back to the basics

[Lt72884]

I have been experimenting in open rocket with stability and fins. I have noticed that if i trim fins down the caliber goes down for stability drastically IE i took off 1.5 inches of height and the caliber went from 5.51 to 3.3.
So, what i want to learn and understand is whats going on with the physics of this.

Why would a fin that has a larger height cause more calibers of stability? is it due to more surface area causing more drag, causing the Cp to be further back? or is there more to it than that? or does the larger fin cause more drag and lift which can affect behavior? What behavior is it changing? pitch? yaw? both? and how do i examine this in open rocket or something? IE, take my rocekt with larger fins, see the data, then compare it to same rocket smaller fins. I see options in plotting the data for pitch, yaw, roll etc. The graphs look interesting.

or do larger fins cause vortex shedding and turbulence which can be a no no?

i am super curious now what is going on.

thanks

any feed back on this subject is wlecomed

 

[cjl]

When you say "height", do you mean span (perpendicular to the tube), or chord (parallel to the tube)? Also, just for some reference, what size of fins are we talking about here? What are your baseline fin dimensions? 1.5 inches would be the entire fin on a small rocket, and totally negligible on a gigantic monstrosity at LDRS or BALLS.

 

[Rex R]

I've understood it to be that the air coming off the body tube can blanket the fins ànd effectively cause them to stall if they have insufficient span to get at least some portion in clean air.

 

[cjl]

While that's true, I'm not sure how well OpenRocket models that, plus boundary layer thickness varies based on a whole pile of factors.
A really easy way to test that would be to make 2 models that are identical except scale, and see if it predicts the same stability margin for both (since the amount of fin within the boundary layer will be smaller for a larger rocket that is identical except a scaling factor).

 

[prfesser]

I have been experimenting in open rocket with stability and fins. I have noticed that if i trim fins down the caliber goes down for stability drastically IE i took off 1.5 inches of height and the caliber went from 5.51 to 3.3.
So, what i want to learn and understand is whats going on with the physics of this.

Why would a fin that has a larger height cause more calibers of stability? is it due to more surface area causing more drag, causing the Cp to be further back? or is there more to it than that? or does the larger fin cause more drag and lift which can affect behavior? What behavior is it changing? pitch? yaw? both? and how do i examine this in open rocket or something? IE, take my rocekt with larger fins, see the data, then compare it to same rocket smaller fins. I see options in plotting the data for pitch, yaw, roll etc. The graphs look interesting.

or do larger fins cause vortex shedding and turbulence which can be a no no?

i am super curious now what is going on.

thanks

any feed back on this subject is wlecomed

My understanding: larger fins move the Cp aft because they produce a greater restoring force.

When the rocket attempts to rotate (around its Cg) away from the direction of travel, larger fins at the aft end have more surface area exposed to the moving air than do smaller fins. The air pushing against that larger area generates more restoring force to bring the rocket back to straight flight.

There are other issues of course but I think that's the main one.

 

[waltr]

As Prfesser said: larger fin area has larger restoring force.

Go look up the old Estes papers of figuring out stability. These are from the 1960's but the physics has not changed.

In these one method to determine stability was to do a flat paper cut-out on the rocket profile. This simulates the aero forces (by AREA) into distributed weight. The CP is then determined by finding the CG of the 2-dimenional cut-out.
From this it should be very easy the "SEE" that large fins move the CP rearward.

Note: this method is not quite as accurate as the Barrowman calcs (OR & rocsim) but for a simple rocket does work to determine if rocket will be stable before being built and flown.

 

[Steve Shannon]

Air pressure against one side of a fin (greater than the other side) creates a force. Pressure times area equals force. So the larger the fin, the greater the force. That force can be reduced to a single point force working through the centroid of the shape of the body part. The shape of the fin determines where the centroid is. All the body parts experience some force through their centroids. The combination of forces can be reduced to another single point of force. The location of that point is the center of pressure.
Because the rocket is a free body, that force forces the rocket to rotate about its center of mass. If the center of mass (gravity) is ahead of the center of pressure, the force makes the rocket rotate into the airflow.

 

[firemanup]

Buy and read this book.

It will answer your current questions and the questions you don’t know you have yet.

 

[Lt72884]

When you say "height", do you mean span (perpendicular to the tube), or chord (parallel to the tube)? Also, just for some reference, what size of fins are we talking about here? What are your baseline fin dimensions? 1.5 inches would be the entire fin on a small rocket, and totally negligible on a gigantic monstrosity at LDRS or BALLS.

my bad. it was very late wheni wrote this post. in open rocekt, i just use the height feature but it doesnt take any off any numbers from any other measurement, so im not sure which edge to trim. Im guessing it would have to be the edge that against the rocket body to drop that height down. Here are 2 pics of what i am doing in OR.
My goal is two fold. Understnad why the caliber drops, and how to trim my current fins to the new fin set. IE, which edges am i cutting because in OR, the only diminsion changing is the height and sweep angle. If i had a "saw/cutting device" what am i cutting? am i actually going to trace a new fin shape onto the old fin and cut it out?

thansk

current fin dims:


New fin dims after dropping the height:

 

[Lt72884]

Buy and read this book.

It will answer your current questions and the questions you don’t know you have yet.
View attachment 569203

ordered and on its way. thanks

 

[Lt72884]

Air pressure against one side of a fin (greater than the other side) creates a force. Pressure times area equals force. So the larger the fin, the greater the force. That force can be reduced to a single point force working through the centroid of the shape of the body part. The shape of the fin determines where the centroid is. All the body parts experience some force through their centroids. The combination of forces can be reduced to another single point of force. The location of that point is the center of pressure.
Because the rocket is a free body, that force forces the rocket to rotate about its center of mass. If the center of mass (gravity) is ahead of the center of pressure, the force makes the rocket rotate into the airflow.

ok, its like me walking outside in a hurricane, for lack of a better example, holding a 8 foot piece of plywood up vs a 2 foot piece in the wind. I can imagine what it would be like to have that extra force. I went back through my fluid mechanics book and found the math for it.

The larger surface area seems to be creating extra lift and drag. The main issue is the turbulance and vortex shedding at the aft end of the fins, it can cause the rocket to start to stall as another poster added.

i would love to see thin in a wind tunnel with vapor to see the affects of it.

thanks for your explanation. very helpfu

 

[Lt72884]

As Prfesser said: larger fin area has larger restoring force.

Go look up the old Estes papers of figuring out stability. These are from the 1960's but the physics has not changed.

In these one method to determine stability was to do a flat paper cut-out on the rocket profile. This simulates the aero forces (by AREA) into distributed weight. The CP is then determined by finding the CG of the 2-dimenional cut-out.
From this it should be very easy the "SEE" that large fins move the CP rearward.

Note: this method is not quite as accurate as the Barrowman calcs (OR & rocsim) but for a simple rocket does work to determine if rocket will be stable before being built and flown.

i do see in open rocekt like i stated that it does move the Cp back further.. So maybe i need to do more research on what the Cp really is besides "a point where all the pressure forces go through" IE, with it further back, what type of behavior would a rocket have vs with the Cp closer to the Cg? the rocket rotates about the Cg, so i wonder if it has something to do with mass moment of inertia as well. IE, further back, the MMI is going to be different than if it were closer to the Cg. Its like trying to balance the rocket via the nosecone in the palm of my hand vs letting it balance via the fins or hang from teh ceiling. The further the Cp is aft, the greate the moment arm becomes and the greater the rotation of the rocket become and what not. interesting

 

[tsmith1315]

If i had a "saw/cutting device" what am i cutting?

You would be cutting the height, and because of the angled forward edge, that would result in a longer tip chord and shorter sweep length.

Print both fin patterns out and superimpose them.

 

[David_Stack]

The other thing which you can do (I believe Open Rocket supports this, but I am not at my PC where the software is loaded to verify) is change the scale of your existing fin shape.

Doing so would retain the leading edge sweep angle, but all other dimensions (root chord, tip chord, height, sweep length) would be adjusted accordingly. Easy way to do upscales/downscales of existing models (or see the effect of change to CP is as a result of a change in fin area)

 

[Handeman]

I think you are starting to understand it. The CP is the aerodynamic balance point like the CG is the mass balance point.
A simple way to look at CP is if all air movement is 90° from direction of flight, straight from the side, the point where the nose cone section and fin section are balance is your CP. If you put a pivot point ahead of the CP, the fin section will move away, if you put the pivot behind CP, the nose cone section will move away.
In flight, the rocket wants to rotate around the CG and that rotation is countered by the two basic air vectors. The vertical vector which is the air the rocket is moving into and gets bigger as the rocket moves faster, and the horizontal vector which is the angle of attach of the fins and and side (horizontal) wind the rocket is encountering. With the CP behind the CG, there is the fin section and the part of the nose section between CP and CG that will want to push away from the horizontal vector so side wind has a stronger effect when the rocket is moving slow and the CP is further away from CG. That is why over-stable rockets tend to weathercock into the wind at launch.

 

[Lt72884]

You would be cutting the height, and because of the angled forward edge, that would result in a longer tip chord and shorter sweep length.

Print both fin patterns out and superimpose them.

perfect, i actually did just this about 10 minutes ago. I glued the fin template to some cardboard so i can do the measurements needed and create new ones in OR.

 

[Lt72884]

The other thing which you can do (I believe Open Rocket supports this, but I am not at my PC where the software is loaded to verify) is change the scale of your existing fin shape.

Doing so would retain the leading edge sweep angle, but all other dimensions (root chord, tip chord, height, sweep length) would be adjusted accordingly. Easy way to do upscales/downscales of existing models (or see the effect of change to CP is as a result of a change in fin area)

ohh, i need to see if i can do this in OR, that would help me understand more.. thanks for that

 

[Lt72884]

I think you are starting to understand it. The CP is the aerodynamic balance point like the CG is the mass balance point.
A simple way to look at CP is if all air movement is 90° from direction of flight, straight from the side, the point where the nose cone section and fin section are balance is your CP. If you put a pivot point ahead of the CP, the fin section will move away, if you put the pivot behind CP, the nose cone section will move away.
In flight, the rocket wants to rotate around the CG and that rotation is countered by the two basic air vectors. The vertical vector which is the air the rocket is moving into and gets bigger as the rocket moves faster, and the horizontal vector which is the angle of attach of the fins and and side (horizontal) wind the rocket is encountering. With the CP behind the CG, there is the fin section and the part of the nose section between CP and CG that will want to push away from the horizontal vector so side wind has a stronger effect when the rocket is moving slow and the CP is further away from CG. That is why over-stable rockets tend to weathercock into the wind at launch.

thanks for more information on this. I grabbed me ol trusty fluids book and Cp is such an interesting concept. Behaviors are different for each fluid IE, water acting on a sluice gate, using the Cp to open said gate when there is enough water etc etc.
Cp does create a moment arm between Cg which is why a longer moment arm, it can cause instability, but with torque, a longer arm is sometimes better such as using a cheater bar..
in case anyone wants an equation. This takes into account the center of pressure in the force equation on a plate

 

[tsmith1315]

My goal is two fold. Understnad what the caliber drops, and how to trim my current fins to the new fin set.

Not sure what you mean here in the first part. A “caliber of stability” is a unit of measurement equal to one airframe diameter in length.

“5.4 cal” in your first fin image of Post #9 means the CP is 5.4 body tube diameters aft of the CG.

Since the airframe diameter isn’t shown, I can’t tell you what that is in inches.

 

[Lt72884]

Not sure what you mean here in the first part. A “caliber of stability” is a unit of measurement equal to one airframe diameter in length.

“5.4 cal” in your first fin image of Post #9 means the CP is 5.4 body tube diameters aft of the CG.

Since the airframe diameter isn’t shown, I can’t tell you what that is in inches.

i ment why not what haha. i was typing to fast.
I know what the caliber means as in body diameter, i wanted to know why the caliber drops when i cut the fins. haha. typo on my end

 

[Handeman]

thanks for more information on this. I grabbed me ol trusty fluids book and Cp is such an interesting concept. Behaviors are different for each fluid IE, water acting on a sluice gate, using the Cp to open said gate when there is enough water etc etc.
Cp does create a moment arm between Cg which is why a longer moment arm, it can cause instability, but with torque, a longer arm is sometimes better such as using a cheater bar..
in case anyone wants an equation. This takes into account the center of pressure in the force equation on a plate

View attachment 569255

That is all well and good, but I think that is where most university teams, and their professors miss the whole point and why mentors are so important.

I doesn't matter if your CP is 1.95 calibers, or 2.09 calibers. All the math is good, but doesn't really matter in the real world. That would be about 2 calibers and the affects on the flight are going to follow a general pattern because in the real world, you can't calculate all the variables. You have to generalize and use approximations. Almost none of the hobby rocketry people go to that detail on parameters of the rocket, yet are very good at knowing if it will fly right or not. Don't get lost looking at the trees instead of the forest.

 

[waltr]

Another source of education:
https://www.apogeerockets.com/
Here is a video demonstrating CP:
https://www.apogeerockets.com/Advanced_Construction_Videos/Rocketry_Video_284

 

[cjl]

my bad. it was very late wheni wrote this post. in open rocekt, i just use the height feature but it doesnt take any off any numbers from any other measurement, so im not sure which edge to trim. Im guessing it would have to be the edge that against the rocket body to drop that height down. Here are 2 pics of what i am doing in OR.
My goal is two fold. Understnad why the caliber drops, and how to trim my current fins to the new fin set. IE, which edges am i cutting because in OR, the only diminsion changing is the height and sweep angle. If i had a "saw/cutting device" what am i cutting? am i actually going to trace a new fin shape onto the old fin and cut it out?

thansk

current fin dims:

(images removed to prevent an obnoxiously long post)

OK, so that makes sense that that would have a huge impact - you're going from 44.5 square inches per fin to 29.3, so your shrunken fins are only 65% of the size of the originals. Of course that is going to move your CP by a large amount.

That having been said, one thing you have to understand is that the CP provided by OpenRocket or any similar sim program is only a rough value. A lot of amateur rocketry is based more on "rules of thumb" and estimation rather than extremely precise calculations, because precise calculations are somewhere between incredibly difficult and totally impossible for an amateur to do.

The reality is, Cp actually depends on mach number, reynolds number, angle of attack, finish quality, and a whole pile of other factors. However, on the other hand, the actual requirement for stability is just that the Cp be behind the Cg. Not by 1 caliber. Not by some arbitrary margin. It just needs to be behind it.

Due to the uncertainty in Cp though, the "1 caliber" rule was developed to basically encompass the uncertainties in Cp estimation, as well as to provide a sufficient restoring moment to make the disturbance response pretty fast (as an aside, I really prefer to use a 10% of body length rule of thumb rather than 1 caliber, as I feel it works better across a wider range of rockets). As others have stated above, at a basic level to just fly rockets safely, it's better to focus on broad principles rather than very detailed specifics, since it's very easy to get bogged down in the details and you can largely get away without them (hell, most L3 projects are based more on rules of thumb than they are of, and I'm trying not to sound too snobby here, actual aerospace engineering). This kind of "rule of thumb" design is also where references like the above referenced "Handbook of Model Rocketry" are excellent, and I will 100% echo the recommendations to read that ASAP.

Despite that though, I'll go into more details in a further post or two in a minute.

 

[cjl]

ok, its like me walking outside in a hurricane, for lack of a better example, holding a 8 foot piece of plywood up vs a 2 foot piece in the wind. I can imagine what it would be like to have that extra force. I went back through my fluid mechanics book and found the math for it.

The larger surface area seems to be creating extra lift and drag. The main issue is the turbulance and vortex shedding at the aft end of the fins, it can cause the rocket to start to stall as another poster added.

i would love to see thin in a wind tunnel with vapor to see the affects of it.

thanks for your explanation. very helpfu

So, yes, kind of?

Fins, ideally, should be acting much more like wings than like a piece of plywood held against the wind. There should be minimal vortex shedding aside from a tip vortex that forms behind the rocket, and the lift slope should be closely approximated by a short aspect ratio wing rather than any kind of drag equation.

This is what really drives rocket stability. In effect, you have a cylinder with a cone at the top and some wings right at the rear. If you imagine perturbing this to some small angle of attack, cylinders and cones don't tend to produce large forces or moments from a slight side flow, but wings can generate relatively large forces even at only a few degrees of angle of attack. Since these wings are placed right at the rear of the vehicle, this means that when the vehicle experiences a small angle of attack, it experiences a fairly large side force, but importantly, that side force is localized very close to the rear of the vehicle, since that's where the fins are. As long as this side force is effectively acting behind where the center of gravity is, this means that the rocket will experience a torque about the CG that tends to push the rear in the direction the rocket was pointing, which will rotate the front back towards the direction of travel. Thus, positive stability.

This is really all the Cp is, is that it's an attempt to calculate where, on average, this lift force when you cause a small perturbation is acting. This is why it needs to be behind the CG - because that's the only way that the torque about the CG will tend to push the nose back in the direction of flight, rather than away from it.

Don't think of this in terms of bluff body aerodynamics or behavior. Think of wings. This is much closer to airplane aerodynamics than it is to airflow around a building or similar.

 

[cjl]

thanks for more information on this. I grabbed me ol trusty fluids book and Cp is such an interesting concept. Behaviors are different for each fluid IE, water acting on a sluice gate, using the Cp to open said gate when there is enough water etc etc.
Cp does create a moment arm between Cg which is why a longer moment arm, it can cause instability, but with torque, a longer arm is sometimes better such as using a cheater bar..
in case anyone wants an equation. This takes into account the center of pressure in the force equation on a plate

View attachment 569255

So, a sluice gate is really not a good analogy here. As stated in the last post, fins behave far more like wings than like bluff bodies.

As for moment arms? As stated, Cp is effectively the "average" location of lift when the rocket is flying at a nonzero angle of attack, so as long as the Cp is behind the CG, the restoring moment is positive. In addition, there's (with a few weird exceptions) no reason to worry about damping coefficient, since there's going to be some inherent viscous dissipation. As a result, as long as your Cp is behind your CG, your rocket is stable (within the caveats mentioned above where you really want a ~10% of rocket length margin due to estimation errors and such).

The reason very large margins are often avoided is because they cause an extremely large restoring moment, leading the rocket to very quickly "correct" into the wind as soon as it leaves the launch rail. This can lead to longer distances from the pad, higher deployment velocities, and lower altitudes, but it's not due to instability, it's often termed as "overstability".

Also, that formula you posted is going to be largely useless for rockets, since again, they act more as wings than as bluff bodies or flow obstructions.

 

[Lt72884]

So, a sluice gate is really not a good analogy here. As stated in the last post, fins behave far more like wings than like bluff bodies.

As for moment arms? As stated, Cp is effectively the "average" location of lift when the rocket is flying at a nonzero angle of attack, so as long as the Cp is behind the CG, the restoring moment is positive. In addition, there's (with a few weird exceptions) no reason to worry about damping coefficient, since there's going to be some inherent viscous dissipation. As a result, as long as your Cp is behind your CG, your rocket is stable (within the caveats mentioned above where you really want a ~10% of rocket length margin due to estimation errors and such).

The reason very large margins are often avoided is because they cause an extremely large restoring moment, leading the rocket to very quickly "correct" into the wind as soon as it leaves the launch rail. This can lead to longer distances from the pad, higher deployment velocities, and lower altitudes, but it's not due to instability, it's often termed as "overstability".

Also, that formula you posted is going to be largely useless for rockets, since again, they act more as wings than as bluff bodies or flow obstructions.

as soon as you said "quickly correct" into the wind, i thought of weather vanes. The larger fins on a rocket can quickly cause it to get back to a straight path. So a stability caliber of 7 would be over stable? If that is true and correct, can an over stable rocekt actually fly safely? Not going to do this, just curious to know.

i will reply to your others later. I have a big tech report for fluids due tonight and i just got back from work... sigh

thank you for taking the time to write your explanations. I have lots of book theory and little application so im trying to relate what i can to things.

I like the 10% rule. Ill see if i can follow that on my current build. Have the cp be 10% body length behind cg.
i do have the book now. and i have a couple of James barrowmens white papers too.

 

[Neutronium95]

as soon as you said "quickly correct" into the wind, i thought of weather vanes. The larger fins on a rocket can quickly cause it to get back to a straight path. So a stability caliber of 7 would be over stable? If that is true and correct, can an over stable rocekt actually fly safely? Not going to do this, just curious to know.

Overstable rockets can fly perfectly safely. They're just going to be more sensitive to the wind, and will arc into it more. There are a bunch of ways to mitigate that, including but not limited to using a longer rail, using a higher thrust motor, angling the rail with the wind so it will correct to vertical, or just waiting for lower winds.

On the other hand, an unstable rocket is never safe to fly, and would require modification or a complete redesign to be made to fly well.

I've often seen people get very worried about their rockets being overstable, when it's not nearly as big of a problem as they think it is. There isn't a hard line where you can say that a rocket is overstable or not. Every stable rocket will tilt into the wind to some degree, you should just have a good feel for whatever design you have, run some sims and make an appropriate choice about whether the configuration you want to fly is suitable for the conditions at the launch.

I'd also suggest that you take a look at this series of lectures about rocket stability. It was very informative for me, and while I haven't really applied any of the math in them they helped inform my understanding of the principles of rocket stability.

 

[Handeman]

What @Neutronium95 said is absolutely correct. You need to know how your rocket flies and then "fly the field". Good judgement in those areas comes through experience. Almost none of the university teams have anyone with the experience to take in those factors when making those decisions which is why a mentor is so important. But then the team has to listen to the mentors advice and not just push forward because their schedule says they have to fly to certain altitude or a certain motor.

 

[Lt72884]

What @Neutronium95 said is absolutely correct. You need to know how your rocket flies and then "fly the field". Good judgement in those areas comes through experience. Almost none of the university teams have anyone with the experience to take in those factors when making those decisions which is why a mentor is so important. But then the team has to listen to the mentors advice and not just push forward because their schedule says they have to fly to certain altitude or a certain motor.

thats the thing.. i dont know how my rocekt flies. I just have the OR file. Trying to find what "I" think is a stable caliber is tough because its my second rocket. 7 seems high, so does 4. I was aiming for 3 since that seems to be a "standard" caliber haha. I have gotten the rocekt on OR to 2.71 and i am assuming that is good based off of information i have read
thanks for the clarity. I will get this... one day

 

[Lt72884]

Overstable rockets can fly perfectly safely. They're just going to be more sensitive to the wind, and will arc into it more. There are a bunch of ways to mitigate that, including but not limited to using a longer rail, using a higher thrust motor, angling the rail with the wind so it will correct to vertical, or just waiting for lower winds.

On the other hand, an unstable rocket is never safe to fly, and would require modification or a complete redesign to be made to fly well.

I've often seen people get very worried about their rockets being overstable, when it's not nearly as big of a problem as they think it is. There isn't a hard line where you can say that a rocket is overstable or not. Every stable rocket will tilt into the wind to some degree, you should just have a good feel for whatever design you have, run some sims and make an appropriate choice about whether the configuration you want to fly is suitable for the conditions at the launch.

I'd also suggest that you take a look at this series of lectures about rocket stability. It was very informative for me, and while I haven't really applied any of the math in them they helped inform my understanding of the principles of rocket stability.

thank you for the video. i will watch it asap. Im wanting to make sure that the stability number i picked for my rocket is actually good. I have been told 7 is to high, and i dont know why, and i have researched that 3 is good. is 7 "overstable" or "unstable"?