When and how to shape LPR fins?

[Kruegon]

My LPR kits are primarily Estes brand. As a kid, I remember instructions telling us to fully airfoil fins. Most now days just say to round a leading edge. Some have full bevels, like the HoJo and Nike Smoke.

I've seen kits with edge bevels, rounded on all sides, square edges, full bevels, and everything else we do to them. If you vary from the instructions here, how do you determine which way you finish fin edges?

 

[neil_w]

Other than scale models which demand a particular profile, it seems to be a matter of personal preference. I round my leading and trailing edges because I think it looks better, and because I feel guilty if I don't (seriously). Someday I hope to be cured of this malady.

Obviously rounding or beveling or airfoiling or whatever can reduce drag as well, but that's not usually a significant factor with LPR.

 

[EXPjawa]

I usually do whatever suites me at the time. Normally, its just rounding the leading edge, or maybe the trailing edge too. If the fins are elliptical (Astron Sprint), I'll go all the way around. There are several where I've put effort into airfoiling, but they're either scale (or scale-like) or they're space plane type rockets (where a wing "ought" to have an airfoilish shape). But for a 3FNC rocket, just rounding is fine for me. With all that said, I will say that I put a lot of effort into shaping the fins on my Semroc Ruskie, where I sanded some contour and shape into the wheel pants / landing gear leg fins, as well as how the cockpit joins the rudder, so that it has some depth to it.

 

[samb]

When ?
Whenever I feel like it. Up till today, that's every build because, like Neil, I think rounded edges on fins are prettier.

How ?
Hand sanding block and OEM Mk1 eyeball. Or sometimes I lay the sandpaper on a flat surface and work the edges at an angle against it. I did use a palm sander on the 1/4 inch ply of my LOC Doorknob with the fin taped to the edge of the workbench.

 

[hornet driver]

As others have said, It's pretty much up to you . Scale builds excepted. From a performance point of view, there's no real( at least not measurable by the human eye), gain in performance in LPR. Your better off shaving weight, making sure the fins are aligned, launching in NO wind, things like that. Personally, I prefer to round my fins. How sharp that round is ,I base on the look I want. At least to me flat leading edges look unfinished. I can live with the trailing edge being flat on a highly swept fin but that's just taste. Also if you really taper the trailing edge to that pretty paper thin look, it's not very durable on recovery and it usually takes about ONE flight to find this out. ---H

 

[Kruegon]

So I seem to fall with most of you. I usually round the edges in general. I'm working on learning to do bevels well. I have several HoJo kits I'm in the middle of and I want to ale them more scale like.

The thing that sparked this is my current build. The Estes M-104 Patriot. The instructions are very clear and specific to leave them square. And it annoys me. But I am working to specifically build the kits stock. I am making one exception. I've started to put rail buttons on every rocket. So I'll be doing that instead of the launch lugs.

 

[K'Tesh]

One problem I see with square edges is that they are easier to show damage.

I typically round my edges, unless there's a reason not to (say for the back edges of sustainer fins that nest with the booster's fins (I leave the booster's leading edge square then too)).

 

[TopRamen]

I only do it for looks.

This rocket goes to an apogee of nearly 15 feet, then hovers perfectly for a solid second before the ejection charge puts an end to it's antics.
The fins are all just paper. I embossed the lines with a pair of scissors and my thumb. It works at any scale, but I don't have time for it right now.
It's paper, so it is essentially free.

 

[Kruegon]

The current build is the M-104 Patriot. It's my best guess that the fins are left square to generate more drag. Thus the fins can be smaller and slightly closer to scale. Not that these fins are scale, but rounded edges may have required even larger fins to adjust the CP. Same with the Bullpup. The aft fins are rounded leading edge only and the canards are to be left square.

 

[samb]

The current build is the M-104 Patriot. It's my best guess that the fins are left square to generate more drag. Thus the fins can be smaller and slightly closer to scale. Not that these fins are scale, but rounded edges may have required even larger fins to adjust the CP. Same with the Bullpup. The aft fins are rounded leading edge only and the canards are to be left square.

That's an explanation I hadn't considered before. I thought that, at least in the Patriot's case, additional sanding instruction was beyond the skill level 1 designation. Needless to say I ignored the instructions and knocked off the leading and trailing edges on mine.



Sanding in those crisp, knife edge sections in 1/8th and 3/16th balsa is a skill I haven't acquired yet. Folded cardstock over internal stringers like Top showed is one method. Chris Michielssen's rocketbuilding blog has several build threads that detail his airfoiling technique. Hardening the edge with CA and then coloring the edge with a marker to give your calibrated eyeballs a visual reference when sanding are two of the tips I learned there.

 

[neil_w]

Sanding in those crisp, knife edge sections in 1/8th and 3/16th balsa is a skill I haven't acquired yet. Folded cardstock over internal stringers like Top showed is one method. Chris Michielssen's rocketbuilding blog has several build threads that detail his airfoiling technique. Hardening the edge with CA and then coloring the edge with a marker to give your calibrated eyeballs a visual reference when sanding are two of the tips I learned there.

I can't quite imagine knife edge sections on balsa surviving even regular handling, much less the rigors of launching. *Removal* of sharp edges is one of the beneficial side effects of rounding IMHO. Even if I wanted the "square look" or just was feeling lazy, I'd still knock the corners off.

 

[dford]

Dude.
Thou shalt airfoil all edges.
Thou shalt paper all fins.
Thou shalt CA all edges.

Unless in competion flights. No CA. No paper

Knife edge fins aren't a problem following certain steps. I've had mids land chute less and turn out ready for launch (minis the chute) because the fins were knife edge/rounded and papered TO the body tube.

It is a matter of preference. Some people don't like to sand more than they have to.

Personally I enjoy sanding a sweet looking fin shape into an awesome aerodynamic wing and finishing it with well deserved strength to remain that way.

 

[caveduck]

As a long time LPR comp flyer I have to respectfully disagree about the performance benefit of airfoils. Fin profile matters a LOT if you want altitude performance. Square fin edges are especially draggy; even rounding them will bring a considerable improvement, and a sharp trailing edge to essentially eliminate base drag from the fins contributes quite a bit more. It's actually not hard to do - with a sanding block and some CA you can make a good set of balsa contest fins in minutes. That said, except for competition models I do a profile that fits with the look of the model, whether squared off, rounded, flat tapers, etc.

 

[samb]

As a long time LPR comp flyer I have to respectfully disagree about the performance benefit of airfoils. Fin profile matters a LOT if you want altitude performance. Square fin edges are especially draggy; even rounding them will bring a considerable improvement, and a sharp trailing edge to essentially eliminate base drag from the fins contributes quite a bit more. It's actually not hard to do - with a sanding block and some CA you can make a good set of balsa contest fins in minutes. That said, except for competition models I do a profile that fits with the look of the model, whether squared off, rounded, flat tapers, etc.

Yeah, I think 100+ years of aerodynamic science has proven that airfoils beat a flat plate in many areas. I think the OP is focusing on getting a good scale simulation on LPR sized models.

 

[Kruegon]

Actually, it was just a burning question that crossed my thoughts. The object of my current endeavor is to build the kits EXACTLY stock according to the instructions. Thus I will leave the Patriot fins with a square edge. I am learning to bevel fins for my HoJo builds, which is supported by the instructions. But for other builds, I often round all edges. Haven't seen a call for airfoil by instructions for a while. A friend in our club always does edge bevels on all sides. I just wanted to know when and why and how you choose to do your edges.

There are only two primary ways to increase performance. Fin finishing / nose cone style or shear power of the motor. Not withstanding a thrust to weight ratio which requires more math and is not quite a beginner area. So understanding how and why you choose to finish your own fins will increase my understanding in this area and, hopefully, allow me to increase my own skills and performance. I'd like to get good enough to enter performance competitions one day.

 

[lcorinth]

I airfoil, or at least round, almost all my fins, except on rockets where I don't feel it looks right and performance is the last issue on my mind (i.e. something hulky/sporty like Der Red Max), or where the fin is an odd shape that might be hard to properly airfoil. And I don't track altitude on most of my rockets, so I don't really have any data on how it's affected my rockets. But I do it for two reasons: 1) I like the way it looks, and 2) even if I'm not tracking altitude, I like to think I've given the rocket the opportunity to be the best rocket it can be.

It only took me a little practice to get pretty good at airfoils. Even my first try at ellipticals went pretty well.

I even decided to try it on my first set of plywood fins on a MPR/HPR project:

[video=youtube;AI2jtedVBwI]https://www.youtube.com/watch?v=AI2jtedVBwI&feature=youtu.be[/video]

 

[Micromeister]

Like most of the other posters I always round the leading and trailing edges of the fins on all models regardless of size Micro, LPR or MPR.

That said: there are other considerations as well. First it is never a good Idea to round the fin tips. Why? because it creates a much wider tip vortex that greatly increases drag on the fins.

Someone earlier said on LPR model round fin edges doesn't show visually an increase in overall performance. We'll to the naked eye perhap that is true, however we have documented proof the simply rounding those leading and trailing edges can improve the performance on any model rocket by as much as 10%. While performance is not always the main concern, it is at least worth noting it can be greatly increased by that simple step. 10% seems like alot but over the years I can say for sure it is very possible.
Squared off fin edges do more easily show damage then rounded fins.

If Scale modeling than, Ture airfoiling is often required, Single and double taper fins are quite often used on sounding rockets and many military missiles. while not required for flight reasons on most LPR or MPR models these profiles were required for supersonic prototype flights.

You'll also notice on many Sounding Rockets partuclularly those that use Nike or HO-JO rocket motors the fins are often also tapers tip to root edge. sometimes to as little as the thickness of the sheet metal (.080") or Plate (1/8"). For those who have not closely examined Nike fins they are actually made from 3 pieces of .125" aluminum two outer skins with either a single of double bent taper with a single of double interior 1/8" Z form center span stiffeners at the fold points. Nike and HO-JO fins also taper for Root to tip edges.

Learning to produce True Airfoil "TearDrop" symmetric fins is almost an art form all it's own. Producing one is hard, producing a set of 3, 4 or more identical airfoiled fins in Balsa or Basswood is extemely difficult. Slight differences in any one or two of the set WILL induce spin in the rocket as it flies reducing the achieved altitude by more then the amount gained by the Airfoiled fins would have added over simply rounding the Leading and Trailing edges.

Surface smoothness, particularly around transitions & external projections, Alignment and Optimal mass are the ways we increase the achieved altitude of model rockets for a given motor thrust class. Fin shape and profile run a distant 5th in this area.
Hope these little tid-bits help a little.