdotal evidence in his sheet that 1.2x the max calculated Flutter Velocity in his spreadsheet is a fair boundary for a shred. However, Duncan:
Then there is AeroFinSim, that is no longer available.
So, I built a spreadsheet and MathCad book (uploaded as TXT, open with Mathcad) with faithful representation of the original NACA TN4197 Flutter Speed Calculations that include Altitude at Max Velocity, and the atmospheric model. They are here for your use.
Caveats
As I tried to understand the math in the flutter spreadsheets that were posted here in the early 2010s, most started with NACA TN 4197. https://ntrs.nasa.gov/api/citations/19930085030/downloads/19930085030.pdf
One from Duncan McDonald, was linked and posted a bunch of times in various revisions (Duncan McDonald FinFlutter), and is relatively accurate and true to 4197 (depending on the version, there were multiple posted). He also has anecdotal evidence in his sheet that 1.2x the max calculated Flutter Velocity in his spreadsheet is a fair boundary for a shred. However, Duncan:
- Assumes ISA at 0' MSL, ignoring altitude (pressure and speed of sound variation), and
- Shows a different shear modulus for G10 and FR4 (confusing the user).
Then there is AeroFinSim, that is no longer available.
So, I built a spreadsheet and MathCad book (uploaded as TXT, open with Mathcad) with faithful representation of the original NACA TN4197 Flutter Speed Calculations that include Altitude at Max Velocity, and the atmospheric model. They are here for your use.
Caveats
- Use your own common sense. I am not representing these Max Flutter calculations as truth. I’m offering these as a faithful articulation of the formula in TN 4197, and a tool for your consideration.
- There is no silver bullet, and these calculations may, in-fact, not be accurate representation of flutter. (In this model, if you hold span and tip cord constant, and reduce root cord incrementally, your safe flutter speed increases…which we know is not true). I do think there is some value, as you contemplate aspect ratio and other factors. If you use it and compare results (shred/no shred), you may find a valuable tool to protect your investments!
- The hardest part of this model is getting the shear modulus. Even if you have this model, but your shear modulus is wrong, the model is worthless.
- I have included the shear modulus for G10, with references for my values on the materials page of the spreadsheet. Since many mid/high power new users are using this material, having good values is great for G10 users.
- This method is probably most applicable to those without much experience, that are doing their own design. Those using production kits and recommended motors, and those that are level 3, 3+ pushing the boundaries of speed and altitude, will NOT likely find much value.
- By increasing G10 fins from 1/8” to 3/16”, I doubled the max flutter speed, but lost the CD/CP battle with larger J and even the small K motors. The heavier fins moved my CD rearward sufficiently to lower my stability margin with the CP movement of larger J and K motors. The design of the Viper, with a small OAL of 55” and an aspect ratio of 1.342, just doesn’t lend itself to larger motors.
- Even if the numbers are “wrong” or “off,” it gave me a starting point to at least be ALERT to the fact that I’m getting close to a potential “flutter” speeds. A 90% margin for my Viper at sea level would be around 316mph, with a flutter speed of 350. Using the anecdotal speed of 1.2x results (McDonald) for actual flutter threshold would be 420mph. This gives me some speed ranges where I can be “Heads Up Aware” to take precautions, or provide heads up at the range as I increase speeds with a new design.
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