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Dad of Sapling

Dad of Sapling

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I have read that Estes are designed to be overly stable so a swing test not needed. While I was waiting for some paint to dry I thought I would try a swing test out anyway. Both my Apha 3 and Generic both swung level as can be but backwards. Do I need to add nose weight? I followed the instructions, the only modifications was painting the Geneic and a coat of Future to both.
 
Dad of Sapling said:
I have read that Estes are designed to be overly stable so a swing test not needed. While I was waiting for some paint to dry I thought I would try a swing test out anyway. Both my Apha 3 and Generic both swung level as can be but backwards. Do I need to add nose weight? I followed the instructions, the only modifications was painting the Geneic and a coat of Future to both.
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You did swing test it in the flight ready configuration, correct? With the new (unused) motor, parachute and wadding installed.

Swing tests are always a good idea. A long string and a decent amount of speed are crucial. Once you do a number of swing tests on various different rockets, you will end up getting a good reading in regard to which rockets are the most stabile. Some point right into the wind, others are a bit wobbly and some have no sense of direction whatsoever.
 
This is perfect timing! I too have finished the Mach1 BT55 Blackbird. OR says I have 3+ calibers of stability, so I said let's do a swing test just to see, and it flew backwards. I was thinking of adding nose weight as I have had other "longer" rockets do somersaults in the past, which is not the flight I had envisioned. Lol So are swing tests accurate, or should I just assume OR is correct?
 
If you did the swing test right you now know the actual CG. Does the CG match the CG noted in the kit?If it does, you should be OK.
Swing test are not always easy to do. I believe that they are just one tool for determining stability. If you are doing the swing test wrong your results will be wrong.
 
lakeroadster said:
You did swing test it in the flight ready configuration, correct? With the new (unused) motor, parachute and wadding installed.

Swing tests are always a good idea. A long string and a decent amount of speed are crucial. Once you do a number of swing tests on various different rockets, you will end up getting a good reading in regard to which rockets are the most stabile. Some point right into the wind, others are a bit wobbly and some have no sense of direction whatsoever.
Click to expand...
Yes this was in flight configuration.
 
bobbyg23 said:
I'm going to get slammed for this, but don't worry about swing tests.
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You do, at the very least, need to confirm where your CoG is with a simple balance test. This, along with the simulated CoP, will confirm your stability margin. Any RSO will want this information.

Edit: Just noticed that OTT beat me to it!
 
I haven't done a swing test in years, decades probably. I remember a lot of rockets that flew backwards during the test and flew fine under power.

My theory is that rockets that are stable have two solutions in a swing test; pointing forwards or backwards. The rocket will point in the direction that it first reaches stability and that is often backwards. If you do the test properly it will fly nose first, but swing tests can be difficult to do properly for some rockets. Speed of rotation, getting it to the proper speed quickly, length of string, etc. are all important.

If you built a kit and the manufacturer says its stable when built properly it should be fine. First flights should always be a heads up flight.
 
Voyager1 said:
You do, at the very least, need to confirm where your CoG is with a simple balance test. This, along with the simulated CoP, will confirm your stability margin. Any RSO will want this information.

Edit: Just noticed that OTT beat me to it!
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Definitely need the cg. 100% correct
 
bobbyg23 said:
Definitely need the cg. 100% correct
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100%? That's a "lofty" goal, and total unnecessary.

I did an Open Rocket sim of an Estes Alpha No. 671-K-25, circa 1967 design. In stock configuration It has a stability caliber of 1.17.

A stability caliber as high as 2.0 or as low as 0.75 would still make for a stable flight, which means the cg could be off + 3/4" to - 3/8" and the rocket would still fly just fine.

This is rocket science... there is a margin of acceptable error.
 
One condition of a swing test is that the rocket starts out facing forward like it would in a real launch (if I remember correctly). I seem to recall watching a video with Tim @ Apogee talking about how it is difficult for a rocket to turn 180 degrees once it gets flying backwards so you need to start it nose first. Keep in mind there is no rear thrust like in a real launch so the forces encouraging the rocket to rotate around the CG are different.

EDIT: Here is the Apogee source (it was a newsletter not a video): https://apogeerockets.com/education/downloads/Newsletter53.pdf

If it starts out nose first and turns around to fly backwards you have a problem.

However, if it just happens to start out backwards and flies stable backwards just stop and swing again (and try to get the nose in the front when it starts swinging at speed).
 
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bobbyg23 said:
Definitely need the cg. 100% correct


lakeroadster said:
100%? That's a "lofty" goal, and total unnecessary.

I did an Open Rocket sim of an Estes Alpha No. 671-K-25, circa 1967 design. In stock configuration It has a stability caliber of 1.17.

A stability caliber as high as 2.0 or as low as 0.75 would still make for a stable flight, which means the cg could be off + 3/4" to - 3/8" and the rocket would still fly just fine.

This is rocket science... there is a margin of acceptable error.
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I took bobbyg23's answer as: 100% correct. You do need the CG. That being said getting the CG is easy to get 100% correct (and should be) just add the weight were needed. Just saying you can adjust the CG were ever you want it. CP on the other hand with a scratch built, unique design would be hard to get 100% correct without a sim program.
 
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@BigMacDaddy is correct; the rocket needs to start the swing test nose forward for it to be valid.

A lot of rocketeers know that CG is not static; it moved as the fuel is burned. A lot don't seem to realize that CP is not static, either. CP varies with angle-of-attack. As angle-of-attack increases, the CP moves forward. Once the airflow over the fin separates (stall condition), the CP will move dramatically forward, as the lift forces drop off, and the only correctional force is the separation drag. The CP is farthest forward at an AoA of 90 degrees.

The fins are sized with the assumption that the rocket starts out pointy-end-first. Stability is defined as the tendency to correct from an upset. It does NOT guarantee that your model will flip pointy end forward from any condition. The recommendations for calibers of stability assume your rocket starts flying nose forward. It is entirely possible a stable rocket can swing test backwards, as the entry parameters do not match the underlying stability assumptions.
 
ZoomieG said:
@BigMacDaddy is correct; the rocket needs to start the swing test nose forward for it to be valid.

A lot of rocketeers know that CG is not static; it moved as the fuel is burned. A lot don't seem to realize that CP is not static, either. CP varies with angle-of-attack. As angle-of-attack increases, the CP moves forward. Once the airflow over the fin separates (stall condition), the CP will move dramatically forward, as the lift forces drop off, and the only correctional force is the separation drag. The CP is farthest forward at an AoA of 90 degrees.

The fins are sized with the assumption that the rocket starts out pointy-end-first. Stability is defined as the tendency to correct from an upset. It does NOT guarantee that your model will flip pointy end forward from any condition. The recommendations for calibers of stability assume your rocket starts flying nose forward. It is entirely possible a stable rocket can swing test backwards, as the entry parameters do not match the underlying stability assumptions.
Click to expand...


That is why it is the norm to start out with at least +1 caliber of stability on a design, not .75 or .5.
 
Swing tests are tricky. I remember posting a similar question to Rec.models.rockets once long ago. when I was a young BAR.

The problem is that the center of pressure varies with the angle of attack. The CP number you get from Barrowman assumes very small angles of attack and is invalid at higher angles. At a 90 degree angle (sideways) you get the cardboard cutout location.

So when you start the rocket moving the CP could be very far from its zero degree angle of attack location. Far enough so it will not assume a stable attitude no matter what you do.

In these days of modern times, the only reason to perform a swing test is if you have a rocket configuration which is difficult or impossible to model in Rocksim/Openrocket. Or VCP even.
 
Zeus-cat said:
I haven't done a swing test in years, decades probably. I remember a lot of rockets that flew backwards during the test and flew fine under power.

My theory is that rockets that are stable have two solutions in a swing test; pointing forwards or backwards. The rocket will point in the direction that it first reaches stability and that is often backwards. If you do the test properly it will fly nose first, but swing tests can be difficult to do properly for some rockets. Speed of rotation, getting it to the proper speed quickly, length of string, etc. are all important.

If you built a kit and the manufacturer says its stable when built properly it should be fine. First flights should always be a heads up flight.
Click to expand...
Concur, swing tests are hard to do. If you built a kit and don't over build it, it should be stable. Their are some ESTES kits that maybe borderline stable and/ or underpowered and users on this forum will mention them. But simple, classic kits that the OOP mention, don't worry
 
My time to get slammed.....
just do your sim. I've never swing tested anything, nor am I ever going to.

First of all: LPR kits are designed to fly with the motors they state. I'm pretty certain I'm not smarter then Estes, so I'm just gonna build and fly it.
Second;y: I've RSO'd probably hundreds if not thousands of rockets If I identify the kit, I'm certainly NOT going to ask you for CP and CG. That's not just low power, either. Wander up to me with a Quark, Wildman JR, Super DX3. I already know it's ok as long as it's stock-built. I bet most other RSO's are the same (but I"m sure the exceptions to the rule will point out the error of my statement.)
Thirdly: I don't know everyone, and if I don't know you and you wander up talking about swing tests, it will just cross my eyes. If I don't know you, I have no idea you can either do the test corretly and/or are able to correctly interpret the test while you do it
Fourth: It's 2022 not 1970. Most computers can do 150-200 million operations per second. that's way fast enough to run these amazing computer programs with way more precision then my eyes have. Why regress to the tests of our grandparents? Do you communicate with telegraphs? Move into this century, it's way more accurate. If you're really worried about it, skip directly to the front of the line, RS-Pro, with it's 6 degrees of freedom analysis.....or go to the middle, Rocksim, open rocket or RS Aero.....
 
There doesn't seem to be much point of knowing the CG without knowing the CP unless the manufacturer indicates. I looked at the Estes website and instructions for the Generic and don't see CG data. RocketReviews has a lot of Rocksim files (that can be used in OR) but a lot of them are crap and need modifications. Apogee has a lot of Rocksim files but no Generic or Alpha3.
 
Ronz Rocketz said:
There doesn't seem to be much point of knowing the CG without knowing the CP unless the manufacturer indicates. I looked at the Estes website and instructions for the Generic and don't see CG data. RocketReviews has a lot of Rocksim files (that can be used in OR) but a lot of them are crap and need modifications. Apogee has a lot of Rocksim files but no Generic or Alpha3.
Click to expand...
One of the nice things about using a sim program like OR or RockSim is that it will give you a pretty accurate CP, and you can get the actual CG from your model, compare with the sim program and override CG if needed and as long as everything is input correct and the CG/CP relationship is correct no swing test needed IMO.
 
rharshberger said:
One of the nice things about using a sim program like OR or RockSim is that it will give you a pretty accurate CP, and you can get the actual CG from your model, compare with the sim program and override CG if needed and as long as everything is input correct and the CG/CP relationship is correct no swing test needed IMO.
Click to expand...
Exactly...
 
cbrarick said:
My time to get slammed.....
just do your sim. I've never swing tested anything, nor am I ever going to.
Click to expand...

Far from a slam, but, well, a partial disagreement! :p

cbrarick said:
First of all: LPR kits are designed to fly with the motors they state. I'm pretty certain I'm not smarter then Estes, so I'm just gonna build and fly it.
Second;y: I've RSO'd probably hundreds if not thousands of rockets If I identify the kit, I'm certainly NOT going to ask you for CP and CG. That's not just low power, either. Wander up to me with a Quark, Wildman JR, Super DX3. I already know it's ok as long as it's stock-built. I bet most other RSO's are the same (but I"m sure the exceptions to the rule will point out the error of my statement.)
Thirdly: I don't know everyone, and if I don't know you and you wander up talking about swing tests, it will just cross my eyes. If I don't know you, I have no idea you can either do the test corretly and/or are able to correctly interpret the test while you do it
Click to expand...

I agree with the above statements 100%.

Unless the kit is pretty heavily modified, she's gonna fly safely. Aside from a rather large set of fins glued to the nosecone, most kits have a safety margin that will cover a multitude of sins, intentional or otherwise.

To the third point, there is some nuance involved in conducting a proper swing test. Failure to understand these things can cause "false readings", but the failure mode is almost always to indicate instability in what is actually a stable rocket rather than vice versa.

The old Estes technical report on stability outlines the proper methods to achieve accurate results. Here is a link to it on Ninfinger's site.

cbrarick said:
Fourth: It's 2022 not 1970. Most computers can do 150-200 million operations per second. that's way fast enough to run these amazing computer programs with way more precision then my eyes have. Why regress to the tests of our grandparents? Do you communicate with telegraphs? Move into this century, it's way more accurate. If you're really worried about it, skip directly to the front of the line, RS-Pro, with it's 6 degrees of freedom analysis.....or go to the middle, Rocksim, open rocket or RS Aero.....
Click to expand...

This is where we differ a wee bit...

You know, I have never been one to toss out old tools just because something newer or better comes along. I have a Dremel and an even smaller micro drill/rotary tool thing, but I also still have an old hand drill and a pin vise in my collection. Sometimes the finesse achievable with the hand drill or pin vise is much more appropriate to the task at hand.

I like to sim everything I can, and I find it to be a modern miracle compared to the old days of longhand Barrowman equations, but a lot of my builds involve odd shapes and configurations that are beyond the capabilities of most sim programs to model accurately, and sim programs that will model them are beyond the capabilities of my wallet. Time to break out the hand drill...

Things like the one in the pic below are fairly easy to swing test and, provided the test is done properly, will provide a reasonable expectation of a safe flight. I couldn't begin to build a sim model of this in either RO or RS...:rolleyes:

Now, where'd I put that roll of string???

1645760287458.png

As much as I like my truck, some jobs just require a horse! 🤠

Mike
 
I do a few swing tests a year on smaller rockets, just so I have an excuse to go out in the yard and play. You might think I'm kidding, but spinning around in a circle as a kid was fun. Now, I spend too much time behind a computer screen. Every now and then, swinging something around in a circle on a string has merit, even if it is unrelated to rocket stability. I simulate a lot more than I experience. I am trying to correct that.

Sandy.
 

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