Leading Edges on Fins Technique

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Woody's Workshop

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Long long time ago, instructions never mentioned anything about beveling or air foiling any part of the fins. (Mostly Estes & Centuri, maybe a quest. To long to remember.)
Then came along you could bevel or round over the leading edge if you wanted to, but it wan't neccessary.
After that came, it's best to round over all "But" the Root Edge. Again, but not neccessary.
Along comes "you should always round off all edges but the root edge.
Then came, you need to double bevel all edges except the root edge and trailing edge.
About the same time comes along air foiling fins like airplane wings. Or long double tapers on all but the root edges.

I don't know about anyone one else. I did a few when I was 10 or so, in a hurry didn't give a crap...Just get it ready to fly.
Little bit later on, I started to take more time and started rounding edges.
Somewhere in all this upgrading fin edge upgrades came along the need to offset leading to root edge to get the rocket to spiral to track better. All I seen was a bunch of wobble.

So I was pretty dead set on double beveling all edges but the trailing and root edges. Now I have a developed a new problem I don't know the cure to.
I could never get the bevels on both sides of the fins at the same angle. So sanded this side, then that until they almost matched angles in the middle of the fin. So I ended up with fins, all of a little bit different size. Which didn't work out well like the making the rocket spiral.

For years I have just been beveling "One" side of the fins edges but the trailing and root edges.
For the most part, the fins remain the same size, along with the angles, the rocket ever so gently gets a very slow spiral counter clockwise and flights have been pretty straight. Except the one's like the Berthas that like to weather cock into the wind. Especially staging the SBB!

So I'm curious how many out there only bevel one edge, those that do double bevel, how do you keep the angles and fin size the same with the point in the middle of the fin; or just do a round over and don't worry about it? Or whom goes to the extreme of air foiling fins. Some kits are made for long double bevels, or single bevels. Even air foiling. But I mean on every build.

Now take in consideration all I've done since 1970 is LPR. That is why I am curious because the next few builds in line are going to MPR.
Mostly 29mm MM's. Maybe a 38mm MM. I have to find out what I can fly being a non-certified flyer, or even purchase. (Later questions on that)

Just wondering what everyone else does?

Thanks for taking the time to share your thoughts and techniques.
 
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I bevel all my fin edges. Just for looks, nothing to do with aerodynamics. I didn't bevel the fins on my Batray because it would have been difficult since all edges are curves. Certainly for MPR, it's not necessary at all. It does affect performance, but not necessary.
 
I radius the front edge of the fin rather than bevel. Rounded shapes are less problematic to wind shear. I don't think there is much to gain on the back edge as the motor is filling in the low pressure area behind.
 
I pretty much just round the leading and trailing edges over, and don't worry about trying to get a bevel or airfoil, except on the 2 gliders I've done. (Well one done, one started.)

The MPR/HPR rocket I've flown the most is a crayon bank with squared off plywood fins. No rounding or beveling. The other 2 MPR birds I have, one has rounded edges (no bevel) and the other is squared off g10. (I didn't want to bother trying to sand / shape fiberglass at time.) I did bevel the fowrad canards on my L1 cert Kraken, but the fins thenselves are tubes, so no beveling there either.

I do remember more than one thread on "beveling jigs" here in the forum to help folks get even bevels. If you're curious about those, you could probably search for them pretty easily.

I guess I'm very much in the "why bother" camp. I'm also not in the "high performance, punch a giant hole in the sky, bust mach, and look great doing it" camp. :) That being said, I do have plans to build a higher performance rocket next year (maybe.) and it will have beveled fins.
 
There is a method you can use to sand in bevels pretty accurately.

Get a heavy piece of plywood which is flat and a fair bit bigger than your fin. Get another somewhat smaller piece.
Spray the smaller piece with a light coat of 3M77. Ditto the back side of a piece of open grit 220 sandpaper (most likely the best choice) let them dry up a little then stick them together. Now you have a nice big sanding block.

Take your big piece of plywood and measure back from a straight edge some distance - perhaps half an inch, perhaps an inch. It depends on the bevel you want to achieve.

Now take your fin blank and tape it down or lightly stick it down or somehow make it stay on the large plywood plate. You want the edge you are going to bevel lined up right at that line. That is, the fin does not extend to the edge of the plywood but is instead parallel to an edge and set back a bit.

Next mark out (gently) a second line. This one goes on the fin itself, back from the edge you want to bevel. This distance should be a little less than the distance the fin is set back from the edge of the plywood.

Take a strip of masking tape and place it on the fin, masking off the area behind where the bevel is desired. One can make this strip longer than the fin, and use it to take the fin down to the large plywood block.

Now take the plywood sanding block, and sand away. Be gentle about it, and patient. Once the masking tape starts to wear at the edge, replace the tape. Once sanding is starting to wear the tape and touching the edge of the plywood base block, then that side of the bevel is done.

One has to be more gentle on the second side of the bevel as it is not supported. Or, one can make a wedge of balsa into a support. Up to you.

Or one could flip the fin, put tape on the fin like it was done for the other half, but put the bevel down. This time, just set the edge back a small distance from the edge of the plywood. Now the bevel is supported but you'll have to determine the setback distance required to produce a fully symmetric bevel.

Gerald
 
After lots of practice, I can hand bevel with a belt sander matching bevels faster than setting up a jig. On rounded fins like our Excel, I just do a nice round over with the random orbital sander and call it good.
 
I've previously run simulation on various bevels for the same rocket using RASAero. For the rocket in question pretty much any bevel would give at least a 30% increase in altitude over a square fin. A good bevel is more like 35% to 38%.
 
There is a method you can use to sand in bevels pretty accurately.

Get a heavy piece of plywood which is flat and a fair bit bigger than your fin. Get another somewhat smaller piece.
Spray the smaller piece with a light coat of 3M77. Ditto the back side of a piece of open grit 220 sandpaper (most likely the best choice) let them dry up a little then stick them together. Now you have a nice big sanding block.

Take your big piece of plywood and measure back from a straight edge some distance - perhaps half an inch, perhaps an inch. It depends on the bevel you want to achieve.

Now take your fin blank and tape it down or lightly stick it down or somehow make it stay on the large plywood plate. You want the edge you are going to bevel lined up right at that line. That is, the fin does not extend to the edge of the plywood but is instead parallel to an edge and set back a bit.


This description is kind'a close to what is going on with the Brady Bevel-Matic. It makes bevels extremely well.
 
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Been hand rounding the Leading and tailing edges of All my sport flying models since the 70's.
For better Aerodynamics Do NOT round fin tips. Leave them square to lessen the tip vortex drag created by rounding. Tapering to Root edge to tip can add a little altitude to your models but takes a lot longer to produce accurately.

Save the symmetric airfoiled and/or tapered fins for High performance competition models where the extra 2 to 5% altitude gain can be worth the extra time involved sanding. It is these High performance competition models that would be the only models needing a super accurate sanding jig.

For General sport flying no-one will tell the difference between a rounded leading and tailing edge model and one with symmetric airfolded Fins. There is however a pretty good visual difference between just rounding these leading and tailing edges and leaving them square which creates a fairly heafty addition to the drag on the model. Not as evident in HPR models but man can you tell with 1/4A through G motor models.
 
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I was looking at the same issues a week or so ago...back in the late 70's / early 80's I remember picking up somewhere the idea of sanding the fins in a "light" airfoil (maybe that should be "lite" airfoil, as in "Lite" beer).

Reading the current Estes kit instructions, there really is no mention of anything, so I was looking stuff up.

Apogee's Peak of Flight newsletters had this:

short article on both fin shape and airfoil --> https://www.apogeerockets.com/technical_publication_16

long article specifically on airfoil shape -- https://www.apogeerockets.com/downloads/Newsletter305.pdf

I'm still working my way through the longer article, but my take so far is, rounding the edges of the fin is better than leaving them square, and you don't have to sand too much to get a benefit. Of course, having a perfect airfoil would be best, but for most people flying for enjoyment, the amount of work is really overkill for the performance/stability gain. Most likely sanding *evenly* is what is desirable.

Still working may way through those references, however...maybe I'll change my mind...
 
I round most of my leading edges. On my NASA Pegasus BT-70 upscale, the fins were left square, as per the original instructions, and it looks OK. When I do the 4" upscale, I'll probably just ease the edges a bit...

Chris
 
Here is my technique:

1) Color the edges (other than root) with a marker
2) Bevel one side of the fin until a bit less than half of the edge color has been sanded away
3) Bevel the other side until you are left with just a thin line of color, which should be very near to the center of the thickness
4) Lightly round over to remove the thin line of color and the sharp transition between the bevel and the flat side of the fin

You can make the bevel very steep, almost equivalent to rounding, or very shallow, a good start on an actual airfoil.
 
Has ANYBODY ever read G. Harry Stine's "Handbook of Model Rocketry"... He covers it extremely well in there.

Leaving all the fin edges square will fly fine, but have a LOT more drag than is necessary. If you're not going for maximum altitude on a given motor, but just ease of construction and sport flying, that's fine... leave 'em square... (course experienced craftsmen rocketeers reserve the right to snicker about your rockets after you go home... LOL:))

Rounding over the leading and trailing edges reduces drag considerably, (better than 50% IIRC of square edged fins) and is pretty easy and straightforward to do... just knock the corners off the leading and trailing edges with a little sandpaper and elbow grease. Doesn't have to be perfect. Less drag=higher altitude on a given motor.

Maximum drag reduction comes (for sub-Mach rockets) with a rounded leading edge (which is easy to do, remember) and a tapered trailing edge, to get a more "teardrop" profile. Tapering the trailing edge DOES take some craftsmanship and a little work, certainly more than just "rounding them over" does... but for maximum altitude and minimum drag, that's what you need to do. You have to decide if the reduction in drag and higher altitude is worth the work, though, I guess... BUT, you can probably reduce the drag another 20-25% over squared off fins cut straight from the balsa sheet...

Sanding the LEADING edge into a taper along with the trailing edge actually doesn't reduce drag and in fact can increase it. While there might be SLIGHT reductions in drag from a perfectly proportioned "convex" shape to the fin, with the leading edge sanded to a knife-edge, curving back smoothly to the "mid-chord" area of the fin, and then tapering back again to a knife-edge at the trailing edge of the fin, the performance gained would be "down in the noise" and virtually undetectable (motor variations, wind, weather, launch angle, etc. would have FAR more influence on the flight than any miniscule drag reductions from such fins...) Plus, they'd be INCREDIBLY hard to make and CONSISTENTLY make the same shape between ALL the fins (it's usually irregularities from one fin to another that induces spiraling, spinning, and other such things... and spin induces drag and costs altitude itself). So the thing is, WHY BOTHER?? So you can gain another 2-4% drag reduction (AT MOST, IF you get them "perfect")??

Wedge and double-wedge fins commonly used on real rockets like the Saturn V, Saturn Ib, various military missiles, etc. LOOK cool, but are designed to achieve their best efficiency at sustained supersonic velocities, sometimes at multiple Mach numbers. Hobby rockets, at BEST, only achieve supersonic velocities for a few seconds (in most cases) and model rockets RARELY break the sound barrier, and even then only for a fraction of a second to a second or so at most. The VAST majority of hobby rockets never come close to the speed of sound, and fin shapes for supersonic flight are draggy and inefficient at subsonic velocities. SO, the ONLY reason to include such fin cross-sections on model rockets is for aesthetic reasons-- for scale, or scale-like rockets, where the desire for it to LOOK like the real thing is more important than the additional drag created by the wedge or double-wedge shape. I've put wedge fins on Saturn V's and IB's before, but simply because they don't REALLY "look right" without them... and just ate the additional drag. Scalers and semi-scale rockets are NOT designed with maximum altitude, speed, or acceleration in mind anyway... or shouldn't be!

Oh, and all this applies STRICTLY to the LEADING EDGE and TRAILING EDGE. OBVIOUSLY, one should NEVER sand the root edge down to a point or round it over-- how will you get the fin attached solidly and straight like that? BTW, having the fins glued on as ABSOLUTELY STRAIGHT AS POSSIBLE will reduce spiraling and corkscrewing/wobbling more than anything-- the fins need to be as perfectly aligned as possible not only down the long axis of the tube (not "twisted" on the tube) but ALSO sticking out at 90 degrees to the tube surface when viewed from directly above and behind... if the fin is "cocked" or "crooked" and its centerline is not aligned with the center point of the tube, it will tend to convert some of its "lift" in correcting the flight into rotational energy, causing the rocket to start spinning and corkscrewing in flight. This, again, wastes energy that COULD have been used to gain altitude, and thus lowers the peak altitude achieved...

ALSO, the TIP EDGE should remain SQUARE, or be sanded down to a knife edge. Rounding the TIP edge of the fin furthest from the tube induces and magnifies fin tip vortices which create additional drag and lower altitude. Squared off or knife-edge fin tips help inhibit (slow down) the airflow around the fin tip from the "high pressure side" of the fin (at an angle of attack, the 'windward' side) to the "low pressure side" (the opposite or "back side" of the fin at an angle of attack) thus reducing the size, strength, and persistence of the tip vortices created, thus lowering drag. Since we've already established that creating a "knife edge" fin edge requires substantial effort and craftsmanship and attention to getting them all as IDENTICAL AS POSSIBLE between fins, then the EASIEST (and most efficient aerodynamically) is to leave the fin tip edge SQUARE AS WHEN IT WAS CUT FROM THE SHEET BALSA.

Now, there is SOME aerodynamic gains to be had in tapering the fin from the root edge out to the tip edge, (making the fin thicker at the root edge glued to the tube, and thinner at the tip edge farthest from the tube) but again, this is for reducing drag to ABSOLUTE MINIMUM to wring every drop of altitude possible out of a given motor... for 99.5% of the rockets out there, it's not worth the time or effort. The gains are there, but small, and require substantial effort to achieve. MUCH easier to go for "lower hanging fruit" with a MUCH higher 'bang for the buck' reward ratio...

In short, in order:

Square-edge fins fly fine, but with high drag.
Rounded leading/trailing edges cut drag a WHOLE bunch.
Rounded leading edge, tapered trailing edge cuts drag a bit more than just rounding the edges.
"Convex" shapes where the whole fin is rounded isn't worth the effort except in extreme cases.
Thinning the fins from root to tip isn't worth the effort except in extreme cases.
Wedge and double-wedge missile shaped fins are draggier than rounded fins, but good for scalers.
Tip edges of the fins should be left SQUARE to reduce vortex induced drag.

That's pretty much it.

On NORMAL builds, I usually just round the leading edge, and taper the back quarter inch of the trailing edge... pretty easy to do and really cuts the drag. Do what works best for you, though!

Later and good luck! OL JR :)
 
So basically you can wad all of these tips in a ball, like a chunk of Play-Doh, and flatten it out. What you get is, round everything off except for the root edge. I also round fin tips just for the uniformity. Makes for an evenly distributed look. Round it all off. No harm done and no one else is going to critique that on the field. Rounded off is indeed less draggier than flat. Bevels....meh.
 
I have another reason besides drag reduction for airfoiling fins--I want them to be reasonably effective as lifting surfaces. (I know the fins are not expected to produce lift in the airplane sense, but hear me out.)

We know that at higher angles of attack (as when launching in the wind), the CP will move forward. If it moves to or beyond the CG, the rocket goes unstable. However, the fins will also produce a lifting force that helps the rocket weathercock into the wind, thus restoring angle-of-attack to zero.

If the fins are airfoiled, they will produce lift at higher angles of attack than a squared-off slab will (no sharp corners to induce a stall). That gives you some extra insurance when launching on a breezy day, when the rocket may find itself at a high angle of attack when leaving the rod/rail.
 
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I got a nice shape on the fins of my 3rd or 4th build, the Estes Cosmic Explorer. Trouble is, it was nearly by accident that I got them all nearly exactly alike, and I'm not sure I could repeat it the way I did it.

Nonetheless, this is the rocket in my fleet which barely rolls at all during flight.

IMG_1693.jpgIMG_1732.jpgIMG_1735.jpg

In the launch video, especially the slo-mo at the end, you can see that it barely spins at all in the roll axis:

[video=youtube_share;Ys8giDCMDfk]https://youtu.be/Ys8giDCMDfk[/video]

I think I've recently come up with a fairly simple way to get a nearly consistent bevel on fin edges for small, balsa/basswood fins on small LPR rockets. So, what I do now, for the most part, is to round the leading edges, bevel the trailing edges, and leave the tip (and obviously root) edges square. I'll try to get my pictures organized and share what I've come up with, but I have a lot of pictures to go through.

Here are a couple from my first scratch build - just the trailing edges in these pictures. You can see that they're not quite perfect, but pretty darn close:

IMG_2011.jpgIMG_2013.jpg

But for larger plywood fins, John Coker has a tutorial on his website where he built a sanding jig that works great. I don't have the tools to build it, but I'm going to try to do it when I get my hands on the right tools.
 
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So basically you can wad all of these tips in a ball, like a chunk of Play-Doh, and flatten it out. What you get is, round everything off except for the root edge. I also round fin tips just for the uniformity. Makes for an evenly distributed look. Round it all off. No harm done and no one else is going to critique that on the field. Rounded off is indeed less draggier than flat. Bevels....meh.

Sorry Gary:
I'll second Ol JR's question: Have you guys never hear or used G. Harry Stine's Handbook of model Rocketry? This subject is covered in detail in each and every edition!
You missed the part about NOT rounding either the root or tip edges to reduce the vortex drag created when you do round the TIP edge.
fins look very uniform without rounding the tips.
 
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Sorry Gary:
I'll second Ol JR's question: Have you guys never hear or used G. Harry Stine's Handbook of model Rocketry? This subject is covered in detail in each and every edition!
You missed the part about NOT rounding either the root or tip edges to reduce the vortex drag created when you do round the TIP edge.
fins look very uniform without rounding the tips.

Well Micro, I honestly have never read GHS's book(s). I had to learn it by trial and error. I know there are better ways to do rocketry than the way I do it, but that's not as important to me as, building, designing and flying stable rockets. Of course you don't round off root edges! What were you thinking????!!! Getting the absolute highest altitude through design, is not important to me either. I do what works for me best. Just because GHS suggests keeping your fin tips flat, does not make it "LAW".

Furthermore, my designs are built around the drag factor. So, if my approach to rounding fins aids in that, it's working. I don't approach rocketry as a "paint by numbers" building method. All I have to do is tweak a design until it looks stable to me. This method may sound way out of the box for most rocketeers, but I grew up, doing it this way.

Can you give a percentage of how many people on this forum, fly their own designs?
 
I've previously run simulation on various bevels for the same rocket using RASAero. For the rocket in question pretty much any bevel would give at least a 30% increase in altitude over a square fin. A good bevel is more like 35% to 38%.

Do you mean degrees? Or % of the total fin length?
I still have my micro planes, which would work well on a jig.
 
I got a nice shape on the fins of my 3rd or 4th build, the Estes Cosmic Explorer. Trouble is, it was nearly by accident that I got them all nearly exactly alike, and I'm not sure I could repeat it the way I did it.

Nonetheless, this is the rocket in my fleet which barely rolls at all during flight.

View attachment 248838View attachment 248839View attachment 248840

In the launch video, especially the slo-mo at the end, you can see that it barely spins at all in the roll axis:

[video=youtube_share;Ys8giDCMDfk]https://youtu.be/Ys8giDCMDfk[/video]

I think I've recently come up with a fairly simple way to get a nearly consistent bevel on fin edges for small, balsa/basswood fins on small LPR rockets. So, what I do now, for the most part, is to round the leading edges, bevel the trailing edges, and leave the tip (and obviously root) edges square. I'll try to get my pictures organized and share what I've come up with, but I have a lot of pictures to go through.

Here are a couple from my first scratch build - just the trailing edges in these pictures. You can see that they're not quite perfect, but pretty darn close:

View attachment 248844View attachment 248845

But for larger plywood fins, John Coker has a tutorial on his website where he built a sanding jig that works great. I don't have the tools to build it, but I'm going to try to do it when I get my hands on the right tools.

Looking at Cokers site, brings tears to my eyes yet again that I lost my wood shop. Woodworking and Rocketry is the only things I've done in my life that I've hung onto. My talents are just wasting away in this little room of this Apartment.
 
Well Micro, I honestly have never read GHS's book(s). I had to learn it by trial and error. I know there are better ways to do rocketry than the way I do it, but that's not as important to me as, building, designing and flying stable rockets. Of course you don't round off root edges! What were you thinking????!!! Getting the absolute highest altitude through design, is not important to me either. I do what works for me best. Just because GHS suggests keeping your fin tips flat, does not make it "LAW".

Furthermore, my designs are built around the drag factor. So, if my approach to rounding fins aids in that, it's working. I don't approach rocketry as a "paint by numbers" building method. All I have to do is tweak a design until it looks stable to me. This method may sound way out of the box for most rocketeers, but I grew up, doing it this way.

Can you give a percentage of how many people on this forum, fly their own designs?

Well, folks can do it their own way, that's cool....

Thing is, though, information is either correct or incorrect... regardless of who says it. Yeah, just because "G. Harry sez so!" doesn't make it "law", nor does it make it incorrect, either. Just like if you or I say something different... it's either correct or incorrect.

Here's the correct information. Rounding the fin tips increases drag, because it allows the air to more easily "round the tip" of the fin from the "high pressure side" to the "low pressure side", whereas a square tip or sharp-edged tip helps impeded the airflow over the tip. The easier the air spills over the tip, the more induced vortex drag. Pretty simple. It's not a matter of opinion or conjecture or "someone who wrote a book said it so its "law" or something... it's been thoroughly researched and documented over the years in aeronautical research.

Cropdusters typically have simple squared-off wingtips for this exact reason. Commercial jets usually have had "sharp ends" to reduce vortex shedding. NEW jets usually have upturned "mini- wingtips" that help minimize wingtip vortex shedding (which aircraft shed CONTINUOUSLY from the wingtips due to the shape of the wing producing continuous lift to hold up the plane). Ever watch "Die Hard 2"?? As McClane is running down the runway after blowing up the bad guys jet, as the other jets are coming in for a landing, you can see the STRONG vortex shed by a wingtip of a landing jet as the wingtip cuts through the pall of smoke. Same thing with rockets.

Anyway, if you WANT rounded tips, that's on you... it's all good. BUT, "just because you've done it that way forever" doesn't mean it's right, or the best way. In fact it produces more drag. If that's less important to you than having rounded tips, that's fine. BUT, don't confuse newbs or others who might think that doing it that way is going to be better for performance, because as a matter of fact, it is NOT.

Whatever floats yer boat... :) OL JR :)
 
Yup as someone who's taken Aerodynamics what has been said above about tip vorticies is correct. There are many ways to mitigate the lift loss, of the high pressure air spilling over the wing tip to the low pressure side, however the vortices will always form somewhere. Winglets, racked tips like the 787, end plates, flow fences, even tip tanks to some degree, etc. all are viable ways to control where the vortices start from. Squared off ends work because fluid flow has a very difficult time with right angles, there were actually be a small regions of "dead air" at the wing tip that will extend the center of the vortex just off the end of the wing.

https://en.wikipedia.org/wiki/Wingtip_device

As for me I usually bevel my TE's and have been know to round or bevel my LE's.
 
Yup as someone who's taken Aerodynamics what has been said above about tip vorticies is correct. There are many ways to mitigate the lift loss, of the high pressure air spilling over the wing tip to the low pressure side, however the vortices will always form somewhere. Winglets, racked tips like the 787, end plates, flow fences, even tip tanks to some degree, etc. all are viable ways to control where the vortices start from. Squared off ends work because fluid flow has a very difficult time with right angles, there were actually be a small regions of "dead air" at the wing tip that will extend the center of the vortex just off the end of the wing.

https://en.wikipedia.org/wiki/Wingtip_device

As for me I usually bevel my TE's and have been know to round or bevel my LE's.

Yep, Stine says as much in HBOMR...

Quoting from memory here, but paraphrased "to this I hear a loud, young voice saying "well, why not just put a plate across the end of the fin, to prevent the air from spilling over". This isn't a solution, because the air will just spill over the ends of the plates, forming two vortices instead of one, and increasing the interference drag at the plate/tip joint."

You can REDUCE the vortices with things like the wingtip winglets as mentioned (on aircraft lift wings) but you cannot ELIMINATE it... minimization will have to do... :)

Later! OL JR :)
 
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