Fin Size and Stability

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Race58

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Are rockets more stable with longer (Vertical) or with wider fins or is it just fin area than makes the difference?
 
Neither and both.
BALANCE, Race58-SAN!

From my experience, the shorter rockets need wider (larger) fins. The CP often moves forward with smaller fins. Longer rockets, tend to have a forward biased CG, and can often get away with a smaller fin. Nose weight can be used to compensate n cases where the CP is forward of CG (lots of tech babble here huh?) , but only to a finite degree.
Bob H48- (Yeah I’m talkin bout chu Bob!), has made 1 or two scale paper rockets that have no fins, but fly quite nicely because of working out the correct CG (BALANCE)
One of my own designs the XLR-18 uses small fins as well,; but this brings in another point- increasing the number of fins can also help with questionable stability. The XLR-18 didn’t need them, but I added an extra fin anyway. I flies very well, and high! Other more notable examples of this- The Estes Cobra 500(?) and the Mean Machine.
The final answer- Depends on whatcha doin man!?? However you do it though- BALANCE!

My 2 pennies worth….

AX’E :thrasher:
 
I'd strongly suggest getting a copy of "the Handbook of Model Rocketry" any edition will do, the current version is available thur Quest, the nar thur NARTS, and most book stores. It has a complete section on stability that is well worth the read.
You might also try to locate a copy of either Estes or Centuri engineerings tech report on stability TR-100 or TIR-100 if memory serves. I seem to recall they are available on-line somewhere but I can't put my finger on it.
 
As with a lot of things, it depends on the situation. It is not only a function of the fin area, but also where the center of that fin area is. That is why swept back fins (think of the Mosquito, Quark and 220 Swift) are efficient - they pull the CP back. A fin of the same area that did not extend back past the base of the rocket would have a CP further forward. And depending on where the CG is, even that second rocket might be stable.
 
use rocksim (or the free trial version) to play with fin design. if you do custom fin sets, you can mess round with fin shape and literally watch the cp and stability numbers change along with the fins. that should give you a good idea of how it works.
 
I'd strongly suggest getting a copy of "the Handbook of Model Rocketry" any edition will do, the current version is available thur Quest, the nar thur NARTS, and most book stores. It has a complete section on stability that is well worth the read.
You might also try to locate a copy of either Estes or Centuri engineerings tech report on stability TR-100 or TIR-100 if memory serves. I seem to recall they are available on-line somewhere but I can't put my finger on it.

Yes, every hobby rocketry person should read the Stine book. Although there are some nasty typos in the stability section.

The "classic collection" of Estes technical reports at https://www.esteseducator.com/content/publications.php are free and, er, um, classics. :surprised:
 
Thanks to all for the inputs. I keep learning every day and at my age that's hard to do.:lol:

I have one particular rocket in mind (BoRocks 3-cluster Crusader)and I am trying to decide which fin layout is better. The fins are exactly the same it is a matter of which way to place them.
Like the pic below. Left or right or would there be no difference at all in preformance? The rocket will be 27" tall with a BT80 BT.

Fin layout.jpg
 
Definately use the ROCK-SIM trial! :)
You really should get a book like the one (same mentioned, or similar if you're going to keep designing your own toys) having a reference within arms reach is great forstudying BEFORE you do something. :)

AX'E- en gang til... :thrasher:
 
I will do that when I get one. This is not my design and is the fins that come with the kit, but you can put them on either way. I was just wondering if there would be any difference in preformance as to which way.
Always look up^:D
 
Thanks to all for the inputs. I keep learning every day and at my age that's hard to do.:lol:

I have one particular rocket in mind (BoRocks 3-cluster Crusader)and I am trying to decide which fin layout is better. The fins are exactly the same it is a matter of which way to place them.
Like the pic below. Left or right or would there be no difference at all in preformance? The rocket will be 27" tall with a BT80 BT.

You didn't say how much of a span the fins had. If the kit says you can mount them either way, it will probably be stable either way. The left side has more frontal area and will probably be more draggy. The right side may limit the maximum motor weight slightly because of a higher CP.
 
Thanks, I think that gave me my answer.:)
Just to add, the span is 3.5" on the left and 4.5 on the right.
You didn't say how much of a span the fins had. If the kit says you can mount them either way, it will probably be stable either way. The left side has more frontal area and will probably be more draggy. The right side may limit the maximum motor weight slightly because of a higher CP.
 
If the fins are plywood then you would be safe attaching them using either configuration.

If the fins are balsa (and the grain direction becomes important) then you should attach them the usual way.

As far as what is better, the Barrowman equations really only address fin area. They don't even consider whether the fin is nicely airfoiled or flat-plate shaped. There are a whole bunch of things that should probably be included in a stability analysis, but most of our tools don't get down to such detailed levels.

Since a fin functions as a wing on our model rockets (explanation below), it would be good to review how wings work. Classic aero theory says that wings with a short chord (distance from leading edge/LE to trailing edge/TE) and a long span are more efficient, so a fin with that shape could deliver the same aerodynamic corrective force while being smaller, lighter, and less draggy. But they are also more prone to breaking under load, and to aero flutter (if your rocket moves fast enough). You have to strike your own compromise between good performance and reasonable durability. And if you are going for a certain style or "look" then classic aero theory just doesn't matter anyway.

If you want to go for high aspect ratio fins, you can improve your odds of success a bit by using different (stronger) fin materials than balsa (like basswood, plywood, fiberglass, or even graphite composite) or by using stronger root attachments (thru-the-wall, fillets, etc).

Back to explaining fins-as-wings: When your rocket moves straight ahead, and if the fins are well aligned with the rocket's long axis, then the fins will be at zero angle of attack to the oncoming airflow. If something disturbs the orientation of the rocket from a desired flight path, and the nose tilts one way and the tail tilts the other, you don't want it to fly sideways, you generally want it to point straight ahead for maximum speed/altitude/etc. So, the fins that are also now tilted are effectively at a small angle of attack to the oncoming flow, and generate a lifting force (perpendicular to the geometric plane that the fin is in, and also perpendicular to the rear end of the rocket) that tries to push the rear end back into a streamwise orientation. When the fins generate this lifting force they act exactly like little wings and all the rules and principles of subsonic aero come into play.

You want a round, smooth LE. A square LE will generally immediately trip the airflow from laminar to turbulent, which adds drag and reduces the effectiveness of the fin. You also want a TE that more-or-less tapers down to a thin edge (it doesn't have to be a paper-thin "knife" edge) to help satisfy the Runge-Kutta assumptions on airflow around wings. You can leave a substantial portion of the chord completely flat, and just shape the LE and TE, and you will get significantly better aero performance from the fins. For competition, this might be important, and could be the difference that gets you the winning advantage in an altitude model, for instance. For sport flying, all this fin airfoiling and planform optimization really doesn't make as much difference. Personally, I believe that most kits have fins that are enough oversized that you can safely leave them square-edged, and let your rocket fly with a bunch of planks. If you don't want to mess with it, don't; if you like to mess with it, go right ahead. Fin shape will probably not make a significant difference unless your design is on the hairy edge of being unstable...but you shouldn't be designing that way anyway...

(And that was probably way more answer than you wanted)
 
Some of the Apogee newsletters have some good articles on this topic. Remember too that the outboard section of the fin is likely in the less turbulent airflow and that's a benefit to extending with width of the fin versus it's length if you are looking for more corrective force. There's also consideration of extending the moment arm by being farther away from the body tube. Lots of interacting considerations and I think RockSim handles them nicely.
 
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