Fin flutter analysis of the 2.6" FG Screech

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

mperegrinefalcon

Well-Known Member
Joined
Jun 12, 2017
Messages
360
Reaction score
75
So I just got finsim so I could check to see if my 2.6" Screeches fins would disintegrate if I flew it on an L935, and these are the numbers it came up with. It's my first time using finsim, and I don't really know what I'm looking at. It's really surprisingly low velocities that fin flutter sets in, but the thing is I've seen videos of this rocket flying, and there is no flutter way past what finsim is saying is the maximum velocity. Heck, I'm pretty sure I've flown it faster then what finsim is saying for the flutter speed and divergence speed. I put it up on a J500, and the sim was within 500 feet of its altitude. I haven't been able to get the flight data off the RRC3 yet, but I'm sure it was close to about the mach 0.7 predicted by Openrocket. Here is the data:

Screenshot (15).jpg
 

Attachments

  • Screenshot (14).jpg
    Screenshot (14).jpg
    86.5 KB · Views: 118
That value seems low. Have you selected the apprioate materials? Usually I've gotten G10 flutter above Mach 1 and CF or 6061T6 fluttering by Mach 2. I was using the NACA 4197TN method reference body tube which isn't the best but well its old school and you can hand compute it. Double check the fin profile entries.
 
Exactly what I was thinking. I have checked to entries, but they are all correct. I assume I'm doing something wrong in the software. I'm positive my rocket has gone above mach .5
 
If you increase the structural dampening number it increases the flutter mach the inputs may be overriding the data on that page for advanced uses. I don't recommend playing with that Entry much. Your first page should have a flutter Mach too. The problem is as a undergrad mech engineer student I do not know how one experimental finds that structural dampening number. It might be a linear vibes problem but I haven't had a linear vibrations class only a lab with an oil filled crank slider which isn't as bad as a rocket airframe. Somehow guys like NACA and NAS in Tulhoma get those technical numbers by testing the fins beyond what we do in amatuer rocketry. In actual wind tunnels at actual Mach numbers of airframe limits. They get that fin to vibrate then they get the data off of it somehow which isn't helping your problems.

The absolutely biggest problems I've had with this program were doing airfoils that tapered. We just had to assume rectangles and fly it experimentally. It was honestly scary to us. In solidworks we could get the CG of the fins. Perhaps you could also get the cg of fin by balancing it off of the ruler it may add about 0.2 Mach to flutter depending on fin. If you are using a custom material this is a point of error in data entry as the properties aren't always fully known on composites compared to metals. There wasn't like a ultimate yield listed but it was labeled something else on composite materials that I wasn't as familiar with. I'm not a composite dork. There's a whole another field of engineering itself. The poisson's ratio isn't always specified. I find that infuriating.

An Airforce study has the span and thickness of fin are the most influential factors on fin flutter. 176 pages after they ripped fins off at Mach 4.
 
When I use the NACA 4197TN reference to body tube on a 29mm MD fin I get Mach 1.38 using G10 and a mach 2.2 at sea level on AS4 CF using the program's values for 1/16" thick material. Still using NACA 4197TN and CN reference body tube. Make sure the fin geometry entry units are inches. There's a lot of little details that might get you. On that same fin, the 3D method shows Mach 4.13 which I "assume" is bullsh*t. And under custom user materials, the elastic modulus is the largest factor. I don't know if you are using a stock finsim material, those properties are in an excel included in program for the NACA flutter, but even the naca flutter calculation can be 20 percent off which isn't great but it beat guessing stuff.
 
Ya, it's all in inches. I don't know what I'm doing wrong. I'm using the NACA 4197, and have selected G10 for the material with .063 as the thickness in the fin shape screen. It's still giving me the .3 mach bull.
Screenshot (16).jpgScreenshot (17).jpgScreenshot (18).jpgScreenshot (20).jpgScreenshot (19).jpgScreenshot (21).jpg
 
The center of gravity of that fin may not align with the value entered by default. You may need to balance the fin and get the length from beginning of root to the CG point and divide by the root length to get a decimal below 1 that will replace the default FinSim value. That will adjust the flutter Mach slightly. Other than that without having a screech infront of me or fin values I do not know. There are better people than I at finsim.

If you want to post up root, tip, span, Sweep length, thickness, and body tube diameter outer maybe somebody can check it. This is really odd too. Because the screech looks to have a sweep angle for Mach number to reduce drag and compressibility effects of Mach plus flight. It looks like you have done it all right that I can see which...

The elasticity value in the material used by FinSim seems to have the highest effect of all values on the flutter mach.
 
Try increasing your CG field from .55 to say .60. See what happens to flutter numbers. They should increase. The farther aft you can get your CG to be, the flutter numbers will increase. If your fins are already mounted on airframe, and you cant measure the real CG (by balance on angle iron edge) then cut another fin the same shape as yours and measure the CG on that one. My fins have a CG of .658. Take the measured CG of your fin from the front edge and divide by the root length, this will give you the number that goes into the CG field. Example.........From fore (front) end of fin to measured balancing point is 9.875 inches. My root length is 15 inches. 9.875 divided by 15 is .658. I have been down the same road you are on. Go to website and read everything you can about it and dont be afraid to tweek numbers one at a time so you can follow what is happening. NACA 4197 in finsim 4.5 is used for supersonic flight. Good luck!
 
We don't know how the program computes flutter. I know it worked for a trapezoidal fin. I'm wondering if the triangle fin profile affects the equation wrongly. If you have time there are other articles to manual compute it to check the program values. Apogee, NACA, or universities may have technical documents. Ch.5 wing design pdf was for airplanes. It should have the way to get aspect ratio of fin. You can then compute the flutter manually using an equation. While your at it that article has the supersonic equations related to Mach number to check the optimial fin angle for Mach number. I think that David guy flew a screech on a Loki K1127 and the fins never ripped off. It might be vibrating slightly but not at full flutter?????? Idk. had different flutter equations for trapezoidal than non trapezoidal. It's tip length at zero is probably screwing with it somehow??!
 
OkAY.
I tried testing a G10 fiberglass 3/32" fin off of a SAAB 05 at sealevel.
root=10"
tip=0"
span= 1.69"
sweep=9.44"
thickness=0.09375"
body OD = 2.26"

The first page indicates the flutter mach is 1.73 using G10 at NACA 4197TN at reference to body tube. Divergence mach at 2.21. First page no CG adjustments. Actual material would reflect something along the AS4 material as it's a CF part, I just wanted to try sim a triangle fin. No issues my end. Fins are already on that rocket, I wanted to give the OP a way to check his finsim against actual number. If he gets numbers I got then its not broken program.

This kit was from Wildman as my L-1 kit. Are the screeches fins too thin for it's application oR span to wide? WEIRD.
 
It looks like you specified the number of fins = 0. 1/16" is uncomfortably thin for a fin of this size.
 
I should specify when I enter the fin specs I test for fin#1 quantity 1 under the profile entry screen.
 
I tried all of that, but the only thing that changed the value is the thickness of the fin, but on the K1127 video it hit well over mach 1, with no noticeable flutter.
 
Could one of you try to enter the values of the for the fin shape and try it on your finsim? The dimensions you need are in on of the attatched screenshots. I have 1" rocketpoxy fillets on the outside (5000 PSI), and internal fillets inside. I do have beveled edges on the fin, but my understanding is that can't be modeled in finsim, correct?
 
1/16" is uncomfortably thin for a fin of this size.
Especially for the body tube diameter and motor options this guy has. The material could be CF for example. On my senior project scratch design the university spent three weeks of CFD wind tunnel modeling on, we used thicker than 1/16" at sustainer root and 3/32" at root of booster with materials better than G10. It didn't flutter in FinSim or M1.5 in reality with tapering airfoil. University gave us a hypersonic prof to help us on aero for drag corfficient. We asked our mechanical engineering department for structural analysis in FEA, they laughed, as senior mech students we couldn't compute the structural analysis of a curved geometry part with what learned at school by hand. We weren't shown how to FEA. Guess what failed, the interstage of horrid hollow geometry at only Mach 1.5. Madcow never had a body tube diameter at that time but I still take blame on the structural but I did all the aero as mech. What we didn't know almost hurt us, the aero work I did was excellent, the motor selection and the interstage design was just horrible. No HPR mentor or any previous experience. We learned a lot that flight, about what you don't know about a rocket is what usually breaks.

That Burt Rutan guy that designed spaceship one had 74 some mechanical revisions on a light airplane called a long EZ. That guy's aero design was top notch. His customers were fixing the bracket designs for controls in garage. It's honestly what the designer doesn't know. That's what hurts. Jim Bede tried a supersonic homebuilt, jet, it killed two test pilots by flutter, vertical tail shear off. He had many designs of slow planes that were fine, and I wonder if he didn't know about flutter. You get a bunch of components together and the entire design gets hard because it takes a lot of experience or a lot of knowledge of at worst both.

If I was selling screeches I'd upgrade those fins to CF or use 3/32" g10. So yah I feel for the design mistake because I've made one before. It was what we didn't know.
 
Ya, before putting it to a mach flight I think I'm going going to do 2 layers of t2t with 2oz FG that I have laying about. Is it okay to use hardware store epoxy for that, or should I get something like west systems ES6209?
 
Man. I have done a total temperature CALORICALLY perfect calculation for a Mach number with specific heat ratio of air at 1.4 at Mach 1.7 and room temp start point assumption it was about 197 Celsius. We spent a lot of money on Cotronics epoxy. Their 4525 IP is a room temp cure epoxy good to 500F and 10,000 psi tensile. We tried that 4700 which was spacecraft oven cure pain the arse stuff, I never had a fin rip off. We didn't know how hot the casing got. Then we flew a second rocket designed for M1.1 on Rocketpoxy G5000 which had about half the temp rating. It's tensile was real good at 7,600 psi. The Aeropoxy denied a sale of epoxy to M1.7 rocket because the temperature exceeded 120F. And the glass transition temp, the TG value is when it is exceeded the glue loses viscosity and fails thermally. I think ES609 is great for subsonic rocket. If it's going much over Mach 1 I'd pick anything else, cotronics high Mach, hydrosol for high Mach, and maybe rocketpoxy if it's like lower than Mach 1.2... The total temperature calculation is from advanced fluids/compressible gas dynamics. There's an oblique shock off the nosecone angle, it drops the Mach "barely" at that point of you want to get technical before you total temp compute it. All the other heat flux rocket thermo articles I read assume subsonic. There's not a exact method... A lot of guys get snotty about data or epoxies here. You can call Aeropoxy and ask if they deny a sale you know not to go with that product. At Mach 1.1 the total temp I got was 96.96 C or like 206 F which is under the rocket poxy Max temp use.

What isn't right is the rocket is cooler than the calculation because the calculation is basically a theory that it's a perfect gas and whatnot. I've heard the excuse the rockets don't get fast enough long enough for practically it to matter for amatuer rockets lower machs. This isn't like a Mach 23 spacecraft rentry. It doesn't last for minutes like the space craft. You can always use a better epoxy. The irony is materials didn't melt, below that total temp calc. Practically it's not hot long enough or something odd. You fin isn't as hot all over as say the very tip of nose. I tried to stay thermal conservative. Some people have flown certain epoxies beyond what I computed.
 
I know proline 4500 is a good epoxy. Would it work for doing T2T layup?

If I were to do a T2T I would forgo FG and use CF. That being the case I would be inclined to use a single layer of 3K 2x2 Twill with AeroPoxy PR2032 and PH3660 hardener under vacuum with a nylon release peel ply and a breather. ProLine 4500 is a good epoxy for some applications but not a layup, it is far too Viscous.

Having said all of this you are going down a road that if not prepared can be a bit challenging as there is some tooling and consumables that you would need to invest in. You can do it without vac-bagging and by all means you can use FG however you will undoubtedly end up with a less efficient result than if it was CF and under vacuum, whcih may be fine for your goals.

Put it this way, a T2T attempt can result in anything from a thick, messy, ugly, clumpy mass that adds very little strength on your fins, to a very efficient thin structurally strong and neat layup.
 
Put it this way, a T2T attempt can result in anything from a thick, messy, ugly, clumpy mass that adds very little strength on your fins.

If I tried it, it would probably end up something like this lol. I've not tried doing any T2T layup work before, and have only ever used rocketpoxy. Where would I start to do research in how to do proper layup work with vacuum bagging? I want to do a project with a minimum diameter M motor to try to reach 50,000 feet, and I will definitely have to do something like that. This project is a long way off due to only being in high school, but I do have my L3 kit in the mail. I would want to launch this at BALLS, and I'm thinking an M2245 would be best for a 50k shot. My L3 attempt will not be a 50k shot thought, I don't think my L3CC would appreciate that very much. If I can manage to reach 50k on a single M, Why not try staging an O3400 minimum diameter booster to the minimum diameter M2245 that made it to 50K?
 
Another option to consider.

If you feel after your analysis that the stock fins cannot handle the velocity, then instead of a T2T you could consider replacing the stock fins with something stronger. That is if you have not assembled the rocket yet. If you wanted to stick with 1/16th you could look at some economy 1/16th dragon plate. https://dragonplate.com/ecart/categories.asp?cID=93. They will even cut the fins for you. That is if your analysis proves that this material is appropriate.

This would simplify the build process and reduce the potential mass where you don't need it.
 
Another option to consider.

If you feel after your analysis that the stock fins cannot handle the velocity, then instead of a T2T you could consider replacing the stock fins with something stronger.
I was thinking the same thing. Here's another option: https://www.aliexpress.com/store/pr...510.html?spm=2114.12010612.0.0.6a073376K1GnGU (They have several sizes, but 250x400mm should be plenty for you.)

Best Carbon on Aliexpress is a favorite among custom quadcopter folks. I've built quadcopter parts with it, but haven't it flown it on a rocket yet. I plan on taking their 2mm carbon plate to Mach 2 soon though.

Carbon fins plus some Proline 4500 (or whatever your favorite is) should get you easily to Mach 2.
 
This rocket is already built, so it would have to be for the next rocket. I have a 38mm CF Mongoose that I'm going to attach the fins to one of these days. That will easily surpass mach 1.
 
I broke Mach on a J510W, K1127, K1127, and a J1026. Flights were within a month or so of each other.
The RRC3 I think reported speed a bit high. One MARSA flight gave me goofy data back, and another on the J1026 reported just a bit over Mach. I’d bet they all just a bit over.

I have a 54mm mounted one, but haven’t flown it yet. I do wish that version had full length tabs and thicker fins. I’ll probably put some CF on mine before it flies.
 
If I tried it, it would probably end up something like this lol. I've not tried doing any T2T layup work before, and have only ever used rocketpoxy. Where would I start to do research in how to do proper layup work with vacuum bagging? I want to do a project with a minimum diameter M motor to try to reach 50,000 feet, and I will definitely have to do something like that.

I applaud your enthusiasm and desire to pick up a skill-set and further your knowledge. I got into working with CF a few years ago and my progress has been evolutionary building upon each layup. I can tell you from my experience as an individual and not having a mentor to draw on, there is a learning curve. I don't know if you have some local resources to draw on or a group that you can work with but if so it would very likely be a good experience for you. If not then you can certainly go down this road on your own as there are a number of experienced people on this forum as well as a lot of videos on YouTube going over the topic.

My first layups were not using CF, they were simply using a light weight FG cloth with AeroPoxy PR2032. This resulted in a successful layup but I quickly came to the conclusion that there are better materials and techniques that would yield disproportionately better results. My first CF layups were very basic and although they arguably yielded better strength they were still relatively heavy and could have been stronger if I got the cloth to resin ratio better. This lead me down the vacuum road.

My first vacuum was a vacuum generator, whcih used the venturi effect connected to a compressor to create vacuum. After a couple of layups using this device with some nylon peel ply and breather I realized two things. Layups under vacuum with the proper ply and breather can reduce the epoxy as well as compress the layup significantly. I also realized that running a compressor continually sucks, pun intended, in that it is noisy, draws a lot more electricity than it needs to, and puts a lot of wear on the compressor motor than is required. This experience resulted in me building a proper vacuum press that cycles up to a preset threshold of vacuum and only turns on as the vacuum drops. Ideally it wouldn't' turn back on but you often do get leaks in the bagging. Resolving leaks has resulted in purchasing better vac-bag attachments, better sealing tape etc. and adopting better vac-bag techniques.

I could go on but the point is you can quickly see how the process evolves if you review each layup and look for ways to build on it and improve the process and results.

Good luck on your journey
 
If you're going to do a T2T layup, I would start small and make it easier on yourself. For a first layup, I'd use a ~3-6 oz glass cloth and a laminating epoxy from whatever local vendor you have. Boat people are nice and have the right stuff, but it may be a little more expensive there. The exact properties of the glass and epoxy aren't really that important for M1.5-ish as long as they are compatible and intended for laminating. The reason I suggest glass is that it's a lot easier to see voids than in carbon. I'd recommend getting a fiberglass roller as well if you can find one--it's a roller with lots of ridges like you were going to cut spaghetti out of a sheet of dough.

Then you're going to do a couple of test layups on plywood. You want to add enough epoxy that the glass is clear with bubbles, but not more than that. You should still see the weave and shouldn't see puddles. That will give you a feel for laminating. Then you're going to do one more test layup on a scrap piece of nonpermeable material (fiberglass if you have it, but glass, Lexan or even ABS pipe will work as well) to simulate working on your fiberglass rocket. Make sure the number of layers you use makes you happy with the new fin thickness. Finally, you'll coat the real thing. Don't worry about vacuum bagging--learn to walk before you run. That will give you a pretty good surface without a huge amount of effort.

YMMV, and there's nothing stopping you from vacuum bagging either if you really want to.
 
If you're going to do a T2T layup, I would start small and make it easier on yourself. For a first layup, I'd use a ~3-6 oz glass cloth and a laminating epoxy from whatever local vendor you have. Boat people are nice and have the right stuff, but it may be a little more expensive there. The exact properties of the glass and epoxy aren't really that important for M1.5-ish as long as they are compatible and intended for laminating. The reason I suggest glass is that it's a lot easier to see voids than in carbon. I'd recommend getting a fiberglass roller as well if you can find one--it's a roller with lots of ridges like you were going to cut spaghetti out of a sheet of dough.

Then you're going to do a couple of test layups on plywood. You want to add enough epoxy that the glass is clear with bubbles, but not more than that. You should still see the weave and shouldn't see puddles. That will give you a feel for laminating. Then you're going to do one more test layup on a scrap piece of nonpermeable material (fiberglass if you have it, but glass, Lexan or even ABS pipe will work as well) to simulate working on your fiberglass rocket. Make sure the number of layers you use makes you happy with the new fin thickness. Finally, you'll coat the real thing. Don't worry about vacuum bagging--learn to walk before you run. That will give you a pretty good surface without a huge amount of effort.

YMMV, and there's nothing stopping you from vacuum bagging either if you really want to.

That all sounds like a good idea. I'm definitely going to try practicing on a piece of plywood and then a piece of plastic.
 
Back
Top