What does it mean when a fin "stalls?"

Ravenex said:

You can simply say that stall is a condition where the high angle of attack causes the fin to become less effective.

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That right there is your best bet---To be honest, this has come up before and the thread was sort of the same with the same mixed results. For the most part, all of you are right---it depends on your point of view !! Question: do fins make lift? Answer : yes they do--on both sides equally---so it's a net wash--get my drift? Question: how in the world do you stall a rockets fins? This assumes the rocket is traveling at great velocity and the fins are aligned properly!Even with a gust of wind--the fins will remain at or near 0 deg AOA relative to the airflow. As the rocket travels up ,it's always making little corrections as the fins try to stay neutral. If the AOA drifts a bit the fin side facing the airstream is under high pressure and deflects the rocket back on course at the same time the side away from the airstream is in low pressure and begins to make more lift that it's opposite side thus righting the rocket. In extreme cases you can see this in rockets that waggle on the way up. So, how do you stall a rocket fin?? First, if the rocket is not dynamically stable to begin with---your cp/cg relationships are way off or the fins are too small to counter act any deviations in the flight path-- this will result in flight departure--- caused by high AOA that the fins simply could not overcome. Slow off the pad can also cause the rocket to want to flip ends---the rocket is stable but not moving fast enough for any of the forces on the fins to generate enough force to keep it headed in the right direction. one side of the fin is deflecting air and by the way making lift as the other side is making a ton of drag since the air has become unattached(stalled) . If you have ever seen a rocket go up a little way and then snap into a loop and the straighten out and go cruise missile or land shark---this is usually the reason--low speed off the rail--the fins are not working yet-- it loops but is beginning to gain speed--as it gains speed the deflection off the fins begins to prevail and equilibrium is restored and the rocket takes off for the next county!!!---What I've told you really is an oversimplification --fin shape--profile--airfoil and thickness ,dynamic stability and thrust and speed all play into the equation, but that's a discussion for another day.

Ravenex said:

You can simply say that stall is a condition where the high angle of attack causes the fin to become less effective.

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If it comes up, I'll probably do that.

I gotta tell ya, man, this rocketry class, which I've been calling Rocket Camp, has been full of drama this week. Day 1 I got there, and they had not gotten the kits I'd ordered (as well as several other items). The kits we had were E2X, and would've taken about 20 minutes to build. I talked to the guy who was running the camps, and he ran around looking for rockets, so I spent the first 3 hour day extemporizing on rocketry (my whole plan for the first three days went out the window), and we worked on the 4 leftover kits from last year - with six kids - as a team. I had to say things like "you sand this fin, and you sand that fin." Toward the end, the kids seemed to be a little bored.

By day 2, they'd gone to the store, and bought out all the Skill Level 1 rockets they had.

So we started with a launch of a couple RTF rockets, then started building.

Problem was, because we all had different rockets, I couldn't build with them. I had to have them each read the instructions, and ask me for help. So I was running around from kid to kid, helping them all. Not a problem, except that because I couldn't build with them, they built so fast. Meaning they didn't let glue dry between steps, and the rockets were done really fast.

My plan was to, say, assemble the motor mount, then say "let's let the glue dry and talk about blah, blah, blah."

Today, we painted. Because they'd all built so fast, we were done early with the building process. So, ironically, because I'd lost a day due to the wrong kits, I actually had more time on my hands to fill, because they all got through building so fast.

The rockets are solid and safe, so I'm not worried about that. But now I have two more days - six hours - to fill with... Something. The weather Friday's gonna be iffy, so we have to launch tomorrow, then I have to come up with something for Friday.

Some of my teacher friends say this is normal. You make a plan for your first class, and things go awry, then it's all trial and error.

Today, though, I think I hit my stride. I seemed to be able to communicate with the students in a way that was more effective. And these kids are great - really sharp, interested, and able to absorb some technical information I threw at them with little problem. Due to the abundance of time and their intelligence, today I decided to throw some more technical information at them, to try and challenge them. And they got it! I think I figured out a way to explain newton/seconds to them so that it made sense to them, and when I threw them a curve ball by saying, "So, if I had two B motors, how many newton/seconds would that be? And so what classification motor would that make two B's?" And they knew it was a C.

Their painting skills leave a little to be desired, but... you know... kids...

Oh, and then, this evening, I got an email that the director of the program (a bunch of educational camps, not just rocketry) had resigned today. I guess that explains some of the SNAFUs from earlier this week.

Next week, things will go better. I have a plan based on what happened this week, and we have enough of the same kit (Estes Viking) that I will be able to go through it with all students, step by step. I think the rockets will turn out better that way too.

I always thought a "stall" was a place where horses slept and deposited things that had to be scooped out with a shovel along with the straw. Who knew?

Daniel, you're doing just fine so press ahead and ignore anything anyone else has to say...including me.

tmacklin said:

I always thought a "stall" was a place where horses slept and deposited things that had to be scooped out with a shovel along with the straw. Who knew?

Daniel, you're doing just fine so press ahead and ignore anything anyone else has to say...including me.

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Thanks for the beer, Ted

In the case the angle of attack is that high, most rockets are going to be weathercocking and the stall will reduce the weathercocking by reducing fin effectiveness. Potentially this could lead to what is technically an unstable condition but if transitory will be a fraction of a second of anti-weathercocking -- some long designs can do that anyway. Conceivably some sort of oscillatory behavior could occur due to, at times, the flow not stalling alternating with instants of stalling, for example during slow flight in turbulent air. That could be the answer to some rare cases of unexplained instability.

Correct rocket design avoids overstability in order to avoid excessive weathercocking, as well as avoiding understability due to any of the many reasons you could end up with that problem, such as wanting to build a lightweight rocket with small fins and a heavy motor in the back.

Imagine a fin is a nice smooth airfoil shape and symmetrical. At zero angle of attack to the relative wind, there is zero lift.

If there is a perturbation of the rocket and the fins are now at an angle, they will act like a wing and there will be some lift generated by the side of the fin that is facing away slightly from the oncoming wind, but there will also be drag on the side of the fin now presenting itself like a barn door to the oncoming wind. I think you will find that the pushing force of drag is what generates the largest component of the fin correcting force.

If the angle of attack is too large, the airflow over the lifting side of the fin will separate and that side will "stall" just like an airplane wing, but at that angle of attack the side of the fin that is being hit by the oncoming air will be experiencing HUGE drag forces. Try sticking your hand out the car window while driving fast and turn your hand so that the palm is facing directly into the wind. The top side of your hand is "stalled" but the drag on the palm side is absurdly high.

Some of my teacher friends say this is normal. You make a plan for your first class, and things go awry, then it's all trial and error.

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I forget which general said that no plan survives first contact with the enemy, but it seems appropriate here.

If you want to get all technical (and I agree that it probably isn't a good idea), I would say that stalling is when the airfoil has gone past its angle of attack of maximum lift. You can see a nice diagram illustrating the effect here: https://en.wikipedia.org/wiki/Lift-induced_drag in the "Calculation of Induced Drag" section. Once you get past 20 degrees of angle of attack, lift goes down and drag keeps going up. There is still some lift, but it's not anywhere near as efficient as at lower angles of attack.

I thought this was a ROCKET forum...

FINS do not provide lift... (they aid stability)

All LIFT is provided by the engine... Duh

Are they aerodynamic flying surfaces... YES

Any deviation from zero AOA or having unmatched airfoils...

Causes pressure differentials which in turn cause...

Movement upon the horizontal axis... Not lift

KidRockET said:

I thought this was a ROCKET forum...

FINS do not provide lift... (they aid stability)

All LIFT is provided by the engine... Duh

Are they aerodynamic flying surfaces... YES

Any deviation from zero AOA or having unmatched airfoils...

Causes pressure differentials which in turn cause...

Movement upon the horizontal axis... Not lift

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Well I strongly disagree! How dare you? I'll have you know that I have done EXTENSIVE research on this very problem and have come to a conclusion. In fact, I've written a paper on this very subject which is at the publisher's office even as I type this. You'd better retract this before I am forced to register a FORMAL COMPLAINT! This is exactly the type of irresponsible behavior that puts OUR HOBBY AT RISK!

Idiots.

When a fin stalls, it just means you need to choke it.

It means you have entered The Twilight Zone.

DizWolf said:

Idiots.

When a fin stalls, it just means you need to choke it.

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Enough of us here are old enough to actually have had vehicles with which you had to do that....

Good to hear the kids are picking it up. Kids can be smarter than you think...but the trick is to figure out how much information to give them over a given time period.

If you want to keep it simple...just say any surface like a wing or fin generates a force with enough speed. How that force is used determines what it is called.

FC

If you really want to keep it simple, show the kids this thread, and invite them to join the Rocketry Forum.
You are likely correct in your assumption that they are pretty great folks, so we should at least get their opinion about what it means when a rocket fin stalls.

If the conversation was without Merit, the Thread would not have made it this far.

tmacklin said:

Well I strongly disagree! How dare you? I'll have you know that I have done EXTENSIVE research on this very problem and have come to a conclusion. In fact, I've written a paper on this very subject which is at the publisher's office even as I type this. You'd better retract this before I am forced to register a FORMAL COMPLAINT! This is exactly the type of irresponsible behavior that puts OUR HOBBY AT RISK!

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No can do...

Aren't you the "Basement Bomber"...

You have a lot of nerve...:grin:

I agree with the general sentiment that stall is an advanced topic probably not worth going into for a kids introduction. The tradeoffs between drag, thrust, and weight are more appropriate for that level.

However, this comment deserves a response:

TopRamen said:

I really don't like this conversation. I feel that terms like "Lift" and "Stall" have no place in rocketry, and that is why the first time in my life I'm hearing them used like this is in this thread.
Even if they are appropriate in relation to model rockets, the hobby and even books on the hobby have done just fine without the use of such terminology.

Neither of these books talk about "Stall" in relation to Missiles or Rockets, and they only reference lift in how it relates to airfoils.
I even re-read quickly over chapter 10 of The Handbook of Model Rocketry, and found nothing.

View attachment 265198View attachment 265197


I think it is safe to say that you can omit any mention of "Stall" as it relates to rocketry from your curriculum.

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I'm not familiar with the Brassey's text, but it appears to be a survey of missile systems rather than a discussion of their design. Stall is most assuredly an issue in missile design and is addressed quite often in works like these:

Tactical Missile Design by E. Fleeman

Missile Aerodynamics by J. Nielsen

Missile Configuration Design by S. Chin

Stall is especially challenging in many air-to-air missiles, where the high angles of attack during high-g maneuvers combined with large fin deflections to accomplish those maneuvers can easily result in flow separation and stall over the fins. This is where odd looking configurations like the Israeli Python and Russian AA-11 come from:





The front canards are fixed surfaces that act as vortex generators to energize the airflow over the trailing, movable canards to improve flow attachment at high angles and improve maneuverability.

This discussion also made me think about missles with long range and extended glide times. A prime example is the AIM-54 Phoenix, a missile that requires at least some lift during the descent/terminal phase to help complete the intercept. And this is for a conventional looking rocket, unlike cruise missles like the Tomahawk.

So, stalls and lift can certainly be topics of discussion when it comes to rockets...though I'd say in advanced stages.

FC

Fins develop a lift force when the apparent wind has a non-zero angle of attack. If they did not, the fins would have a no effect on the direction of flight of a rocket. Angle of attack is angle between the rocket velocity vector and the wind velocity vector. When the angle of attack is zero, the rocket flies in a straight line. When the angle of attack is not zero, the lift force will turn the rocket into the wind to make the angle of attack zero.

Fins are wings. And develop more lift as the angle of attack increases, however the effect depends on the airflow across the wind surface. That's what turns the rocket into the wind. As long the airflow follows the wing surface, you get more lift as the angle of attack increases, however in the real word, if the angle of attack get too large, the airflow separates from the wind and the lift decreases. If the angle of attack continues to increase, at some point the lift decreases to zero and the fins cease to generate lift and stall, and no longer generate corrective forces so the rocket will begin to tumble.

https://exploration.grc.nasa.gov/education/rocket/rktstab.html from https://exploration.grc.nasa.gov/education/rocket/rktaero.html

https://www.nakka-rocketry.net/fins.html

https://rocketcontest.org/media/stem_lesson_ld04_aerodynamics_sarradet.ppt from https://rocketcontest.org/pdf/stem_model_rocketry_curriculum_sarradet.pdf

Bob

I have to say that you have gotten some really good answers here and some really bad ones. You decide.

Honestly, and I apologize for even thinking this, it's probably my age but the first reply I thought of when I saw the title of this thread:

What does it mean when a fin "stalls?" Was:

It means six more weeks of winter!!

Sorry,

Keep working with the kids. It's important.

KidRockET said:

I thought this was a ROCKET forum...

FINS do not provide lift... (they aid stability)

All LIFT is provided by the engine... Duh

Are they aerodynamic flying surfaces... YES

Any deviation from zero AOA or having unmatched airfoils...

Causes pressure differentials which in turn cause...

Movement upon the horizontal axis... Not lift

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No disrespect intended, but motors provide thrust, not lift. "Lift" doesn't necessarily mean "up."

There are four forces acting on a rocket: Gravity, thrust, drag and lift. Drag and lift are at 90 degrees to one another.

As a matter of fact, for all the comments that this is too advanced a topic, and for the mentions that nothing is stated about it in The Handbook, here's what I found, in The Handbook, on the chapter on stability:

When a moving stream of air strikes a surface broadside (at an angle of attack of 90 degrees) or even at a slight angle, it produces a high pressure on one side of the surface and a low pressure on the other, as shown in figure 9-4. This pressure difference creates a drag force opposite to the motion of the airstream and a lift force that is at right angles (90 degrees) to the surface. The higher the angle of attack, the greater the lift and drag forces - up to the angle of attack where the surface stalls. At the stall point, the air breaks away from the low-pressure surface, the lift force decreases drastically, and the drag force increases tremendously. By properly positioning the fins on the model rocket, by making them the right size, and by giving them the right shape, we can use this lift-drag force as a stabilizing force to offset pitch-yaw rotational disturbances.

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I knew I wasn't crazy.

It's actually been rather surprising how much technical information these kids have absorbed. I have them three hours a day for five days, and they've learned a ton. I thought for sure they'd not get even the basics of total impulse, center of pressure, etc. but I had to fill the time with something (especially since the weather's been too bad to launch most days). They really got a lot of it. Several parents have approached me to tell me how surprised they are at how much of the physics of rocketry the kids have learned.

Besides, learning often happens with repetition. If they don't get it the first time, the second or third time, it will start to make sense. Gotta start with the hard stuff some time. I'm trying to give them the tools they need so they don't just launch a few RTF models, get bored, and quit.