jahall4
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Your last graph shows an upper velocity bound of CNa also now!
It sure seems to, that's why I posted it.
Your last graph shows an upper velocity bound of CNa also now!
What is more concerning are the oscillations in the Pitch force (is really a moment or a torque that rotates the model about the c.g.) and wind angle of attack. If you get a wind angle of attack, you want the pitch force to restore you, and that is good static stability. But you don't want a pendulum effect where the pitch force and angle of attack continue to oscillate, you want dynamic stability. It appears that the oscillation does eventually damp out, but you might want to make some design changes to increase stability so that oscillation does not extend so far or the frequency of it slows down. That sim curve looks like the rocket is going over a washboard road. The sim curve is technically stable, but I would be curious as to the caliber measure of stability.
I am not familiar with details of any of the popular sim tools. What this looks like, is some kind of compressibility correction applied to CN when the Mach number increases. At V=473 ft/sec you are around Mach 0.42, the Cn (and Cd-drag) compressibility corrections are applied gradually when you pass Mach 0.3, not just an on-off switch when you hit Mach 1. (The sim might be using the theoretical Prandtl-Glauert rule, perhaps?)
I am not sure of the notation they are using, CNa might be the slope of the CN versus alpha curve, assuming a linear straight line near the low-alpha range. At zero-alpha, you have zero force, so CN would be zero at zero alpha, but the slope of the line from zero alpha to a small alpha < 10-deg would basically be constant.
Do you really thing the oscillation is worrisome? After the initial 2deg deviation off the rail, its next highest peak is barely .2deg at 1 second and damps quickly.
Interesting thought about a gradually applied compressibility factor. I still find the discontinuous behavior unnatural though lol.
Do you really thing the oscillation is worrisome? After the initial 2deg deviation off the rail, its next highest peak is barely .2deg at 1 second and damps quickly.
Below Mach ~ 0.42 you don't really have significant effects of compressible flow, you are assuming, the flow is totally subsonic around all parts of the vehicle, so no theoretical corrections for compressibility are required there. Not a bug, just using mathematics that properly simulate the flow physics.
I am not concerned about the low amplitude, but the high frequency. That is what we call "flutter" and that can potentially rip a rocket to shreds. But, since this is a proven design, it is probably just an exaggerated effect of the high winds used in the sim.
Yes, I think that is consistent with the typical theory for low-speed flows.So you would expect to see CNa as a constant below Mach 0.42?
... but the frequency is only 2-3 cycles/sec. and then its gone.
Yes, I think that is consistent with the typical theory for low-speed flows.
Did the sim report the stability in calibers? I imagine it is between 1 to 2?
I not familiar with RasAeroII, but I'm not surprised as I never have considered RockSim a supersonic tool. This will interest you...
View attachment 326587
As there is the low end limit, there is an upper limit as well, probably like around Mach 0.9. The Prandtl Glauert factor is 1 at Mach zero, and goes to infinity at Mach 1, so most numerical codes put a practical limit in the factor between the range of Mach 0.9 to 1.1 or so. That is why you have the two different constant CNA portions of the curve in this example sim.
The Prandtl Glauert factor is 1 at Mach zero, and goes to infinity at Mach 1, so most numerical codes put a practical limit in the factor between the range of Mach 0.9 to 1.1 or so.
As there is the low end limit, there is an upper limit as well, probably like around Mach 0.9. The Prandtl Glauert factor is 1 at Mach zero, and goes to infinity at Mach 1, so most numerical codes put a practical limit in the factor between the range of Mach 0.9 to 1.1 or so. That is why you have the two different constant CNA portions of the curve in this example sim.
I'm designing a 3 fin version of a proven 4 fin rocket.
As a starting point, I would scale the fins by 115%, that comes from the square root of 4/3, which is used as an area ratio of an individual fin. This would preserve the total fin area, the total fin area of the three fin version would match the total area of the four fin version.
Three fins will have a little less aft weight, and also less drag, compared to four fins. So you are moving the c.g. forwards a little along with the c.p. With 1.76 calibers you have some wiggle room to work with in how you want to deviate from the original and still have some margin.
Beware, I don't think silly oddroc mindsiming is going to fly in this quadrant of the forum. Only Vulcan Science Academy approved discourse based on pure logic, mathematics and science. If you can't sim it, don't fly it! You must do the math.
But... Engineering is the science of good enough. A professor told me that, so it must be true! I suppose everyone's level of good enough is different.
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