Fin Beveling

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Brian-Tampa

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Hello!
I am currently building a modified LOC Precision EZI-65. I have read many posts on how to bevel the fins, but I am looking for info on how far back on the trailing edge to start the bevel.
I have seen bevel angles vary from 10 to 15 degrees on many posts. And also do you taper the fins to an absolute sharp edge or leave the trailing edge ever so slightly flat.
Any info would be greatly appreciated!
 

JohnCoker

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I don't think there are any absolute rules. Unless you're modeling a specific prototype (which the EZI-65 is not), it's purely a matter of cosmetics. I generally don't bevel trailing edges as it makes the tips of the fins fragile, and they tend to get banged up anyway during transportation and landing.

Since you said "bevel", I assume you are not talking about airfoil shapes for reducing drag. Those are not simple bevels and you would have to run a simulation to determine the optimum shape for the size of fin and speed you're trying to optimize for.
 

Brian-Tampa

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John,
Thanks for your reply. I was kind of "on the fence" about beveling the trailing edges. I have rounded the leading edges, but now thanks to your input I will leave the trailing edges stock.
When I built my first EZI-65 a year ago, I left the fins stock with no rounding of the leading edges or beveling of the trailing edges.
I had many good flights from this rocket. The last successful flight was on Nov. 16 at the Annual Buc Blast in Plant City, Florida. I hit 3579' on an I289DM, max speed 517mph.
Unfortunately, she was lost on Dec. 21 at the same launch site. I used the same engine but my Jolly Logic chute release pulled off at apogee when the delay charge blew and the chute deployed prematurely, probably at about 3500'.
The rocket drifted pretty far and I never found it. I like the EZ design and have begun building my second one. It should fly within a month. My first EZ was aptly named "SEPARATION ANXIETY", this new will be named "30 DOLLARS A SECOND"
I appreciate your input, as I have read many of the articles you have on your website.
Thanks,
Brian
 

rharshberger

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I use a router table and an 11° (might be 11.5°) bearing guided bit, and then stick my fins to a larger block of wood using double sided carpet tape so my fingers are farther from the bit. The 11°is convenience not anything technical.
 

Mike Helm

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Just a layman here. Here's a photo of an Orion sounding rocket fin tip. You'll notice the leading edge is beveled and the trailing edge is flat and square. Can anyone here explain this? I like it and build my recreational rockets like this because they suffer little to no damage on landing.
 

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JohnCoker

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Just a layman here. Here's a photo of an Orion sounding rocket fin tip. You'll notice the leading edge is beveled and the trailing edge is flat and square. Can anyone here explain this?
My guess is that it reduces drag to have a tapered rather than a blunt leading edge and it likely depends on the speed for which they're optimizing.
 

Mike Helm

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My guess is that it reduces drag to have a tapered rather than a blunt leading edge and it likely depends on the speed for which they're optimizing.
I get the leading edge taper. I'm questioning if anyone knows why the trailing edge is blunt (or square) on the Orion fin in my photo. I've Googled this without results. I like it for durability of the blunt trailing edges but I'm wondering if the blunt trailing edge lends to stability because of drag or something like that. I'm just curious and don't mean to hijack the thread but the question does seem to fit nicely here.
 

JohnCoker

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I get the leading edge taper. I'm questioning if anyone knows why the trailing edge is blunt (or square) on the Orion fin in my photo.
Ah, good question. I would have thought that at least rounded would reduce drag, but maybe the savings isn't worth the extra manufacturing cost.
 

blackjack2564

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Generally the shock wave created by high mach flights, collapses behind the fin, not on it.
So rear edge taper has no value.
 
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