L2 build, flutter analysis/design help...

[MarsLander]

Building a 4.1x EAC Viper (link in sig). Using Wildman tubes and 1/8" G10 for fins. In trying to do a bit of flutter analysis to see what happens with a K motor. The closest fin pattern that I can find is the Wildman Cherokee 98 (WM Cherokee Link). LE, TE and tip cord are all swept (TE and tip inward), see diagrams. WM fin is longer and a smaller aspect ratio (AR). First review (by @Ken McC) on the Wildman site for the Cherokee 98 says, "It has been up to 10,794 feet and achieved Mach 1.29 on an CTI L935" When I run flutter analysis, Cherokee 98 Vf comes out to about 596 and 477mph with 80% margin. Viper Vf comes out to about 490 and 392mph with 80% margin. Makes sense given the AR. I'm using a thrust plate, so I'm less concerned about the fin can and more concerned with Flutter. (Fin area is not going to match between planform and flutter calc because of the sweeps. The relative difference is reasonable for the accuracy here)

1. What do you make of running 1/8" G10 at speeds above these Vf? (i.e. how does M1.29 NOT flutter?)
2. There are 5 J motors that exceed 390mph. The lowest K runs 469. If I want to run these higher speeds, do I need to go to 3/16" G10? If I change to 3/16", Max Safe Speed goes to 720mph!!!
3. The viper fins are dumbo ear fins relative to all the other 4" kits, and the viper is on the small end for OAL at 55", so I'm less drag, less mass, larger AR, and faster speeds with a K. Do I punt running on Ks and let this be an I/J rocket and build something bigger/heaver with more appropriate fins to run K motors?
4. Any other thoughts to consider?

 

[James Keller]

I'm by no means an aerospace engineer, that being said I have ran 1/8" G10 to Mach 1.3 and a lot of people have on this forum. This absolute mad man here flew it to over mach 2. Although his fin shape was a little different. However, contrary to my inital assumption, according to finsim the flutter speed you calculated appears to match. Calculating for .25" fin you would begin to futter around mach 1.5 and with a 3/16" fin you flutter around mach 0.8. I don't think you should move down to a 3/32" fin though I think you meant 3/16". If you want to run bigger engines in this I would just upsize the thickness as long as you don't care about altitude. Or of course you could always build something bigger, never can build to many rockets. Another idea is to do a few layers of carbon fiber/fiberglass tip to tip. Either way goodluck on the project!

 

[MarsLander]

I don't think you should move down to a 3/32" fin though I think you meant 3/16".

Yes, flubbed it and corrected.

If you want to run bigger engines in this I would just upsize the thickness as long as you don't care about altitude.

Yeah, pisser. That is what I think I'm concluding. Don't care about altitude. LOL.

according to finsim the flutter speed you calculated appears to match.

I should have asked if anyone had finsim and could run it for me. Thank you for this!

Either way goodluck on the project!

Thanks!

 

[MarsLander]

@James Keller Is this closer to the finsim results with 1/8" G10? (Fired up mathcad!)

 

[MarsLander]

Bumping to 3/16" gives me a much better margin for those K motors! The slowest K is 470mph!

 

[MarsLander]

Oh, those knock on effects of rocket design. I have too much to learn.

Flutter is the least of my problems.

The #1 constraint with this rocket is Fin Design. Given the size of the fins, weathercocking is the largest constraint due to the size and shape of the fins. This will be a fair weather flyer. 8-10mph gives me an initial flight angle off the rail of <10. Anything over and I exceed 10. Doesn't matter punchy, or rail speed.
The #2 constraint is the CG.
At 55", this is a short rocket for a I/J/K rocket. Going up to 3/16" G10 brings my CG back even further. The higher impulse K motors pushes my CP forward to the point I get to unstable, sooner.
The #3 constraint is Flutter.
Even if I fix flutter issues with 3/16" G10, I exacerbate the CG issue.

I don't think there is a way to stay within a 4.1x upscale (+/- 5%) without adding nose weight to alter CG for stability, even then, while it gives a larger margin for stability, weather cocking takes the lead.

Fair weather flyer it is...and design lessons learned.