Round the Leading Edge of the Fin and the Trailing Edge, or Just the Leading Edge?

brockrwood

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The instructions for this rocket kit say to round the leading edge of the fin. The instructions do not say to round the trailing edge of the fin. I have always rounded the leading edge and the trailing edge.

What do you think I should do?

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Paul Howard

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I agree, do what you want. Me personally? I'm a total nerd from RC Glider and freeflight endeavors and usually compulsively airfoil my fins and I've taken a liking to turning skill level 1 Kits into level 2 or 3 by adding hardwood leading and trailing edges and then sanding in airfoils with fairly sharp trailing edges. I could be wrong, but I do think the rockets fly a little straighter/smoother due to the airflow being much smoother around the fin with less vortexes as a result. And a little higher as one would expect. I also try to fly on calm days and use parachutes that will give a soft enough landing.

That said, just rounding the edges will be much better/cleaner aerodynamics than squared edges.

Just my 2 Cents-Paul
 

bjphoenix

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Older rocket books and instructions will tell you to put airfoils on the leading and trailing edges of fins. I have some very old rockets done that way but they get banged up with launching, storage and travel. I think a square edge would also be somewhat fragile, at least the corners would be, and it would be hard to do and keep it uniform. A rounded edge is fairly easy to do and I think it the most resistant to damage that would make it look bad.
I don't know how much difference this makes to aerodynamics, probably not enough to matter unless you are going for altitude.
 

Alan15578

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Older rocket books and instructions will tell you to put airfoils on the leading and trailing edges of fins. I have some very old rockets done that way but they get banged up with launching, storage and travel. I think a square edge would also be somewhat fragile, at least the corners would be, and it would be hard to do and keep it uniform. A rounded edge is fairly easy to do and I think it the most resistant to damage that would make it look bad.
I don't know how much difference this makes to aerodynamics, probably not enough to matter unless you are going for altitude.
All rockets get banged up in use, transport, and storage. However, air foiled three ply aircraft plywood holds up surprising well, and can even be transplanted to new rockets.
 

neil_w

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When talking about airfoils or beveled fins: in my experience basswood holds a sharp edge pretty nicely. Forget balsa; even the slightest handling is going to ruin the edges.
 

GlenP

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round LE and sharp TE is the lower drag option that is an approximation of an airfoil on a flat fin

sharp TE tend to get dinged up easily on landing

http://sargrocket.org/documents/Resources/Centuri/dm-1.pdf

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depending on the kit and fin thickness, the aesthetics may look better with some flat edges. Cardstock or fiberboard fins don't really shape that well and are usually left with square edges.
 

Lord Rory Gin

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Pretty sure rounding the trailing edge is worse aerodynamically than leaving it square. I would just do what you think looks best.
I'm not an aeronautical engineer but that statement just sounds dead wrong. A tapered leading and trailing edge and tapered outward edge would seem to be the best aerodynamically.
 

brockrwood

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round LE and sharp TE is the lower drag option that is an approximation of an airfoil on a flat fin

sharp TE tend to get dinged up easily on landing

http://sargrocket.org/documents/Resources/Centuri/dm-1.pdf

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depending on the kit and fin thickness, the aesthetics may look better with some flat edges. Cardstock or fiberboard fins don't really shape that well and are usually left with square edges.

The old Centuri Design Manual is a great resource book! Thanks for the link!

I also note, from flipping through the book, that Centuri once made a 1/2 A engine that was a "shorty". It was still an 18mm engine, but it was not as long as a standard 18mm engine. Estes' 1/2 A engines are mini, 13mm engines. Interesting!

I am going to build a clone (or semi-clone) of the Centuri Cloud Buster that is shown at the end of the Design Manual. I note, for the record, that the rocket separates at the first transition. Interesting! Basically it breaks in half. You could put a narrow payload in the top or middle section. The ejection charge would have to be pretty strong to pop the whole top of the rocket off, including the weight of a payload.

Gotta learn how to make a boat tail...

It's just a transition pointed the "wrong" way, no?
 
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Mike Helm

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I saw an Orion in a museum and it had pointy leading edges and square trailing edges. I was wondering if this aids in unguided stability by creating drag?
 

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Mike Helm

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modeltrains

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Air is a rather less dense fluid than water but this from boating comes to mind,
If you are going fast enough the trailing edge will cavitate ( if square) and at a certain speed this will create LESS drag than trying to keep the water attached BUT will affect the load the rudder can take.
I think the rule of thumb is water doesn't like filling at more than 12 degrees hence all the work on NACA profiles.
 

Woody's Workshop

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I do what I want. I knife edge everything but the root edge, then laminate the fin with one piece of paper leaving mounting tabs on both sides of the root edge. But, it's only done on balsa. It's kinda hard to knife edge layered card stock, such as the Viking. As a rule I will cut out replacements from balsa.
And by knife edge I mean single bevel. The fin will slice and reroute air on one side of the fin only.
I have no idea what advantages or disadvantages this has, don't care. I just like flying rockets, and enjoying the kids "WOW's" and looks on their faces (especially the first time they see one go up) that attend and retrieve my rockets for me.
 

Paul Howard

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As mentioned multiple time above, tapered "sharp" trailing edges are more fragile. A good "minimum" fix is to put Cyanoacrylate on the leading, outer and trailing edges to harden them. Also, that's one of the reasons I and other posters add harder woods like Sitka Spruce or Basswood, and whatnot that comes in strips or thin sheets that match your fin balsa sheet thickness. One thing I really like is natural wood finishes on all wood fins, especially if there is "character" to the wood, I recently got some nice looking walnut and mahogany strips for my leading and trailing edges. "Papering" as mentioned above works well also. Copy Paper adds a LOT of strength, good Freeflight aircraft Tissue or Silkspan and thinned yellow-glue to apply also works well and the nice thing about them is you often can see the wood through the tissue or silkspan somewhat, and if you get colored material, then it reduces the need to paint.
 
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Paul Howard

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How do you get all sides of the foil to be exactly the same? Do you have a jig?

If they are not the same, the rocket will have some roll.
I often intentionally airfoil my fins with a semi-symmetrical airfoil intentionally to induce v-e-r-y s-l-i-g-h-t spin for more strait-off-the-direction-of-the-launch-rod flights, it seems to work really nicely and it cancels out the almost unavoidable other un-noticed mis-alignments that can happen.
 
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boomtube-mk2

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I have a question? What is the flight speed of an unladen swallo . . . oops! wrong forum.

Which would be better, squared leading/trailing edges or poorly sanded leading/trailing edges? That is, edges that might lead to miss-matched airflow over the fins, possibly causing wobble or spin?

I always sanded by edges except on some models that had really small decorative fins wherein sanding them would drastically alter their shape.
 

Paul Howard

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I saw an Orion in a museum and it had pointy leading edges and square trailing edges. I was wondering if this aids in unguided stability by creating drag?
I'm guessing that Orion is flying more than Mach 1, and to quote Tim Van Millican(sp?) at Apogee "Speed changes everything", and our Low Power rockets fly considerably slower than that. We're comfortably in the subsonic rounded leading edge, tapered trailing edge zone. Cool pictures! Thanks for posting that!
 
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