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  #11  
Old 05-06-2009, 12:41 PM
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Quote:
Originally Posted by Doug Sams
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.
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  #12  
Old 05-06-2009, 12:54 PM
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Quote:
Originally Posted by BEC
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!

Frank
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  #13  
Old 05-06-2009, 01:28 PM
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Quote:
Originally Posted by BEC
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.
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  #14  
Old 05-06-2009, 02:23 PM
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Quote:
Originally Posted by BEC
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

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  #15  
Old 05-06-2009, 03:17 PM
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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
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  #16  
Old 05-06-2009, 03:22 PM
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Quote:
Originally Posted by Nuke Rocketeer
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.
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  #17  
Old 05-06-2009, 03:24 PM
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Quote:
Originally Posted by 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


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 .
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  #18  
Old 05-06-2009, 04:04 PM
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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.
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  #19  
Old 05-06-2009, 04:59 PM
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Talking To square, or not to square...

Quote:
Originally Posted by Doug Sams
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 ) that rounded edges cause more drag than square ones, 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.

As always, your mileage (or airspeed) may vary,
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  #20  
Old 05-06-2009, 06:10 PM
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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.
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