Free Form Fins - CP Issues - OpenRocket

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Aquaman33
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Free Form Fins - CP Issues - OpenRocket

Postby Aquaman33 » Mon Sep 30, 2019 8:22 am

I am working on an experimental concept in OpenRocket.

But I am having a few issues with the Centre of Pressure jumping around when I make tiny adjustments to the free form fins.

I sketched out these fins and I got a CP of 126cm and a stability of 1.93 cal. Very happy with that.

But then I started to drag the top left corner (see green arrow).

Image

If I drag that corner about 1cm in any direction the CP jumps forward 13cm and the stability drops to 1.25 cal.

I am happy with 1.93 cal. I am not happy with 1.25 cal.

Image

These are 20cm fins so a 1cm adjustment to 1 corner seems tiny in proportion to the fins.

So there are only 2 explanations that I can think of:
1. There is a tiny sweet spot in the corner of the fin that provides significantly more drag.
2. There is a bug in OpenRocket that gives false numbers.

I think it would hard to make the fins to exactly match the ones in the sweet spot, so I guess I should be conservative and use the lower stability.

Has anyone else experienced this?

Any explanations for what is going on here?

I have always found OpenRocket very reliable, but this is making me nervous.

Thanks.

joeman
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Re: Free Form Fins - CP Issues - OpenRocket

Postby joeman » Mon Sep 30, 2019 8:38 am

I haven’t experienced this.

Did dragging away from centre line also cause a reduction? (Not that you would want to in real life). I would expect this to increase area of fins and so increase stability.

Have you performed drag/stability analysis? Ie over range of speeds... rather than just at mach 0.3? Not that this helps...but it might provide some insights.

Cheers

Joe


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SpaceManMat
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Re: Free Form Fins - CP Issues - OpenRocket

Postby SpaceManMat » Tue Oct 01, 2019 12:29 am

I’ve not looked into how free form fins are calculated in OR but pretty sure it’s not going to be straight forward and there will be a whole bunch of assumptions as any shape is possible. I expect you are coming up against a threshold and it’s causing OR to change its calculation method. I suspect it’s not a bug so much as a simplification of an extremely complicated problem, but would have to really dig into the code to figure it out.

Have you tried not using free form fins? I suspect you’ll get better answers from it, however this then introduces issues related to the fin form being different to the actual fins.

I guess the next question is how critical is your rockets performance? If you’re building a Mach buster then getting stability right is very critical, but it looks like your building a LPR. In which case I would assume a worst case scenario and then possibly add a bit of nose weight after you build it and have verify where the CG actually is.
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Lamp
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Re: Free Form Fins - CP Issues - OpenRocket

Postby Lamp » Wed Oct 02, 2019 9:45 am

If your rocket is short and squat I think your design is just in the "sweet spot", but if your rocket is longer, SpaceManMat might be right. You can test for this if you make small progressive changes in each direction and graph the results of point location Vs. stability margin (say moving the point 2mm each time). If you get a progressive curve, your fin shape is probably just really sensitive but if you get a sharp jump it is probably a change in calculation method.

You could also run your design in another simulation program like Rocksim but if you do not have it and don't want to pay for it, RasAero would do the job, although more designed for supersonic flight, it is free and would give you a good check... :wink:
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Aquaman33
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Re: Free Form Fins - CP Issues - OpenRocket

Postby Aquaman33 » Thu Oct 03, 2019 9:28 pm

Thanks all for the advice.

I decided to change the fin design significantly to something that doesn't give me these anomalies. I don't want to take a chance.

If you are tweaking fins in OpenRocket just be aware that a sudden increase in stability from a small change may be too good to be true.

Aquaman33
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Re: Free Form Fins - CP Issues - OpenRocket

Postby Aquaman33 » Thu Oct 03, 2019 9:30 pm

SpaceManMat wrote: it looks like your building a LPR.


:D It's actually part of my Level 2 project. That body tube is 152mm! :D

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OverTheTop
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Re: Free Form Fins - CP Issues - OpenRocket

Postby OverTheTop » Fri Oct 04, 2019 7:36 am

Make sure you fly with a decent size chute, as fins out the back are more prone to damage on landing.
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