Do you sand the leading edges of fiberglass fins?

Some kits with balsa or other wooden fins make a big deal of sanding the fins, and even provide a "scale" option (which is presumably accurate), and a "sport" option (which is just rounding out the leading edges, and easier). I have the latest Black Brant II from Estes in mind.

My first fiberglass kit however, doesn't say anything about sanding the fins. It's the mini Black Brant II from Madcow. Should I round out the leading edges, or just leave them as they are?

I'm just wondering what most people usually do.

I would recommend at least rounding the leading edges. Going a little further and getting a aerofoil profile is even better, that includes profiling the trailing edge as well. However, depending on how thick the fins are, there might be a limit to how much you can remove with sanding.

Either way, leaving the leading edges flat does introduce extra drag that will decrease your potential performance.

I bevel some, round some, airfoil some and leave some squared. It varies. But at least rounding it slightly makes it easier for painting. Trying to sand the edges will burn through the paint faster on squared edges.

I like to sand all edges of the fins. whether it is beveled, or rounded, not only does it help with peerformance, I think it looks much better.

I usually bevel the leading and trailing edges, but don't make such a sharp edge that it is easily damaged. Leave a little thickness there .

OverTheTop said:

I usually bevel the leading and trailing edges, but don't make such a sharp edge that it is easily damaged. Leave a little thickness there .

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Not being so familiar with fiberglass, I was actually concerned that bevelling fibreglass fins might amount to sharpening blades. I’m all for performance, but not to that extent.

Funkworks said:

Not being so familiar with fiberglass, I was actually concerned that bevelling fibreglass fins might amount to sharpening blades. I’m all for performance, but not to that extent.

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Are you using a round or pointed nosecone? You'll never feel the knife edge.

technically, if you're going over mach 1 then sharp leading and trailing edges are better otherwise an airfoil shape is better. However, fiberglass fins are usually so thin that your effort is better spent on a very smooth and polished body tube+fillets. The finish would have to be pretty much perfect in order for the leading/trailing edge on a fiberglass fin to be the performance bottleneck.

Chad said:

technically, if you're going over mach 1 then sharp leading and trailing edges are better otherwise an airfoil shape is better. However, fiberglass fins are usually so thin that your effort is better spent on a very smooth and polished body tube+fillets. The finish would have to be pretty much perfect in order for the leading/trailing edge on a fiberglass fin to be the performance bottleneck.

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...very true. And I have found the sharper you make the edge, the quicker they get banged up in the field, car, shop etc. In my unscientific opinion makes it worse than starting a little rounder in the first place.

shonc182 said:

Are you using a round or pointed nosecone? You'll never feel the knife edge.

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It's conical, but the point isn't sharp. I haven't tried to sharpen it but I do wonder a little how close I could get.

Chad said:

technically, if you're going over mach 1 then sharp leading and trailing edges are better otherwise an airfoil shape is better. However, fiberglass fins are usually so thin that your effort is better spent on a very smooth and polished body tube+fillets. The finish would have to be pretty much perfect in order for the leading/trailing edge on a fiberglass fin to be the performance bottleneck.

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I'll just round them off a bit.

Ha. I'm building the exact same rocket (1.6" fiberglass Madcow Black Brant II kit). And was wondering exactly the same thing... The fins on this kit are only 1/16" thick.

So, is the consensus to just round them, or leave them as-is (square)?

Beveling them by hand to have a nice uniform sharp leading/trailing edge would be hard. And I'd be more than likely to mess 'em up.

I found this article by John Coker for building jig for disk sander (and/or router). But I guess those would be mostly for thicker fins?

I also have this burr router bit, which should work well on thick 1/4" plywood... But not sure how well or if at all it would work with 1/16" thin fiberglass.

Sanding fins by hand iss super easy. You CAN get a pperfect bevel. i use an 18" block with a full sheet of sand paper thumb tacked in place. I like to place the fin on the edge of a table, using a fiber hobby board as support.. it takes very little effort to remaove material with 60 grit. it is easy to see the bevel starting, all you need to watch for is the "white" part is the same width. Flip it ooover and do the other side. you can leave a bit to round off if you do not want them sharp.

I mounted a Dremel under a router table (cheapie from Aldi) and made an angled fence for sanding fin bevels. You could make something similar out of wood.


Further details here
https://forum.ausrocketry.com/viewtopic.php?f=6&t=4758&p=58844
and here
https://forum.ausrocketry.com/viewtopic.php?f=6&t=5019&start=45

Very consistent bevels are produced using this method.

For 1/8" and below I round off mainly the leading and trailing edges, number one reason is because I've cut myself on that edge and have also seen shock cords cut by it.

double post

double post again

amiliv said:

Ha. I'm building the exact same rocket (1.6" fiberglass Madcow Black Brant II kit). And was wondering exactly the same thing... The fins on this kit are only 1/16" thick.

So, is the consensus to just round them, or leave them as-is (square)?

Beveling them by hand to have a nice uniform sharp leading/trailing edge would be hard. And I'd be more than likely to mess 'em up.

I found this article by John Coker for building jig for disk sander (and/or router). But I guess those would be mostly for thicker fins?

I also have this burr router bit, which should work well on thick 1/4" plywood... But not sure how well or if at all it would work with 1/16" thin fiberglass.
View attachment 397714

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Personally, I prefer quiet hand tools these days, and I value safety since I’ll be showing these off to some kids, so rounding off is what I’ll do.

GaryT said:

For 1/8" and below I round off mainly the leading and trailing edges, number one reason is because I've cut myself on that edge and have also seen shock cords cut by it.

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Really? Thanks for confirming it can cut. I mean... it stinks to be injured but by telling us, I’d say it was not in vain. ... But if the injury was permanent and serious, let me know and I'll just shut up.

My point: This confirms I only want to round off the edges (for this particular project anyway).

Funkworks said:

Personally, I prefer quiet hand tools these days, and I value safety since I’ll be showing these off to some kids, so rounding off is what I’ll do.



Really? Thanks for confirming it can cut. I mean... it stinks to be injured but by telling us, I’d say it was not in vain. ... But if the injury was permanent and serious, let me know and I'll just shut up.

My point: This confirms I only want to round off the edges (for this particular project anyway).

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Without jumping the gun I'll ask polity, what was your point again ?

So. I ended up doing what David suggested above and hand beveled leading and trailing edges. Wrapped some 60 grit sandpaper over a long "sanding block" (a nice looking flat and square scrap piece of wood), and beveled them. Once I got them beveled, I simply dulled the edges just enough so that they are not knife-sharp. It doesn't look perfectly perfect like John Coker's bevels made using a jig for disk sander. But they are passable. Not really 10% bevel either, I kind of eyeballed it and ended up with what looks visibly larger angle, and if you look really hard not totally consistent from fin to fin, but I declared 'em good enough. One fin glued in last night, epoxy curing on the second fin since this morning, third one going on sometime in the afternoon (once epoxy is cured enough on the second fin).

I didn't bevel the tip edge, to get that fractal percentage of performance (reducing drag induced by air flowing over tip edge from one side of the fin to the other when there's pressure differential between the two fin surfaces).

GaryT said:

Without jumping the gun I'll ask polity, what was your point again ?

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I just hope the cut wasn’t too bad and everything is fine now.

lol Ahhhh, oh it was nothing serious. When sanding by hand and especially fin edges I just ran my finger right down that sharp edge is all. thanks

I cannot count the number of times I have cut myself on fins. On rockets that are going for altitude records I make them as sharp as I can get them. Otherwise I leave a bit to round off.

double post, sigh

double post, yeah I'm clumsy today

GaryT said:

lol Ahhhh, oh it was nothing serious. When sanding by hand and especially fin edges I just ran my finger right down that sharp edge is all. thanks

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Just to recap: I'm new to fiberglass, I was wondering if is could be dangerous, your case answers my question better than I could ever hope for.

Result: since I don't need peak performance, and I will be showing my work to kids, I will optimize safety and round the edges instead of risking sharp edges and cuts.

Also, I'm kind of bored today. Taking breaks from working overtime. It's just sooo quiet on Sundays.

David Schwantz said:

I cannot count the number of times I have cut myself on fins. On rockets that are going for altitude records I make them as sharp as I can get them. Otherwise I leave a bit to round off.

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Well, injuries suck, but thanks for confirming the danger and allowing newbies like me to learn from your experience. I used to be in charge of a lab where security was a concern, so I still have that mindset around materials.

I draw file a bevel on the leading and trailing edges but I never take it to a sharp edge. Instead I round it and smooth it. If I ever received a set of fins with such a sharp edge I would blunt them and round them over.
The simple reason why is that sharp edges don’t really help at the velocity most of our rockets fly. The greatest percentage of each flight is at subsonic velocities where there’s just no benefit to a sharp leading edge, although a sharp trailing edge is beneficial.

Just to clarify, just nicks. Nothing that ever needed ER

Square leading edges have huge drag across a wide velocity range. Just rounding them over produces a noticeable performance increase. Square trailing edges also add a lot to the base drag. An 0.125 thick fin with 4" span on a 4-fin rocket adds 2 sq. in. of base area, a 28% increase on the base area of a 3.0" rocket. For sport flying it doesn't really matter, but if you're trying for altitude, fin profiling is not optional.

Keeping the edges of the fins that run parallel to the airframe square helps slightly in the fins providing stability. Not a large factor, but real.

OverTheTop said:

Keeping the edges of the fins that run parallel to the airframe square helps slightly in the fins providing stability. Not a large factor, but real.

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I think keeping that edge of the fin square actually reduces the drag. This applies to both rocket fins and airplane wings. Whenever angle of attack is non-zero, there will be pressure differential between the two sides of the fin/wing. The air flowing on the side with higher pressure will be pulled over the tip edge towards the side with lower pressure. Long story short, this manifests as increased drag. It's much more of an issue for the airplane wing, because they normally have non-zero angle of attack in level flight. Hence those winglets on modern airliners, they block air flowing over the tip, thus reducing total drag and resulting in measurable fuel savings. It's likely less important for rocket fins, because ideally fins would have zero angle of attack. Reality is not ideal, so the rocket fins will experience varying but still small angle of attack. Since it is harder for the air to flow around square edge than around rounded or beveled edge, keep those tip (parallel to body tube) edges square. Does it make sense to put winglets on the rocket fins to reduce drag further? Probably not... After all you don't see winglets on vertical/horizontal stabilizers of an airplane either.