CP Calculations

[Elapid]

I wanted to verify that the CP would be the same on my 4.7x mosquito as it is on the original, relatively...

Using the formula from the Handbook of Model Rocketry by G. Harry Stine, i started by writing down all the variables, then a simple "plug & chug" to the finish...

sounds simple, but it took me a couple hours due to constant interruptions during critical mathematical operations.

i finally came up with a reasonable answer after the third re-check of my work.

it was a nice exercise in rocket science.

 

[shockwaveriderz]

It might be a little easier and faster if you use VCP......

https://v-serv.com/vcp/

"visual" CP......

has alot of other useful functions besides calculating cg/cp

and it freeware.....

 

[Elapid]

but where's the CHALLENGE in that?

 

[Micromeister]

Congratulations Elapid!
Nice to see others who want to work the gray matter rather then rely on punching buttons... nothing wrong with the latter.. but it's much more satisfying to do the work yourself. if only to see the process
Very well done

 

[shockwaveriderz]

hey I still like to draw my new rocket designs on paper with pencil and ruler... prior to using rocksim ......does that count? besides the "old gray matter" has had a serious seepage problem for 20 years now....I'm lucky I can still "push buttons"........

 

[powderburner]

I know that the hallowed Barrowman equations have helped many people get their rockets designed, and is a solid part of our technical reference material. And I know it is hard to criticize something without offering something better, but there are some features to the Barrowman equations that I just can't stay comfortable with.

It doesn't give any credit (or penalty) for the body tube. The effect of that aerodynamic item has to be at least as influential as the nose cone. Applying the Barrowman methods to an oddball all-tube configuration can leave you with the odd conclusion that the design is unstable, when I have flown several and know that they are indeed quite stable.

It doesn't include any assessment of the effectiveness of the fin airfoil, whether it is a flat plate or a nicely airfoiled shape. And a good round nose and good airfoil shape will tremendously improve the effectiveness of the fins. The 'thin flat plate' assumption is very conservative.

Probably the thing that bothers me most is that the Barrowman method is supposedly based on the assumption that the rocket experiences only small variations in angle-of-attack, and then models the stability characteristics as though the rocket is flying sideways.

I am trying to get some of my 'stability and control' co-workers to help me take a look at those 1966 equations and see if we can come up with a better method.

 

[shockwaveriderz]

There are some "extended" barrowman equations here:

https://www.cmass.org/member/Robert.Galejs/sentinel39-galejs.pdf

as to "What Barrowman Left Out"

Heres an interesting webpage about real life versus the Barrowman Equations:

https://www.apogeerockets.com/education/instability.asp


Additional Barrowman Cp equations:

https://www.ninfinger.org/~sven/rockets/ModelRocketry/Model_Rocketry_v03n02_11-70.pdf

page 39

Elliptical Fin CP Equations

But I do agree with you that these equations could use an updating and overhaul to a certain degree.....as to the assumptions made...

 

[powderburner]

Yes, the old Barrowman methods need a review, at a minimum. But it is difficult to find a way to quantify surface roughness (as it impacts NC drag and fin effectiveness) and fin airfoil shape, at least in a way that is practical to the average model rocket hobbyist. I have been dabbling with this for years, and am likely to be several more years before (if?) I ever come up with anything.

 

[Elapid]

of course the calculations have their limitations, but the bottom line to me is not whether the CP is precisely located, but whether the rocket will go up into the sky, or into the crowd...a silly millimeter one way or another isn't gonna make any difference in the real world.

IMO

 

[Micromeister]

. I have been dabbling with this for years, and am likely to be several more years before (if?) I ever come up with anything. [/B][/QUOTE]

Precisely, Mr engineer:
We all are aware of the simplifications used here. and I'll agree with Elapid's statement,adding

If you or anyone ever come up with a better way. I'll be happy is hear it, until that time....I'll just say what we hear all the time "everyone has a better idea, until its time to use/prove it".

But back to the original Kodo, it is still nice to see folks actually "doing the work" not just relying on the answer spit from a computer progams, Use them if you like but Verify!