Sooner Boomer
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I concur tube fins spin very little.
Except for the Corkscrew - a rocket with a single, off-center tube fin. It doesn't just spin, it spirals.
I concur tube fins spin very little.
It does if let's say the rocket was built unevenly or has an imbalance in weight or drag and would otherwise veer to one side...then sufficient spin would prevent that imo.
Maybe. I'd buy that for weathercocking. Assuming same net surface area spread among three fins vs 4*, at any given time the average cross sectional area of the fin can (in 2 dimensions perpendicular to net wind vector) will be greater for 3 find than 4, as the three fins each have a longer hemi-span. But even if on fin is slightly larger, as the rocket rotates it goes from one side to the other, so rotational forces should balance out.I've read that 4FNC rockets have less tendency to spin, all else being equal, than 3FNC rockets. I believe I understand why, and it's pretty simple. A cross wind perpendicular to one fin "sees" more area on that side than on the other, so that side is pushed downwind wind more, and, voila, spin. So for the minimum spin, in addition to symmetry and all the other good stuff, if you're using "regular" fins, use four rather than thee.
Spin balancing, to align the principle inertial axis to the airframe, is a very good thing.Something I've noticed, esp. with rockets that have heavy electronics packages (batteries) - the rocket will rotate on its longitudinal axis (direction of flight) around the "vertical" center of gravity. The further the mass is from the center, the worse the rotation (spin).
Not quite. Intentional roll or spin is used to get more predictable trajectories, not to improve stability. The spin tends to average out the effects of asymmetries. For most of us, the factors you cited are small potatoes, compared to thrust misalignment, which can be alarmingly large and uncharacterized for sport rocket motors.It does if let's say the rocket was built unevenly or has an imbalance in weight or drag and would otherwise veer to one side...then sufficient spin would prevent that imo.
Actually my response was to a question about stabilizing "the flight" of a rocket vs. making the rocket itself more stable. I still believe that is the case that a roll or spin can prevent an unevenly built or imbalanced rocket have a straighter flight (what I interpreted to be "stable flight". This may be a matter of semantics.Not quite. Intentional roll or spin is used to get more predictable trajectories, not to improve stability. The spin tends to average out the effects of asymmetries. For most of us, the factors you cited are small potatoes, compared to thrust misalignment, which can be alarmingly large and uncharacterized for sport rocket motors.
Yes, regarding canted motor mounts. By I think that what Alan15578 was talking about is defects in the motors' nozzles, which occur randomly. Sometimes radically, which can cause skywriting in an aerodynamically stable rocket, but sometimes only slightly.Regarding thrust misalignment (with the axis of the rocket), that can be used in a positive way as well if used to cancel out the imbalance...I believe the fliskits Duece can still fly on one canted motor. There are other examples of imbalanced rockets that fly fine due to spin or angled thrust, etc.
For most of us, the factors you cited are small potatoes, compared to thrust misalignment, which can be alarmingly large and uncharacterized for sport rocket motors.
It seems like decoupling the camera's (potential) spin from the rocket body's (potential) spin would mostly only help if all the same things are done to keep the camera from spinning that should be done for the whole rocket if they are not decoupled. It would help regarding nozzle defects, but otherwise I don't see the point.Here's a wild thought...regarding the OP's initial inquiry on how to reduce spin for a camera, how about having it mounted on a roller/thrust bearing sleeve over the bt or nose with two fins added to it? I believe that should help keep it more stable and less rotation even if the rest of the rocket rotates.
I remember that one. That was a great project.I actually investigated using such bearings for a spin-can rocket I was building years ago, but that's a separate story.
Actually I believe it would make a significant difference. The reasoning is that creating two small fins on a collar should be much more precise and able to be built straight vs the rest of the rocket since there are significantly less build variables involved. Basically mostly the two fins in the airstream vs. everything on the rocket and the nozzle (good point about the nozzle...never really thought of that).It seems like decoupling the camera's (potential) spin from the rocket body's (potential) spin would mostly only help if all the same things are done to keep the camera from spinning that should be done for the whole rocket if they are not decoupled. It would help regarding nozzle defects, but otherwise I don't see the point.
As usual, you are probably right, I just don’t conceptually understand why, that’s on me!That reminds me of a point I forgot to make yesterday. Since asymmetric airfoils will cause spin, one method to prevent spin would be don't airfoil the fins at all. Maybe round the corners, but that's it. If you don't try then you can't mess it up. And that is an OK strategy if you're not too concerned with altitude.
Also, you do have a point, using my 3 fin vs. 4 fin reasoning, that the rotational torque would switch sides as the rocket rotates. That means I may have the explanation wrong, but doesn't change what I've read, that most sounding rockets have 4, and that's for the purpose of reducing spin. (Note the word most, so please don't bother raising the counterexamples that are unquestionably out there.)
I think a free spinning can would likely benefit from a gyroscope. May be difficult to accomplish on a LPR rocket.Yes, regarding canted motor mounts. By I think that what Alan15578 was talking about is defects in the motors' nozzles, which occur randomly. Sometimes radically, which can cause skywriting in an aerodynamically stable rocket, but sometimes only slightly.
It seems like decoupling the camera's (potential) spin from the rocket body's (potential) spin would mostly only help if all the same things are done to keep the camera from spinning that should be done for the whole rocket if they are not decoupled. It would help regarding nozzle defects, but otherwise I don't see the point.
I remember that one. That was a great project.
Yes, and it takes very little spin to accomplish that.It does if let's say the rocket was built unevenly or has an imbalance in weight or drag and would otherwise veer to one side...then sufficient spin would prevent that imo.
I'd have to devote some thought to it, but that doesn't match my intuition. My expectation would be that the fin area on each side of the rocket, taking angle into account, will be the same. It doesn't seem like you have to worry about the upwind fin shading the downwind, since the crosswind will never be great enough in comparison to the speed of the rocket for that to happen.I've read that 4FNC rockets have less tendency to spin, all else being equal, than 3FNC rockets. I believe I understand why, and it's pretty simple. A cross wind perpendicular to one fin "sees" more area on that side than on the other, so that side is pushed downwind wind more, and, voila, spin. So for the minimum spin, in addition to symmetry and all the other good stuff, if you're using "regular" fins, use four rather than thee.
Not sure that is correct.Except for the Corkscrew - a rocket with a single, off-center tube fin. It doesn't just spin, it spirals.
Ah, very interesting! I never questioned the usage nor definition of "spiral" since it's so commonly used. Based on the main definition I got off Google, it seems incorrect, however then that also means what we call spirals in the tubes to be incorrect and "helix" is more appropriate.Not sure that is correct.
I think it ”Precesses”
from wikipedia
Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In other words, if the axis of rotation of a body is itself rotating about a second axis, that body is said to be precessing about the second axis. A motion in which the second Euler angle changes is called nutation. In physics, there are two types of precession: torque-free and torque-induced.
View attachment 459209
“A rose by any other name is still a rose.”Ah, very interesting! I never questioned the usage nor definition of "spiral" since it's so commonly used. Based on the main definition I got off Google, it seems incorrect, however then that also means what we call spirals in the tubes to be incorrect and "helix" is more appropriate.
"
adjective
noun
- winding in a continuous and gradually widening (or tightening) curve, either around a central point on a flat plane or about an axis so as to form a cone.
"a spiral pattern"
"
- 1.
a spiral curve, shape, or pattern.
"he spotted a spiral of smoke"
However, I think our common usage is similar to "spiral staircase". In this case, we commonly refer to a rocket Precessing as spiralling due to the spiral smoke trail left.
I'm wondering if anyone knows anything about preventing a rocket from spinning, and if so, what are the tricks. It probably helps to have fins with perfectly symmetrical airfoils, but I'm wondering if a square or triangular body shape would help (instead of a standard cylindrical body tube). Maybe there are other ways but nothing comes to mind yet (excluding gyros and electronics for now).
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