Are Fins Over-Engineered?

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sr205347d

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I am a BAR with no experience with HPR, or even MPR. I have read lots of posts here showing how to mount fins TTW with epoxy, and supported with epoxy fillets. Way back, when building Estes kits, fins were merely glued to the paper tube with Elmer’s glue, and they held on just fine. I am wondering if the reason for the extra security is to keep the fins on: 1) in flight; or; 2) on landing.

Certainly you don’t want the fins shredding off during boost, but if not going supersonic, at what point is fin flutter an issue?

It is also not nice to have fins damaged on landing. But wouldn’t it be nicer to have them pop off in a way that allows an easy field repair rather than having to take it back to the shop for major rocket surgery?

For eons, model airplane guys have attached wings with things like nylon bolts, or even rubber bands, that would break in order to prevent major structural damage in crashes. Such means could be used for rockets too. Or, maybe there is a glue that is strong enough for flight loads, but separates cleanly during a smack down with a failed parachute.

Or, maybe fin breakage is not a problem. Let me know.
 
This is just one guy's opinion who's done lots of tip to tip--

It's usually done to keep the fins on for high acceleration and/or high speed. Like above 50 Gs or Mach 2.5, if I had to put a number on it.

It was all the rage for a couple decades. But it really creates as many problems as it solves. The faster you go, the worse the problems become.

A good fillet with a high quality adhesive is really all that's needed, and tip to tip is going out of style.

Edit:
I should also point out that the above only applies to minimum diameter rockets where the fins are surface mounted, the through-the-wall.
 
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I am a BAR with no experience with HPR, or even MPR. I have read lots of posts here showing how to mount fins TTW with epoxy, and supported with epoxy fillets. Way back, when building Estes kits, fins were merely glued to the paper tube with Elmer’s glue, and they held on just fine.
This is not entirely true. I've been building rockets for a long time and I've had plenty where the fins came off. Usually the fins come off on landing, or during handling or storage. I have not had a problem with fins coming off in flght but I've seen them come off of other peoples rockets. I've also had balsa fins break, both parallel to grain (the weak direction) and perpendicular to grain (the strong direction). These days when I build Estes-class rockets I paper the fins so they are stronger parallel to the grain and I put on fairly large glue fillets so they will have good adhesion to the body tube.
When I've built larger rockets, 2.6" to 4" diameter, I've used plywood and through the wall construction because I didn't want fins breaking off while landing.
I have not built rockets to break mach, except 1, so I haven't had to worry about fin flutter. I'm an engineer but not an aerospace engineer so I'm not equipped to know that much about fin flutter. I've seen videos of a few rocket projects that went well past mach 1 and they suffered from high temperatures. If there is enough air drag to cause high temperatures, I can only imagine what forces it would put on the fins.
 
I am a BAR with no experience with HPR, or even MPR. I have read lots of posts here showing how to mount fins TTW with epoxy, and supported with epoxy fillets. Way back, when building Estes kits, fins were merely glued to the paper tube with Elmer’s glue, and they held on just fine. I am wondering if the reason for the extra security is to keep the fins on: 1) in flight; or; 2) on landing.

Certainly you don’t want the fins shredding off during boost, but if not going supersonic, at what point is fin flutter an issue?

It is also not nice to have fins damaged on landing. But wouldn’t it be nicer to have them pop off in a way that allows an easy field repair rather than having to take it back to the shop for major rocket surgery?

For eons, model airplane guys have attached wings with things like nylon bolts, or even rubber bands, that would break in order to prevent major structural damage in crashes. Such means could be used for rockets too. Or, maybe there is a glue that is strong enough for flight loads, but separates cleanly during a smack down with a failed parachute.

Or, maybe fin breakage is not a problem. Let me know.
Fins can begin fluttering at nearly any velocity depending on size and shape but the probability is much higher beginning at 0.8 Mach.
If you build a rocket fin designed to pop off on a hard landing there’s a good possibility it won’t survive flight. Low and slow would be the exception.
I had a violent lawn dart where the fins survived and remained attached to pieces of body tube. Nothing else survived; the motor case was belled at the front and components actually sheared off the altimeter. I still have the fins. Someday I’ll build a new rocket with them. 😁
 
I am a BAR with no experience with HPR, or even MPR. I have read lots of posts here showing how to mount fins TTW with epoxy, and supported with epoxy fillets. Way back, when building Estes kits, fins were merely glued to the paper tube with Elmer’s glue, and they held on just fine. I am wondering if the reason for the extra security is to keep the fins on: 1) in flight; or; 2) on landing.

Certainly you don’t want the fins shredding off during boost, but if not going supersonic, at what point is fin flutter an issue?

It is also not nice to have fins damaged on landing. But wouldn’t it be nicer to have them pop off in a way that allows an easy field repair rather than having to take it back to the shop for major rocket surgery?

For eons, model airplane guys have attached wings with things like nylon bolts, or even rubber bands, that would break in order to prevent major structural damage in crashes. Such means could be used for rockets too. Or, maybe there is a glue that is strong enough for flight loads, but separates cleanly during a smack down with a failed parachute.

Or, maybe fin breakage is not a problem. Let me know.

When in doubt... Build Hell-for-Stout. Preferred method: Through the Wall and at least papered, better yet, plywood. I even do this now with LPR, I use a coupler inside the body tube at the motor mount and slot the body tube with oversized holes so the fin glues to the coupler, then use wood glue to make a 3 layer fillet. makes for a very strong attachment and reinforces the body tube at the fin attachment area.

If you are not building a competition rocket to win altitude events, why not build the rocket so it's durable enough to last for many flights?

That being said, since you are a LPR guy... In high school, my senior year, back in the 70's, the freshman class was building rockets in "Industrial Arts" class. I had been building rockets for years at that point and was friends with the shop teacher. I did a scratch build and used cardboard fins that hung down below the rocket. After the class finished launching their rockets, the shop teacher asked me to launch my franken-rocket. When launched, those carboard fins fluttered so badly you could hear them make a "brrrrttt" noise and the flutter slowed the rocket to a speed at which the flutter stopped... then it would speed back up and flutter again. The rocket got a round of applause from the freshman bystanders.

Uhm, well, yeah, of course I knew it would do that. (Not).

I'd suggest you build one though. When you experience something like this, it's a much better learning experience than reading about it.

Happy (smoke) Trails,

John
 
Most kit fins are pretty okay. Many people copy ideas they've seen before, doubt one or more parts of it, and go looking for another way to beef it up.

Hence the occasional AV bays you see with 2+ pieces of 3/8 all-thread; 5/8 plywood fins with three interlocking rings and three sets of epoxy fillets with fiberglass overwrap, etc.

Each of these techniques has a place and a purpose. But ( at least for me ) it's a lot of fun ( and cheaper! ) to try and match the materials and the motor to the situation.

Those S6 FAI birds probably wouldn't survive an A10 ( I keep meaning to check ), but a minimum diameter L2050 looks pretty sweet off the 50 pads at Argonia.
 
For eons, model airplane guys have attached wings with things like nylon bolts, or even rubber bands, that would break in order to prevent major structural damage in crashes. Such means could be used for rockets too

I don’t know that I’d trust rubber bands, but nylon bolts (aka shear pins) are doable. Motor thrust curves vary wildly and there are obvious choices to avoid.

I’ve pondered mounting fins similar to a strap-on booster, but reverse. Force downward on the fins would tend to lock them in place, while upward force, like a fin-first landing, would push them out of the locked position. There are a couple of upscales in my OpenRocket files that may get something like that, or shear pins, or simply bolt-on fins that can be replaced without surgery.
 
Rubber bands work great for trainer
rc aircraft. Have flown thousands of flights with them. Look at Sig Kadet. But for fins on a rockets, I think not.
 
Fin flutter is an issue, and is often destructive. The mechanism is the same as flutter in wings so its been extensively studied since the dawn on aviation. NACA did a good report on it in the 50s (NACA Tech Note 4197) which is still relevant today. Copies can be found online.
Flutter is usually defined as a 'flutter speed' beyond which flutter becomes likely. It depends on a number of factors include the span and chord length of the fin, its thickness, and the stiffness of the material from which its made.
My rules of thumb for reducing flutter are:
1. Minimize span and tip chord.
2. Use either a thick material or one with good stiffness.
 

John Cipolla's Fin Flutter programs are no longer available for download.

The Fin Flutter equation in Newsletter #291 is believed to be incorrect. If you go
to Cipolla's website he has a document that points out the issues with #291.
The equation in #291 overestimates the fin flutter velocity. John's document
is attached.

I have also attached NACA-TN-4197. This is the document that people refer to
when they work with the fin flutter equation; Equation No. 18.
 

Attachments

  • POF-291_FLUTTER_ERROR.pdf
    235 KB · Views: 0
  • NACA-TN-4197.pdf
    9.9 MB · Views: 0
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Last winter I had a Goblin shred basswood fins. They were through-wall with tabs to the motor mount, and most of the material broke where it met the body tube. When rebuilding the rocket, I tried to break off the remaining pieces but it was very difficult to snap them. There must have been a lot of force during flight. It was an interesting flight and figured I'd share that.

Goblin 2.6", CTI G127-9 - 2160' @ 820kmh

426401-0c2cbeac2c7e60258434aa4ff68f03ba.png

fflutter.png

426402-7a8154ab56bed24295aa5d6a4acb1883.jpg
 
the bond between a flat fin surface and treated tube surface is unsound, and the idea of building a rocket that is intentionally 'just okay' structurally can't coexist with my mrocd. saw a guy poke holes in the fin so the glue could have a little footing, and i thought that if you can cut a slot in a tube without a stability issue, then doing little poke holes in the tube as well as the fin would have no adverse effect. i do this on every build now.

20220815_051431.jpg
 
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Today I saw what looked like a Der Red Max shred its fins on an H motor. Looked like plywood fins mounted TTW. Bummer, but it points out that some fin planforms may be more susceptible to flutter.

I also saw this:

2023-02-25 13.37.00.jpg2023-02-25 13.37.22.jpg2023-02-25 13.38.26.jpg2023-02-25 13.38.47.jpg

Level 3 guy, L motor. He said it was drag separation. I saw the bottom section come down without benefit of parachute. Certainly not a field repair. Likely not repairable at all. But hey, the fin didn't break! Nylon bolts may not look pretty, but this doesn't either.

Lessons: Fins need to be stout and stiff. Some kind of frangible attachment method could save you a few bucks.
 
Last winter I had a Goblin shred basswood fins. They were through-wall with tabs to the motor mount, and most of the material broke where it met the body tube. When rebuilding the rocket, I tried to break off the remaining pieces but it was very difficult to snap them. There must have been a lot of force during flight. It was an interesting flight and figured I'd share that.

Goblin 2.6", CTI G127-9 - 2160' @ 820kmh

View attachment 565417

View attachment 565420

View attachment 565418

Single ply basswood or basswood plywood?
 
It was single ply 3/32" (0.095") basswood. I have since rebuilt it with 3 ply lite-plywood (0.108") and will try a similar size motor next winter.
That'll work. I'm betting if the original fins had been papered, using wood glue, they would have survived with no issues also.
 
Three methods I’m looking at:
-Create fin roots that stick out of the airframe, bolt to that.
-Twin roots could provide a slot for the fin to slide into, and will be symmetrical for less OCD-type annoyance.
- Break away tips for the part that is the most susceptible to snap. Much easier to replace a tip that was designed for replacement, than an entire fin at the root.

The old Vashon liquid rockets had an aluminum motor/airframe. They used a plastic U-channel, similar to the little red ones that hold the wings on cheap balsa airplanes, to hold the fins with just friction. The little plastic channel was attached to the metal airframes with contact cement. It worked pretty well, but I would do high thrust like that.
 
Keep your fins and motors at the back of the rocket. Surface prep is key. Good technique on fillets. DO NOT OVERPOWER YOUR ROCKET WITH TOTALLY AWESOME, HIGH AND QUICK THRUST MOTORS!

Wing warping fins is very bad.
FB_IMG_1626664241093.jpg
Shaping fins in flight with fire is very bad.
20210621_184045.jpg
Keep on the straight and narrow path of sound engineering and your Low and Mid Power fins will be fine. Over engineering for high and ultra high power as your motor sizes slide down the backside of the alphabet is...
TOTALLY AWESOME! :)
 
Keep your fins and motors at the back of the rocket. Surface prep is key. Good techine on fillets. DO NOT OVERPOWER YOUR ROCKET WITH TOTALLY AWESOME, HIGH AND QUICK THRUST MOTORS!

Wing warping fins is very bad.
View attachment 565922
Shaping fins in flight with fire is very bad.
View attachment 565924
Keep on the straight and narrow path of sound engineering and your Low and Mid Power fins will be fine. Over engineering for high and ultra high power as your motor sizes slide down the backside of the alphabet is...
TOTALLY AWESOME! :)

I'll vouche for him.. He knows "warped" ;)
 
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Three methods I’m looking at:
-Create fin roots that stick out of the airframe, bolt to that.
-Twin roots could provide a slot for the fin to slide into, and will be symmetrical for less OCD-type annoyance.
- Break away tips for the part that is the most susceptible to snap. Much easier to replace a tip that was designed for replacement, than an entire fin at the root.

The old Vashon liquid rockets had an aluminum motor/airframe. They used a plastic U-channel, similar to the little red ones that hold the wings on cheap balsa airplanes, to hold the fins with just friction. The little plastic channel was attached to the metal airframes with contact cement. It worked pretty well, but I would do high thrust like that.
I drove to Dayton today to pick up an AT Aerreauxbee-Hi from Randy at eRockets. Since I was nearby, I drove down the street to the Air Force museum to get inspiration for how to bolt on fins. I think this AIM-9 technique might work:

2023-02-27 12.23.32.jpg

Here are some others:

2023-02-27 11.56.31.jpg2023-02-27 12.25.00.jpg2023-02-27 12.28.24.jpg

BTW, here is a real Aerobee:

2023-02-27 11.57.42.jpg
 
Fin tabs in low power rocketry rock.
Low power Sunward SU 47 kit:
20230227_161644.jpg
Super strong over many flights! Now if we could get durable water slide decals that could withstand multiple fin flexing, even with clear coat they will eventually fail.

Even some strakes from an old TLP kit.
20230227_162050.jpg
Strong as iron!
 
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John Cipolla's Fin Flutter programs are no longer available for download.

The Fin Flutter equation in Newsletter #291 is believed to be incorrect. If you go
to Cipolla's website he has a document that points out the issues with #291.
The equation in #291 overestimates the fin flutter velocity. John's document
is attached.

I have also attached NACA-TN-4197. This is the document that people refer to
when they work with the fin flutter equation; Equation No. 18.
I have V4.5 of AeroFinSim and a spreadsheet circa 2001 by Duncan McDonald. The spreadsheet comes out with lower number than FinSim and I tend to trust FinSim more since the science behind it is solid. It's a shame the latest version of FinSim isn't available, but I expect the version I have (which I bought) is close enough for what we do with it. I will have some rockets going over Mach in the next year and will see if they survive.
 
Fin tabs in low power rocketry rock.
Low power Sunward SU 47 kit:
View attachment 565947
Super strong over many flights! Now if we could get durable water slide decals that could withstand multiple fin flexing, even with clear coat they will eventually fail.

Even some strakes from an old TLP kit.
View attachment 565950
Strong as iron!
Rollerons for the win
 
View attachment 565901

This is a snip of my fin flutter calculator and the variables that go into it. Altitude at max expected speed is one of the biggest variables. I like to do comparisons of different materials, different thicknesses, and different geometries. I don't use plywood unless basswood shows a low shred speed. No fiberglass unless plywood will shred. No carbon fiber unless fiberglass shows weakness. I run these numbers in tandem with the rocket simulator drag outputs of fin shape and thickness. I like altitude so I use this calculator to get the minimum sized fins (least weight) I need to do the job.
Too thin = shred. Too short = not enough control surface. Too thick = excess drag/lower altitude.

Your needs may vary, but to me, fins have the most calculator time.
(Most of my calcs are based on "NACA TN 4197 FLUTTER VELOCITY ANALYSIS BY JOHN CIPOLLA, VERSION-2")
In my post above, this is the spreadsheet that (usually) comes out with lower velocities than AeroFinSim. If you build your fins to this spec, they should be OK. The differences are quite varied, though. Most of the AeroFinSim results are somewhere between 1.2 to 2 times higher than the spreadsheet. I have two instances where AeroFinSim comes out lower (.77 to .9). Fin geometry is what it is all about. From all of my fiddling with fin geometry, The shape in the attached pdf is the most resilient.
 

Attachments

  • Fin Set.pdf
    64.2 KB · Views: 1
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