Rounded fin edges out of fashion??
 

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?

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.

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.

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! :)

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

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

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

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

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

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




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