Flutter Tools for You.

[MarsLander]

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:

1. Assumes ISA at 0' MSL, ignoring altitude (pressure and speed of sound variation), and
2. Shows a different shear modulus for G10 and FR4 (confusing the user).

Another one is this one from about 2014 (Apogee Flutter Speed Excel) (with a link to download a flutter spreadsheet) that used the equations from Apogee Newsletter 291 Flutter. I am not a rocket scientist, but in both the original article, and the same article in Sport Rocketry, the original author offers no explanation of the changes to TN 4197. After reading the following and running the math myself, I believe the apogee calculations are in error, so does the AeroFinSim author: https://www.aerorocket.com/FinSim/NACA_4197/POF-291_FLUTTER_ERROR.pdf.

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.

LEARNINGS: I found it useful in My analysis for MY upscale/design. I learned two things:

• 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.

Stay Safe out there!

 

[OverTheTop]

Thanks for your efforts on those tools.

Remember that increasing thickness increases stiffness by the cube of the ratio. So adding 50% extra thickness gives you almost 3.4 times the stiffness.

 

[Blast it Tom!]

You have my appreciation. To my way of thinking, Mathcad should be on every engineer's computer. Though TBH I'm still a Mathcad 15 guy. Even since I've retired 28 months ago, the company has stayed with 15.

 

[MarsLander]

For whatever reason, this issue stuck in my head and I finally had a chance to resolve it.

In Duncan's spreadsheet, he has a dataset on estimated flutter speed and actual speeds above and below which rockets did and did not shred. He identified a boundary to the shred speed relative to the flutter speed using TN-4997 math and real world speed data. As a result, he stated:

So 120% of the flutter velocity is a working number for the "Shred Velocity".

On its face, I would take this to mean I could fly my rocket up to 120% of predicted Vf and be reasonably assured it will not shred.

However, his calculation was:

(Shred speed(actual) - Vf predicted)/Vf predicted = Shred Speed as a % of flutter velocity (SSaa%oFV).

However, this calculation measures the difference between the two speeds as a % of Vf; meaning that his statement should have been:

Vf + (120%*Vf) = a working speed for the "Shred Velocity"

That's a big difference!

A better way to calculate and communicate this is:
(Shred Speed / Vf) = Shred speed as a % of flutter velocity
In this case, all the examples of rockets that DID NOT SHRED, were flown <=220% of Vf.

I have updated the spreadsheet, and MathCad files to include a 220% shred speed calculation.

To use the Mathcad file, change the extension to .mcdx

Enjoy!

 

[Blast it Tom!]

Thanks! Downloaded along with your explanation. I'll ad that to the list of stuff I'd love to explore if I ever have the time!!!

 

[MarsLander]

A user reported the inability to download the Mathcad sheet. I'm attempting to correct that with this post.

Start Mathcad. Download the file, find it using file explorer, right mouse click, select "Open With" and select mathcad. I have confirmed this works on this file. Having Mathcad running should put mathcad in the selections when you "open with."

If that does not work, go back to file explorer, right mouse click, select "Open With," then "choose another app," scroll to the bottom and then choose "choose another app on your PC, then navigate to "C:\ProgramData\Microsoft\Windows\Start Menu\Programs\PTC Mathcad"
and select the Mathcad program.

 

[atestani]

That works... thanks!

 

[MarsLander]

That works... thanks!

This version only works for symmetrical fins. I scratched up the calculations on your asymmetrical fins in real time the other day. Should have saved it.

 

[atestani]

This version only works for symmetrical fins. I scratched up the calculations on your asymmetrical fins in real time the other day. Should have saved it.

You posted a graphic of it in my "stiffening" thread... Posted here as well. Any chance you could put the changes into an mcdx file and post that?

 

[MarsLander]

Any chance you could put the changes into an mcdx file and post that?

Mathcad is pretty easy...give it a whirl. If you can't figure it out, I'll update. Otherwise it might be a while until I can get to it...

 

[MarsLander]

Any chance you could put the changes into an mcdx file and post that?

Here you go!

 

[atestani]

Here you go!

Wow! Thanks so much!