Rounded fin edges out of fashion??

[BEC]

In the three and a half months or so of my having become a BAR I've noticed something that makes me wonder a little. I see many pictures of rockets here, on makers' web sites and elsewhere that have square-edged fins. Has trying to reduce drag by rounding leading edges and tips, or even shaping fins to a thin symmetrical airfoil section fallen out of favor, or has someone just demonstrated in the past 35 years that it's wasted effort?

In the few model rockets I've built in these past few months I haven't been able to bring myself to leave LEs in particular square, I had to round them. I've generally left trailing edges square because I know that TEs on small model airplanes also can be left square at no real performance loss...even though the fly rather more slowly.

So....is it building simplicity, or aerodynamic research, or what that has led to this trend of square fin edges?

[Der Red Max]

...what that has led to this trend of square fin edges?Laziness!
(at least in my case) :chuckle:

[Leo]

You are not the only one that noticed it.

I'm rather disappointed that some real nice builds are "tarnished" because the leading edges have been left square. I'm sure in most cases the instructions state otherwise.

[Rocketflyer]

In the three and a half months or so of my having become a BAR I've noticed something that makes me wonder a little. I see many pictures of rockets here, on makers' web sites and elsewhere that have square-edged fins. Has trying to reduce drag by rounding leading edges and tips, or even shaping fins to a thin symmetrical airfoil section fallen out of favor, or has someone just demonstrated in the past 35 years that it's wasted effort?

In the few model rockets I've built in these past few months I haven't been able to bring myself to leave LEs in particular square, I had to round them. I've generally left trailing edges square because I know that TEs on small model airplanes also can be left square at no real performance loss...even though the fly rather more slowly.

So....is it building simplicity, or aerodynamic research, or what that has led to this trend of square fin edges?


Good question. I too have noticed this. I can't leave mine squared. At the minimum they have to be rounded and somewhat tapered at the trailing edge. I dislike sanding, but will do it on the fins.

[motley16]

I have also noticed the trend. Lazy or just different to be different. Not sure. I follow instructions and at times still round the leading edges. Do what YOU like and feel looks best! :)

[Nuke Rocketeer]

I have noticed that too. I think it is laziness

I round and taper the fins on all my rockets, unless there is a scale reason not to. I just have to get that lst bit of efficiency out of it, or the engineer in me will nag me awake at 3AM.

[jetlag]

I have noticed that too. I think it is laziness

I round and taper the fins on all my rockets, unless there is a scale reason not to. I just have to get that lst bit of efficiency out of it, or the engineer in me will nag me awake at 3AM.

Plus, it looks so much nicer when all is filled and finished. Though when I was a tyke, I never filled the grain and spent little time rounding; just get 'er finished by the weekend!
Allen

[GregGleason]

Fin symmetry has been a pet issue for me for last couple of years. On my latest mid-power build, I bought some carbon rod with the same OD as the fin thickness and glued it to the square leading and trailing edge, then filled in the gap with a mix of epoxy and microballoons for ease of sanding. The nice thing about carbon rod is that it is uniformly round and straight, along with being dent/ding resistant. It turned out pretty well, too.

My dream is to get my own CNC machine and mill my fins to +/- 0.01 inch tolerances. That way I can get those nice contours for V2 fins or multiple faceting for Astrobee D fins. Now as to how I plan to seal/paint to keep those tolerances … :eek:

Greg

[Jerry Irvine]

The actual optimum shape for strength and performance is ogive on the fins. Both leading and trailing. Especially trailing.

Just Jerry

http://v-serv.com/usr/pubs/air10.htm

http://v-serv.com/usr/kits/micro-sentraSRB.htm

[Doug Sams]

I've generally left trailing edges square because I know that TEs on small model airplanes also can be left square at no real performance loss...even though the fly rather more slowly.My understanding is the opposite - the trailing edge affects drag more than the leading edge. While the blunt leading edge certainly presents more drag than a taper, the blunt trailing edge causes swirling airflow that significantly adds to the drag.

That said, I'm guilty of squaring mine off. Trying to recall why I went to that, I believe it's because I wanted the fins to be stronger and less susceptible to damage along the edges.

At one time, I would not only round the edges, but put aggressive tapers on them. The result was fins that were very thin over much of their surfaces which left them weaker and more prone to landing damage. Also, somewhere I learned that clipped tips were more aerodynamic than rounded (or elliptical) tips which tend to cause swirling airflow and thus drag. Once I started leaving the tips alone, it was not long before I began leaving the entire fin completely square along the edges.

About that time, it also occurred to me that, since I'm not a competitor, I didn't care about performance, so there was really little motivation for airfoiling anyway, and skipping that step saved some time and kept some dust out of my lungs http://forums.rocketshoppe.com/images/smilies/smile.gif

OTOH, I hate blunt launch lugs - they make me cringe. So I always bevel the leading and trailing ends of the lugs to make them look more aero. I don't really care if they are more aero, but they gotta look like it http://forums.rocketshoppe.com/images/smilies/smile.gif

Here's a pic which illustrates my point about the lugs. Doug

http://www.doug79.com/haulin-mail/HaulinMail1-4.jpg


.

[BEC]

My understanding is the opposite - the trailing edge affects drag more than the leading edge. While the blunt leading edge certainly presents more drag than a taper, the blunt trailing edge causes swirling airflow that significantly adds to the drag.



This depends on the Reynolds number regime you're in and is in part why I posed the question. For a small model airplane - say 6-8 inch wing chord and flying between 15 and, say, 45 mph, a 1/8 inch thick and squared off trailing edge is lower drag than thinning to a minimum edge or simply rounding it - well-defined edges, just as you want on the step of a float for a float plane, turn out to be lowest drag. This is based on tests done by a modeling friend of mine who is also a designer and a test engineer at Grumman, Tom Hunt. And, as you noted, it's also far less fragile.

But a model rocket has a lower chord typically and flies rather faster (at least in part of the flight) so the Reynolds numbers are different and the squared-off trailing edge may or may not be better.

I have certainly noticed apparently superior performance on my old Astron Constellation where I followed the instructions 30-odd years ago and shaped the fins to a nice symmetrical airfoil shape than I see with a similar weight rocket on a similar motor that has squarish fins.

I guess some research is in order.

The considerations of handling and landing damage, as well as the aforementioned "laziness" or just not wanting to deal with sanding dust are also practical considerations where maximum performance isn't needed..... design (and building) of flying machines is always a series of tradeoffs. That's part of what makes it so much fun.

[cas2047]

So....is it building simplicity, or aerodynamic research, or what that has led to this trend of square fin edges?

Well you shamed me into it. I was wrapping up a Q-Modeling Mega-Viper build and hadn't planned on tapering the fin edges. I'd actually already applied a coat of gloss white paint, but it didn't go on well so I'm sanding that off.

I always defer to the instructions, and when the square option is given I sometimes go the path of least resistance. The Mega-Viper instructions give you the option of leaving the fins square or sanding them down. I had elected to leave them square, but I've now changed my mind.

After reading your post I've decided to do the right thing. The first fin is now tapered and looks great. Seven more to go!

Thanks! :D

Frank

[Nuke Rocketeer]

This depends on the Reynolds number regime you're in and is in part why I posed the question. For a small model airplane - say 6-8 inch wing chord and flying between 15 and, say, 45 mph, a 1/8 inch thick and squared off trailing edge is lower drag than thinning to a minimum edge or simply rounding it - well-defined edges, just as you want on the step of a float for a float plane, turn out to be lowest drag. This is based on tests done by a modeling friend of mine who is also a designer and a test engineer at Grumman, Tom Hunt. And, as you noted, it's also far less fragile.

But a model rocket has a lower chord typically and flies rather faster (at least in part of the flight) so the Reynolds numbers are different and the squared-off trailing edge may or may not be better.

I have certainly noticed apparently superior performance on my old Astron Constellation where I followed the instructions 30-odd years ago and shaped the fins to a nice symmetrical airfoil shape than I see with a similar weight rocket on a similar motor that has squarish fins.

I guess some research is in order.

The considerations of handling and landing damage, as well as the aforementioned "laziness" or just not wanting to deal with sanding dust are also practical considerations where maximum performance isn't needed..... design (and building) of flying machines is always a series of tradeoffs. That's part of what makes it so much fun.

A few years ago I used to help my kids' elementary school 6th grade do a science priject using model rockets every year. One year one of the kids questioned why he needed to airfoil the fins (We were building Alphas). I told him why, and proposed that half the class airfoil the fins and half leave them alone. We measured the altitude the rockets got on A8-3 engines and the airfoiled Alphas got significantly better alitude than the square edge Alphas.

[Doug Sams]

But a model rocket has a lower chord typically and flies rather faster (at least in part of the flight) so the Reynolds numbers are different and the squared-off trailing edge may or may not be better.Good points. I didn't think about the chord length much. (swag) I can see where, with a short chord, the divergent airflow at the leading edge is well past the trailing edge before it recombines, so there's gonna be some swirl no matter what the shape of the trailing edge. Sound about right?

Doug

.

[Bill]

In my first life as a rocketeer, I used to taper the trailing edge and round the leading and tip edges. After getting tired of the damage to trailing edge, I would round all but the root edge.

Now, I round the leading and trailing edges and the tip edge only when it is not generally parallel with the root.


Bill

[BEC]

A few years ago I used to help my kids' elementary school 6th grade do a science priject using model rockets every year. One year one of the kids questioned why he needed to airfoil the fins (We were building Alphas). I told him why, and proposed that half the class airfoil the fins and half leave them alone. We measured the altitude the rockets got on A8-3 engines and the airfoiled Alphas got significantly better alitude than the square edge Alphas.

That's great! Now I don't have to have some students perform this experiment.

Actually, my daughter and I have both built FlisKits Triskelions and she left hers square and I did rounded LEs on the fins and the tip plates (but not tapered TEs). We plan to have a few drag races to see how they compare after we get 'em painted.

[BEC]

In my first life as a rocketeer, I used to taper the trailing edge and round the leading and tip edges. After getting tired of the damage to trailing edge, I would round all but the root edge.

Now, I round the leading and trailing edges and the tip edge only when it is not generally parallel with the root.


Bill

Which goes to show that the lovely little "pods" at the tips of the Astron Constellation's fins serve a function - to protect the tips of the airfoiled fins from breakage on landing :D.

[tbzep]

One of the tricks used for Estes/Centuri catalog models to look really good in photos was to leave the edges square.

I shape the fins on some rockets, and leave others square. Like Doug, I don't compete, so I don't give a hill of beans about the drag. Some fins look just fine square, such as the Big Bertha. Others need some shaping in a big way, such as the Astron Sprint. In my latest batch of builds, I have left some square, some rounded, and a few airfoiled.

[Tau Zero]

While the blunt leading edge certainly presents more drag than a taper,
[SNIP]
That said, I'm guilty of squaring mine off. Trying to recall why I went to that, I believe it's because I wanted the fins to be stronger and less susceptible to damage along the edges.I noticed in the 3rd version of Stine's Handbook of Model Rocketry (it's what my public library has :rolleyes: ) that rounded edges cause more drag than square ones, :eek: BUT airfoiled fins have the least amount of drag.

So when it comes to resistance, ranked from least to most, this is *apparently* how fin edges stack up:

1. Airfoiled
2. Square
3. Round


But yeah, it's because I'm lazy. *And* flat edges are easier to do if I've already attached computer label paper to the fins instead of using wood filler. Again, laziness triumphs. :o

As always, your mileage (or airspeed) may vary,

[scigs30]

I only build Estes kits and try to make the build look like the picture in the catalog. I notice a lot of times the directions say to sand LE and TE round, but the picture on the package shows square. My last couple of builds have been square, such as the Astron Scrambler and the 4 new Estes kits. Once again I was copying what I see on the picture. My Blue Bird Zero that I have completed painting today has rounded edges just like the picture. I don't fly for competition and my rockets go into the air just fine round or square.

[BEC]

Jay,

Hmmmmmm...... I have the most recent addtion of the Stine manual - 8th edition(?) (one of my BAR purchases)....I should've looked there! :eek: If rounded LEs only are worse than square then I have a Quest Astra (ready for fill and paint) and my Triskelion (in primer) and a FlisKits Whatchamacallit and dooHickey, Semroc Boid and an original (all in flying status) that are all set up "worst of the three". :eek: again.

scigs30,

I've started on a Star Stryker and noticed that the instructions say to round fin LEs but the pictures show square. Starting that build is what prompted me to finally "vocalize" this question after wondering about it for some weeks.

[Jerry Irvine]

I noticed in the 3rd version of Stine's Handbook of Model Rocketry (it's what my public library has :rolleyes: ) that rounded edges cause more drag than square ones, :eek: BUT airfoiled fins have the least amount of drag.

So when it comes to resistance, ranked from least to most, this is *apparently* how fin edges stack up:

1. Airfoiled
2. Square
3. Round


But yeah, it's because I'm lazy. *And* flat edges are easier to do if I've already attached computer label paper to the fins instead of using wood filler. Again, laziness triumphs. :o

As always, your mileage (or airspeed) may vary,

If it really says that then it is wrong.

Airfoiled, rounded, square.

I spent a year doing wind tunnel tests. Also I refer you to numerous reports published by the MIT Roc Soc.

Jerry

[BEC]

Anyplace those tunnel test results were published (kind of like Mike Selig's model airplane Reynolds number airfoil work in the Soartech journals, perhaps)?

[GregGleason]

If it really says that then it is wrong.

Airfoiled, rounded, square.


Jerry

I believe Jerry is correct.

In my copy of G.H. Stine's 7th edition it mentions that a square fin tip, not leading edge, will have a lower induced drag than a round one.

Fin optimization for a given airspeed would take a LOT of CPU time. There a lot of variables that come into play:


fin geometry
airframe OD
airspeed


Greg

[Jerry Irvine]

I believe Jerry is correct.

In my copy of G.H. Stine's 7th edition it mentions that a square fin tip, not leading edge, will have a lower induced drag than a round one.

A rounded fin tip promotes vortex formation due to the ease of pressure differential equalization from surface to surface, and thus vortex drag which is induced drag since it is only attached to the body by a boundry layer and flow field.

A square edge reduces vortex drag, and an airfoiled tip makes the VD considerably lower and also lowers frontal area.

Fins like 2250 and Goblin with intentionally downward shaped, flat edges are designed to reduce vortex drag by sucking the vortex more into the wake of the body itself.

Airplanes use anti-vortex tips like little tails to break up most of the vortex power, but ask any pilot who has gotten too close behind a large plane, vortecies are still there and strong.

Just tech Jerry

[Bill]

One of the tricks used for Estes/Centuri catalog models to look really good in photos was to leave the edges square.


Is that because square edges stand out better in pictures, thus highlighting the fin shape?

I had always thought it was easier to make round edges look perfect whereas slightly unsquare (slanted) edges are easily noticed.


Bill

[o1d_dude]

I round all the edges but don't bother with tapering the trailing edges.

Save that effort for my Hand Launch and Catapult Launch Gliders. =)

[snaquin]

Fins like 2250 and Goblin with intentionally downward shaped, flat edges are designed to reduce vortex drag by sucking the vortex more into the wake of the body itself.

Just tech Jerry

Jerry,

I noticed many of the early sounding rockets seemed to favor this design. Mark Mayhle used this in all of the Lasor series kits and the NCR Hypersonic kits I had were also very similar in design. USR Stinger Two also comes to mind as a real drag cheater.

I prefer to sand an airfoil on the root, leading and trailing edges with a belt sander leaving the tip squared with the 2250/2650 design. It just looks right .....

My best performing rockets use this proven design :)

.

[STRMan]

A few years ago I used to help my kids' elementary school 6th grade do a science priject using model rockets every year. One year one of the kids questioned why he needed to airfoil the fins (We were building Alphas). I told him why, and proposed that half the class airfoil the fins and half leave them alone. We measured the altitude the rockets got on A8-3 engines and the airfoiled Alphas got significantly better alitude than the square edge Alphas.

Actually, these days, unless an airfoil or rounded shape is specified, I leave my fins squared for this very reason. I want my rockets back! I'm usually trying to hold down the altitude. Lower altitude usually means less chance a RET will get my birds. :D

If I was entering a competition, I would definitely make an airfoil shape on my fins.

[rokitflite]

I pretty much always round at least the leading and trailing edges of my fins... As a matter of fact, the ONLY ones I can remember leaving square are the ones on the "Pirates Booty" rocket I just built. The only reason I left them square was because the "blocky" look fit in with the scheme... I find it harder to do a nice, clean square edge than it is to round them :eek:

[snaquin]

In my latest batch of builds, I have left some square, some rounded, and a few airfoiled.

Same here ..... depends on the rocket design and the material. If I'm using plywood I generally sand an airfoil on the leading and trailing edge leaving the tip square. If I'm using basswood I generally round them. Balsa I tend to leave square on all edges because I usually cover them with label paper and ca the edges and it's easier to finish.

.

[Jerry Irvine]

USR Stinger Two also comes to mind as a real drag cheater.

I prefer to sand an airfoil on the root, leading and trailing edges with a belt sander leaving the tip squared with the 2250/2650 design. It just looks right .....


Me too.

Those new Quest motors are going to be a dream for 2250/2650 owners. We're back baby!

Jerry

http://v-serv.com/usr/kits/stinger2.htm

[Nuke Rocketeer]

Actually, these days, unless an airfoil or rounded shape is specified, I leave my fins squared for this very reason. I want my rockets back! I'm usually trying to hold down the altitude. Lower altitude usually means less chance a RET will get my birds. :D

If I was entering a competition, I would definitely make an airfoil shape on my fins.

Feed the RET's!!! They have to live too......They have little ones who need the wholesome balsa, plastic and paper we provide with our rockets...... :D

[STRMan]

Sorry, but I am actually a proponent of RET genocide. I say kill em all! :chuckle:

Better yet, grind them up and print copies of new editions of "The Handbook or Model Rocketry" from paper made from their wood. Now THAT would be justice! :D

I might actually try to make concave leading edges on my next rocket's fins just to deny RETs their bounty. Maybe I can launch my rockets w/a streamer already deployed on the launch pad. Hummmmm...

[Leo]

My fins are always rounded on leading and tailing edges with only very few exceptions. For me one of the more easier steps when building a rocket.

Example: My Maxi Icarus

http://www.leo.nutz.de/images/rockets/forums/Maxi_Icarus_Fin.jpg

[Bill]

Maybe I can launch my rockets w/a streamer already deployed on the launch pad. Hummmmm...


With a single long one, it is called a Saint Louis Arch.

Someone (Jim Flis?) attempted a massively clustered one.


Bill

[Mark II]

I might actually try to make concave leading edges on my next rocket's fins just to deny RETs their bounty... Hummmmm...
Or you could mount the fins so that the root edges are perpendicular to the long axis of the rocket (and to the direction of travel) ... ;) :chuckle:

Like these two models:

http://www.rocketreviews.com/images5/sun_umbrella.jpg http://www.squirrel-works.com/catalog/pie/pie2.jpg



I DOUBT that you will ever find either one hanging from the branches of an RET, unless it was put there intentionally!

:D :D :D

MarkII

[Jerry Irvine]

Or you could mount the fins so that the root edges are perpendicular to the long axis of the rocket (and to the direction of travel) ... ;) :chuckle:
Like:
http://www.rocketreviews.com/images5/sun_umbrella.jpg
MarkII

Someone should make a top ten list of rockets NOT to build and put this as number one with a bullet and the one to beat for 2-10 positions.

It may avoid trees, but also it avoids performance, low drag, motor usage efficiency, flight cost savings, and sanity.

May I buy you a drink?

Jerry

BTW :D

[Mark II]

Someone should make a top ten list of rockets NOT to build and put this as number one with a bullet and the one to beat for 2-10 positions.

It may avoid trees, but also it avoids performance, low drag, motor usage efficiency, flight cost savings, and sanity.

May I buy you a drink?

Jerry
Yes, there ARE some trade-offs to employing the perpendicular fin arrangement that I mentioned, of course... ;)

Thank you for the offer, but I don't drink.

MarkII

[Jerry Irvine]

Yes, there ARE some trade-offs to employing the perpendicular fin arrangement that I mentioned, of course... ;)

Thank you for the offer, but I don't drink.

MarkII

It doesn't show.

Just Jerry

[Royatl]

Someone should make a top ten list of rockets NOT to build and put this as number one with a bullet and the one to beat for 2-10 positions.

It may avoid trees, but also it avoids performance, low drag, motor usage efficiency, flight cost savings, and sanity.

May I buy you a drink?

Jerry

Problem is, Jerry, that there are plenty of people who *enjoy* flying these types of rockets. I do myself every once in awhile, but there are people who do it in excess. I think it is due to the smoke trail that is generated, as well as the reduced walk for recovery.

[STRMan]

Problem is, Jerry, that there are plenty of people who *enjoy* flying these types of rockets. I do myself every once in awhile, but there are people who do it in excess. I think it is due to the smoke trail that is generated, as well as the reduced walk for recovery.

OK, I HAVE to make that model. Is it a scratch built? Does someone kit it? Just the ticket for a backyard flight!

[pantherjon]

OK, I HAVE to make that model. Is it a scratch built? Does someone kit it? Just the ticket for a backyard flight!

That is a Sunward kit..The Flying Umbrella..

Available at finer rocket shops, including:

Uncle Mike's (http://www.unclemikesrocketshack.com/Sunward.html)
It is down towards the bottom of the page..

[jflis]

With a single long one, it is called a Saint Louis Arch.

Someone (Jim Flis?) attempted a massively clustered one.


Bill

Did more than "attempt" :D

It was at NARAM in 2007 during the imagination celebration. It was a UFFO with a different color streamer under each of 12 foam cups.

I kept it hidden inside a box so that no one could see it until the boost brought it above the edge of the box.

It was quite dramtice :) I called it the St. Louis Rainbow

Here are some pix. Others have better pix.

[cas2047]

It was at NARAM in 2007 during the imagination celebration. It was a UFFO with a different color streamer under each of 12 foam cups.



Jim where do you come up with this stuff! That is VERY cool! :D

[wilsotr]

It doesn't show.

Just Jerry

OK ... that was funny. :)

[wilsotr]

Did more than "attempt" :D

It was at NARAM in 2007 during the imagination celebration. It was a UFFO with a different color streamer under each of 12 foam cups.

I kept it hidden inside a box so that no one could see it until the boost brought it above the edge of the box.

It was quite dramtice :) I called it the St. Louis Rainbow

Here are some pix. Others have better pix.

There out to be a whole forum category: "Weird Stuff to do with Rocket Motors."

Or maybe not, now that I think about it .....

[j.a.duke]

My fins are always rounded on leading and tailing edges with only very few exceptions. For me one of the more easier steps when building a rocket.

Example: My Maxi Icarus

http://www.leo.nutz.de/images/rockets/forums/Maxi_Icarus_Fin.jpg

I've done both square and rounded but haven't been able to get my rounding to be symmetrical.

What's your secret?

Thanks.

Cheers,
Jon

[Mark II]

It doesn't show.

Just Jerry
OK ... that was funny. :)
I'm not quite sure how to take that... :confused:

If you are saying that I have the online persona of a drinker, then I then I would take that as an insult (and ask how you came to that conclusion?).

If you are saying that I don't need any alcohol to help me write either dull, funny, or at least weird off-the-wall, posts, then I would say, "Well, duh!"

If you are saying that no one ever finds their wit or their identity in a bottle, and that in any event, my disinterest in alcohol consumption is totally irrelevant to this or any other rocketry-related discussion, then I would like to reach across cyberspace and shake your hand. :cool:

BTW, I do get high on a regular basis, but I do it with the help of black powder and composite propellant. :D

But truthfully, Jerry, I should recognize your friendly gesture for what it is, and take you up on it sometime if I get the chance. Just make mine a Diet Pepsi, though. :)

And now, back to our regularly-scheduled thread...

MarkII

[Bill]

Did more than "attempt" :D

It was at NARAM in 2007 during the imagination celebration. It was a UFFO with a different color streamer under each of 12 foam cups.


I wish I was there to see that. Someone told me about it and thought it did not work as you planned? Did you try it twice? And I thought it was at NARAM-50 in 2008.


Bill

[Bill]

I've done both square and rounded but haven't been able to get my rounding to be symmetrical.

What's your secret?



I hold the fin with one hand with the edge to be rounded on top. With sanding block in the other I begin sanding with the sandpaper perpendicular to the flat side of the fin and slowly tilt it as I sand until it is parallel with the flat side. Then repeat on the other flat side. Then do it again. And again. And again

Using a sanding block is essential to get consistency along the edge.

The main secret is to take my time and remove only a small amount of wood with each pass. Once the corners of the edge are gone, I start paying attention to how much I sand on each side of the fin so that the curve is symmetrical.

In truth, my edges end up more like the leading edge of an airfoil with a gradual taper ending in a round "point" rather than a strict semicircular cross section that "round" implies. The taper makes any slight asymetry less noticeable. But I am OK with that.


Bill

[Mark II]

I always round the edges of balsa fins. I haven't bothered doing it with plywood, though. I don't use basswood very often, but I attempt to round the edges of basswood fins, too. It can be a real pain with that material, though. I'll probably do more rounding on basswood and plywood when I finally get a belt sander or disk sander someday.

I use the same technique that Bill described, except that I start with the sanding block at 45° to the flat side of the fin (which is also 45° to the fin edge, too, I guess. :rolleyes: ) I work on just taking the sharpness off the right angled edge on one side, and then do the same on the other. Then I gradually blend in the shape from there.

A couple of tricks to getting the shape even on both sides of the edge:

Draw a pencil line down the middle of the edge, halfway between each side. For thin material, this can be tricky. I carefully measure the thickness with a machinist's ruler and mark the half thickness with a fine point mechanical pencil. Then I find something flat with a smooth, straight edge (a CD case works really well for this) and lay the fin next to it. I stack up sheets of cardstock under the fin until the marks I placed on the edge line up with the edge of the case, and then I use the case edge as a guide to draw a line down the fin edge halfway between each side. This provides a great visual reference to help me gauge my sanding to keep it even on both sides.

Another trick, which works best on thicker fins, is to draw a line on each side parallel to the edge to be sanded but half the fin thickness away from the edge. Then put a strip of easy-release masking tape along that line. When you round the edge, you will want to sand in a curve that goes evenly from the middle of the fin's thickness (described above) to that masked line. Carefully sand that curve in along one side of the edge, and then turn the fin around and sand it into the other side of the same edge.

As Bill, said, sand gently, stopping frequently to check your progress by sighting down the edge. Sand by starting at one end of the fin edge and making light passes with the sanding block (always use a block), in one direction only, down the edge all the way to the other end, sort of like you would if you were sharpening a long blade. Always go down the entire length with each sanding pass; otherwise, you risk sanding in a scallop or "saddle" into the edge. Be careful not to over-sand. It is easier to take a little bit more off than it will be to put it back on. I actually had to build a fin edge back up once, early on, when I sanded a saddle into it, so I know! :chuckle:

Keep the old saying in mind: less is more. :D As in, do less total sanding on the edge than you think you will need to do. You will usually get better results that way.

MarkII

[KD5NRH]

Using a sanding block is essential to get consistency along the edge.

I've been using a 320 grit sanding sponge for rounding: the slight give lets it curve the surface just a bit with each pass, and a couple of swipes along the pointed edge left after both sides are done rounds it down slightly as well. It's sort of like using a slack-belt sander to put a convex curve on a knife edge. (Except, of course, that you wouldn't then sand off the edge when making a knife.)

It also seems just about right for slightly radiusing the edges of some other balsa pieces, like the various bits of the Screaming Eagle I'm working on. IMO, it looks a little more finished if the corners aren't quite so sharp.

[jflis]

I wish I was there to see that. Someone told me about it and thought it did not work as you planned? Did you try it twice? And I thought it was at NARAM-50 in 2008.


Bill

It worked the first year I tried it (launched it twice, in fact). The next time I tried it (NARAM 50, I think?) I had an E9 blow a nozzle and it never left the box...

Hoping to try it again this year :)

[Bill]

It worked the first year I tried it (launched it twice, in fact). The next time I tried it (NARAM 50, I think?) I had an E9 blow a nozzle and it never left the box...



NARAM-50 must have been the "attempt" I heard about and did not realize you had already flown it before.


Bill

[luke strawwalker]

In the three and a half months or so of my having become a BAR I've noticed something that makes me wonder a little. I see many pictures of rockets here, on makers' web sites and elsewhere that have square-edged fins. Has trying to reduce drag by rounding leading edges and tips, or even shaping fins to a thin symmetrical airfoil section fallen out of favor, or has someone just demonstrated in the past 35 years that it's wasted effort?

In the few model rockets I've built in these past few months I haven't been able to bring myself to leave LEs in particular square, I had to round them. I've generally left trailing edges square because I know that TEs on small model airplanes also can be left square at no real performance loss...even though the fly rather more slowly.

So....is it building simplicity, or aerodynamic research, or what that has led to this trend of square fin edges?

Like a LOT of the craftsmanship skills and building habits, I think it's fallen out of favor because 1) RTF is "in"; nobody wants to 'take the time' to build a model rocket "properly" anymore 2) what building they DO have to do, folks want to go as quickly as possible and the minimum possible, eliminating "unnecessary steps" of which airfoiling the fins is DEFINITELY in that category, and 3) much less of the 'science' behind rocketry seems to be on the minds of modern builders, especially noobs, and so a lot of folks don't even know a) fins can/should be airfoiled and b) why they should be airfoiled.

Of course us 'old farts' that 1) prefer to build a rocket and generally 2) don't like RTF rockets still airfoil the fins. Of course we fill and sand the fins/nosecones too and generally try for a nice paintjob/finish. For some folks flying is more important and they don't particularly care how it looks. They may not even be interested in gaining a little performance from drag reduction through airfoiling, given the extra work required.

Just depends on the modeller... I'm into 'semi-scale' stuff and usually do whatever 'looks right' for the model in question, even if it's aerodynamically sub-optimal. On sport models though, rounded leading edges and tapered trailing edges is still order of the day...

That's just my observations and take on it, though... Later! OL JR :)

[GregGleason]

It takes a craftsman to appreciate craftsmanship.

Mass hobbies are about instant gratification.

I am not saying that one is better than the other; it is just my "It is what it is" statement. The age, talent, and character of a person will often show in their work.

With J.S. Bach or Beethoven you get one thing, with novices something else. But most people were a novice once.

Greg

[Solomoriah]

It's a style thing for me. Performance isn't that important to me; fun flights are. But when I build a rocket, if it looks like a "performance" rocket, I round or airfoil the fins, but when I build something like a Baby Bertha, I leave 'em square.

[LeeR]

I've been using a 320 grit sanding sponge for rounding: the slight give lets it curve the surface just a bit with each pass, and a couple of swipes along the pointed edge left after both sides are done rounds it down slightly as well. It's sort of like using a slack-belt sander to put a convex curve on a knife edge. (Except, of course, that you wouldn't then sand off the edge when making a knife.)

It also seems just about right for slightly radiusing the edges of some other balsa pieces, like the various bits of the Screaming Eagle I'm working on. IMO, it looks a little more finished if the corners aren't quite so sharp.

A sanding sponge sounds like a great idea, I'll have to try that. I like putting the fin at the edge of my workbench, so I can put my hand down around the end or edge of the workbench, with my fingers curled up over the edge and over the top -- across the surface of the fin (with the edge to sand right at the edge of the workbench.) To me, it is a natural position and feel to get a really good "read" on how well the rounding and tapering is going.

I tend to do airfoils across the fins, with more rounding on the leading edge, and more of a sharper taper on the trailing. I've gotten out of marathon build sessions, and if it takes me a few evenings to sand the fins, so be it. And at my pace, I'll finish all my kits just prior to my 187th birthday ...

[mrhemi1971]

I'm so crazy that I used to airfoil those 69 cent mosquitos, before stuffing A10's in them.....

[tfischer]

I didn't read through all 6 pages of this topic, so maybe this was already brought up...

I'm recently "back" into this hobby, and I've built 4-5 new Estes kits over the past few weeks. One thing I noticed right away was the lack of instructions on putting an airfoil on the leading edge of the fins... I was particularly surprised with this on my new Big Bertha build -- since this was an older rocket, and I had remembered this in the instructions on all my rockets as a kid. In fact, I went back to one of the rocket plans site to look up the older Bertha instructions -- sure enough, Estes dropped the instructions to round off the fin edges.

Frankly, I build my rockets to fly, not to display, so I've never been vary particular about filling the balsa grain, although I do round off the edges a bit. My typical method of "fill" involves a couple coats of spraypaint, sanded off, which gets the smaller grain but not the larger grain. If I keep at it, I might try the "fill and finish" approach. But I'm impatient and I've never liked waiting for multiple coats of sealer to dry and having nothing to do while waiting :)

[Solomoriah]

I build rockets to fly and to display, though flying takes precedence. I rarely round the fin edges, because it really doesn't matter to me if they get maximum performance or not. They go plenty high as is. But lately I find myself applying thinned filler with a brush, sanding when dry, then priming and sanding repeatedly until the spirals and wood grain are filled.

Then I go screw up the paint job...

:D

[ga1ba2]

If the instructions say to airfoil than that's what I do. If the instructions don't mention it I usually just ease the edges a little.

[Bob H]

Frankly, I build my rockets to fly, not to display, so I've never been vary particular about filling the balsa grain, although I do round off the edges a bit. My typical method of "fill" involves a couple coats of spraypaint, sanded off, which gets the smaller grain but not the larger grain. If I keep at it, I might try the "fill and finish" approach. But I'm impatient and I've never liked waiting for multiple coats of sealer to dry and having nothing to do while waiting :)That's why I use self stick label paper to cover my fins. It only takes a few minutes per fin and there is no filling and sanding required.

Now, if I could only get label paper to conform to compund curves, I wouldn't have to fill and seal my balsa nose cones and transitions.

[Solomoriah]

I get along so well with brush-on filler that I rarely feel the need to paper fins. Though, for my 3x18mm Ultimatum, I papered the 3/32" fins with 110# paper. Them are some TOUGH fins, boy. :D

[foose4string]

Sometimes I round the leading and trailing edges, many times I don't. To suggest a model is tainted somehow, or question someone's modeling skills because they haven't rounded the edges is ridiculous. I used to round everything, but some models just don't look right with rounded edges IMO. I think fin shape or rocket design often dictates whether or not I choose to round them or not. For instance, I happen to think Sci Fi designs look better squared ( and are generally not designed for high performance anyway ). Sometimes I think to myself, "why bother air foiling fins when you've already added so many draggy, components(half a dozen decorative tubes, ping pong balls, forward fins, plastic what-nots, nose weight, etc.) to the rocket?" If you want to build a rocket that goes out of sight, then you leave all that junk off, be more considerate of the fin shape, mind the tube diameter and length, and round or taper the fins. All or nothing baby! :D

I'm a sport flyer and not a competitor, so it doesn't make a hill of beans to me whether or not my model flies 500 ft.....or 637 ft. As long as it goes up safely, comes back down under chute or streamer(or some other fancy means of recovery), and I can retrieve it, then I'm happy. :)

[rosko_racer]

I am in line with Craig. In my book, airfoiling or rounding of fins are for gliders or hig-performance / competition rockets. Most of my rockets have squared or semi-rounded fin edges. Unless it is a scale rocket (like the V-2 or a Nike) where the fins call for some special "tappered or rounded" finish the I will do it. As for the fins' finish, it depends on the mood I am at the time... paper laminate if I feel lazy, or F-n-F if I have the time. Both work for me. All NCs get treated with wood hardener first, then a week later, F-n-F for a smooth look. I build for display, to fly, and have fun with friends and family

I am a sport builder / flyer who is not really interested in the competition side of rocketry. As long as it looks good on the pad and on the display stand, it flys and can be recovered, I am happy.

[Maniac BAR]

I am one to agree with the notion that the rocket dictates what the fin shape should be. I built an Estes Deep Space Transport and the fins needed to be rounded on the LE with a very slight taper to the TE. It looked like a rocket/plane and that was the fin shape to use.

I did a Qmodeling Snooper and left the fins mostly square. Only gave a very slight edge break to the LE and left the TE square. With all the tubing attached to the fins and their angular shape, putting a rounded LE on the fins would have looked out of place. It is the kind of sized model that doesn't need max height to give it a nice flight profile, either.

On the Semroc Texas Firefly I rounded and carefully shaped the sustainer fins to accent their long sweepback shape. However, the booster fins were only very slightly rounded. I may have done that to help the booster slow down at staging but the finished bird just looked right! Ok, the recovery walks have been a bit much with the sustainer but man, that bird just looks so good on display. :)

I have also done competition rockets where fins shape was a major importance. I even spent a major amount of time putting an airfoil shape on some .016 G10 for a very small competition rocket. Talk about a PITA :eek: Did it help? Don't know, I placed fourth or fifth with it. It just depends on the rocket and what you expect from its flight profile.

But with that said, if the plans call for a taper of some kind or rounding of the fin edges, that is usually what I will do, as others have agreed on as well. I have, also now reailized that some of the smaller kits I am now building have square edge fins. Like others, I like to get my rockets back. :D

Personally, I think to "round or not to round" is not the question. To "fly and recover or not to fly" should be. :chuckle:

[Mark II]

With the Astron Sprite, the fin LE's, trailing edges and the fin caps all had to be rounded (and blended, in the case of the caps), or else it just didn't look right. A bit of a PITA, because everything was so small, but it really needed to be done. Goonies, on the other hand, just can't be designed and built with anything but big square-edged slabs of balsa for fins. They need those kind in order to keep the max. altitude below 200 feet. ;)

MarkII

[luke strawwalker]

I am in line with Craig. In my book, airfoiling or rounding of fins are for gliders or hig-performance / competition rockets. Most of my rockets have squared or semi-rounded fin edges. Unless it is a scale rocket (like the V-2 or a Nike) where the fins call for some special "tappered or rounded" finish the I will do it. As for the fins' finish, it depends on the mood I am at the time... paper laminate if I feel lazy, or F-n-F if I have the time. Both work for me. All NCs get treated with wood hardener first, then a week later, F-n-F for a smooth look. I build for display, to fly, and have fun with friends and family

I am a sport builder / flyer who is not really interested in the competition side of rocketry. As long as it looks good on the pad and on the display stand, it flys and can be recovered, I am happy.


Yes, very good points rosko and foose... and I agree, for the most part... Looking 'proper' for the kit IS important and streamlined fins DON'T FIT every situation... an X-wing or a Colonial Viper with streamlined fins WOULD look stupid! (grins)

Where I part company I guess is that I've seen how things are judged at my nephew's 4-H fair, where workmanship really counts for NOTHING. I taught my nephews a lot of tricks for getting really good finishes and getting really NICE looking rockets-- yet whomever is judging obviously knows little or nothing about craftsmanship because rockets that are basically just slopped together with glue runs, TOTALLY unsanded, unfilled, unpapered, UN-ANYTHINGED balsa fins not even on straight, nosecones with the fuzzies and balsa acne, and bad paint jobs with runs, decals on crooked, etc. end up being scored higher because it's a cooler looking rocket design that caught the judge's fancy.... NONE of which has ANY bearing on the quality of what the kid is supposed to be doing or learning, unless guessing which design the judge will find "coolest" is the primary quality being judged and main purpose of the excercise... :rolleyes:

Anyway, IMHO a nicely finished rocket just does more to impress me than coming out to the field with some sloppily glued up balsa-acne'd looking botch job and shoving a big motor into it... which, seems like a lot of the low HPR/MPR guys are content to do at times... just a difference of opinion I guess but IMHO just about ANYBODY with the cash to spend on MPR/HPR motors and big kits can shove a big motor into something and fire it off-- what impresses me is someone able to make the thing look good on the table AND in the air!

JMHO! OL JR :)

[stefanj]

I've built many clones / semroc kits based on old Centuri models.

Many of these have compound double-swept fins. Like the Hustler, or the Centauri.

Sanding the leading and trailing edges of these fins is a real pain. It's really hard to get a good even shape at the V-shaped intersection.

Then it occurred to me, as I started building a Centauri . . . sand the leading and trailing edges of the fins' components BEFORE gluing them together.

[snaquin]

I've built many clones / semroc kits based on old Centuri models.

Many of these have compound double-swept fins. Like the Hustler, or the Centauri.

Sanding the leading and trailing edges of these fins is a real pain. It's really hard to get a good even shape at the V-shaped intersection.

Then it occurred to me, as I started building a Centauri . . . sand the leading and trailing edges of the fins' components BEFORE gluing them together.

I agree ..... the Hustler and Lil' Hustler were difficult so I decided to only slightly round all the exposed edges because of the fin shape. The EnerJet Egg Crate had a short flat edge between the leading edge and tip and if I wasn't careful could have easily been sanded too much. I chose to only slightly round all the edges on this model as well to maintain the original look of those fins that has a more square than rounded look in the catalog images.

I have another Semroc Lil' Hustler in the bag and when I build that one I may just leave square fin edges because personally that particular fin shape looks pretty good that way.

Good idea about sanding before gluing them together!


:)

.

[BEC]

Then it occurred to me, as I started building a Centauri . . . sand the leading and trailing edges of the fins' components BEFORE gluing them together.

Which is the way to handle the fins on the Astron Sprite, too. Do most all the shaping before installation.