What does it mean when a fin "stalls?"

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lcorinth

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I'm teaching a rocketry camp for kids, and I have a plan for tomorrow, where I talk about when it's OK to fly and when not. I know that in high wind, because of the high angle of attack, fins can stall.

Now, I understand that this means that the fins "stop working," but what does this technically mean? I've never actually read a definition. What, in aerodynamic terms, is a stall?

I know the kids may not need to know this, but some of these kids are pretty smart, and they'll ask what that means. I want to give them the right answer.

Thanks!
 
A stall means there is airflow separation at the surface of the flying surface (wing, flight control, fin, etc). Basically the angle between the chord of the fin and the airflow is so great that the air going over the "top" does not reattach properly, with a loss of lift (effectiveness) being the result.
 
At low angles of attack, air flow is "attached" to the surface of the fin. If you were to draw streamlines around the cross section of the fin, they would follow the shape of the fin. Basically, the friction forces of the air against the surface are sufficient to "turn" the flow around the shape of the fin.

At high angles of attack, the friction forces can no longer turn the air enough to follow the surface. The streamlines separate from the surface, creating a bubble of turbulence where air is no longer flowing smoothly along it. This is known as a stall. Wings/fins require smooth airflow to generate lift, so a stall rapidly destroys this lift force.

Stall is is mostly an issue for airplane wings (loss of lift) and control surfaces (if a rudder/aileron is stalled, it doesn't work effectively). It is rarely an issue for rockets. The main issue for a rocket would be if a forward fin or body tube stalled in such a way that the separated bubble enveloped a rear fin, "shadowing" it and never letting it get an attached flow started.
 
Fins can "stall"? I had never heard that before. The main issue that I've heard of with wind is that the rocket may weathercock and point into the wind. The fins have not lost effectiveness --- they've done their job and pointed the rocket into the slipstream. But before the rocket is up to speed, the slipstream is at an angle to verticle, and the rocket takes off in that direction.
 
I find it hard to believe it can be much of an issue for rockets as they are not exactly relying on the Fins to generate lift.
Even if the control surfaces(fins) are inadequate, the momentum is determined by duration of thrust and unless it is an unstable design, the trajectory of a ballistic arc.

I'm pretty sure there is a reason we don't see it discussed here, unless I may have missed it.
 
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The effect that fins have on stability is from lift which is neutral at zero angle of attack and towards the axis of air flow at noon zero angle of attack. It's this lift which pushes the tail back toward the axis of flight and stabilized the rocket.
 
Fin stall can affect the direction of a rocket as it leaves the rail. For example if you angle a rail away from the wind to avoid a non-optimum trajectory (like towards property you do not want to fly near), and the wind is fresh, the angle of attack is very high. This fins will stall and it will appear that the rocket takes a hard turn upwind as soon as it leaves the rail.
 
Fins can "stall"? I had never heard that before. The main issue that I've heard of with wind is that the rocket may weathercock and point into the wind. The fins have not lost effectiveness --- they've done their job and pointed the rocket into the slipstream. But before the rocket is up to speed, the slipstream is at an angle to verticle, and the rocket takes off in that direction.

I have never heard this term used in rocketry either.

Everyone here has tried to explain it in very technical terms. If I were you, I would avoid using this term when trying teach kids (or anybody new to rocketry).

The simplest way to explain what fins do is to tell them that fins are there to "stabilize" the rocket by keeping it going in a straight direction. This can be affected by wind. The bigger the fin area, the more the wind can cause "weather cocking". The rocket's speed, especially off the rod is a major factor. The slower the speed, the less effective the fins are to stabilize it.

Jerome :)
 
Fin stall can affect the direction of a rocket as it leaves the rail. For example if you angle a rail away from the wind to avoid a non-optimum trajectory (like towards property you do not want to fly near), and the wind is fresh, the angle of attack is very high. This fins will stall and it will appear that the rocket takes a hard turn upwind as soon as it leaves the rail.

But is this actually due to stall? I'm no expert on this, but it seems to me that if the fins are in a stall, then the fins have lost effectiveness, and the rocket would be unstable. But what you are describing sounds to me like the fins are being very effective and are pointing the rocket into the wind, despite the fact that that might not be the direction you want it to go. The function of the fins, as I understand it, is to reduce the angle of attack. And in the case of wind and low launch speed, the rail is aimed at a high angle of attack. So if the rocket comes off the rail and immediately corrects to fly into the wind at a low angle of attack, then haven't the fins been effective and have done what they are supposed to do?
 
The effect that fins have on stability is from lift which is neutral at zero angle of attack and towards the axis of air flow at noon zero angle of attack. It's this lift which pushes the tail back toward the axis of flight and stabilized the rocket.

^^^ this.
 
I have never heard this term used in rocketry either.

Everyone here has tried to explain it in very technical terms. If I were you, I would avoid using this term when trying teach kids (or anybody new to rocketry).

The simplest way to explain what fins do is to tell them that fins are there to "stabilize" the rocket by keeping it going in a straight direction. This can be affected by wind. The bigger the fin area, the more the wind can cause "weather cocking". The rocket's speed, especially off the rod is a major factor. The slower the speed, the less effective the fins are to stabilize it.

Jerome :)

I agree. I would avoid discussion of stall. Stick to the topic of CG, CP, and stability. A stable rocket self corrects and points itself into the flow of air past the airframe. In calm air, when the rocket launches, that direction will be the direction that the rod or rail is pointed, and a stable rocket will fly in the direction it is pointed. In wind, the motion of air over the airframe is partly from the side, and unless the rocket is moving much faster than the speed of the wind when it leaves the rail, a stable rocket will tend to aim itself into the wind --- weathercocking.
 
The effect that fins have on stability is from lift which is neutral at zero angle of attack and towards the axis of air flow at noon zero angle of attack. It's this lift which pushes the tail back toward the axis of flight and stabilized the rocket.

I'm thinking the lift most be perpendicular to the air flow to force the tail back, not toward the axis of air flow. In any case, the point is the lift corrects the angle of attack toward zero.

So, how does stall actually affect that? I thought a stable rocket would always correct back to zero angle of attack, no matter how high the angle of attack. If it was flying completely sideways, or even backwards, there's really no point where the fins have lost lift and won't tend to force the rocket back toward stable, straight flight through the flow of air, is there?
 
Keep in mind that CP is the center of the total lift on the rocket (the average). In a stall the lift is reduced greatly so the rocket will have increasing stabilizing forces as the angle of attack increases until you reach the stall angle and the stabilizing forces decrease with increased angle of attack. As the angle of attack increase the body and nose also generate lift, some in ahead of the CG and therefore in a destabilizing manner, if the stall of the fin causes the lift behind the CG to decrease the CP will move forward meaning the rocket will become less and less stable.
 
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.

Stall Idiocy 2015-06-10 001.jpgStall Idiocy 2015-06-10 005.jpg


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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Please see question #7 of the L2 certification test study guide:
https://www.tripoli.org/LinkClick.aspx?fileticket=kyIgDEN7biM=&tabid=316

The center of pressure (CP) of a rocket is generally defined as:
a. The balance point of the rocket without the motor.
b. The total area of the fins, airframe and nose cone divided by two.
c. The point at which aerodynamic lift on a rocket is centered.

The answer is C. Lift and stall are used to calculate the CP vs. Angle of attack of a rocket or missile and are therefore fundamentally built in to everything we do.
 
I believe the OP said he was teaching kids basic model rocketry, not trying to prepare them to get their L2 or work for NASA on Glide Recovery.
 
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that E=ThirstyBarbarian;1466469]But is this actually due to stall? I'm no expert on this, but it seems to me that if the fins are in a stall, then the fins have lost effectiveness, and the rocket would be unstable. But what you are describing sounds to me like the fins are being very effective and are pointing the rocket into the wind, despite the fact that that might not be the direction you want it to go. The function of the fins, as I understand it, is to reduce the angle of attack. And in the case of wind and low launch speed, the rail is aimed at a high angle of attack. So if the rocket comes off the rail and immediately corrects to fly into the wind at a low angle of attack, then haven't the fins been effective and have done what they are supposed to do?

Yes.
 
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I'm teaching a rocketry camp for kids, and I have a plan for tomorrow, where I talk about when it's OK to fly and when not. I know that in high wind, because of the high angle of attack, fins can stall.

Now, I understand that this means that the fins "stop working," but what does this technically mean? I've never actually read a definition. What, in aerodynamic terms, is a stall?

I know the kids may not need to know this, but some of these kids are pretty smart, and they'll ask what that means. I want to give them the right answer.

Thanks!

You could always prepare a demonstration using a weathervane style model to explain weathercocking to them.
I high wind, the fins don't technically "stop working", rather technically, they are working just fine, only not in the intended fashion.
 
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.

Party Pooper...

It is much more fun to listen...

To the experts completely...

Complicate a very simple concept...

There, I said it...

Fins don't provide lift? Who knew?
 
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I'm teaching a rocketry camp for kids, and I have a plan for tomorrow, where I talk about when it's OK to fly and when not. I know that in high wind, because of the high angle of attack, fins can stall.

For kids at rocketry camp, I think you can forget about stall for the purposes of discussing wind and when it is not OK to fly. That's getting too far into the weeds. You should talk about the perils of weathercocking in high wind --- cruise missile flights, and delays calculated for straight flights being too long for arcing flights, resulting in high-speed deployments and/or crashes. You could also discuss rockets floating away on the wind under chute. If you are following NAR or TRA safety guidelines for wind speed and launch rod angle, my guess is you are probably not going to be launching with a rocket past its stall angle. (Someone may correct me on that.) Regardless, the safety issues related to wind don't need to discuss stall, just the consequences of flying in wind.
 
There are more than one question in my post, and it is not clear which one you are answering "yes" to.

I counted 2 questions in your post. Yes the fins did what they are supposed to do and Yes stalled fins are less effective than fins with attached flows on both surfaces. I think that stalled fins with a high AOA will cause a direction correction that can easily overshoot (over correct). Fins with attached flows will overshoot less, I think.
 
The principles of aerodynamics do not change simply because the direction of flight is vertical.
 
Hmm. I'm not sure why a few people are getting grumpy about this question.

So, here's why I asked: I'm trying to impart safe flying in these kids, and one of the principles I've heard (possibly not in The Handbook, but definitely in some Apogee newsletters) is that in high wind, with high angle of attack, fins can stall.

Now, I've never witnessed this, but I thought I'd maybe toss off a line like "in high winds, your fins can stall" because I have read it several places related to model rocketry.

That's it. I was just going to say, "don't fly in high winds - the fins could stall, and the rocket could go unstable." Seemed like an easy sentence to toss off.

But then I thought about that sentence. I've got some smart kids here, and I can just see that if I toss off that expression, one of them will ask "what does 'stall' mean?" They're curious, and if they want to know something, I think I'd like to tell them, and since I didn't know the technical definition, I thought I'd ask.

I do like the picture of the stalling plane wing from wikipedia - very visual and cool. But I guess I'll just avoid saying it.

I am trying to keep things basic for, but I'm actually surprised at some of the technical information I've given these kids which they remember the next day. And so I'm seeing if I can give them some more information to chew on. What they don't retain or care about, I set aside.

Edit: Another factor here is that I have these kids for three hours a day for five days - kind of a lot of time. I'm finding I have way more time than needed to just build and launch, so I'm trying to give them a taste of some of the sciencey aspects of rocketry, because a lot of them seem to like it. And I have time to fill.
 
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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