Fin/fillet question

[Target]

I notice most lp guys do not round over or airfoil the fins. I do notice an abundance of time is spent on fillets. Why? Square/round fins/fillets all deal with air flow correct? I am not even asking in relation to acheiving altitude. Is there a reason to need to to be so precise on a fillet when you do not round or airfoil the fin? Why spend that amount of time on them when you are not flying for altitude attempts? Surely an answer will be....because that is how fins attach, they arent thru the wall.

if you are going for looks which I am sure many are....why not take one more simple stemp and round over? There's been many gadgets posted about with photos simplifing this process? Not trying to start a pi$$ing/b!t*hing match. I have several 3-4 inch flying with ttw and I do add some fillets just not to the extent I see here for low powers kits. I built a 'home engineered" fin jig. Just asking out of curiosity because I see no reason to dedicate this much time to fillets if you dont round the fins....you've likely put elmers fill and finsh or papered them correct? Please dont answer....it causes spin.

 

[Gary Byrum]

Funny, I rarely see peoples rockets in my area that don't at least round off their fins or shape them in one fashion or another. I do all of mine. As far as these classy smooth rounded perfect looking fillets, I believe it's more to the aesthetics if the truth be known and found mostly with mid to HPR models. I could stand corrected on that note though. I would assume that they do play a part in cutting down wind resistance. Generally on LPR models, I won't go the distance on that. Usually 1 or 2 Tightbond fillet applications work for me.

 

[goldlizard]

I fly low and mid power rockets, and I always wedge the leading edge and round over the trailing edge. My biggest problem is when I paper the fins the paper wrinkles. I am not sure if I an using to much white glue or not enough. I lay a bead on one edge, then take one of those fake credit cards the banks send you to get you into more debt, and spread the glue evenly across the fin.
Anyway I digress, I agree with you, you go to all the trouble sanding and filling fillets why not take another 5 minutes and wedge or round over the leading edge.

 

[Bill Gibson]

Depends on look you are trying to achieve...some scale models have little to no rounded edges on the fins.....some of mine are shaped, some are square....as far as performance, size is a much of an issue as shape...drag is not a good thing for performance! Fillets do add strength, but i try to get rid of as much excess material as possible....i also use epoxy and micro-ballons, even on low power models...epoxy simply is not that heavy. if used with care!.....Cant remember the last time i lost a fin......!

 

[JStarStar]

1) I usually round the leading/trailing edges with a quick pass with a Dremel. Very rarely do I spend a lot of time crafting a 'symmetrical' airfoil (round leading edge, sharp trailing edge). Few if any of my rockets are ever going to be entered in any kind of altitude/performance competition so incremental performance improvement due to painstaking airfoiling is not relevant to me.

2) The amount of time I usually spend on fillets is the amount of time it takes to run a line of glue, smooth it with my fingertip, and clean off any excess or bubbles. I only spend an 'abundance' of time on fillets when they are a significant part of the design (i.e., the V-2).

3) The practical reason of course for fillets on LPR rockets is to serve as reinforcement for fin attachment at the body tube/fin junction point, to provide more gripping surface than only the fin root edge.

4) Although point 3) is effectively obviated by the use of TTW fin construction, even minimal sloppiness in slot cutting or alignment can produce gapping beween the fin surface and the (outer) body tube, usually making fillets necessary on TTW rockets as well to ensure any such small gaps are sealed.

 

[Target]

I fly low and mid power rockets, and I always wedge the leading edge and round over the trailing edge. My biggest problem is when I paper the fins the paper wrinkles. I am not sure if I an using to much white glue or not enough. I lay a bead on one edge, then take one of those fake credit cards the banks send you to get you into more debt, and spread the glue evenly across the fin.
Anyway I digress, I agree with you, you go to all the trouble sanding and filling fillets why not take another 5 minutes and wedge or round over the leading edge.

Goldlizard,
I have had the issue with wrinkles so many times I despise papering. I have found with papering fins thining the glue just a bit more than my comfort level. Apply w/ brush. Paper one side the weigh it down quickly. Let is set for while then do the opposite side weigh and leave overnight. No warpping yet or wrinkles so far anymore. Large scratch 3-4 inch rockets I use ply from Hobby Lobby Elmers fill and finish then several coats of primer sanding between. I have used index card stock before for my mid powers. Good finish!

 

[samb]

I do it on every rocket because that's how I was taught by Mr. Estes and Co. from their classic '60s catalogs:



Definitely a personal preference but square leading/trailing edges just look unfinished to my eye.

 

[soxfan121]

Definitely a personal preference but square leading/trailing edges just look unfinished to my eye.

That's really the only reason I do it on my low power and lower mid power rockets. The only time I do it on my rockets for performance reasons is when it's high mid power to high power rockets. I also really like the look of a nice neat fillet so I usually use painters tape and make perfect squared fillets.

 

[goldlizard]

Goldlizard,
I have had the issue with wrinkles so many times I despise papering. I have found with papering fins thining the glue just a bit more than my comfort level. Apply w/ brush. Paper one side the weigh it down quickly. Let is set for while then do the opposite side weigh and leave overnight. No warpping yet or wrinkles so far anymore. Large scratch 3-4 inch rockets I use ply from Hobby Lobby Elmers fill and finish then several coats of primer sanding between. I have used index card stock before for my mid powers. Good finish!

How much do you thin the glue?

 

[dave carver]

I round the edges after the fin is glued to the rocket, usually during the filling process. I've tried to do a few with tapered trailing edges, a Flis Rhino comes to mind. After getting 3 of the 4 fins to sand to a taper rather easily the last fin was of much harder stuff. I spent as much time on the last fin as I did the other 3 fins put together The fins sure came out looking good

 

[Target]

How much do you thin the glue?

For me to learn this technique years ago I had to learn by mixing twice as much water as glue. I made some awful messes. It was elmers back then. I use titebond now and I do prefer the original. Still mix it double water to glue with titebond and I apply very little on the fin then stick the paper and very quickly weight it down with 10 lbs. I am sure guys like sodmiester, bradycos or qquake can better inform you than me. They CAN build. Another trick I used because I wasn't and am not that great a builder is heavy stock paper. (I think that's what its called over here...heavy stock paper?) Have a friend that calls it resume paper. It's thick and hasn't wrinkled very much from me, Try index cards. 3"x5" and 5"x7" w/ lines turn in.


I actually thought that elmers was THE only glue around. Model car or model kits were very boring so never used testor's much. Beside when you got bored elmers made the best peel off finger prints. Yep I of course had to show the grandkids!

 

[T-Rex]

Sometimes I do, sometimes I don't... I tend to prefer the look of a clean square edge to the uneven, misshapen edge that exists after my attempts to round them over. I generally have grown up believing that the fillet is there to reenforce the joint, My fillets are just a line of TBMT with a wipe of the finger. Before I learned about TBMT, it was TBII,

 

[Target]

Funny, I rarely see peoples rockets in my area that don't at least round off their fins or shape them in one fashion or another. I do all of mine. As far as these classy smooth rounded perfect looking fillets, I believe it's more to the aesthetics if the truth be known and found mostly with mid to HPR models. I could stand corrected on that note though. I would assume that they do play a part in cutting down wind resistance. Generally on LPR models, I won't go the distance on that. Usually 1 or 2 Tightbond fillet applications work for me.

About what I thought Gary. Aesthetics. Thanks

 

[cobra1336]

IMO...fillets are to make the fins stronger to the body tube. Why do you do it on LPR is to learn how to do them so when you move up to the next level you have an idea what to do and why. The idea is to move from one level to the next and understand the benifits of each build. That's how I learned. One step at a time

 

[JStarStar]

IMO...fillets are to make the fins stronger to the body tube. Why do you do it on LPR is to learn how to do them so when you move up to the next level you have an idea what to do and why. The idea is to move from one level to the next and understand the benifits of each build. That's how I learned. One step at a time

Many people have no intention or desire to "move up to the next level."

 

[Area66]

IMO...fillets are to make the fins stronger to the body tube.

Externals fillets are made to mainly reduce the drag, you can achieve structural reinforcement with only internal fillets or /and fins to fins fiberglassing on the motors tube like I like to do.

 

[Gary Byrum]

Many people have no intention or desire to "move up to the next level."

Good point! This is not a graduation process. It's a friggin hobby. We all have a level we feel comfortable with. There is no competition regarding finishing jobs here.

 

[luke strawwalker]

I fly low and mid power rockets, and I always wedge the leading edge and round over the trailing edge. My biggest problem is when I paper the fins the paper wrinkles. I am not sure if I an using to much white glue or not enough. I lay a bead on one edge, then take one of those fake credit cards the banks send you to get you into more debt, and spread the glue evenly across the fin.
Anyway I digress, I agree with you, you go to all the trouble sanding and filling fillets why not take another 5 minutes and wedge or round over the leading edge.

You're using too much white glue... basically you want JUST ENOUGH glue to completely and evenly and VERY THINLY coat the entire surface of the fin... burnish the paper down onto the fin with the round barrel of a sharpie marker and squeeze out ALL the excess glue from under the paper out toward the edges of the fin... basically the thinner the glue layer, the better... (stronger).

Also, most of the folks I fly with at least round over the fins... I usually streamline mine-- rounded leading edge, tapered trailing edge, which is the most efficient shape. For scale models where a wedge shape is on the prototype, I do a wedge shape... although aerodynamically at the speeds our rockets fly at, it's not ideal aerodynamically...

IMHO, the mark of a craftsman builder is making the fins aerodynamic... square cut fins with poor/no filler and lots of balsa grain showing, just looks like a sloppy build or a newbie... Unless there's a reason for not doing it, like trying to minimize spin on ascent by not streamlining the fins (because a slight difference in the airfoil on one side or the other, on one or more fins, can induce spin on ascent) for a camera rocket or something along those lines...

Later! OL JR

 

[luke strawwalker]

How much do you thin the glue?

Thinning the glue adds water, which is the enemy of papering fins... excess glue causes wrinkles because the water from the glue is absorbed into the paper, and causes the fibers to swell... causing the paper to "warp" and soften and wrinkle.

Use straight white glue, spread very thinly and evenly over the entire surface of the fin. Apply the paper, then burnish it with a round sharpie marker to squeeze ALL the excess glue out from under the paper, starting from the center of the leading edge of the fin and working towards the back and the outer edges...

If done correctly, the fin will turn out very light, very smooth, and with no wrinkles or bubbles... less glue actually makes a stronger joint, though it sounds counterintuitive...

Fillets basically don't have to be very big, nor do they add a lot of strength (some, not a lot). I fillet with Titebond Moulding and Trim Glue and smooth it with my finger-- instant fillets, can do all the fins at once, and no pits or voids to fill like with yellow wood glue or white glue, and no waste from mixing or having to don gloves to apply it, taping, and all that other nonsense (which is total overkill for LPR and most MPR-- epoxy definitely has its place, but for LPR/MPR it's just overkill)

If you want fins attached so strongly the paper tube wall will separate before the glue joint holding the fin on breaks, use a double glue joint-- apply a THIN layer of white or better yet yellow wood glue to the fin root and to the tube wall where the fin will be glued on. ( lightly sand the glassine layer of the tube on the fin line first if you want to enhance the joint strength by giving the slick glassine "tooth" for the glue to grab onto). Let this first layer of glue dry with the fin and rocket tube still apart. Once the glue layer on the tube wall and fin root are dry or just about dry, apply a SECOND thin layer of glue to the fin root, then join the fin to the tube... start at one end or the other, carefully align the fin, and rotate it down flush against the tube, and hold it for about 20-30 seconds... the yellow glue will "grab" almost instantly, so make sure your alignment is good when the fin hits the tube, otherwise the glue will "lock up" and you won't be able to move the fin. Hold the fin aligned and 90 degrees to the tube wall for about 20-30 seconds, and the fin will be set up and glued to the tube... let them dry awhile and they're ready for the Titebond Moulding and Trim Glue fillets...

Works like a champ...

Later! OL JR

 

[Sooner Boomer]

I usually round leading and trailing edges. I think it looks better, and helps reduce stress or helps you find splinters in balsa. The most effort I spent on a rocket was a Centauri Nike Smoke. Those were a real challenge, but I got the shape right. I use fillets to give extra strength to the root edge of the fin. I'm sure it helps aerodynamics too, but the stuff I fly, it's only going to contribute another ten feet or so at best.

 

[Zeus-cat]

I round over the leading edge and taper the trailing edge on all fins UNLESS the rocket is meant to have blunt edges or is so draggy that it really makes no difference.

 

[chrism]

Some companies will have you round the edges and some companies such as Quest and Custom have you leave the edges square. I usually build per instructions. I noticed in the newer Estes kits, that they just round the leading edge and leave all others square.

 

[wfcook]

Fillets are mainly to strengthen the fin/tube joint on fins that don't go through a slotted tube.

It is not clear to me at all that they are aerodynamically advantageous. I would be more inclined to remove volume from the tube at the joint rather than add it (in yacht design we call this a "dillet"). This takes the form of a depression in the tube, but structurally that would be a nightmare. The best solution is arguably a through-tube fin glued to the motor tube with internal fillets, but that makes little sense on, say, and Estes Quark.

There is a brief mention of dillets on page three of this Burt Rutan plane description: https://www.oshkosh365.org/saarchive/eaa_articles/1988_10_04.pdf

Other than that, googling brings up some...interesting things. But in way-oversimplified layman's terms, you don't reduce drag by putting more area and volume in the way of the fluid. The dillet gives the fluid a place to go when it runs into the fin without excessive pressure buildup.

 

[JohnCoker]

IMO...fillets are to make the fins stronger to the body tube.

This is definitely true for HPR. But when using white glue, I don't think the fillets add much if any strength since white/yellow glue (aliphatic resin) has little gap-filling ability. Has anyone tested this?

 

[Aero B]

I have always at least rounded the fins.
On some rockets I actually airfoil, but, the airfoil is flat bottomed. I leave one side of the fin flat then airfoil the other side making the old Clark Y airfoil. I makes the rocket spin at a slow rate.
I always fillet, too.

 

[Winston]

This is definitely true for HPR. But when using white glue, I don't think the fillets add much if any strength since white/yellow glue (aliphatic resin) has little gap-filling ability. Has anyone tested this?

You're increasing the bonding area vs having that area just limited to the thin edge of the fin. I've seen unfilleted fins separate at that fin/BT bonding point whereas I've seen filleted low power rocket fins break off in the fin material instead of at the joint from a greater impact.

 

[Maxx Mayhem]

You want not to thin the glue, this will make the wrinkling problem worse, as the glue has to shed much more moisture as it dries, but instead, to use a very thin (but even) film of glue. The less, the better. I have used a vacuum bag to clamp the fins, one time it worked perfectly with tracing paper, one time it did not, I had too much glue.

Fillets provide a larger area of adhesion between fin and tube. So once you have reinforced your fin with the paper, all that extra strength does little for you if your fin falls off, so...

Filleting is really not done to reduce drag, it is just for strength. it actually slightly increases the frontal area of the rocket, the issue of turbulence is complex...Feathering, rounding, airfoiling the fins all help to reduce drag and thus may increase performance, but this reduces the strength of the fin, which should then be reinforced, and then filleted for optimum strength.

 

[maxvelocity]

I always take the time to cut a bevel on the leading and trailing edges of my fins. I think it makes it look so much better as well as perform better. I was surprised to find my Shadowaero Blackbird did not have any bevel on the fins. He says they found little gain so they no longer waste the time to bevel.

 

[JeromeK99]

Good point! This is not a graduation process. It's a friggin hobby. We all have a level we feel comfortable with. There is no competition regarding finishing jobs here.

I agree! IMHO smaller LPR birds can be more challenging and harder to build than MPR or HPR. Keeping them light weight and more than 3FNC or 4FNC. Over the many years I have been going to launches, I have seen far more "pretty" crafted LPR birds than most of the slapped together HPR. HPR just means you need stronger (and heavier) components and stronger glue. Parts are bigger and much easier to deal with. Weight is more so you need bigger motors. All of this adds to the cost.. but hey.. if that's your thing cool! It is a hobby! Just don't confuse the size of somebody's rocket with their skills.

but I have digressed...

Nice fillets add strength and look pretty.. With LPR I seriously doubt much is gained with aerodynamic fillets or fins. Do I really care if my "B" motor bird goes 500 ft. or 515 ft?? Besides, at 15 ft away on the pad, they all look good!

Like I said (and others too). Its just a hobby. We get out of it what we put in! A pretty rocket does impress me knowing what it takes to accomplish that.. though I don't think any less of a person if their rocket isn't "museum or shelf queen" quality.

Jerome