Fin Thickness and Flutter

[MarkABrown]

I'm designing a Level 2 project and have a question about fin thickness. This rocket will have 2 close proximity fin sets with the rear fins swept forward. I know that this configuration greatly increases the stress on the fins so I want to design these fins to have VERY great strength. My first choice was to make them out of 1/8" G10 fiberglass but, that stuff is so expensive! I could easily have $100 stuck into just fins! So, I started looking at 1/16" G10. This is more reasonably priced but, I'm concerned about fin flutter at high speed.

So, my questions are... Do I need to be concerned with fin flutter with 1/16" G10? Am I overbuilding using G10 (ie. Can I get away with using 3/16" Aircraft Plywood for the fins if I glass them)?

 

[Oliver Arend]

As long as you're not getting close to the speed of sound (vmax <= 250 m/s), both 1/16" G10 and 3/16" ply+glass should hold up.

Oliver

 

[PGerringer]

The AMRAAM4 has 1/16" fins and they are very solid. I can't imagine them fluttering. Take a look at all the styles and sizes PML has to offer, you can probably find something that is adequate. 8 fins at $4 comes to (wait, I need to take my shoes off) $32 which is slightly less than $100.

 

[MarkABrown]

Well now I'm torn. Should I use the G10 or the plywood? I think the plywood fins look cooler since they're thicker and they would be easier to round the leading edge. However, the 1/16" G10 would probably weigh alot less.

 

[Austin]

Mark,

It's your choice; you have to figure out what you are more comfortable working with. Ply fins do have to be thicker than the equivalent G10, as previously mentioned, but either one will work fine. You don't necessarily have to glass plywood as it can be sealed or coated, but it's something I would do on my own rockets, as you well know.

You could save alot of money on G10 fins if you cut them yourself. A sheet of .062(1/16") 6" x 24" is about $8.99; I just ordered 3 sheets for my 4" Harpoon as it will have G10 fins. If you have a Dremel Drill, which you should, you can cut your own fiberglass fins from this sheet stock with an inexpensive investment in material and a whole lot cheaper than having someone else cut them. It just takes a little extra time, a little patience and several asbestos cutting wheels. Best of all, you get the satisfaction that you scratch built the entire rocket!

So, good luck on your decision bro, and most of all...

HAVE FUN!!!

Carl

 

[MarkABrown]

Cutting the fins is no problem. I've got a 9" band saw with a metal cutting band. I've got to think about this some more.

 

[John Coker]

Originally posted by MarkABrown
So, my questions are... Do I need to be concerned with fin flutter with 1/16" G10? Am I overbuilding using G10 (ie. Can I get away with using 3/16" Aircraft Plywood for the fins if I glass them)?

Fin flutter is hard to nail down. There seem to be no hard-and-fast rules of thumb about when it will occur. There has to be harmonic vibration along the fins induced by the air. This means (at least) that fin shape, material, stiffness and air speed all play a part.

Unless you're planning a high-performance rocket, I don't think G10 thickness will make that much of a difference with fin flutter. However, since your fins are going to be swept back, you may want to go with the thicker G10 to avoid damage on a rough landing.

If you plan a rocket which will break Mach, then fin shape and stiffness become very important. Fins should not be swept back and the length at the tip should be different from the length at the root. (The traditional clipped delta is the most common.)

Also, plywood laminated with fiberglass, or better yet carbon fiber, will be stiffer than G10 (and probably stronger). Plywood and fiberglass make an amazingly good fin material and are quite reasonably priced. You can also get a good shape on the fins (before 'glassing) without breathing fiberglass dust (from the G10).

 

[MarkABrown]

Originally posted by John Coker


However, since your fins are going to be swept back, you may want to go with the thicker G10 to avoid damage on a rough landing.

Actually, the fins are swept forward. I'm leaning more and more towards the plywood and glass. It's easier to fabricate and its cheaper. Thanks for the help.