Fin Airfoil

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ascastil

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On low power to medium rockets, no scale or competition, do they have to be perfect? I guess you could call them sport builds, but I just want to sand the edges and make em look nice.
 

OverTheTop

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For such rockets I usually just round the leading and trailing edges. Sometimes I don't even do that. Depends on the rocket. They still look great.

Here is an example:
cestristopside.jpg DSC05644.JPG

You don't even notice the edges are only rounded. Flies really well.
 

OverTheTop

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Round them before assembly where it makes sense! I just use a cork sanding block with some fairly fine 200 grit paper. The balsa doesn't need much effort, so be gentle and don't rush the job.

Remember if you sand them to a sharp edge they are not very robust and easily dinged during handling or landings. That's one of the main reasons I don't airfoil.

If you were going for a record then airfoil or other shaping would be needed. Not what I go for.
 

Nytrunner

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I've been on an airfoiling kick recently because I dislike square edges facing the wind.
I do balsa fins by hand, and prefer to use a sander of some sort with 10 or lower grit on plywood fins.

I don't have a sander currently so I had to do my Leviathan's by hand. (BroncBuster II in the Mid-power forum)
Definitely was an opportunity to practice consistency. I apply thin superglue to the trailing edge in order to stiffen them up.
I start by sanding the green (~10-20 deg off vertical), then the yellow (~10-20 off horizontal), then the red (~30 off horizontal), and finish by working it into a curve. Trying to get as close to en ellipse's profile as i can by eye.
IMG_20161126_223111655.jpgIMG_20161126_223225524.jpgfoiling.png

I offer no illusion that my foils are perfectly consistent on a fin to fin basis, but they work perfectly well for general sport flying.
And in an ideal case you may get up to 70% drag reduction! (SG force corresponds to rearwards force on the fin)
Foil difference.png
 

OverTheTop

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The difference in drag (as shown in your sim) could also be greatly mitigated by a root fillet. That is one of their major improvements to the aerodynamics.
 

Buckeye

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I've been on an airfoiling kick recently because I dislike square edges facing the wind.
I do balsa fins by hand, and prefer to use a sander of some sort with 10 or lower grit on plywood fins.

I don't have a sander currently so I had to do my Leviathan's by hand. (BroncBuster II in the Mid-power forum)
Definitely was an opportunity to practice consistency. I apply thin superglue to the trailing edge in order to stiffen them up.
I start by sanding the green (~10-20 deg off vertical), then the yellow (~10-20 off horizontal), then the red (~30 off horizontal), and finish by working it into a curve. Trying to get as close to en ellipse's profile as i can by eye.
View attachment 306992View attachment 306993View attachment 306994

I offer no illusion that my foils are perfectly consistent on a fin to fin basis, but they work perfectly well for general sport flying.
And in an ideal case you may get up to 70% drag reduction! (SG force corresponds to rearwards force on the fin)
View attachment 306999
Nice use of CFD! What is your code?
 

Nytrunner

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I just use the Solidworks Flow Simulation toolbox (or as I like to call it: Crash Roulette Software!). Its not a dedicated CFD program, but people that know what they're doing can get it do amazing things.

My uncle and his friend are PROfessional CFD guys (I'm talking jobs like shuttle SRBs, Orion capsule descent, etc...), so occasionally I show them results, and they go "That looks valid" or "Mmmm, I don't know about that...."
I played with SimFlow, but the free version only allowed 10k or 100k mesh nodes.

I should really revisit that fin model though. The more I think about it as a whole (not just those two snaps), I want to change the setup and check for sanity violations.
As my fluids/heat transfer prof used to say "Fluid and heat transfer modeling is refined guesswork. Study and work hard enough at it and you may become a good enough guesser that someone will pay you for it".
 
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ascastil

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Theres some hardcore fin airfoilers in here, lol! I did my best to make them look even. I more do it for looks than performance. I dont like the look of a fin that hasnt been rounded or sanded somehow.
 

lcorinth

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I've been on an airfoiling kick recently because I dislike square edges facing the wind.
I do balsa fins by hand, and prefer to use a sander of some sort with 10 or lower grit on plywood fins.

I don't have a sander currently so I had to do my Leviathan's by hand. (BroncBuster II in the Mid-power forum)
Definitely was an opportunity to practice consistency. I apply thin superglue to the trailing edge in order to stiffen them up.
I start by sanding the green (~10-20 deg off vertical), then the yellow (~10-20 off horizontal), then the red (~30 off horizontal), and finish by working it into a curve. Trying to get as close to en ellipse's profile as i can by eye.
View attachment 306992View attachment 306993View attachment 306994

I offer no illusion that my foils are perfectly consistent on a fin to fin basis, but they work perfectly well for general sport flying.
And in an ideal case you may get up to 70% drag reduction! (SG force corresponds to rearwards force on the fin)
View attachment 306999
I did the same with my Leviathan fins, using a sanding block. I think I used 150 grit paper, so it did take a little while, but wasn't any more difficult than doing it on balsa - just a little slower.

I try to airfoil most of my fins. I just can't help myself.
 

Nytrunner

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I did the same with my Leviathan fins, using a sanding block. I think I used 150 grit paper, so it did take a little while, but wasn't any more difficult than doing it on balsa - just a little slower.

I try to airfoil most of my fins. I just can't help myself.
And now I see a major typo....that was supposed to be 60 grit not 10.....

Now I've done it once and have the experience to say its not something I want to repeat.
Especially since I see a 4" Phoenix in my future that has ~ 6 ft of 1/4" leading edges.....


There's a guy in my club that makes fun of me for airfoiling all my fins now when I leave tape ridges on masked paintjobs.
I'm about reducing drag in some ways, but really don't care about others :eek:
 

rocket_troy

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And now I see a major typo....that was supposed to be 60 grit not 10.....

Now I've done it once and have the experience to say its not something I want to repeat.
Especially since I see a 4" Phoenix in my future that has ~ 6 ft of 1/4" leading edges.....
It's also worth pointing out that trailing edges are just as important as leading edges, especially for subsonic flight.

Troy
 

Nytrunner

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It's also worth pointing out that trailing edges are just as important as leading edges, especially for subsonic flight.
He speaks truth ^^

Once the elliptical front edge parts the flow with minimal separation, the taper allows the streamlines to rejoin with (theoretically) no vortex formation.
You could just round the back edges, but, in my mind, Rounding them is harder then just making a trailing taper which is more flow efficient.

What would be really great is a comparison analysis (Beyond my skill to analyze as of now):
Added weight of wax/polish to get micro surface roughness vs Precision foiled fins.

For me, the hassle of hours of sanding to the 2k grit then polish/wax doesn't have sufficient return on investment. Plus gravel and tree branches don't unfoil a fin. (unless they unfin the rocket)
 

rocket_troy

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That’s true, but the primary motivator of my point was that (for flat surfaces on the leading and trailing edges with equal slope angles): the increase in pressure on the leading edges with an increase in velocity is accompanied with an equal decrease in pressure on the trailing edges ie. There’s drag on the leading edges from static pressure + dynamic pressure and there’s also drag on the trailing edges from static pressure – dynamic pressure. That is to say, there’s less nett pressure on the base of the fins pushing the rocket in the forward direction.
Adding/cutting streamlined surfaces to these faces reduces drag via 2 different mechanisms – on the leading edges it reduces the momentum shift of the molecular collisions via reducing the directional change.
On the trailing edges, it provides more time (via distance) for molecules to impact the surface to impart their (upward directional) momentum onto the surface.
So, by this reasoning, it’s not strictly correct of me to say (or imply) the impact of modifying the leading vs trailing mods are “equally important”, nevertheless, the importance of the trailing edges can easily be understated and as Nytrunner mentioned, there are additional contributors add weight to the importance of modifying them.

Troy
 

OverTheTop

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On the trailing edges, it provides more time (via distance) for molecules to impact the surface to impart their (upward directional) momentum onto the surface.
It is actually a turbulent pocket that forms behind the square edge of the fin, effectively where the air has to fill the gap (coming in from either side) as the fin moves forward. Turbulence equates to wasted energy. The thinner the edge the smaller the patch of turbulence.
 

BDB

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And now I see a major typo....that was supposed to be 60 grit not 10.....

Now I've done it once and have the experience to say its not something I want to repeat.
Especially since I see a 4" Phoenix in my future that has ~ 6 ft of 1/4" leading edges.....


There's a guy in my club that makes fun of me for airfoiling all my fins now when I leave tape ridges on masked paintjobs.
I'm about reducing drag in some ways, but really don't care about others :eek:
For larger projects like the 4" Phoenix, you could use a router with a curved bit to round the edges. I did it with my Terrordactyl fins a few months ago. It's a far cry from a real airfoil, but it looks a lot better than a square edge.
ImageUploadedByRocketry Forum1481848300.976001.jpgImageUploadedByRocketry Forum1481848322.985695.jpgImageUploadedByRocketry Forum1481848342.339782.jpg
 

rocket_troy

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It is actually a turbulent pocket that forms behind the square edge of the fin, effectively where the air has to fill the gap (coming in from either side) as the fin moves forward. Turbulence equates to wasted energy. The thinner the edge the smaller the patch of turbulence.
Most of the effect is not turbulence, it's basic physics. Exactly the same mechanisms that provide us drag reductions in boat tails.

Troy
 

OverTheTop

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You are right with pressure as the speed goes up. Both come into play. The taper helps the flow come together from both sides with minimal loss of energy and lower aft pressure drag.
 

Nytrunner

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@BDB, I wish I had a router, but I think I'll get the apartment fully furnished before investing in shop equipment lol.

I think part of troy's point is illustrated with my rough simulation, but I want to push back on the idea that Pressure increases as flow velocity increases. The increase in pressure is the result of Deceleration of the flow as it is diverted by the fin. Since we're talking airfoils, we're in the subsonic realm of incompressibility. I have a feeling Mr Bernoulli may back me up here. We're you referring only to the dynamic pressure?

Plus, I have a hard time grasping that molecular impacts to the trailing edge of a Rocket's fin will have any practical effect. If we were talking Lift from an airplane's airfoil, I could see your point, but the streamline model is better for such cases than a particle model.
 

rocket_troy

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@BDB, I wish I had a router, but I think I'll get the apartment fully furnished before investing in shop equipment lol.

I think part of troy's point is illustrated with my rough simulation, but I want to push back on the idea that Pressure increases as flow velocity increases. The increase in pressure is the result of Deceleration of the flow as it is diverted by the fin. Since we're talking airfoils, we're in the subsonic realm of incompressibility. I have a feeling Mr Bernoulli may back me up here. We're you referring only to the dynamic pressure?

Plus, I have a hard time grasping that molecular impacts to the trailing edge of a Rocket's fin will have any practical effect. If we were talking Lift from an airplane's airfoil, I could see your point, but the streamline model is better for such cases than a particle model.
Yes I was referring to dynamic pressure.

The atmospheric pressure applied to your rocket (approximately 14.7 Psi at sea level) is from the result of molecular collisions from molecules within the atmosphere on your airframe. If stationary this is all form static pressure. So, you have molecules colliding on the base trying to push your rocket upwards, and you have molecules colliding with the top trying to push your rocket downwards. They all equal out if the rocket is stationary.
Now, let's assume flat leading and trailing edges with zero AOA on both edges (a rectangular fin) for simplicity.
Move the rocket forward at a certain velocity and the molecules are impacting your leading edges harder because the surface is also moving into them. This produces more downward force (drag) as F=ma and a (or delta v) increases with fin velocity for obvious reasons.
No arguments or confusion at this point.
Now focus on the trailing edges: again, at a stationary position there's 14.7 Psi of static pressure pushing up against these edges trying to lift the rocket or fins. This is force that assists the motor. Now accelerate the rocket to a given velocity. Coz these trailing edges are now moving away, the average velocity of the collisions is reduced by the velocity of the rocket. This results in less pressure (and in turn) less force on the trailing edges - again F=ma and a or delta v or the change in momentum from each individual molecular collision is reduced. The result of this is a *drag* force, just like the leading edges - the only difference is the leading edges are providing drag from harder collisions, and the trailing edges are producing drag from softer collisions. The nett result is the same coz for a rectangular fin with zero AOA, the increase in molecular momentum shift on the leading edges is equal to the reduction of molecular momentum shift on the trailing edges. Well, up to a point. Once you achieve a certain rocket velocity, these flat trailing edge will effectively see a vacuum and can't provide any additional drag (beyond what it already has) with any increase in rocket velocity.

Troy
 

BDB

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Good explanation. I like to think of drag as a vacuum that is created behind a moving object. The vacuum naturally tries to "suck" the object backwards. By tapering the trailing edge of a fin or an airframe we are creating a path for molecules that were displaced by the leading edge to back-fill into that vacuum space.
 

Nytrunner

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First off, much apologies to Ascastil for threadjacking. Didn't predict a surprise fluid mechanics discussion.

Alright, now I see where you're coming from. I haven't heard fluid pressure described as "particle collision" since freshman physics. As far as the physical mechanism of pressure, that's accurate. I find it easier to zoom out to the flow level for analysis and application.

I still hold the position that boundary layer reduction and streamline management have more influence than merely the presence of the varying fluid pressure (ie. particle collision) and change of momentum.
https://galileo.phys.virginia.edu/classes/311/notes/fluids2/node11.html
https://www.princeton.edu/~asmits/Bicycle_web/separation.html

The second link illustrates the turbulent vortex formation of rounded trailing edges. In my BroncBuster II thread (MPR section) someone mentioned that you'll get just as good results by simply rounding the trailing edge. I'll admit that its better than a square trailing edge, but its just trading one inefficient flow for a slightly less inefficient flow.
 

Winston

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I've been on an airfoiling kick recently because I dislike square edges facing the wind.
I do balsa fins by hand, and prefer to use a sander of some sort with 10 or lower grit on plywood fins.

I don't have a sander currently so I had to do my Leviathan's by hand. (BroncBuster II in the Mid-power forum)
Definitely was an opportunity to practice consistency. I apply thin superglue to the trailing edge in order to stiffen them up.
I start by sanding the green (~10-20 deg off vertical), then the yellow (~10-20 off horizontal), then the red (~30 off horizontal), and finish by working it into a curve. Trying to get as close to en ellipse's profile as i can by eye.
View attachment 306992View attachment 306993View attachment 306994

I offer no illusion that my foils are perfectly consistent on a fin to fin basis, but they work perfectly well for general sport flying.
And in an ideal case you may get up to 70% drag reduction! (SG force corresponds to rearwards force on the fin)
View attachment 306999
About 3D printed leading edges glued to leading edges (this was before I ditched my crappy Kickstarter M3D 3D printer):

https://www.rocketryforum.com/showthread.php?66976-Possible-product-for-3D-printer-owners

I also made a 3D printed sanding block over which was applied wet sanding sandpaper (for paper backing strength although later I just applied box packing tape to the back of regular sandpaper) allowed to fit into the elliptical grove in the sanding block whose dimensions were adjusted to take the thickness of the sandpaper into account. Bad results because the sanding process needs to be progressive, going from a low aspect ratio ellipse sanding block groove to a 2 to 1 aspect ration one (the shape with minimum subsonic drag). Still haven't tried that as I always have multiple projects in work. Plan eventually to also try that with epoxy adhered ultra-fine beach sand (from Hobby Lobby) then, if that works, some more durable fine abrasive obtained somewhere else. Main possible issue is heat buildup during sanding which can deform the PLA I prefer to use. ABS would work better temp resistance wise and more exotic filament types definitely would.
 

OverTheTop

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I still hold the position that boundary layer reduction and streamline management have more influence than merely the presence of the varying fluid pressure (ie. particle collision) and change of momentum.
I agree. Pressure is only a small component of the equation.

If you just look at pressure when analysing these sort of problems the result can be a bit simplistic. Early fluid dynamics were somewhat like this, treating the flow as irrotational and inviscid, not accounting for turbulence and viscous effects. One result of this was D'Alembert's paradox: https://en.wikipedia.org/wiki/D'Alembert's_paradox
Aerodynamic losses were far in excess of those predicted at the time. Since then tools have been developed to be able to deal with rotational and viscous layers more effectively.

Boundary layer control is one of the cornerstones of airfoil design. That's what it is mainly about, especially keeping it attached at varying angles of attack.

Interestingly, experiments have shown that a flat trailing edge (1% of chord approximately) can be beneficial in some circumstances! :facepalm:

If you want a good book check out Fundamentals of Aerodynamics by Anderson. A great read and takes you from the basics to hypersonics for a good basic understanding. Search for the title and author and include "free pdf"

I am half way through a textbook on elementary fluid dynamics currently. The more I read the more I realise it is not a very intuitive subject. It is able to be learned, but not intuitive just out of the box.


I also made a 3D printed sanding block over which was applied wet sanding sandpaper
Great idea!
 
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Nytrunner

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Interestingly, experiments have shown that a flat trailing edge (1% of chord approximately) can be beneficial in some circumstances! :facepalm:

The more I read the more I realise it is not a very intuitive subject. It is able to be learned, but not intuitive just out of the box.
I find the flat trailing edge interesting. Although it would be about .09" on my leviathan, so I'm not sure how consistent I can get that by hand :confused:.
Maybe the restriction of width still keeps the trailing stagnation point ~close enough~ to the fin to reduce losses.

You can't be more right. Basically, once calculus gets involved, I stop trying to rely on instinct. Only for simplified cases like laminar incompressible flow or Bernoulli's (after you've satisfied 4 restrictions) can I begin to say things like, "this pressure behavior makes sense" , or "I can safely say this without working it out formally".
 
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