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Fin dimensions and effect on stability?

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Senior Space Cadet

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I guess I still have a lot to learn.
I almost always have the fin as far back as it can go, without the root extending past the body tube. Assume that for the following.
I noticed a long time ago that, while keeping the tip chord relative (I usually make it half the root chord), shortening the root chord moved the CP back.
At first that seemed counter intuitive, since the surface area of the fin was being reduced. It still seems a little counterintuitive, though I realize it must be because the CP of the fin, by it's self, has moved back.
My spinner rocket design was proving problematic so I was just working on another design with six fins.
I'd picked a root chord and, normally, I would just make the height the same or pick something that looked good.
But this time I was watching the stability number as I increased the fin height and noticed that at some point the stability started to go down.
I'm at a loss for understanding why the stability would go down with an increase in fin height.
 

n27sb

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More than likely the mass of your fins has increased and shifted the CG aft.
 

Senior Space Cadet

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More than likely the mass of your fins has increased and shifted the CG aft.
I wouldn't have that would be possible, with balsa fins, but I was using 1/4" balsa that I intended to make into a wing shape for maximum spin.
I'll go through the same process with 1/8" balsa and see what happens.
Since density isn't changing with fin height, there must be a point of diminishing return as far as fin height or area. There must be a point where increasing height/area isn't increasing stability enough to compensate for the increase in mass. I never would have guessed, but now that I know, it's one more thing I'll watch for in designing my rockets.
 

jqavins

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I believe that n27sb hit the nail on the head, except for one thing:
I noticed a long time ago that, while keeping the tip chord relative (I usually make it half the root chord), shortening the root chord moved the CP back...

I'm at a loss for understanding why the stability would go down with an increase in fin height.
First you mentioned the CP, then the static margin. I know you know those are not the same, obviously.

If the fins are REALLY tall, the CP of a fin alone can pass forward of the full rocket CP. Once you're there, making them any taller will bring the rocket's CP forward at the same time that it brings the CG down. Making thinner fins reduces the effect on CG (as of course you realize) but not the CP issue, if your fins are that tall.

Many of us have made the same discovery, and usually in silico, glad to say. For me it was with a rocket that I just couldn't make stable no matter how big I made those darn fins. Watching the CP and CG as I changed the fins size, and taking time to think things through instead of the flailing trial and error I had been doing, I realized that what I needed to do was make the rocket taller. That moved the CG forward a lot and the CP only a little. So I increased the length a little more, and decreased the fins size a little, and things were still good. This I repeated and continued until I reached something near optimum. (I never either calculated of diddled until I found the exact optimum, just stopped when it seemed like I was close.)

I ended up with a quite "normal" looking rocket. Then I had the epiphany that that approximate look seems normal because it's what we see a lot, and it's what we see a lot because it's near optimal. That was a valuable "well duh" moment for me.
 

Senior Space Cadet

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I believe that n27sb hit the nail on the head, except for one thing:First you mentioned the CP, then the static margin. I know you know those are not the same, obviously.

If the fins are REALLY tall, the CP of a fin alone can pass forward of the full rocket CP. Once you're there, making them any taller will bring the rocket's CP forward at the same time that it brings the CG down. Making thinner fins reduces the effect on CG (as of course you realize) but not the CP issue, if your fins are that tall.

Many of us have made the same discovery, and usually in silico, glad to say. For me it was with a rocket that I just couldn't make stable no matter how big I made those darn fins. Watching the CP and CG as I changed the fins size, and taking time to think things through instead of the flailing trial and error I had been doing, I realized that what I needed to do was make the rocket taller. That moved the CG forward a lot and the CP only a little. So I increased the length a little more, and decreased the fins size a little, and things were still good. This I repeated and continued until I reached something near optimum. (I never either calculated of diddled until I found the exact optimum, just stopped when it seemed like I was close.)

I ended up with a quite "normal" looking rocket. Then I had the epiphany that that approximate look seems normal because it's what we see a lot, and it's what we see a lot because it's near optimal. That was a valuable "well duh" moment for me.
I have no idea what a static margin is so, no, I'm not confusing it with CP.
I was using six fins in order to create as much drag as possible, so this might have something to do with it.
Let me play around with it for a bit and if it isn't related to having six fins I'll post a file, or screen shot, so you can see what's going on.
 

Senior Space Cadet

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Ok, this should be enough to show what I'm talking about.
In this screen shot, the rocket has a stability of 1.59 caliber with a fin height of 42mm.
Without changing anything else, if I increase fin height to 43mm, the stability drops to 1.58 and will continue to drop if I continue to increase the height of the fin.

Screenshot 2021-01-25 13.19.49.png
 

jqavins

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Ah. The static stability margin (sometimes just "static margin", or "stability margin", or "margin") is the distance between the CP and the CG, expressed as multiples of the maximum forward diameter, which is usually the body tube diameter, but different if you've got something like an Honest John. That "multiples of the diameter" measure is what's called calibers.

So when larger fins move the CP aft, if they move the CG aft by more, the two "centers" get closer together, so the static margin is reduced.

Now I'm worried that I may have insulted you, that the reference to a "Well duh" moment may have seemed like it referred to you. On the contrary, "well duh" is what I said to myself when it all came together that day.

-----------------------

Your last post ninjad me, so here's a response thereto.

1611606533222.png

The leading corner of the fins has passed the CP. Further increasing the height of the fins will have very little effect on the CP, and will eventually start to bring it forward. All the while moving the CG aft.

Your rocket does not seem to be nearly as short as the one I learned my lesson on. Still, more height would help. Or a little nose weight. Both of those move the CG forward. Smaller fins would probably help more, but if these are the fins you want, for aesthetic or other reasons, then the length and/or nose weight would be the way to increase the margin. Or you could leave it as is; a static margin of 1.5 is really quite adequate for a medium length rocket.
 
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Senior Space Cadet

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Ah. The static stability margin (sometimes just "static margin", or "stability margin", or "margin") is the distance between the CP and the CG, expressed as multiples of the maximum forward diameter, which is usually the body tube diameter, but different if you've got something like an Honest John. That "multiples of the diameter" measure is what's called calibers.

So when larger fins move the CP aft, if they move the CG aft by more, the two "centers" get closer together, so the static margin is reduced.

Now I'm worried that I may have insulted you, that the reference to a "Well duh" moment may have seemed like it referred to you. On the contrary, "well duh" is what I said to myself when it all came together that day.

-----------------------

Your last post ninjad me, so here's a response thereto.

View attachment 447870
The leading corner of the fins has passed the CP. Further increasing the height of the fins will have very little effect on the CP, and will eventually start to bring it forward. All the while moving the CG aft.

Your rocket does not seem to be nearly as short as the one I learned my lesson on. Still, more height would help. Or a little nose weight. Both of those move the CG forward. Smaller fins would probably help more, but if these are the fins you want, for aesthetic or other reasons, then the length and/or nose weight would be the way to increase the margin. Or you could leave it as is; a static margin of 1.5 is really quite adequate for a medium length rocket.
No, not insulted at all. I know I don't know as much as most of the people that are active on this forum and I'm just a retired baker.
I worked up another example with a shorter root chord and I was able to make the fins really tall before I stopped seeing an improvement in stability. It never went down, but it stopped going up. Now, this could be a limit of Open Rocket.
I was going to insert the screen shot but, apparently I messed it up. But it would have shown a rocket with a short, really tall fin at it's maximum stability. Making in any longer didn't add stability.
The lesson I've learned is I can make my fins smaller than I have been. I've always wondered why I'd see photos of guys launching high powered rockets out on some dry lake and their rockets would be really long with tiny little fins. Now I know why.
 

cbwho

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Remember an arrow is very stable.

I like big fins, that I cannot deny.
 

Nytrunner

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I've always wondered why I'd see photos of guys launching high powered rockets out on some dry lake and their rockets would be really long with tiny little fins. Now I know why.
Other factors for the extreme fliers, the faster you go, the more you have to take fin flutter into account. That's why you see the strong long root chords and short tips (less mass waiving around in the airstream). They also have their recovery harnesses, parachutes, and electronics up in the front of the rocket which can move the CG forward.
 

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