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HPR Fins: Rounded vs. Straight-Edged

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jcsalem

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I like the look of fins with smooth curves rather than straight edges.

However, as I build my first curved-fin HPR rocket (H engines) I'd like to know if I'm going to run into trouble. Of course, my biggest concern is avoiding shredded fins. Here are my questions:
* Will curved fins have more or less flutter than similarly shaped fins with straight-edges?
* Will curved fins have more or less drag than straight-edged?
Bonus question:
* Is there any way to have RockSim estimate the amount of fin flutter?

My fin plans in a nutshell:
- 2.6" dia rocket, probably about 45" long. Phenolic airframe.
- Designed for G and H engines
- fin shape will be something similar to the Estes Executioner or Big Bertha but with curvy sides rather than the straight edges.
- Attachment will probably be TTW with Epoxy.
- Planning to use 1/8" poplar lite-ply which I have on hand

I've had great success with elliptically-shaped fins on a tall F/G engine rocket I made with a long fabric tube as the airframe.

-- Jim
 

firemanup

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From what i remember your most aerodynamic fin design is basically tear dropped.

Similar to the wing on a plane, leading edge is rounded and trailing edge is wedged down basically like a knife edge.

I round all fins anymore regardless of what they're made of, and i don't know for a fact but my educated guess is that the rounded or air foiled fins would have less flutter problems....

I've got 3, well actually 4 birds with the eliptical fins and I like them, there's pics on my page of them, (bsd sprint and scratch built spiderman and green goblin)

The bird you're describing building sounds great but be forwarned...on an H it's gonna go outta sight.

My spidey and goblin rockets are 2.6 diameter birds 51 inches long with three eliptical fins and they go nearly out of sight on G64's...i'm pretty nervous about flying them on H power as I would want to recover the rocket and hardware.

For your rocsim questions i don't have it so can't help ya there.. good luck...
 

Nite Builder

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Jim;
Its kinda nice to see questions that I once had also be the same questions other rocketeers are experiencing. First of all, when it comes to fin flutter, the one thing that seems to cause this the most is how "far" they extend off the end of the rocket. The farther they extend, the more flutter you get. Fin flutter however can be minimized with the addition of small "feet" epoxied to the end of the fins. This creates a little more end drag, but tends to reduce flutter by as much as 50%. You might notice these feet being used on larger rockets, but works on smaller ones as well, at least from my experiences.

As for rounded fins vs. straight edge fins, I think G.Harry Stein said it best when he said that, any coefficient drag that can be prevented is "always" a good idea. I say go for it. Design a new, cool looking fin and come back here and tell us all about it.
 
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