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  #1  
Old 03-31-2011, 03:23 PM
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76gunner 76gunner is offline
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Default To round or not to round.....

Okay, I need the general consensus of the rest of you.... way back when, the kit instructions used to tell you to round, or put an edge on the fins. I just recently started building a Baby Bertha, and no where could I find anything that said to do this. Is this a lost art, or is it just not done anymore? Help me Obi-Wan Kenobi, your'e my only hope
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Old 03-31-2011, 03:34 PM
jetlag jetlag is offline
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I nearly always round the edges unless it is a scale model of a rocket with square-edged fins.
It is more aerodynamic and aesthetically pleasing. I'm sure there are other opinions, but I have never been sorry for rounding edges, usually leading and trailing edges only.
Allen
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Old 03-31-2011, 03:57 PM
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Quote:
Originally Posted by jetlag
I nearly always round the edges unless it is a scale model of a rocket with square-edged fins.
It is more aerodynamic and aesthetically pleasing. I'm sure there are other opinions, but I have never been sorry for rounding edges, usually leading and trailing edges only.
Allen


Thanks Allen
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Old 03-31-2011, 04:20 PM
BPRescue BPRescue is offline
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Quote:
Originally Posted by 76gunner
Okay, I need the general consensus of the rest of you.... way back when, the kit instructions used to tell you to round, or put an edge on the fins. I just recently started building a Baby Bertha, and no where could I find anything that said to do this. Is this a lost art, or is it just not done anymore? Help me Obi-Wan Kenobi, your'e my only hope

Whether or not the instructions tell you to round the leading edge/taper the trailing edge depends on the rocket. I believe the Bertha’s and some others do not instruct this because it is an entry level/level 1 type build, whereas if you get into some of the harder builds and or higher performance the instructions do tell you to do it. It may also me a shape thing, since the Bertha’s are of a certain fin design.

I have also seen the double glue thing and fillets described in some builds, but not others. It has to be a basic rocket/level one thing, but like others, I just do-em all that way…

Last edited by BPRescue : 04-01-2011 at 10:54 AM.
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Old 03-31-2011, 08:13 PM
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tbzep tbzep is offline
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Sometimes I round, sometimes I airfoil, sometimes I taper, sometimes I leave square. It just depends on the model and my mood. Rounding is not really going to help performance, so if you do it it's just for your own satisfaction.

BTW, squared fins photograph better. Check out all of the old Estes/Centuri catalogs with actual photos and you will see very few that have been airfoiled or rounded.
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Old 03-31-2011, 09:00 PM
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Thanks, its been awhile but some habits die hard, my stuff is far from perfect, but what I have done looks pretty darn good. I had forgot how much fun this stuff is! Booyah!

Gunner
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Old 03-31-2011, 09:00 PM
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cas2047 cas2047 is offline
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Quote:
Originally Posted by tbzep
Sometimes I round, sometimes I airfoil, sometimes I taper, sometimes I leave square. It just depends on the model and my mood. Rounding is not really going to help performance, so if you do it it's just for your own satisfaction.

BTW, squared fins photograph better. Check out all of the old Estes/Centuri catalogs with actual photos and you will see very few that have been airfoiled or rounded.


Plus 1 to what he said...
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Old 03-31-2011, 10:23 PM
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The change in aerodynamic qualities is virtually nonexistent. The only thing you can do is reduce drag (since there is no lift) and a flat plate that is sanded with a flat edge is almost as clean as a rounded edge sanded irregularly. If you are finicky the trailing edge os probably as important in the reduction of drag. The molecules have to decide at some point which way they are going to travel; left or right. By impacting a flat surface it will create a relatively small amount of drag due to those molecules that do not move forcing others to go around them. It is actually less than the attendant parasitic drag of a rounded surface. Besides, if you talk to a mathematician they will tell you that there are no curves, only very small line segments

It isn't as if you are dealing with that much of a measure of boundary layer on a model with a fin chord less than six inches. Airflow is probably pretty well laminar within an inch of the LE on models we fly. And it is a lot easier to geneate a smooth regular finish on a flat plate.

The best compromise that I can think of is to bevel both sides and leave a small section along the centerline flat.
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Old 04-01-2011, 01:06 AM
chrism chrism is offline
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When I started building rockets in the late 70's, both Centuri and Estes would have you round the edges of the fins. When I came back to rocktery a few years ago, I noticed that Quest's instructions would have you leave all edges square, and the new Estes intructions would have you just round the leading edges of the fins. I try to follow the intructions of each manufacturer in how to sand the fins, but I did round both the leading and trailing edges of the Estes re-release of the Star Trooper. The original Centuri version would have you sand the fins in a proper airfoil shape.
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Old 04-01-2011, 08:14 AM
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76gunner 76gunner is offline
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Quote:
Originally Posted by jharding58
The change in aerodynamic qualities is virtually nonexistent. The only thing you can do is reduce drag (since there is no lift) and a flat plate that is sanded with a flat edge is almost as clean as a rounded edge sanded irregularly. If you are finicky the trailing edge os probably as important in the reduction of drag. The molecules have to decide at some point which way they are going to travel; left or right. By impacting a flat surface it will create a relatively small amount of drag due to those molecules that do not move forcing others to go around them. It is actually less than the attendant parasitic drag of a rounded surface. Besides, if you talk to a mathematician they will tell you that there are no curves, only very small line segments

It isn't as if you are dealing with that much of a measure of boundary layer on a model with a fin chord less than six inches. Airflow is probably pretty well laminar within an inch of the LE on models we fly. And it is a lot easier to geneate a smooth regular finish on a flat plate.

The best compromise that I can think of is to bevel both sides and leave a small section along the centerline flat.


Wow, impressive Young Skywalker Its all moot when you forget to put wadding in and the chute melts together and your bird prangs on impact Not that that has EVER happened to me!

Gunner
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