fin flutter

[rocketsonly]

This link was circulating on the DARS forum, so I decided to bring it here for those who have not seen it yet.

The link contains photos of in flight fin flutter from fins made of G10! Pretty amazing.

https://www.videorocketry.com/XPRS_2004

 

[BlueNinja]

Holy.....

Makes fiberglass look like it doesn't have any adhesive on it!

 

[rocketsonly]

Originally posted by rocketsonly
This link was circulating on the DARS forum, so I decided to bring it here for those who have not seen it yet.

The link contains photos of in flight fin flutter from fins made of G10! Pretty amazing.

https://www.videorocketry.com/XPRS_2004

It also contains video.

 

[Stymye]

same effect as taking a pencil between your fingers and wiggling up and down ,,, makes the pencil look like rubber.. in this case I think it's the camera wiggiling. really cool effect tho!

 

[loopy]

I think you're right, stymye - how could they flutter that bad, and not affect the paint that's on them? There seem to be no cracks in the finish at all. Dry paint doesn't take well to motion like that, and would have cracked and peeled with that amount of flutter, I would think. Granted, I'm not a chemist.

Loopy

 

[BlueNinja]

I don't think it was the camera wiggling... If it was, then the fins would probably be just waving. In the slow-mo video, you cna see that they are bending sometimes at the bottom and sometimes at the top.... Anyways, this is a cool anomaly.

 

[jflis]

Originally posted by stymye
same effect as taking a pencil between your fingers and wiggling up and down ,,, makes the pencil look like rubber.. in this case I think it's the camera wiggiling. really cool effect tho!

Problem is, why does not the rest of the rocket get distorted the same way. Put fins on the pencil and do that trick such that one part looks like rubber while the other doesn't. That;s the *real* corker with that vid.

Also, concerning paint cracking, if that *is* fin flutter it could be explained by heating in the transonic region perhaps... interesting though

 

[powderburner]

That is definitely flutter.

Speaking as an aerospace engineer by training, with a year of experience watching Boeing flutter models self-destruct, with additional exposure to watching flight test films of aircraft and weapons, I can tell you that what you see in those pix and vids is flutter.

 

[Smokin' Rockets]

Originally posted by stymye
same effect as taking a pencil between your fingers and wiggling up and down ,,, makes the pencil look like rubber

That was my first thought too.

As others have said, the paint was not damaged. Every severe case of flutter I have seen had paint chipping/cracking.

......Bill

 

[Mike]

Originally posted by stymye
same effect as taking a pencil between your fingers and wiggling up and down ,,, makes the pencil look like rubber.. in this case I think it's the camera wiggiling. really cool effect tho!

Looking at the stills from the video thats a lot of bending. Surely the camera would have to move a *lot* to get that effect, much more than a slight vibration?

Also the website shows some paint chipping from the base of the roots...if it were just a camera annomoly that wouldn't have happend.

As was said on the site, its seems kinda hard to beleive it is fin flutter but personally I find it harder to beleive its anything else.

Maybe the camera exaggerated the fin flutter somehow?

Just a few thoughts

 

[thomasrau]

Originally posted by Smokin' Rockets
That was my first thought too.

As others have said, the paint was not damaged. Every severe case of flutter I have seen had paint chipping/cracking.

......Bill

Not always the case. I recently exceeded the speed of G10 by lofting my Eclipse on an AMW K800, the flutter was bad enough to tear the bottom 3 inches off the fins but the paint stayed on. Oddly only the two fins near the lug were damaged. You should have seen the wiggle when it went supersonic.

 

[GL-P]

Not always the case. I recently exceeded the speed of G10 by lofting my Eclipse on an AMW K800, the flutter was bad enough to tear the bottom 3 inches off the fins but the paint stayed on. Oddly only the two fins near the lug were damaged. You should have seen the wiggle when it went supersonic.

Probably interference drag off the lug. Also a lug would turn a rocket a little off course, showing the lower parts of the fins to the oncoming air. It was probably going so fast that the fins cracked off.

 

[bobkrech]

Tom

If I had to guess, the launch lug either generated a shock wave or tripped the boundary layer that impinged on the fin and caused a enhanced loading severe enough to break the G-10. Ouch!

I did a quick sim of the PML Eclipse with a similar motor, and again want to point out that this appears to be another flight where the rocket barely went supersonic and encountered severe fin flutter. The transonic flight regieme is the most turbulent because some of the flow is subsonic and some is supersonic generating large moving pressure gradients around the vehicle that only occur when traveling near Mach 1. It's called Mach buffeting. Flow conditions get much steadier above and below this velocity range so if your fins are going to flutter, they are most likely to flutter in the transonic portion of the rocket flight.

Check out the following references which are easy to read and explain the transonic flight problem really well. This is truely the best web-based aerodynamic reference site I have found.

https://aerodyn.org/HighSpeed/highspeed.html

https://aerodyn.org/HighSpeed/buffet.html

https://aerodyn.org/HighSpeed/shock.html

Tom, were the fins upsized to 3/32" G-10 and reinforced by another layer of glass as recommended by PML for K-motors, or were they the stock 1/16" fins?

I really think that the 2 threads on fin flutter are showing that 1/8" G-10 fins are the minimum required for K and above flights on PML kits.

Bob Krech

 

[thomasrau]

Bob,

Shock waves off the lug is my theory as well. The fins survived the rapid climb through mach while under power. Although the fins must have fluttered since the rocket did a little tail wiggle. After burn out during coast is when the major event occured, since it stayed in the transonic region longer and the fins were more than likely already weakened. From the ground we could see it do a much bigger wiggle and for a longer duration which then straightened out when it was back to subsonic.
It had previously flown on the K570, K650 and 670 loads and stayed subsonic. I neglected to take into account the lighter weight and drag (this flight was minus the added video payload bay since I was trying to break 8000').
This Eclipse was "beefed up", it is the glassed phenolic version although the fins were the stock thickness. They did have some glass reinforcement as well as being foamed in place.
This winter the fin-can section will get rebuilt. A 75mm mount as well as being extended and thicker reinforced fins. Next years motto, 10,000 or bust.

 

[vcp]

Posting to this old thread because it was referenced by recent one -and- this one still perpetuates the myth.

I am not speculating. It was certainly an example of fin flutter, but the 'wave' in the fins -is- an artifact of an imager that is using a 'rolling shutter'.

The paper here,
https://ieeexplore.ieee.org/iel5/16/26588/01185163.pdf explains:
"Most CMOS image sensors feature the rolling shutter
architecture to control exposure time. However, imaging
of fast moving objects or using pulsed illumination is a frequent
task in industrial and scientific applications, which cannot be
done with the rolling shutter architecture."

The camera does not take an instant 'snapshot' of an entire scene, rather the image is taken progressively, one imaging line at a time. Each line is a new view of the subject at a different time, so if the subject is moving, it appears distorted. Fluttering fins become wavy fins.

Figure 1 in that paper is a distorted photo of a rotating fan taken with a rolling shutter camera. Somewhere I've seen an example video, but can't seem to come across it now.

GC

 

[Graham Orr]

I'm an engineering student at HMC (and a life-long rocketeer), so take what I say with a grain of salt:

Flutter (especially in transonic/transitional zones) exists. It is strong enough to destroy just about anything if in a shockwave. I think everyone agrees with this.

However, in this video, the fin flutter you see is an aliased version of mechanical resonance ocurring at 28,32, 58, or 62 (maybe higher) Hz if the video is recorded with NTSC standard. Similarly, if PAL was used, the flutter would be 22,26,46,50 etc.

I'm confident that what you see is NOT what you get for various reasons:
1) Last semester, I build a rocket with some professors with embedded dynamics strain gauges in the fins and airframe of a rocket (with G10 fins) and the resonance modes shifted between 50-60Hz in flight. The data can be found at https://www.eng.hmc.edu/NewE80/ResultsOverall.htm .

2) I had three professors of mine (one PhD MIT in chemical and materials engineering, second on team that designed SSME and X43 engine, third HMC->Caltech physics) take a good look at that video. They concluded that steady-state or transient resonance as expressed in the video seems unlikely - that is, they all think we are seeing aliased data.

3) The Nyquist-frequency of a standard video device is 14.985Hz (NTSC is 29.97Hz). If you have tapped and audibly been able to hear your fins make a tone, it is likely you are above the Nyquist-frequency and therefore you will experience aliasing/folding in a standard video.

Anyways, what I (and my professors) find very interesting is the distortion that occurs in the transonic region. This may be a digital glitch but it is likely that the compression wave from the boundary layer changed the local index of refraction of the air, (and by the Archimedes Principle) creates an optical illusion.

We'll be launching a rocket, Mudd II on May 12 that will take very careful dynamic measurements of a carbon-fiber rocket's resonance (with 2 video camera to look for flutter). After a quick test flight on an H (rocket weights about 2 lbs), we're planning a J415. In the summer (at LDRS), with a higher-performance fincan (and CFD/nonlinear stress analysis) we'll push it to the breaking point with a full K/baby L... giving us Mach 2.5 (and with a custom 600MHz blackfin flight computer) sampling on 12 sensors at 30kHz.

Given the 200kpsi tensile strength of the airframe/fins, I'm not quite sure an L will destroy it...we only have 3ft x 54mm for motor space....

 

[Graham Orr]

A correction to my first point:
The body resonance was 50-60Hz. The fins had a recorded resonance around 10Hz, which we think is heavily aliased since our sample rate was only 50Hz (RDAS). It is more likely to be in the 40 or 60 Hz range...

The natural resonance observed on ground was clearly not 10Hz... just by looking at it or plugging it into a standard continuum mechanics model. In flight, the system is driven, but not heavily, so the modal resonance still should be prominent. If we were to achieve sonic velocities (as the rocket in the video did) we would more heavily drive the fins but the modal artifact should remain.

 

[Graham Orr]

Not always the case. I recently exceeded the speed of G10 by lofting my Eclipse on an AMW K800, the flutter was bad enough to tear the bottom 3 inches off the fins but the paint stayed on. Oddly only the two fins near the lug were damaged. You should have seen the wiggle when it went supersonic.

Ahh. Looks like your launch lugs created an oblique shock that destroyed your fins. Based on the geometry of it, you should be able to determine your maximum mach#.