square leading edges on fins?

[lr64]

Can anyone explain why there are so many fins with square leading edges? Is there some reason that alternating vortex shedding and high drag are desirable?
Is it a phobia about sanding blocks, drawknives, spokeshaves and planes? Is it an aesthetic? Is it because people want to use bigger motors without getting higher apogees? I just don't get it.

 

[Scott_650]

Many/most of my rockets have square leading edge (and trailing edge) fins because:

A. It's easier for me to paper them that way.

B. I don't really care about maximum performance, I get plenty of steps in at a launch as is.

C. I have better things to do than more sanding.

 

[lr64]

Do you have better things to do than working additional hours to pay for significantly larger motors? If your rockets are of any size, and you fly a few times, that's a lot more hours than half of one with a plane and a sanding block. BTW, I don't like making paper go around right angles, so I prefer airfoil shapes when I'm papering something. I'll bet I could do one fin with one piece of paper if the trailing edge was sharp. (Ok, generally, when I'm papering something, it's a wing, not a fin.)

 

[neil_w]

Optimizing for altitude (or more generally, "performance") is not a priority for many builders. Each pursues the hobby in their own way, with their own priorities, to maximize their own enjoyment. It's as simple as that.

I usually round my edges (and paper them!) but it's more for aesthetics than for performance.

 

[krislhull]

Do you have better things to do than working additional hours to pay for significantly larger motors?

For me it all depends. On my low and mid power, I tend to at least round the leading edges over, and on my high power builds, if the fins come beveled, great, if not, they tend to stay that way, especially on the larger builds. I do not have a sanding table or a router table, and I do not trust my table saw for bevels. My L3 build had 3/8" plywood fins, and they stayed squared, I even did a tip to tip fiberglass build up on them. On an M1550, I saw 7,500ft, and 650mph, I think Ill take it! I did run various simulations, and if I had beveled the fins, I would have maybe gotten 2-300ft higher, and maybe 20-30mph faster.... in other words, not worth it in my mind. I currently have another, larger build coming up, and I don't plan on beveling the fins either as I want it to be a relatively low (less than 10,000ft) flyer.

As Neil says above, everyone has their own style and preferences. To each their own.

 

[Hobie1dog]

+1 what Scott said

 

[SolarYellow]

I also hate square. It just looks like a heck of couldn’t be bothered.

 

[mh9162013]

BTW, I don't like making paper go around right angles

I've never heard of (or seen) a papering technique involving square fin edges where the paper goes around the edges.

I keep my fin's edges square for aesthetic reasons...as well as OCD reasons. When I see people airfoil or round their edges, they're usually (not always) ending up with rounded fin edges that are all a little bit different. I can't stand that look, but then again, I don't build for high performance. To each their own.

 

[sriegel]

Sometimes its for performance.

A couple years ago, one of my ARC teams left the edges square to reduce altitude as the chosen motor regularly put them high. They didn't want to add weight as they wanted a faster liftoff to mitigate weathercocking. Our home field is Pueblo, CO, at about 5200 MSL.

At finals, they had to round the LE or both LE/TE to get to the target altitudes due to the thicker air in Virginia.

 

[Antares JS]

I usually round fins, but occasionally leave them square if I have a reason to blunt performance. The Estes Sasha is a good example. A rocket that small on two 24mm motors is just more likely to get lost if it goes higher.

 

[lr64]

Optimizing for altitude (or more generally, "performance") is not a priority for many builders. Each pursues the hobby in their own way, with their own priorities, to maximize their own enjoyment. It's as simple as that.

I usually round my edges (and paper them!) but it's more for aesthetics than for performance.

Are we talking about optimizing for performance, or about getting the same performance with a cheaper, lighter motor?

Just a few minutes with a sanding block or plane will reduce drag. I wouldn't be surprised if getting rid of the separated flow that square leading edges cause would actually make the fins lift curve steeper, allowing them to be smaller and still provide adequate stability.

People who want it to be pretty should study this:
https://charlesriverrc.org/articles...ithout-templates/markdrela_airfoilshaping.pdf
I've done this with a sort of elliptical planform, but I bet it's a lot easier with straight leading edges.

It's often handy to use a flexible ruler to draw a straight line with a pencil over the contour of what you're shaping. Look from the side, and you can get a good idea of the cross sectional shape. I found this handy when reconstructing a destroyed wingtip. Casting shadows from something straight, using a single light source, can also be helpful.

People who don't trust their judgement could at least round edges consistently with a router bit. Or glue on half-round mouldings.

People who want beveled leading and trailing edges could glue trailing edge stock together.

If you just want to bludgeon the air, there are always those hideous flying saucer things. Or Jeeps. ;-)

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[Antares JS]

Are we talking about optimizing for performance, or about getting the same performance with a cheaper, lighter motor?

Just a few minutes with a sanding block or plane will reduce drag. I wouldn't be surprised if getting rid of the separated flow that square leading edges cause would actually make the fins lift curve steeper, allowing them to be smaller and still provide adequate stability.

People who want it to be pretty should study this:
https://charlesriverrc.org/articles...ithout-templates/markdrela_airfoilshaping.pdf
I've done this with a sort of elliptical planform, but I bet it's a lot easier with straight leading edges.

It's often handy to use a flexible ruler to draw a straight line with a pencil over the contour of what you're shaping. Look from the side, and you can get a good idea of the cross sectional shape. I found this handy when reconstructing a destroyed wingtip. Casting shadows from something straight, using a single light source, can also be helpful.

People who don't trust their judgement could at least round edges consistently with a router bit. Or glue on half-round mouldings.

People who want beveled leading and trailing edges could glue trailing edge stock together.

If you just want to bludgeon the air, there are always those hideous flying saucer things. Or Jeeps. ;-)

------------

 

[lr64]

Ok, ok, I guess I'll stop whining and just observe what people write. Not that I'll understand it.

 

[Art Upton]

Do you have better things to do than working additional hours to pay for significantly larger motors? If your rockets are of any size, and you fly a few times, that's a lot more hours than half of one with a plane and a sanding block. BTW, I don't like making paper go around right angles, so I prefer airfoil shapes when I'm papering something. I'll bet I could do one fin with one piece of paper if the trailing edge was sharp. (Ok, generally, when I'm papering something, it's a wing, not a fin.)

It is not Significantly larger, it's not even a worry for me.

 

[tsmith1315]

Are we talking about optimizing for performance, or about getting the same performance with a cheaper, lighter motor?

Most of my flights have no goal beyond safe flight and recovery. I’m happy just to fly a rocket.

That’s not to say I don’t typically bevel fins, but if I decide not to bevel for aesthetics or laziness, I wouldn’t consider the difference in performance as a factor.

A few percent lost in altitude won’t be visible from my perspective, and is of no consequence to my joy, so I have no impetus to compensate with impulse.

That said, I do usually stuff in the biggest motor that I feel comfortable with, just for maximum noise and excitement at the flight line.

 

[Art Upton]

People who want beveled leading and trailing edges could glue trailing edge stock together.

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Please post links to where I can buy this edge stock for a BT-60 or BT-80 rocket's fin leading and trailing edge. It would rock for a scale look, as it is not going to Affect the price of the motor I need to use.

Edge stock I have used on RC planes is much too big for Mid-to smaller High power and lower rockets.

 

[Pem Tech]

Many/most of my rockets have square leading edge (and trailing edge) fins because:

A. It's easier for me to paper them that way.

B. I don't really care about maximum performance, I get plenty of steps in at a launch as is.

C. I have better things to do than more sanding.

Give that man a harumph.
Sometimes it just isn't that important. For my sci-fi designs a square leading edge fits the aesthetic. For some of us it isn't about performance, it is about the joy of watching them fly.

 

[Pem Tech]

People who want beveled leading and trailing edges could glue trailing edge stock together.

If you just want to bludgeon the air, there are always those hideous flying saucer things. Or Jeeps. ;-)

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Back on up from dissing Jeeps bro.
Jeeps rock.

 

[SolarYellow]

Jeeps are not meant to go fast. The more squarer, the more betterer.

 

[Rschub]

I find that trees like the taste of really streamlined fins.

 

[KenECoyote]

Many reasons. Most of all, while I do appreciate others liking and admiring my rockets, I build it the way I want to that makes me happy.

For some rockets like the Madcow 4" Patriot, because of the small fins, it's unstable without a lot of nose weight. In this case, I want as much drag as those little fins can make and a square edge does that.

Others and some kits have just made the Patriot fins bigger for more stability, but imho that looks much worse than square edges.

Honestly, who can tell or cares when it's on the pads and launching or coming down?

 

[WizardOfBoz]

I just put two rockets together. First rockets in twenty five years or so, since my son had an Estes Alpha. The two rockets were a Mad Cow Super DX3 (with an apogee blue tube Avbay) and an Estes Argent (I bought the parts and constructed the rocket using the instructions.)

A couple of things before we address the OP's fin topic.
1) Me and West 105/205 is like a three year old and maple syrup. Oy.
2) Me and West 105/205 with colloidal silica is like a 1st grader and library paste. Vey.

I sanded and profiled the fins extensively. The DX3, at least, will be my L1 cert attempt rocket. I think I erred in sanding the fins. It all boils down to failure modes and the strength of a fin.

Performance failure mode: Poor performance due to drag and to fin flutter
1) Fins should be tapered root to tip (reduces weight and drag and if I am not wrong eliminates a resonant frequency which limits flutter).
2) Fins should have an airfoil shape to minimize drag, appropriate for the rocket (rounded front and tapered back for subsonic, wedge for supersonic, if I am not wrong - if you have superior knowledge please correct me).

Certification Failure Mode: Certification failure for the fins is a fin breaking (in landing) or shearing off (in flight), another failure mode is losing the rocket
1) The more meat you have on the fin, the less likely will be a break upon landing
2) Fins shearing off is unlikely for any rocket for L1 or L2 if the fins are properly filleted.
3) Square (low aerodynamic performance) fins keep the rocket low and slow and increase the chance of finding the thing after the flight.

So if I had been thinking about L1, I would not have tapered an profiled the fins so excessively. I'm an engineer, so I think that it was my natural prediliction towards aero performance that had me do so much work. We'll see how much of an error I made.

 

[WizardOfBoz]

I build it the way I want to that makes me happy.

... I want as much drag as those little fins can make and a square edge does that.

Honestly, who can tell or cares when it's on the pads and launching or coming down?

Point 1: Agree - we do this because we find it interesting and enjoyable.

Point 2: Had not considered how square edges might change Cp. Do they? Can this be quantified?

Point 3: No one else would know but I would, and .... well, refer to Point 1.

 

[SolarYellow]

Do alternating shedding vortices promote flutter? The video in the OP sure makes it seem like they might.

 

[tsmith1315]

Point 2: Had not considered how square edges might change Cp. Do they? Can this be quantified?

Do you use simulation software? In OR, you can make those changes and see the effects.

 

[WizardOfBoz]

Do alternating shedding vortices promote flutter? The video in the OP sure makes it seem like they might.

I think that flutter is caused when the resonant frequency of the fins matches the aerodynamic resonance (which i think is the vortex shedding frequency). If the frequencies (fin resonant frequency and fin vortex shedding) I don't believe that you get flutter. So by itself, I suspect that the answer is no. If you have a frequency match, yes.

 

[WizardOfBoz]

Do you use simulation software? In OR, you can make those changes and see the effects.

Ah, I didn't know that OR or Rocsim modeled that. And yes, I do simulations (it's my job). Thanks for updating my understanding of the software capability.

 

[neil_w]

Ah, I didn't know that OR or Rocsim modeled that. And yes, I do simulations (it's my job). Thanks for updating my understanding of the software capability.

Speaking for OR only: base drag is modeled, but is not factored into CP. So fin profile settings affect drag only.

 

[SolarYellow]

It's suggested above that square leading edges may inhibit fins' performance in their role as a wing, which is most of how they generate stability, rather than by their contribution to base drag.

 

[samb]

For me it's so simple. Square edges are ugly, round edges are pretty. Pretty's better.