The pitiful altitudes predicted for this rocket appears to be due to the propellers. These seem to have little affect on the stability of the model, but REALLY impacts its performance. Hmmmm.... GIGO?
Thursday, November 20, 2008
You can slap in any mount you want, but if your sim isn't accurate, it won't help!
The pitiful altitudes predicted for this rocket appears to be due to the propellers. These seem to have little affect on the stability of the model, but REALLY impacts its performance. Hmmmm.... GIGO?
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4 comments:
Can your simulation give you the drag coefficient on the propellers?
Or compute Cd with and without propellers to see what the Sim really thinks the drag coefficient is.
Good idea. The Cd without the propellers varies but is under one for the majority of the powered flight. With them, the Cd is over 8 during the same period. I still don't know if this is right, but it does show that there is a reason RockSim is acting that way. I've seen through the years that it just went bonkers with some of my odd designs. I think V8 may be more solid that way but in that past it didn't always like very thin nosecones or many, many transitions.
Anyway, I'm really thinking the propellers will be for show only and it will fly without them.
Thanks again for the insightful comment.
I doubt the Cd calculation has changed in Rocksim v8. They use the traditional formulas.
What they do "differently" is that they spin the rocket around the long axis and get a cross-sectional area for the CP computation.
I doubt the propellers create such a drag. If you have any aerodynamics reference you will see that even a flat block of a given reference area will not has that kind of drag. So either Rocksim is not prepared to deal with flat parts or their "Mach compensation" is just wrong.
I don't think Rocksim v8 will do anything different.
If you look at an aerodynamics book you will see that a flat block of a given reference area will not have a Cd that high. I bet the problem is that Rocksim is just not computing flat parts or their "Mach compensation" code is wrong.
BTW, your blog is fantastic.
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