#1
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Fin Alignment
My Physics class in school is doing a lab where we are trying to create model rockets. I have read up on fin alignment and such, and it seems that the ideal scenario is one in which the fins are slightly angled with respect to the long axis of the rocket. That is, if you were to put the nose of the rocket up to your eye and look down the body, the front edge of each fin would be a little to the right or left of the back edge. We're wondering what the ideal amount of angling for the fins is. We would like our rocket to remain stable on the flight upward, but we don't want it to spin out of control, and we don't want to create too much unnecessary drag. I figure most of you on this forum are experts at this stuff, so if any of you have any tips about this particular problem I'd really appreciate it.
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#2
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I don't mean to contradict you and I don't know where you read that but...but the ideal angle is zero!
You will have much better luck and a much better flight if the fins are on straight.
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"I'm a sandman. I've never killed anyone. I terminate runners when their time is up." Logan from "Logan's Run" http://sandmandecals.com/ |
#3
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Quote:
Canting the fin as you suggest introduces spin to the rocket. Spin creates drag. Drag reduces the altitude the rocket achieves. So, the canting being the ideal scenario is dependent upon what scenario you are trying to achieve. If you are trying to achieve a super stable rocket (an effect of the spin), then canting them slightly will achieve that (1/32 to 1/16 inch should be sufficent). If you are looking at acheiving the highest altitude possible on a given motor impulse, then you should be looking at the straightest posible fin alignment.
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Don NAR 53455 "Carpe Diem" |
#4
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To Sandman:
I must say, all the diagrams I've looked at agree with what you're saying. But it did make sense to me that adding spin to the rocket would help to stabilize it. I mean, I know footballs and tops and such use spin to keep stable, so I didn't question the logic when I read it. But yeah, straight fins would certainly be much easier to put on, and this angle business would no longer be a problem. Thanks a lot! Last edited by necrodom : 05-30-2008 at 05:57 AM. |
#5
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To Barone:
Hmmm... I don't know whether or not we prefer stability to height, but I'll be sure to pose the question to my class on the morrow! Again, thank you very much for your help. PS. Also, thank you for the terminology help. I didn't know the correct term for this procedure, but "canting" sounds good, and I'm sure you know what you're talking about. |
#6
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Quote:
Football and golf balls too have the dimpled and pebbled surfaces that also induces some lift (lift also induces drag) but without fins the drag isn't as bad. The fins are basically like large paddle slapping at the air so that's probably where the extra drag comes from. The angle of attack is constanly changing especially if there is a cross wind...etc, etc... Slight spin would offset any stability imbalance caused by fins that may be slightly off, but. It just gets very complicated aerodynamically. It's just easier if you keep them straight.
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"I'm a sandman. I've never killed anyone. I terminate runners when their time is up." Logan from "Logan's Run" http://sandmandecals.com/ |
#7
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Model rockets become stable very quickly, the length of the launch rod will do it.That is why we have fin alignment guides. Any slight angle to the fins will effect the flight. Gimbled fins might be good for NASA type rockets using gyro's, but, for the average mdel rocket, I would go with straight fins.
Imagine what would happen if airplane wings were canted??? |
#8
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Quote:
For what it's worth, it's not like there's a tradeoff between height (actually altitude) or stability--we try to achieve both. Stability is a must-have, then altitude's a nice-to-have. A well designed "performance" rocket is both stable and capable of relatively high altitudes given the motor choice. Given that you're studying physics, the math of stuff like Barrowman equations (wiki it) should not intimidate you. Using the equations, or a simulator such as Rocksim, can help you figure outwhat fin size and shape/body length/etc. will make your rocket stable. Good luck! Hope you have fun, get hooked, and come back here for more help. --Chan Stevens Last edited by chanstevens : 06-20-2008 at 10:47 AM. |
#9
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Most model rockets have stable liftoffs on 3-4 foot rods with zero cant.
Sounding rockets launched at Wallops need cant for spin stability because the upper stages will stage with very little air for stability. Even single stage sounding rockets at Wallops have some minor cant for spin for stability once the air gets thin past 60-100K But again, model rockets are best with straight fins, unless you want the spin effect for fun
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Art Upton I am an original AR; pronounce like a pirate says "R" An AR is an Always Rocketeer, in differance to BAR; although I like to visit BARs now and then. |
#10
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Isn't there something in the Handbook of Model Rocketry about a type of instability mode called roll-pitch coupling? I think it has to do with the rolling (spin) sort of synchronizing with the pitching (the "wiggle" as the rocket ascends) to cause a nutation-like effect (where the tip of the nose starts to "draw" an ever-expanding circle), causing the rocket to go unstable. And the best way to avoid that is to avoid spin.
Not that I've ever seen that happen. Has anybody else? Comments? I've been wanting to get a Fliskit Deuces Wild kit and sand cambered airfoils into the fins so that a spin would be introduced, simultaneously demoing Bernoulli's Principle and getting a cool winding smoke trail. But I've been chickening out because of worrying about roll-pitch coupling. |
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