Fin Flutter Occurs at Higher Speed With Increased Span Length?

I used the fin flutter calculator through info central and found that as the span length increased on my proposed clipped delta fin design, the speed at which fin flutter occurred increased. I thought the opposite, and is this true?

View attachment Diablo[1]-1.rkt

This is the link to the calculator if you want to see for your self.

Go to the column on the left and click on "Data Set"
Mine is the first clipped delta fin on the list.

View attachment 138-finflutter.xls

It appears to me that something is awry in the file. That shouldn't happen. I even put in some figures for a trapazoidal fin that I know works, and it gives me a relatively low number, so when I thickened the material it didn't even change the answer. That tells me there is something not working correctly.

John

kokodog said:

I used the fin flutter calculator through info central and found that as the span length increased on my proposed clipped delta fin design, the speed at which fin flutter occurred increased. I thought the opposite, and is this true?

Click to expand...

After all the questions in recent weeks on this forum, I have to ask, what is your cert level? And just how many L/M 75mm motors have you flown? After answering that we can discuss how you should correctly model flutter...

2 L motors, one 54mm one 75mm

Timely thread. I'm working on modeling fin flutter for a rather extreme project I'm working on (100k ft apogee while staying under Mach 3)...my starting place has been the NACA flutter analysis equations, I think they're described on the website that you're looking at.

I developed this spreadsheet for this project in particular in about 15 minutes...it's nothing spectacular and wasn't intended to be publicly released, but it's actually the best estimate that I've found.

I've found two spreadsheet calculators for fin flutter, and the difference between the two was pretty substantial. One of which is the one that you're using, and another one is linked below. I'm not sure who the original author is on the 3rd part spreadsheet.

Mine returns the middle of the road estimate compared to the other two, and the results that it produces are pretty reasonable based on the fin designs that I've seen be successful vs unsuccessful.

Shown are two examples: my rocket and an Extreme Wildman. I've never heard of an Extreme Wildman shredding from fin failure, and this spreadsheet proves that. Simming it on the CTI M2020 gives me a peak velocity of around 1250 mph, and this spreadsheet says fin flutter would occur at 1668 mph.
The best fin designs have a long root chord in comparison to their span, and have an appropriate thickness.

I'd like to next explore how maximum dynamic pressure (Maxq) (usually the driving pressure for structure design on full scale launch vehicles) impacts our rockets at the high end of HPR. I think flutter would destroy airframes long before the maxq would crush them. However, maxq+a nonzero angle of attack has been responsible for some shreds at BALLS in years past.

David Harris

kokodog said:

I used the fin flutter calculator through info central and found that as the span length increased on my proposed clipped delta fin design, the speed at which fin flutter occurred increased. I thought the opposite, and is this true?

Click to expand...

G2Rockets said:

It appears to me that something is awry in the file. That shouldn't happen. I even put in some figures for a trapazoidal fin that I know works, and it gives me a relatively low number, so when I thickened the material it didn't even change the answer. That tells me there is something not working correctly.

John

Click to expand...

I don't know why this would be surprising to anyone, though perhaps there's some confusion about which dimension "span" is. But there's a diagram on the worksheet that shows which dimension it is, so...:confused2:

Yes, if you make skinny fins with a short root chord and a long span, like an airplane wing, they're going to be more flutter-prone than if the span is short compared to the root chord.

But thanks for the link to the spreadsheet. I've heard of these calculations but haven't done them before.

Update: I played with the spreadsheet, and found that although the relationship to span is as I would expect, the flutter speed goes down with increasing root chord, holding everything else constant, which doesn't seem right to me.

Awesome. I wish I had this spreadsheet before I flew my big Bertha upscale on a k1440.

I find the one posted by KoKoDog to be a substantial underestimate of flutter speed for the record. It says the Extreme Wildman fins shred at 492 mph, which just isn't right.

Adrian- I think that's because if you don't also increase the sweep length, you're just increase the effective area of the fin by increasing the root chord- you're just making the rectangular/trapezoidal back half of the fin longer but not making the triangular front part of the fin any longer. Bigger fin= more flutter, but if you make the sweep length longer as well, you end up with less fin area=less flutter.

I cannot see the spreadsheet formulas on my iPhone, but look at the underlying equation for divergence velocity and you will see it decreases with increases in either fin surface area or chord length (at varying rates though as the chord length factor is actually a differential).

Adrian A said:

I don't know why this would be surprising to anyone, though perhaps there's some confusion about which dimension "span" is. But there's a diagram on the worksheet that shows which dimension it is, so...:confused2:

Yes, if you make skinny fins with a short root chord and a long span, like an airplane wing, they're going to be more flutter-prone than if the span is short compared to the root chord.

But thanks for the link to the spreadsheet. I've heard of these calculations but haven't done them before.

Update: I played with the spreadsheet, and found that although the relationship to span is as I would expect, the flutter speed goes down with increasing root chord, holding everything else constant, which doesn't seem right to me.

Click to expand...

Marsman said:

I find the one posted by KoKoDog to be a substantial underestimate of flutter speed for the record. It says the Extreme Wildman fins shred at 492 mph, which just isn't right.

Adrian- I think that's because if you don't also increase the sweep length, you're just increase the effective area of the fin by increasing the root chord- you're just making the rectangular/trapezoidal back half of the fin longer but not making the triangular front part of the fin any longer. Bigger fin= more flutter, but if you make the sweep length longer as well, you end up with less fin area=less flutter.

Click to expand...

I have mine up to 780. No way this is correct.

Marsman said:

I find the one posted by KoKoDog to be a substantial underestimate of flutter speed for the record. It says the Extreme Wildman fins shred at 492 mph, which just isn't right.

Adrian- I think that's because if you don't also increase the sweep length, you're just increase the effective area of the fin by increasing the root chord- you're just making the rectangular/trapezoidal back half of the fin longer but not making the triangular front part of the fin any longer. Bigger fin= more flutter, but if you make the sweep length longer as well, you end up with less fin area=less flutter.

Click to expand...

The sweep length doesn't enter into the calculations at all in that first posted spreadsheet that I was referring to. There's no input for it. The results from that one aren't worth looking at, IMO. I haven't tried the other ones yet.

I think fin sweep also provides a stabilizing effect that can reduce or eliminate flutter, all else being equal. From what I understand of the mechanism for flutter, it's caused when a small twist in the fin amplifies the twisting moment, and the natural frequency of the fin matches the vortex shedding frequency. When a fin is swept, a twist in the fin will be resisted by the increase lift toward the tip, caused by the twist. I did a little web searching, and checked out the web page for finsim, which uses calculations that take this into account. I'm tempted to buy it, especially because it also has some calculations for spin stability, which I'm interested in as well.

I've used finsim--the results seem quite reasonable.

I figured I needed to refine my designs better after having a circular chunk of fin come loose--looked like a shark took a bite out of it!

Have you used Fin Sim supersonic? From glancing through the description, it seems like Fin Sim is pretty confident in subsonic predictions, but they only mention supersonic analysis in passing. I think most of us that are interested in this level of analysis are flying super or even hypersonic rockets.

I've been using an earlier version before they added the supersonic analysis.

Adrian A said:

I don't know why this would be surprising to anyone, though perhaps there's some confusion about which dimension "span" is. But there's a diagram on the worksheet that shows which dimension it is, so...:confused2:

Click to expand...

Adrian, what is the fin span:wink:, that is why i said something was awry. I haven't had time to sit down and look at the formulas on the sheet, but it looks to me like there is something that throws some figures off. you know how math is, sometimes when you have a zero in the equation it makes things figure incorrectly and that is what I think may be happening here. It is basic that If the span increases the speed at which flutter exists goes down. The figure that throws me off is the one for g10 on the trapazoidal fin that doesn't change at all.