Stability effects of vertical surface at the end of a fin

[Duderino]

How much stability does a vertical surface at the end of a fin impart? An example of what I am talking about would be the Jay Hawk rocket.
https://www.apogeerockets.com/Rocket_Kits/Skill_Level_3_Kits/Jay_Hawk_AQM-37A
I am trying to model a new design this in Open Rocket with small vertical fins like this, but it doesn't look like it is supported. I am thinking I could add a fin of an equivalent stabilizing effect to the body, but my Google-Fu is failing on how much effect it would have.

thanks

 

[neil_w]

Fins on fins have a bit less effect than fins on the body tube, at least based on the OR modeling for the upcoming version that supports pods.

 

[Nytrunner]

Fins on fins have a bit less effect than fins on the body tube, at least based on the OR modeling for the upcoming version that supports pods.

Fascinating. Is this based on your comparison of the new version's tip fins vs same sized fins on body tube?

I'd think a tip fin would have even more stabilizing authority since it's further in the airstream

 

[neil_w]

Fins on fins: CP = 14.085"

Fins on body: CP = 14.024"


Apparently the reason is some fin/body interaction (described in Sampo's original paper) that increases the effect on CP when the fin is on the body. I don't know the physics, and cannot vouch for the accuracy or correctness of this, but that's the way it will be implemented in the coming OR. We went through a good bit of discussion recently and the folks who wrote the code feel that the modeling is reasonable based on what they know. It would be good to do the above experiment on Rocksim and see how the results compare.

 

[Duderino]

Thanks, That is helpful.
I will be looking forward to that new OR version.

 

[Cape Byron]

Fins on fins have a bit less effect than fins on the body tube, at least based on the OR modeling for the upcoming version that supports pods.

Upcoming version? Now I'm excited.

 

[neil_w]

Upcoming version? Now I'm excited.

It's still going to be a while, so don't get *too* excited. The good news is there are more developers actively working on it now then there have been in a long time, and stuff is getting done. There's still plenty more work to do. Release target right now is "sometime this year", which is doable if everyone keeps plugging.

If anyone knows Java and wants to get involved, please do.

 

[Cape Byron]

I'm patient. I can do excited long term.

Sorry, can't help with Java except drinking it...

 

[OverTheTop]

Looking at the effect of winglets on aircraft there is a marked improvement in fuel consumption. They reduce the amount of energy in the wingtip vortex, by attempting to slow the mixing of the flows from over the top and underneath the wings.

Having plates on rocket fins will have a similar effect, but the difference is that a rocket is not flying at a non-zero angle of attack constantly. Lift from the fins constantly restores the AoA to nearly zero, and at zero AoA there will be no vortices generated.

Their area will contribute slightly to stability, but not much I think, for reasonably sized plates.

 

[John Kemker]

It's still going to be a while, so don't get *too* excited. The good news is there are more developers actively working on it now then there have been in a long time, and stuff is getting done. There's still plenty more work to do. Release target right now is "sometime this year", which is doable if everyone keeps plugging.

If anyone knows Java and wants to get involved, please do.

Convert it to Python, and I'll give it a shot. [grin]

 

[Maxwelljets]

Fins on fins: CP = 14.085"
View attachment 415198
Fins on body: CP = 14.024"
View attachment 415199

Apparently the reason is some fin/body interaction (described in Sampo's original paper) that increases the effect on CP when the fin is on the body. I don't know the physics, and cannot vouch for the accuracy or correctness of this, but that's the way it will be implemented in the coming OR. We went through a good bit of discussion recently and the folks who wrote the code feel that the modeling is reasonable based on what they know. It would be good to do the above experiment on Rocksim and see how the results compare.

Correct me if I'm wrong here, but doesn't that show the exact opposite of what you're saying? The CP of the fins on fins is farther back than the CP of the fins on body, meaning fins on fins have a slightly larger effect.

 

[neil_w]

Correct me if I'm wrong here, but doesn't that show the exact opposite of what you're saying? The CP of the fins on fins is farther back than the CP of the fins on body, meaning fins on fins have a slightly larger effect.

You are correct... and now I'm confused. So: ignore everything I previously wrote in this thread. I will try to go find out the real story.

Can someone do this experiment with Rocksim to see how it compares?

 

[Maxwelljets]

You are correct... and now I'm confused. So: ignore everything I previously wrote in this thread. I will try to go find out the real story.

Can someone do this experiment with Rocksim to see how it compares?

In terms of actual physics, I suspect small fins on fin tips basically just work like winglets on airplanes, and basically just increase the effectiveness of the existing fin. Once you get into fins on fins that have similar area to the regular fin, they start acting like regular fins as well. Are these fin-on-fin equations in the existing Openrocket documentation/thesis paper?

 

[neil_w]

In terms of actual physics, I suspect small fins on fin tips basically just work like winglets on airplanes, and basically just increase the effectiveness of the existing fin. Once you get into fins on fins that have similar area to the regular fin, they start acting like regular fins as well. Are these fin-on-fin equations in the existing Openrocket documentation/thesis paper?

I would tend to doubt it but I don't know. However, it is important to note that OR isn't aware that the fins are actually fins-on-fins, because the model is not constructed that way (unfortunately).

I can't really speak for Rocksim, but in general these are not CAD programs. They don't "understand" arbitrarily complex geometries. As the geometry gets more complex, the likelihood of accurate simulation goes down.

 

[OverTheTop]

In terms of actual physics, I suspect small fins on fin tips basically just work like winglets on airplanes, and basically just increase the effectiveness of the existing fin.

Winglets only assist lift at non-zero AoA, which is where rockets are most of the time. You pay for that performance with added mass and frontal area. In a rocket the trade is probably not worth it most of the time.

 

[Maxwelljets]

these are not CAD programs. They don't "understand" arbitrarily complex geometries. As the geometry gets more complex, the likelihood of accurate simulation goes down.

Right, I get that it's not doing any CFD or anything, and the equations are just simplified models. I'm still interested in seeing the equations used to model the fins-on-fins.

Winglets only assist lift at non-zero AoA, which is where rockets are most of the time. You pay for that performance with added mass and frontal area. In a rocket the trade is probably not worth it most of the time.

They still act to increase the slope of the lift vs AoA curve, which will result in increased fin effectiveness and therefore will pull the CP backward. It's just like increasing the aspect ratio or surface area of the fin. They all have the same tradeoff between moving the CP aft and adding more mass/surface area.

 

[OverTheTop]

They still act to increase the slope of the lift vs AoA curve, which will result in increased fin effectiveness and therefore will pull the CP backward. It's just like increasing the aspect ratio or surface area of the fin. They all have the same tradeoff between greater stability and more mass/surface area.

Correct.

 

[BABAR]

In a rocket the trade is probably not worth it most of the time.

Compared to typical 3 or 4 fin and nose cone models probably true, you are better off from drag and CP effect by using wider span fins.

But for style points with “winged” rockets like the Jayhawk, they provide a stability surface in an orthogonal direction, so for some models they may be a good option.

 

[OverTheTop]

We quite often trade style for performance

 

[JoePfeiffer]

You are correct... and now I'm confused. So: ignore everything I previously wrote in this thread. I will try to go find out the real story.

Can someone do this experiment with Rocksim to see how it compares?

I'm confused too! Could you send me the OR files you created for this test?

 

[neil_w]

PM sent.

 

[jqavins]

Compared to typical 3 or 4 fin and nose cone models probably true, you are better off from drag and CP effect by using wider span fins.

But for style points with “winged” rockets like the Jayhawk, they provide a stability surface in an orthogonal direction, so for some models they may be a good option.

The Jayhawk that Duderino cited in the OP is the extreme example that proves winglets can contribute meaningfully to stability. Without the winglets the two fins are coplanar and should provide stability in the roll axis but do little or nothing for yaw. The winglets are the only yaw fins.

I'll try to remember to run a test case in RS tonight.

 

[BABAR]

The Jayhawk that Duderino cited in the OP is the extreme example that proves winglets can contribute meaningfully to stability. Without the winglets the two fins are coplanar and should provide stability in the roll axis but do little or nothing for yaw. The winglets are the only yaw fins.

I'll try to remember to run a test case in RS tonight.

Thanks, L”colplanar” was the word I should have used instead of “winged”

 

[vcp]

Added interference drag of the fin/tip plate joint?

 

[jqavins]

RockSim has its own improved formulae for computing the CP, and can also use Barrowman.

I defined three cases. All three have the same three large fins and the same three small fins. Once case places the six fins evenly around the body tube, alternating the large and small.

The second case alternates the fins around the body tube, but each small fin is orthogonal to one of the large fins.

The third case uses the small fins as winglets at the tips of the large fins.

The CP numbers are inches from the tip of the nose cone, so a larger number is more fin effectiveness.

Case​	CP (RS formulae)​	CP (Barrowman)​
Evenly spaced on tube​	17.90​	17.56​
Orthogonal on tube​	17.92​	17.56​
Winglets​	17.96​	17.52​

By the RS formulae, the winglets are slightly more effective than the small fins on the body tube. Oddly, these formulae also show the stability better with the weirdly spaced fins on the tube than the evenly spaced fins. I’m not sure I buy that, and it makes me question just how much better the RS equations are than Barrowman. But I can prove nothing (since I don’t have a wind tunnel).

Barrowman shows the winglets as less effective than the small fins on the tube, opposite from what the RS equations show. It shows the same results for the two arrangements of fins on the tube, which is what one would expect from simple formulae.

And finally, with either the RS or Barrowman formulae, there is damn little difference between winglets and tube mounted small fins, whichever one looks better.

 

[Nytrunner]

Excellent comparison. For basic 3/4fnc models, its good to know that substituting body mounted fins is sufficient.

Also its kind of nice to have my theory born out that winglets are "a little" better than body mounted, and it makes sense that openrocket's narrower range of assumptions would have trouble with it.

How kuch this translates to using OR substitutes for somwthing likenthe Orbital Transport.....thats another story

 

[neil_w]

Also its kind of nice to have my theory born out that winglets are "a little" better than body mounted, and it makes sense that openrocket's narrower range of assumptions would have trouble with it.

Except, as pointed out above, I was wrong... in its current (uh, current-future- whatever) form OR actually gives slightly more weight to the winglets. The difference is slight, though, as in Joe's Rocksim results there.

 

[jqavins]

Another caution regarding my results: they only cover one sided winglets at right angles to the fin. Other configurations may well give different results. But any differences are probably still slight. Probably.

 

[Nytrunner]

Except, as pointed out above, I was wrong... in its current (uh, current-future- whatever) form OR actually gives slightly more weight to the winglets. The difference is slight, though, as in Joe's Rocksim results there.

Oop, you're right. I was only remembering your initial position

 

[jqavins]

Oop...

Is that the singular of "Oops"? For when you've only made one mistake?