Fiberglass leading edge fin bevel thickness?

[scanlin]

Question: How sharp of an edge is optimal for the leading edge of a beveled fiberglass fin on a rocket projected to go mach 1.3-1.5?

My fins are 4.8mm fiberglass at the root. The kit I got has the leading edge pre-beveled but only to the point where the leading edge is 2.5mm thick. Seems too thick, but I'm not sure.

Should the leading edge be thinner for reduced drag? Or is that a bad idea in case it lands hitting something hard on the beveled trailing edge?

This 100" long rocket is simulated to go mach 1.3+ with an M motor (my level 3 cert attempt). I know it will work okay with a semi-blunt leading edge thickness of 2.5mm, but should I bevel it more and get the leading edge thickness down to 1mm or thinner? (I'm relatively new at this and trying to develop good habits for when I make bigger performance projects later) I know if I make it too much of a knife point then I won't be able to paint it well.

Also, should I slightly round the leading edge?

The first fin cross section below is what I got with the kit. The 2nd and 3rd are ideas I'm considering. Thanks for any advice... (this is a 4" Darkstar Extreme, in case that matters)

 

[Hobie1dog]

round over the leading edge. I think it's unnecessary to have a sharp edge.

 

[Neutronium95]

I wouldn't worry about tweaking the bevels on already beveled fins. It will make a negligible difference in drag. Unless I'm building a highly optimized minimum diameter rocket I see fin bevels as a purely aesthetic choice.

 

[fyrwrxz]

Well, you CAN put a knife edge on them. BUT, they are then subject to dings and fraying, leading to much worse drag or erratic aerodynamics if the fins "hunt" for clean airflow or flutter. I'm not gonna throw anybody under the bus, but a chap here had a shovel recovery and sliced himself up pretty good on his own razor fins. He had to do first aide on the playa as he was leaking pretty good. I don't do that any more, no matter how sexy they look. Sliced my thumb once and it hurts like hell, not to mention if you get a nice little carbon thread in it as a bonus.That said, I can't stand ' square ' fins. We don't have flat nose cones, do we? Bevel 'em, all edges, but stay safe and sane. Straight smoke and good chutes!

 

[rfjustin]

IMO, the middle option would be best if you can pull that off...

 

[SolarYellow]

I'd go 0.5mm with rounding and ~4.5-degree taper per side.

 

[scanlin]

Thanks, everyone. I ended up buying a cheap router table and router from harbor freight for $120 and this 11.25 degree router bit from Amazon for $13:

https://www.amazon.com/dp/B075GXVJLR
Got a piece of melamine board and attached it to the router fence with some countersunk screws. Made a little hole where the router bit can stick out a little bit. Then just slowly slide the fiberglass fin across the bit. Worked great and the bevels are super smooth (better than some table saw versions I've seen), and consistent. Once dialed in (which took some practice runs on 1/8" plywood) I could do all 6 fins in about 10-15 minutes. Ended up leaving about 1mm thick on the leading edge.

Appreciate everyone's suggestions and help.

 

[thzero]

I did something similar, but I used a straight bit and then 3d printed a jig that I can replace the plates with different angles.



The left backet just snugs up against the material, then you just slide it through. Came out pretty decent.

 

[thom9894]

I did something similar, but I used a straight bit and then 3d printed a jig that I can replace the plates with different angles.

View attachment 576622

The left backet just snugs up against the material, then you just slide it through. Came out pretty decent.

View attachment 576624

Are the STLs for this jig available? Looks amazing!

 

[thzero]

Are the STLs for this jig available? Looks amazing!

Here you go..
https://www.rocketryforum.com/threads/bevel-jig.179610/

 

[Donnager]

Thanks, everyone. I ended up buying a cheap router table and router from harbor freight for $120 and this 11.25 degree router bit from Amazon for $13:

https://www.amazon.com/dp/B075GXVJLR
Got a piece of melamine board and attached it to the router fence with some countersunk screws. Made a little hole where the router bit can stick out a little bit. Then just slowly slide the fiberglass fin across the bit. Worked great and the bevels are super smooth (better than some table saw versions I've seen), and consistent. Once dialed in (which took some practice runs on 1/8" plywood) I could do all 6 fins in about 10-15 minutes. Ended up leaving about 1mm thick on the leading edge.

Appreciate everyone's suggestions and help.

Just a question. I have a Bosch Router table and use a burr bit for bevels.

My problem is as I drag the fin across the bit, the recessed plastic (junk) flexes just a little, and I get a stop at the end of the recess hole when I get back on the table, which gives me a bodywork booger to get rid of. I've considered building a notched aluminum channel to cover the span, and hold everything flat during processing, and that's probably going to be how I handle it.

Have you had this problem and/or done anything to prevent this, other than technique?

 

[SolarYellow]

After reading all the reviews on the affordable router tables, I bought the Rockler insert plate for my Bosch router and will be making my own table.

You could do what you're thinking about with any continuous sheet of thin flat stock. Cheapest may be a chunk of steel from your local metal supply house (you have one of those, right?). Will be slower to wear any grooves in it than aluminum or fibrous stuff.

 

[scanlin]

Just a question. I have a Bosch Router table and use a burr bit for bevels.

My problem is as I drag the fin across the bit, the recessed plastic (junk) flexes just a little, and I get a stop at the end of the recess hole when I get back on the table, which gives me a bodywork booger to get rid of. I've considered building a notched aluminum channel to cover the span, and hold everything flat during processing, and that's probably going to be how I handle it.

Have you had this problem and/or done anything to prevent this, other than technique?

I used 3/4" melamine wood as my fence (which I attached to the default plastic fence that came with the router), and then carved a little recess in the melamine just slightly bigger than the bit (see my photos above). I didn't get any flex inward toward the bit as the fin was being *slowly* pushed across the bit.

My fins are 5" and 10" long on the beveled edges, and the hole for the bit is 1.5" at the bottom (widest part), so there is a good amount of fin in contact with the melamine fence on either side of the hole. I suppose you could build an opposing fence as well so there is just a little channel for the fin to slide through, although my results were really good with just the one fence.

Another thing I might try next time is to do smaller, partial bevels at first to remove some material (with the same bit but lower), and then raise the bit and do another pass. Just do all of the smaller cuts on all fins first, so once you set the final final bit height for the last pass you can do all the final bevels in one shot (for uniformity).