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  #21  
Old 04-01-2011, 02:01 PM
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Bill Bill 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.



I only airfoiled fins once. It was not easy; it was not fun; and the trailing edges take a real beating landing on anything but the finest grass surface. Never again!

I only photograph rockets from more than 15 feet; at that distance, who can tell? But the next time I build a prototype for a facecard photo, I will bring it up.


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  #22  
Old 04-01-2011, 06:35 PM
nvrocketeer nvrocketeer is offline
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I agree with the comments about doing whatever's necessary for scale or competition.

I also agree with the comments about doing whatever feels right.

Me, I like to round the leading edge, but I don't mess with the others. Life's too short. Also, for me it'll be a bigger improvement if I can get my fins straight and perpendicular than any edge treatment will ever give me.
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  #23  
Old 04-02-2011, 04:33 PM
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Stuart Lodge, a British model rocketeer, is a long-time international model rocket competitor and contest judge. In his book, "The Model Rocketry Handbook: 21st Century Edition" (available from AbeBooks www.abebooks.com and Alibris.com www.alibris.com ), he wrote the following (about fins *and* nose cone shapes with regard to drag and stability) on pages 87 and 88:


FINS

It's been accepted for generations that elliptical fin shapes induce less drag than other shapes and broadly this is true. However, at international space model events, where most of the disciplines are performance-based, a wide range of fin shapes is observed. Fin shape seems to be a lot less critical than theory suggests and connected with the fin's Reynolds Number and the fact that the fins are not always called upon to provide restoring forces - and so not creating lift, which contributes to induced drag. This is a very technical subject, made more so because fins are found at the tail of a rocket, where high speed airflow will have broken up and may be quite turbulent, such that only the tips of the fins are flying in relatively undisturbed air.

The cross section of each fin is also a matter for debate. Most authorities state that a symmetrical aerofoil section is best and ensures that restoring forces are created more effectively. Once more, the international contest arena provides little confirmation of this theory, with many successful fliers choosing thin flat plates as the basis for their fins. The truth lies in the fact that model rockets - especially FAI contest rockets - are relatively small birds and the fins are tiny compared with the wings of model aeroplanes. The result is that whatever aerofoil section and thickness chosen, works fairly inefficiently. In addition, although aerofoil sections may be more efficient, sanding six identical - in a three fin set up - surfaces introduces a human factor....inaccuracy! Crucially however, fin materials must be very rigid - tip flutter is ruinous - and warp-free fins attached accurately to the rocket's rear body with smooth glue fillets are essential. Skill and experience seems to matter more than the theory.


On page 88, he described how the nose shape can affect the fins' ability to keep a rocket aerodynamically stable:


The phenomenon of laminar airflow becoming turbulent, resulting in only the outer regions of fins being effective has been covered already. Some shapes - notably conical noses - promote this effect more markedly than others and can mess up the airflow to a point where the fins cannot properly exert restoring forces, resulting in the missile going unstable towards the end of the propellant burn. Scale models with conical noses, such as Atlas-Centaur, Proton, Tsiklon, Intercosmos et al, can be found wanting if propellant characteristics result in high accelerations and burnout velocities. Often much more fin area, provided by clear acetate add-ons is the only cure.
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Last edited by blackshire : 04-02-2011 at 04:39 PM. Reason: This ol' hoss done forgot somethin'.
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  #24  
Old 04-02-2011, 05:21 PM
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I can't leave 'em square (not leading edges anyway) - it wars too much with my aviation background (miniature and full scale). But I also know that an engineer and designer I respect did some testing and found for small model airplanes at least (relatively low speed, chords of less than a foot) a wing trailing edge better left squared off at about 1/8 inch thick or so and a number of his designs take advantage of that.

I have been following that example for years in my RC airplanes, and since I became BAR'd I have been doing the same with my rockets. Except for an Astron Constellation clone I did (where airfoiled fins are part of the look as well as the performance), I leave the TE's square. Whether I round or leave tips square kind of depends on my mood and the look of the rocket.

Now on model rockets I wonder where the crossover is with the higher speeds but generally smaller chords - where the Reynolds number gets high enough for trailing edge tapering to actually reduce drag. But I have too many research projects already to go chasing that one.

For what it's worth, the one "contest" type rocket I have is a FlisKits Cougar 660. Its fins are elliptical in planform and made from 1/64 inch thick plywood....one can hardly tell if I managed to round the LE on those!
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  #25  
Old 04-03-2011, 03:05 PM
Ltvscout Ltvscout is offline
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Quote:
Originally Posted by blackshire
Stuart Lodge, a British model rocketeer, is a long-time international model rocket competitor and contest judge. In his book, "The Model Rocketry Handbook: 21st Century Edition" (available from AbeBooks www.abebooks.com and Alibris.com www.alibris.com ), he wrote the following (about fins *and* nose cone shapes with regard to drag and stability) on pages 87 and 88:

Stuart is a member here on YORF.
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  #26  
Old 04-03-2011, 03:51 PM
scigs30 scigs30 is offline
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I try to make my Rockets look like the Catalog picture, so sometimes they are square and other times I kiss the edges with sand paper. My opinion the square fins are tougher to finish than rounded. Firs the edges have to be perfectly square and second air bubbles appear in the paint on the edges if not sealed well.
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  #27  
Old 04-03-2011, 09:42 PM
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Quote:
Originally Posted by Ltvscout
Stuart is a member here on YORF.
That's good to know--I wish he would post on here more often, but I know he wears many "avocational hats," including in soccer and cycling.
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