Video of fins flapping before Shred

[constevens]

Well I ended up making a fin that has a more diamond shape with the outer point cut off. I have pics of it in my L3 thread. I have thicker G10 and am layering it with Kevlar/CF/glass. It is going from fin tip to fin tip under the Bodytube across the motor mount tube. Then I will do a slight amount of carbon or class laminate on the outer most fillet. Should handle everything I could possible throw at it.

 

[cls]

I'd like to say something snarky like "why do we only see G10 fins flapping like that" but plenty of us have shredded (unglassed) plywood fins too. but still, I think it takes a bit more to shred plywood than G10.

there is nothing to do about it except run some experiments!

 

[belfert]

I highly recommend the fiberglass/nomex composite for fins that will survive mach and beyond. I did 900 MPH on a set of composite fins with no additional strengthening. The fins will be very light too.

Brian Elfert

 

[DAllen]

Originally posted by Fishhead
I'm still trying to figure out the message that you were trying to write in the sky. Your penmanship is terrible.

That wasn't sky-writting that was sky-scribbling



-DAllen

 

[denverdoc]

Originally posted by cls
I'd like to say something snarky like "why do we only see G10 fins flapping like that" but plenty of us have shredded (unglassed) plywood fins too. but still, I think it takes a bit more to shred plywood than G10.

there is nothing to do about it except run some experiments! [/B
]

May want to visit info central and look at the flutter spread sheet--for most fins, g10 offers about 3x plywood flutter velocity. These are only approximate figures but based on some good NACA research. My guess is we see only g10 fins flapping is that plywood would have shredded long before, and folks with cameras know this and tend to use more robust materials--course 1/2 inch ply is bulletproof.
John S

 

[constevens]

Ok John Dont make me get my gun.. JK..LOL

 

[denverdoc]

Originally posted by constevens
Ok John Dont make me get my gun.. JK..LOL

You used Kevlar right?
J

 

[cls]

Denverdoc, with all due respect I disagree because my experience doesn't match the flutter spread sheet. my Callisto's 1/8" G10 fins would visibly flutter on ~H200 -ish motors, and I161 was miserable. however, my Caliber ISP's 1/8" plywood fins are solid through J350, J415, K185.

 

[denverdoc]

Originally posted by cls
Denverdoc, with all due respect I disagree because my experience doesn't match the flutter spread sheet. my Callisto's 1/8" G10 fins would visibly flutter on ~H200 -ish motors, and I161 was miserable. however, my Caliber ISP's 1/8" plywood fins are solid through J350, J415, K185.

Interesting. I know another very knowledgeable individual who suggests that keeping the TE's square will also render a fin unflutterable. But I think he is talking about a different type then the flex/twist resonance that most of us are talking about.

Think hard about how you sanded the fins--for plywood, I often use a palm sander and taper the hell out of them, then glass. G10 aint so amenable to sanding, and when i do, it is with a belt.

Vibrations in thin plates is a pretty complicated subject. Some suggest poking holes to break up standing waves, others taper the thickness so as to change stiffness/mass ratio as a function of span. Others just buy the biggest hunk of wood and hope for the best.

Guess I'm asking is there is any difference in how you treat the different materials, before we conclude anything?
John S

 

[cls]

the G10 fins were untouched, except 60 grit on the bonding parts, so had square edges. as you said G10 is really tough to sand! the plywood fins were just rounded on leading and trailing edge; square on tip. I (generally) don't taper them because they get a beating anyways and don't want delicate edges getting dinged.

it is true that to reduce drag, trailing edges should either be square, or perfectly tapered to a one molecule thick edge.

my feeling is the flutter calculator is on the right track but needs a tune-up.

 

[bobkrech]

Originally posted by cls
Denverdoc, with all due respect I disagree because my experience doesn't match the flutter spread sheet. my Callisto's 1/8" G10 fins would visibly flutter on ~H200 -ish motors, and I161 was miserable. however, my Caliber ISP's 1/8" plywood fins are solid through J350, J415, K185.

Cliff

Fin span and shape have a lot to do with flutter. The Callisto fins have a large span with little taper, a form factor that is more likely to flutter than the minimal span, swept back fins of the LOC Caliber ISP.

The resistance to flutter depends more on stiffness than strength, and I believe that 1/8" plywood is stiffer than 1/16" G10. I'm not surprised that the fins on your Caliber don't flutter in the 0.95<M<1.14 region that the Caliber sims to, especially since they have the proper swept back aerodynamic shape.

The standard PML fin thickness for the Callisto is 1/16", and like many PML stock fins, will readily flutter in the transonic region. You must have upgraded the fins, and I'm really surprised that 1/8" G10 fins would flutter at the M=0.77 max that wRASP predicts for a I161 flight.

A classic case of PML fin flutter was vigorously discussed here. https://www.rocketryforum.com/showthread.php?s=&threadid=12440

Another interesting fin flutter failure involving shock wave interactions from a launch lug can be found here. https://www.rocketryforum.com/showthread.php?s=&threadid=12444

Bob

 

[denverdoc]

Agreed, the flutter spreadsheet is an approximation and if an individual or club could take the baton, and help to deliver its terrific promise, it would be boon to HPR.

Esp with regard to various composites formulations. I ran Art's fins thru the sheet and came up with something like 580MPH. Never mind what plywood would have done. That they did their dance at between 700 and 800 confirms the eqns are conservative, and likely a good predictor to 1.5x. Beyond that, proceed at your own risk.

But if you read the entry article to the spread sheet, and play with the various planforms (or your own), you'll see what Bob is talking about at a click of a mouse. Double thickness, huge increase, reduce Aspect ratio likewise, in fact the site goes to great lengths to show which are the sensitive variables.

In the end, a fine resource with some edges, but worth spending time at. Glad i did. MyL3 project made of g10 .125 felt good, but with exertion could be flexed. Consulted the spread sheet--said oh-oh, and reinforced with tip to tip CF, no way to flex that, and survived a flight of M1.5 in accordance with the predictions.
John S

 

[OverLord]

WOW!

Sorry for the loss, but that is one of the best videos I ever saw!
Keep at it!

Mike

 

[Apollo Leader]

I saw this flight, including its onboard footage, on the Rockets magazine DVD set. Pretty spectacular shred!

If Mr. Upton sees this topic again, I would like to ask him how his fins were secured to this rocket. Was the entire fins' roots secured through-the-wall to the motor mount? How were the fins filleted internally - doesn't look like much in the way of filleting on the outside of the rocket.

 

[quickburst]

Wasn't this at the last LDRS? If so I was there and saw the flight. I remember meeing you not to long after this happened.

Best of luck to you.

Great video.

 

[UKrocketeer]

oooooooops. man dose fins EXPLODIFIED i was surpised at how much the camera moved. Sorry

 

[Art Upton]

Originally posted by Apollo Leader
I saw this flight, including its onboard footage, on the Rockets magazine DVD set. Pretty spectacular shred!

If Mr. Upton sees this topic again, I would like to ask him how his fins were secured to this rocket. Was the entire fins' roots secured through-the-wall to the motor mount? How were the fins filleted internally - doesn't look like much in the way of filleting on the outside of the rocket.

Hi Apollo Leader. No fillets on the outside of the rocket, it is a slip over AeroSkin design like many real rockets use.

Here is an example of how we have been building the Nuke and Bruiser rockets. Glass cloth on the mmt to the fins and fins in slots on the centering rings.

https://www.rocketreviews.com/reviews/mods/loc_big_nuke_3e.html

The real issue is that large Bruiser fin design and without tip to top cloth, just really is not a good idea for G10 in transonic and higher speeds. That is how I live and learn.

Bob Krech and I did some frame by frame and using the RAW data determined they started the first movements at transonic speed ~ 648mph; the 800mph came just after the fins flew off, less drag

The Big Nuke fin design is more suited for less fluter IMHO, and took an M1400 in that rocket easy.

Notice the Big Nuke fin area in the photos on the EMRR website above and then look at the Bruiser fin area in this photo

https://k8xg.com/tap3/image008.jpg

 

[Art Upton]

Originally posted by QuickBurst
Wasn't this at the last LDRS? If so I was there and saw the flight. I remember meeing you not to long after this happened.

Best of luck to you.

Great video.

Hi Bob, that was at LDRS and I won some of your twiggys BTW