Need CG of Stock Built Fatboy

[Fuddrucker]

Can anyone tell me the CG from the nose of their "built per kit instructions" FatBoy WITHOUT the motor inserted.

Thanks!

...Fudd

 

[JRThro]

I don't have a Fat Boy, but that's an interesting question.

Why without the motor?

 

[sandman]

EMRR has the CP listed...My guess is that the CG is up to the builder since that's all he can change without changing the design.

Go here. https://www.rocketreviews.com/cgi-bin/rocksim/cp.cgi?Estes%Industries&&bymanf

 

[Fuddrucker]

I made a new MM for mine with 4-18mm and 1-24mm. Since that alone is heavier than the stock mount it has already altered the CG/CP relationship from a standard build. If I can get the CG from a standard build, I can then load up the rocket with all five motors, and add enough nose weight to bring the CG/CP relationship back into the original spec.

...Fudd

 

[Fuddrucker]

Originally posted by sandman
EMRR has the CP listed...My guess is that the CG is up to the builder since that's all he can change without changing the design.

Go here. https://www.rocketreviews.com/cgi-bin/rocksim/cp.cgi?Estes%Industries&&bymanf

Thanks.

I got that, but I am just trying to get an approximate CG point from a stock build. I realize that no two builds are going to be the same because of build factors like how much glue is used, paint, and finishing techniques. However, the differences should not be that much.

...Fudd

 

[JRThro]

Originally posted by Fuddrucker
Thanks.

I got that, but I am just trying to get an approximate CG point from a stock build. I realize that no two builds are going to be the same because of build factors like how much glue is used, paint, and finishing techniques. However, the differences should not be that much.

...Fudd

Don't you really need the stock CG with the largest recommended motor installed, rather than without a motor? That's the point you'll be trying to get back to in your modified Fat Boy... which sounds pretty darned cool, btw.

 

[Fuddrucker]

Normally yes.

BUT, since I have already added weight by a heavier motor mount setup, I am just trying to get the relationship of CG/CP from the stock mount. If I took the CG of a stock kit with say a C6-7 loaded, it would not be nearly the same as mine with a C6-7 loaded since my mount is already heavier without a motor.

See here...https://www.rocketryforum.com/showthread.php?s=&threadid=11571

and here...https://www.rocketryforum.com/attachment.php?s=&postid=125437

...Fudd

 

[teflonrocketry1]

The CG of an Estes FatBoy built stock, with no mods, painted and decaled, with no motor installed is; 7 inches from the tip of the nose cone (or three inches aft of the body tube nose cone joint). I measured this on my son's recently completed FatBoy that has been flown once.

Bruce S. Levison, NAR #69055

 

[JRThro]

Originally posted by Fuddrucker
Normally yes.

BUT, since I have already added weight by a heavier motor mount setup, I am just trying to get the relationship of CG/CP from the stock mount. If I took the CG of a stock kit with say a C6-7 loaded, it would not be nearly the same as mine with a C6-7 loaded since my mount is already heavier without a motor.

See here...https://www.rocketryforum.com/showthread.php?s=&threadid=11571

and here...https://www.rocketryforum.com/attachment.php?s=&postid=125437

...Fudd

Still, don't you need the CG of the stock Fat Boy in its launch configuration, fully loaded with motor, wadding, chute, etc.? What you're trying to do is to get the CG of your modified Fat Boy, fully loaded with all 5 motors, in the same place as the stock Fat Boy loaded with, say, a C6-7.

Let's say you place the CG of your modified Fat Boy without any motors loaded into it at the same location as the CG of the stock Fat Boy without a motor loaded into it. This is what you say you're doing. Then, ready to launch with a motor loaded, the CG of the stock Fat Boy will move somewhat toward the rear. Ready to launch with *5* motors loaded, the CG of your modified Fat Boy will move a *lot* further to the rear, your CG will no longer be in the same place as the CG of a stock Fat Boy, and the CG/CP relationship of your modified Fat Boy will *not* be the same as that of a stock Fat Boy.

I'm sure I know what I'm talking about, but I'd sure appreciate it if someone else would chime in and confirm what I'm saying?

Thanks.

 

[powderburner]

And when I to back to Fudd's last post and try to understand it, it makes my head hurt.

Fuddrucker, I am not sure (from your description) if you are doing things correctly or not. I can tell you this much: you MUST have a positive margin of stability by getting the center of mass (or gravity, in common parlance) at a suitable distance forward of the center of pressure.

The CG that we are referring to is for the 'flight' configuration of the rocket, including motor(s). Anything else just does not matter.

If you are building a Fat Boy that is 'scale' for all external configuration features then the CP will not change. And the recommended stability margin will not change. Therefore the CG must be AT LEAST as far forward as the baseline Fat Boy, if not further. That is to say: the longitudinal CG of your new FB (with old NC, old BT, old parachute, old fins, new MMT, new motors, new nose ballast) must be at (or forward of) the longitudinal CG of the old FB (with old NC, old BT, old parachute, old fins, old MMT, old motor).

Check out:
https://www.rocketryforum.com/showthread.php?threadid=3379&highlight=CG+CP+101

Please PM or email me if you want some help walking through this.

 

[JRThro]

Whew! Thanks, powderburner.

Once in a while I'm absolutely sure I'm right about something, and then it turns out that I was full of it. So I appreciate the reality check.

 

[vjp]

Reader's Digest condensed version:

You want the FLIGHT C.G. of the clustered version to be at or ahead of the FLIGHT C.G. of the stock version.

...measuring the empty C.G. doesn't give you any useful data - you have to have the motors in place to check the flight C.G.

 

[powderburner]

If you want to build your new version once, and put in the ballast and not mess with tweaking it between flights, then:
You will need to get the CG of your new version at least as far forward as the original when your new version has **THE BIGGEST LOAD OF MOTORS THAT YOU EVER INTEND TO FLY**