Is it a given that the CP of an upscale or downscale will remain at the same relative position on the rocket, assuming all dimensions are accurately scaled? Does it matter whether the rocket is relying on base drag for stability or is oddly shaped?
Does up/down scaling affect CP?
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My thoughts....
Doing an upscale or downscale, keeping everything dimensionally correct, should maintain the same same relative CP.
Should be able to take a design in Rocksim or Open Rocket, scale it and see.
I would expect the CG will move. Towards the rear for an upscale and possibly forward for a downscale. This is due to the difference in how the mass/volume/size changes for the fins vs the tubes. Also potentially due to materials used, such as changing to heavier plywood instead of balsa for an upscale.
In terms of the base drag hack, it depends on the diameter of the body tube. Since the area increases by the square of the radius, I would expect it to have a bigger impact on an upscale and less impact on a downscale
Doing an upscale or downscale, keeping everything dimensionally correct, should maintain the same same relative CP.
Should be able to take a design in Rocksim or Open Rocket, scale it and see.
I would expect the CG will move. Towards the rear for an upscale and possibly forward for a downscale. This is due to the difference in how the mass/volume/size changes for the fins vs the tubes. Also potentially due to materials used, such as changing to heavier plywood instead of balsa for an upscale.
In terms of the base drag hack, it depends on the diameter of the body tube. Since the area increases by the square of the radius, I would expect it to have a bigger impact on an upscale and less impact on a downscale
lakeroadster
When in doubt... build hell-for-stout!
CP will stay the same, in the simulation. Base drag will not change the CP if it is also scaled at the same rate.
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The CP will change as it scales up or down due to the effect of a higher Reynolds Number.
Center of Pressure as calculated by OpenRocket (and probably Rocksim, but I haven't checked) will not change as the rocket is scaled.
Center of Gravity will mostly not change when you scale a design in OpenRocket (mass objects and overrides will throw things off), but a design scaled in OR is not realistic. By the time you clean it all up to reflect a real rocket that you might build, CG will almost certainly move quite a bit.
As mentioned by @les above, it takes only a minute to test this stuff for yourself with any design in the software.
Center of Gravity will mostly not change when you scale a design in OpenRocket (mass objects and overrides will throw things off), but a design scaled in OR is not realistic. By the time you clean it all up to reflect a real rocket that you might build, CG will almost certainly move quite a bit.
As mentioned by @les above, it takes only a minute to test this stuff for yourself with any design in the software.
Your italics are key here. I'm asking about reality, not sims.
For rockety-shaped rockets, the sims and reality match (close enough for us, anyway).
For other rockets, it seems they don't. The base drag hack allegedly corrects for this, but there is still plenty of controversy about how accurate it really is.
Suppose I have a rocket I know is stable, but it's an odd shape so I can't fully trust the calculated CP. In other words, I don't really know where the CP actually is. (I do know where the CG is, so I can at least say the CP is behind that point.) If I upscale this rocket and keep the CG at the same relative location, can I be reasonably confident it will be as stable as the original?
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I would say yes.
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Odd-shaped rockets spend a lot of time at an angle-of-attack beyond the limit of the Barrowman equations. Also, they can be dominated by aero forces not included in the models. For standard rocket shapes, all the sims should scale well up to sub trans-sonic speeds. After that point, the CP will move rearward and could become unstable. RASAeroII handles trans-sonic and Mach speeds. Some experimenting with the sim programs will give you a feeling for the effects and differences.
lakeroadster
When in doubt... build hell-for-stout!
If the odd-roc isn't stable, it will "spend a lot of time at an angle-of-attack beyond the limit of the Barrowman equations".
If the odd-roc is stable it'll be a flight just like any other stable 3/4FNC.
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Correct, but the aerodynamic forces are balanced in a way that Barrowman doesn't handle in many cases of oddrocs. And the Reynolds number is always low.
lr64
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The Reynolds number of what? If the rocket has all kinds of short features on it that stick out beyond the boundary layer of the rest, the Reynolds numbers on those might be low. A 4 inch feature on a rocket that's moving at 300 feet per second will have a Reynolds number of more than 600,000. Weird stuff is seen most often at 100,000 and below.
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For smooth surface the transition from laminar to turbulent flow is around 400K or so. Rough surfaces, much lower. Once you look at a complex shape and not a simple nosecone+tube, all of the models and rules of thumb go out the window. Flight testing becomes the only sure way to check stability of an oddroc.
The other important point for scalability is the longitudinal moment of inertia of the rocket. The pitch damping factor will increase as the rocket is scaled up and denser materials are used. A cardboard Alpha will be quick to recover from a pitch disturbance; it will also cone at a higher rate. A large fiberglass kit of the same shape will react more slowly to disturbances and likely "stick" with a pitch offset when stabilized. Then, for a "fly the motor casing" Space shot, the longitudinal moment of inertia is magnitudes greater than the Alpha. Any disturbance in alignment, flex, non-symmetry, shifting mass internally, wind shear, or a flexing rail/tower... will recovery slowly if at all. Spinning up the rocket helps keep the disturbances within a small cone, but the spin rate has to be well above the roll-coupled resonance or the aero forces will overcome the strength and stiffness of the rocket. I hate when that happens!
lr64
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Pitch damping and pitch inertia are NOT the same. OTOH, the behavior you describe sounds right.
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The dynamic pitch response is related to both. Aerodynamic forces (fins, body, nose, base drag) plus the initial driving force (wind shear, etc), are limited by the longitudinal moment of inertia (and the current momentum plus roll coupling at the time of the disturbing force). The system response will be (over or under) damped based highly on the moments of inertia, velocity, and fluid properties of the air.
There, I got to use a lot of big words today!
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A lot will depend on the materials you use. If scaling from a balsa fins, Estes tubes, plastic nose cone to an upscale that uses LOC tubes (or blue tube or fiberglass), plywood fins and heavier plastic or fiberglass nose cones, and epoxy vs. white glue, the CG can change a lot. You have to design with the materials you choose and see where the CG goes. It's an iterative process.
I started in rockets building small Estes models, mostly BT20 and then BT50. The rocket is usually very light with respect to the motor weight so CG is pretty far back. Years later when I started building LOC midpower kits I noticed that fins could be much smaller relative to the size of the rest of the rocket. A few years ago I built an Estes HiFlier XL. It has a lot more fin area than necessary for stability, I kit bashed one to an Aerobee-Hi with much smaller fins and it flies great. My theory is that the original HiFlier being BT20 needs big fins for stability, the XL version scaled up to BT60 doesn't need the fins that large but the aesthetic part of the design does.
lr64
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Look at the fins on streamer duration rockets. They're pretty small, even though the rocket isn't large.
lakeroadster
When in doubt... build hell-for-stout!
Funny how the OP never once mentioned CG in his original post, yet the responses keep talking about it? 
Trying to focus on CP in upscale/downscale... that's the point.
Trying to focus on CP in upscale/downscale... that's the point.
Indeed.Funny how the OP never once mentioned CG in his original post, yet the responses keep talking about it?
Trying to focus on CP in upscale/downscale... that's the point.
The fact that CG moves is obvious, and it's trivially easy to measure its location and adjust it if needed.
On the other hand, I don't want to discourage anyone from participating
. If nothing else, it "bumps" my post! 
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Back to the original question about CP remaining the same, I think it might be useful to note that it may remain the same percentage, make sure you realize that the MEASUREMENT in inches, mm, furlongs, whatever will definitely change.
Obvious but easy to misinterpret...
Obvious but easy to misinterpret...
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No, it doesn't. Same holds true for the XL. The fins on both of those rockets are stupidly oversize, and being stupidly oversize moves surface area of the fins farther forward than it should be, moving the CP forward. The original Hi-Flier is known for going unstable when someone loads a C6, if they didn't also use all the provided clay for nose weight.
You can significantly improve the performance of both rockets by making the fins smaller. The CP shifts rearward and drag is reduced. Good results are obtained with just "Little John" style offset trapezoidal fins. Go to something like a Black Brant type planform, and you gain further still.
https://www.rocketryforum.com/threads/improving-the-estes-hi-flier-hot-wash-lessons-learned.180435/
I understand what you are saying, maybe HiFlier was a bad example. They probably came up with that design and then worried about making it work. It would be interesting to see if you could optimize the HF design and stay reasoably close to the design aesthetic.
I have a HiFlier XL built stock, and I have another one that I converted to an Aerobee-Hi. I cut the Aerobee fins from the fin stock in the kit, probably retaining only about half the fin area. Even at that my sim shows 2.5 cal. of stability. However if I do what we said you shouldn't do and just scale it down to BT20 size, it shows 1 caliber of stability. The CP stayed in the same relative position on the body which somewhat answers the OP's question, the CG moved farther back. If I fix the sim by making all of the components correct I think it will gain a little bit of stability but my point was that it is easier to make MPR and HPR stable than LPR. This also illustrates that it can be a lot of work to use the scale function in OR because you then have to go back and edit each and every component to make it correct.
I have a LOC-based MPR that looks a lot like your little john style design.
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Thank you for this. I’ve always felt “Hi-Flier” was a a misnomer for this rocket. It seemed that it was anything BUT an altitude performance bird with far more fin surface (translated “drag”) than needed. It’s a cool looking rocket, almost iconic, but the name was wrong. Of course, nobody is gonna buy a rocket named “Mediocre-Flier”.
lakeroadster
When in doubt... build hell-for-stout!
I think you're confusing "Hi-Flier" with "High Flier"...
hi /hī/
interjection
- Used to express greeting.
- A friendly, informal, casual greeting said when meeting someone.
high /hī/
adjective
- Having a relatively great elevation; extending far upward.
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Given your location, I was expecting you might also consider the John Denver interpretation of "High Flier." although at least one article claims , "Denver insisted that the song was actually about experiencing the euphoria of nature."
I'm trying to remember the Air Force safety rules, something about not smoking for 24 hours prior to flight and not drinking within 50 feet of the aircraft.
lakeroadster
When in doubt... build hell-for-stout!
I suppose relatively speaking the design isn't optimized for altitude but my XL with D12 goes as high as I want, more or less the limit of vision depending on conditions. I've seen them launched with E12 and they probably always go out of sight. The little one will get plenty of altitude with a C, actually plenty with a B.
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