"Intall a CG"?????????

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Skywise - 07 Apr 2010 04:50 GMT

I just got around to scanning my April AMA magazine and found
some very curious statements in the "Safety Comes First" article.

The author starts the article talking about a rogue group of
modelers who are building and flying aircraft that have no CG
at all, and that everyone must be sure to install a CG in their
aircraft.

Just how do you "install" a CG? Unless he's talking about something
other than "Center of Gravity", I don't see how an aircraft can
NOT have a CG, nor how you could install one if it doesn't.

Or should I just assume this is an April Fool's joke?

Brian

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Martin X. Moleski, SJ - 07 Apr 2010 05:01 GMT

> ... should I just assume this is an April Fool's joke?

Definitely! :-O

Marty

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icerinkdad@gmail.com - 07 Apr 2010 13:13 GMT

> I just got around to scanning my April AMA magazine and found
> some very curious statements in the "Safety Comes First" article.
>
> Or should I just assume this is an April Fool's joke?
>
> Brian

It is after all April... but it immediately got me thinking about
controlling a model with a variable CG. A weight shift control would
work for a model that was essentially a FF design. Response might be
kinda slow in some attitudes but it could still be fun to play with
the idea.
Bob

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MJKolodziej - 07 Apr 2010 14:44 GMT

>I just got around to scanning my April AMA magazine and found
> some very curious statements in the "Safety Comes First" article.
[quoted text clipped - 11 lines]
>
> Brian

That's funny, I didn't see that.

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MJKolodziej - 07 Apr 2010 14:51 GMT

There are times that the space shuttle does not have a CG!
mk

>>I just got around to scanning my April AMA magazine and found
>> some very curious statements in the "Safety Comes First" article.
[quoted text clipped - 15 lines]
> :)
> mk

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Martin X. Moleski, SJ - 07 Apr 2010 18:13 GMT

>There are times that the space shuttle does not have a CG!

Hmm.

It operates within earth's gravitational field (you have
to go along way toward the moon to reach the point where
the earth's gravity is precisely counterbalanced by the
moon's).

The shuttle does not become massless when it possesses
a velocity and a trajectory sufficient to offset the
pull of gravity.

The fellows who designed the thrusters certainly had
to think about the center of mass of the shuttle.

There is no center of lift in the vacuum, of course,
so our ordinary anxieties about the relationship
between center of lift and center of gravity (or
center of mass) is not an issue.

Bottom line for me: a mass in motion within a
gravitational field will have a calculable center
of mass and center of gravity even if its motion
is such that it is not accelerating toward the
barycenter.

Marty

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Joe - 08 Apr 2010 08:07 GMT

<snip>

> Bottom line for me: a mass in motion within a
> gravitational field will have a calculable center
[quoted text clipped - 3 lines]
>
> Marty

Yikes! Fr. Marty must be studying to be an IT tech. (Some of those ppl
seem to enjoy confusing mere C.S. mortals).

These are honest questions:

1. What is the difference between center of mass and center of gravity?
(I think I know, but I want to see what Fr. Marty is emphasizing).

2. This is the first time that I have seen THREE terms for a single(?)
concept - what on earth, or in the universe is "barycenter" and how does
it differ from center of mass or center of gravity?

3. "...will have a calculable center of mass and center of gravity..."
Calculable? Maybe just say it will HAVE a center of mass and center of
gravity.

4. "...even if its motion is such that it is not accelerating toward the
barycenter."
Stationary objects (on earth, wrt the earth) have a center of mass and
center of gravity. I therefore believe what you said about "not
accelerating", etc., but what is your point?

5. Are you a physics teacher? I have been a mathematics teacher, so I
love lucid explanations. Thanks for any clarification.

--- Joe

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Martin X. Moleski, SJ - 08 Apr 2010 12:35 GMT

>Yikes! Fr. Marty must be studying to be an IT tech. (Some of those ppl
>seem to enjoy confusing mere C.S. mortals).

I'm root admin on a couple of websites and webmaster on
several others. I'm co-chair of the Big-8 Management
board. So, yes, there is a bit of IT in me. ;o)

>These are honest questions:

>1. What is the difference between center of mass and center of gravity?
>(I think I know, but I want to see what Fr. Marty is emphasizing).

The center of mass is a point located in 3-dimensional space.
The body in question is balanced along all 3 axes around that point.

The center of gravity as we use it with flying machines is concerned
only with the fore-and-aft axis of an aircraft. The location of the
CG with respect to the center of lift will affect how stable the
aircraft is in pitch. The other two axes (roll and yaw) are
neglected for the purposes of that calculation.

>2. This is the first time that I have seen THREE terms for a single(?)
>concept - what on earth, or in the universe is "barycenter" and how does
>it differ from center of mass or center of gravity?

The barycenter is the point around which two bodies rotate (e.g.,
the earth and the space shuttle or the earth and the moon). The
SYSTEM's barycenter is not at the center of mass of either body.

>3. "...will have a calculable center of mass and center of gravity..."
>Calculable? Maybe just say it will HAVE a center of mass and center of
>gravity.

OK.

>4. "...even if its motion is such that it is not accelerating toward the
>barycenter."
>Stationary objects (on earth, wrt the earth) have a center of mass and
>center of gravity. I therefore believe what you said about "not
>accelerating", etc., but what is your point?

The original question was whether the space shuttle has a CG.
I say it does have such a point even though there may not be
much need to worry about it when the shuttle's trajectory and
momentum hold it in equilibrium against the tug of gravity
so that it is in a stable orbit. We speak about it as
"weightlessness" because of the balance of forces but it is
not "masslessness" nor is it "gravitylessness."

>5. Are you a physics teacher? I have been a mathematics teacher, so I
>love lucid explanations. Thanks for any clarification.

I am a systematic theologian by trade. I am interested
in theological epistemology, which, in turn, gets me
involved with philosophy and especially the philosophy
of science. I co-authored the first biography of Michael
Polanyi, who was a medical doctor, physical chemist,
economist, social analyst, philosopher and theologian
(of sorts).

Marty

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Ralph Jones - 15 Apr 2010 21:13 GMT

[snip]

>1. What is the difference between center of mass and center of gravity?
>(I think I know, but I want to see what Fr. Marty is emphasizing).

Nothing. Center of gravity is simply an imprecise name for center of
mass.

>2. This is the first time that I have seen THREE terms for a single(?)
>concept

Obviously not raised Catholic or Protestant...;-)

> - what on earth, or in the universe is "barycenter" and how does
>it differ from center of mass or center of gravity?

The barycenter (literally, center of heaviness) of a single rigid
object is simply its center of mass. The barycenter of a system of two
or more objects is the center of mass of that system, in their mutual
orientation at the instant it is evaluated.

The barycenter of the Earth is at the center of the Earth.
The barycenter of the Moon is at the center of the Moon.
The barycenter of the Earth-Moon system is on the line that joins
their centers of mass, about 4670 km from that of the Earth (which
means it's well inside the Earth).Actually, "barycenter" is hardly
ever used in connection with a single object, because "center of mass"
is perfectly adequate.
The "orbit of the Earth" about the sun is actually the orbit of the
barycenter of the Earth-moon system.

Pellets coming out of a shotgun have a barycenter, too. In the absence
of gravity and aerodynamic forces, their barycenter would fly straight
along the extended centerline of the bore.

rj

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Robert Roland - 16 Apr 2010 09:39 GMT

>Center of gravity is simply an imprecise name for center of
>mass.

I once read that there is a theoretical difference:

Imagine a very, very tall object standing on the earth. The earth's
gravity will be weaker at the top of the object than at the bottom.
Therefore, the center of gravity will be closer to the earth than the
center of mass.

In other words, the center of gravity will move depending on the
object's location in the gravity field(s), while the center of mass
will remain fixed relative to the object.

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RoRo

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Model Forum / Radio Controlled / Air Models / April 2010

"Intall a CG"?????????