Center of pressure in sim vs the kit's manual

[Pariah Zero]

I'm building a LOC Phoenix 4" to certify L1 (and maybe L2) with. Pretty much the only step left is to add nose weight and place the CG, and making sure it's in a "good enough" place versus the CP.

And here I run into a bit of a problem, and I'd like to know what's going on... the kit's instruction manual shows the CP at 27" +/- 0.5". OpenRocket and RockSim, in contrast, show the CP as 29" (using LOC's RockSim file)

So my question is: Why the discrepancy? Is it that the Phoenix's less common fin arrangement that doesn't play nice with the Sims? Or does LOC just have an error in the manual? (I've also sent an email to LOC, but they mention they have a skeleton crew this week... so I figure I might ask here as well.)

Without any further input, my guess is the most conservative thing is to go with the 27" figure, as it's the farthest forward -- and then set my CG so it's 4" ahead of that (ie. ~23").

But can anybody explain whether (or not) OpenRocket & RockSim have limitations for calculating the center of pressure?

Thanks.

 

[Nytrunner]

The CP varies with AoA, so it's hard to tell which conditions the manufacturer and OR used to arrive at their figures. Do a stability plot in Openrocket and look at the shift during flight

(Compare a couple different windspeeds too)

 

[Pariah Zero]

So if I'm doing the right graph... It appears that the Cp really is about 27 +/- 0.5" at launch, and then it rapidly moves rearward to 29".

If my understanding is right, then I've been informed quite a bit, and I thank you!.

 

[Nytrunner]

That stability vs time graph is exactly what you want. Looks like you have a good assessment now, and I think we know where they got their 27 number from!

A side note of the Phoenix: When I built the Madcow 4" Phoenix. I did a rough CFD flow simulation of it and a curiously enough, the results from different AoA showed the resultant CP moving backwards (more stable) as the angle increased instead of more forward like traditional rockets. Fun flying machine