CP of an egg?

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iter

HPR Glider Driver
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Where is the CP on an egg shape, assuming it flies pointy end first? Trying to figure out if one can load that shape with enough nose weight to make it stable without fins.

Ari.
 
Assuming egg is oval, its CP will be in the middle of the oval (according to Barrowman equations).

This article has CP summary for different shapes.

In reality, it will be more stable due to drag stabilization but I don't think there is an easy way to calculate it.

-Alex
 
It would be easy to make an egg out of a cone and tail cone in RockSim. I don't know how the tail cone section will effect base drag(?) Don't remember if Bruce Levison addressed this in his series of articles in the Apogee Newsletter.

This can always be tested experimentally.
 
What is this, Ari-some kinda yolk? Eggs aren't all they crack up to be ya know....
(Don't deploy until you see their whites....)
(How would you like your recovery, sir? Scrambled or apex over easy?)
( I knew an egg once-grew up to be a fine breakfast.)
(The answer is: The rooster)
okay, I'll stop now....
 
It would be easy to make an egg out of a cone and tail cone in RockSim. I don't know how the tail cone section will effect base drag(?) Don't remember if Bruce Levison addressed this in his series of articles in the Apogee Newsletter.

This can always be tested experimentally.

I don't have RockSim, but Open Rocket surprises me. This surprise prompts me to post here.

An oval NC by itself shows CP about where I expect it:

Screen Shot 2013-09-25 at 11.46.42 AM.png


Adding a transition pushes CP way forward according to OR:
Screen Shot 2013-09-25 at 11.46.57 AM.png

Ari.
 
Rocksim's CP with the rear transition is even further in front of the nose. I knew the transition would hurt stability but I can hardly believe what I saw either.
 
For a conical shape, OR says CG and CP coincide for a thin wall, and as the wall thickness increases CP stays put and CG moves backwards. That's what I'd expect.

For other shapes, including ellipsoid, OR says CG is well behind CP even for a very thin wall (and CG moves backwards while CP stays put as the wall thickness increases). That's not what I'd expect.

For any shape, with a thin wall, CG and CP should coincide. I think. Including a half ellipsoid with a transition stuck on the back. Thicker wall should move CG back while CP stays in the same place (for shapes like cones, ellipsoids, or anything increasing in diameter toward the back). OR's placement of the CP looks completely nuts.
 
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When discussing CP it might help to know that our typical analysis is based on simplifications that have an assumption of at least 10:1 length to diameter.

And while I can't find direct work, a flat plate has its CP approximately 2.2 times diameter behind it when dynamic forces are taken into account. This is how spool rockets remain stable.


Kirk
 
A boattail reduces stability. However in this case it might be OK to ignore it and just use the front portion -- which is terrible enough -- although a cut-off shape would have less turbulence issues. I think in this case, it can't be stable without an extra aerodynamic device, it's too much like a ball. One thing misunderstood about finless rockets is it's the balance of forces, even though the total aero forces are smaller. But this just doesn't give you anything to work with.
 
May I highjack this thread for a related moment?

I've just completed a "Scrambler" with a clear payload. I have the distinct impression that this is supposed to transport a raw egg...with the intent that you will recover it intact.

Barring the fact that you might test it first with a hard-boiled egg, isn't the amount of weight in an egg significant, and possibly going to cause problems with CG and CP?

Any advice appreciated.... quick, I'm thinking of flying it at the next and final club launch of the year!
 
Yes, the weight is significant but probably not in the way you are thinking. The egg will act as a lot of nose weight and will actually help the CG and CP relationship (i.e. bring the CG farther forward). What you have to watch for is having a big enough motor to get it off the ground with a large enough thrust to weight ratio for stable flight. Maybe some experienced egg lofters can suggest motors for you. It also depend on what kind of egg you are lofting:

https://www.youtube.com/watch?v=6R-K59H1U0k&feature=youtu.be
 
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I can tell you for sure it not only took a lot of Nose weight to get an Egg to be stable but some sizable FAR aft swept fins to make it fly right.

As for the Scrambler: It's just a later re-release of the old Eggspress for the late 1980's, I still have one of each in the fleet, Actually the Scrambler was my daughters which she used often to get qualified flights in C-Egg Loft Competitions back in the 90's Best motor flying with a raw Grade-A Large egg is C6-3. C6-5's puts a terrible load on the chute, if your not using over the canopy shroudlines it chute will not survive.
I seem to recall she once flew it with a B6-2 loaded with and egg and it just barely deployed with enough time not to break the egg. Would Not recommend the practice.

014-sm_Eggspress_03-15-88.jpg

975a-sm_Scrambler (kathleens-1994)_09-16-06.jpg
 
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Warning: The following is pure heresy, may be considered Dark Side and worthy of placement on the Far Far Away pad with plenty of certified supervision.

I would go with the centroid approach, ditch the silly software, cant the motors and move them up as far as possible, the very top being the best. Better to pull with virtual flame fins via canted motors up front than to push with a traditional motor in back with big fins. Put the long launch rod through the center with ideal conditions and let her rip. The rocket scientists will have shock and awe when it works. You do not need rockets scientists, what you do need is a good electrician to light those motors all at once. Crazy fin-less odd rocs can sure turn out to be stable flying machines, it is just an ignition issue. Eggs away!
 
Warning: The following is pure heresy, may be considered Dark Side and worthy of placement on the Far Far Away pad with plenty of certified supervision.

I would go with the centroid approach, ditch the silly software, cant the motors and move them up as far as possible, the very top being the best. Better to pull with virtual flame fins via canted motors up front than to push with a traditional motor in back with big fins. Put the long launch rod through the center with ideal conditions and let her rip. The rocket scientists will have shock and awe when it works. You do not need rockets scientists, what you do need is a good electrician to light those motors all at once. Crazy fin-less odd rocs can sure turn out to be stable flying machines, it is just an ignition issue. Eggs away!

Sorry Daddyisabar:
You are DEAD WRONG on this one. Please don't cause people to do potentially HOBBY harming Silly things with absolutely no understanding of the aerodynamics involved in the matter. Push or pull with canted motors, we can't get this shape to fly finless without getting it spinning several hundred revs per minute and even than there are concerns.
Tis better to say/type Nothing then to interject BAD heresy which can cause harm.
 
It's OK, Micromeister...

I'm committed with my current configuration of the Scrambler.
I only want to loft one raw egg successfully.

However, my son just gave me a Loadstar II for Christmas, and I see that also has a clear payload chamber. So, if my scrambler fails, I'll be following up with a kit for the new year...
 
It's OK, Micromeister...

I'm committed with my current configuration of the Scrambler.
I only want to loft one raw egg successfully.

However, my son just gave me a Loadstar II for Christmas, and I see that also has a clear payload chamber. So, if my scrambler fails, I'll be following up with a kit for the new year...

That's fine Kirk G:
Only recommendation I can make is while flying a raw Egg is to carefully tape it into a plastic sandwich bag. Not as zip-loc as the seams take up way to much room but a standard thin plastic sandwich bag. I try to use Magic tape to seal the bag before inserting in the capsule with some Cotton balls above and below the Egg if you don't have the old 1" foam pads.

It's also a very good Idea to have at least a 1" spill hole in the apex of your chute canopy using over the top shrould lines. this helps pervent chute Oscillation that slam the model and capsule into the ground.
OH! after recovery it's a good idea to open the Egg capsule over a trashbag lined container:) Egg Loft is truely a fun event.
 
I was planning on saving one "raincoat" so that clean up would be a breeze....
 
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