Aerodynamic question for you engineers

wrote in news:c3beb705-9fa5-424c-a65e-
:

On Jan 27, 5:12 pm, Tina wrote:
Sorry. Rigid bodies do NOT rotate around their cg if an external
force
is applied whose vector goes thru it.

Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
hits a table. The center of rotation as a reaction to that force is
the table edge.

You may write an equation that descibes rotation around its cg, and
another that describes translation, but a center of rotation, to many
who deal with such things, is that point on a rotating body whose
translational motion does not include rotation, the body appears to
rotate around it.

In the case I just described, such a point is at the end of the
yardstick.

You are obviously defining center of rotation differenrtly than I am,
but my American Institute of Physics Handbook on page 2-9 talks about
rotation "in which some axis or point remains fixed in space". That
is
the center of rotation. In the several examples I've given that axis,
the center of rotation, is not at the center of gravity.

I am sure the math and classical physics folks use the same
definition. It's perfectly fine to talk abou other ways of
describing
rotation, but engineers who think about it a little, even if they are
pilots, would tend, I expect, tend to agree with AIP handbook if they
are trying to communicate with other engineers.
.
As I claimed earlier, if allowed thusters on a rigid body, I can make
it rotate around ANY point. The table edge in my example could be
replace by such a thruster.

Now, if the forces are removed, you will get no argument from me that
rotation is about the CG. The forces are not removed in the OP's
question.

I think I see where Bertie's coming from. Rotation is
about the CG, while that CG is moving along some line due to external
forces.


What I mean is, is tht the CG is the CG.no matter what. While other
forces may alter the center of rotation ( my spin example, for example)
by application of an eccentric force, the CG is stil the center of that
mass's universe and is ultimately the governer of the rotation.
The spin is a perfec example. the airplane is not sliding down it's CG,
but a point smewhere out along the stalled wing, more than likely.
However, it is still, at the same time, rotating about it's CG.
The earth rotates about it's axis, bvut it also rotates along many
others dictated by their mass. We wobble consderably because of the
moon's pull, for instance, but we're still hinged on our own CG.

Sorry, I;m making it sound more compicated than it is.



Bertie

Your making it sound more complicated and you are, I am afraid, wrong.

Did someone steal Berite's id? Are you Mx hiding in Bertie garb?

Of course the cg remains the cg, it's a convenient way of describing
the mass as a point, not unlike assuming a spherical cow or MX, for
example. But airplanes are extended bodies. Point masses are ok for
translattional movements, but one needs to be careful about making
assumptions as to what happens to them if forces operate off center.

I continue to assert it's a matter of defining 'rotation' rather than
cg, and you choose to define it with what could be called from a
physics point of view an interesting frame of reference. That's fine,
in pilot to pilot speak, but not so fine if you actually want to crank
some numbers (but it is pretty good for cranking someone's tail!).

Now here's the other thing, and it will shock you. You are arguing
with a woman, and you can have the last words, even if they are not
(as I am sure you have been trained to say) "Yes, dear."





On Jan 27, 10:15*pm, Bertie the Bunyip wrote:
wrote in news:c3beb705-9fa5-424c-a65e-
:







On Jan 27, 5:12 pm, Tina wrote:
Sorry. Rigid bodies do NOT rotate around their cg if an external
force
is applied whose vector goes thru it.

Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
hits a table. The center of rotation as a reaction to that force is
the table edge.

You may write an equation that descibes rotation around its cg, and
another that describes translation, but a center of rotation, to many
who deal with such things, is that point on a rotating body whose
translational motion does not include rotation, the body appears to
rotate around it.

In the case I just described, such a point is at the end of the
yardstick.

You are obviously defining center of rotation differenrtly than I am,
but my American Institute of Physics Handbook on page 2-9 talks about
rotation "in which some axis or point remains fixed in space". That
is
the center of rotation. In the several examples I've given that axis,
the center of rotation, is not at the center of gravity.

I *am sure the math and classical physics folks use the same
definition. It's perfectly fine to talk abou *other ways of
describing
rotation, but engineers who think about it a little, even if they are
pilots, would tend, I expect, tend to agree with AIP handbook if they
are trying to communicate with other engineers.
.
As I claimed earlier, if allowed thusters on a rigid body, I can make
it rotate around ANY point. The table edge in my example could be
replace by such a thruster.

Now, if the forces are removed, you will get no argument from me that
rotation is about the CG. *The forces are not removed in the OP's
question.

* * * * * * *I think I see where Bertie's coming from. Rotation is
about the CG, while that CG is moving along some line due to external
forces.

What I mean is, is tht the CG is the CG.no matter what. While other
forces may alter the center of rotation ( my spin example, for example)
by application of an eccentric force, the CG is stil the center of that
mass's universe and is ultimately the governer of the rotation.
The spin is a perfec example. the airplane is not sliding down it's CG,
but a point smewhere out along the stalled wing, more than likely.
However, it is still, at the same time, rotating about it's CG.
The earth rotates about it's axis, bvut it also rotates along many
others dictated by their mass. We wobble consderably because of the
moon's pull, for instance, but we're still hinged on our own CG.

Sorry, I;m making it sound more compicated than it is.

Bertie- Hide quoted text -

- Show quoted text -

Bertie the Bunyip wrote:
Tina wrote in news:ba6ee109-c6e5-4633-ae25-
:

Sorry. Rigid bodies do NOT rotate around their cg if an external force
is applied whose vector goes thru it.

Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
hits a table. The center of rotation as a reaction to that force is
the table edge.

You may write an equation that descibes rotation around its cg, and
another that describes translation, but a center of rotation, to many
who deal with such things, is that point on a rotating body whose
translational motion does not include rotation, the body appears to
rotate around it.

In the case I just described, such a point is at the end of the
yardstick.

You are obviously defining center of rotation differenrtly than I am,
but my American Institute of Physics Handbook on page 2-9 talks about
rotation "in which some axis or point remains fixed in space". That is
the center of rotation. In the several examples I've given that axis,
the center of rotation, is not at the center of gravity.

I am sure the math and classical physics folks use the same
definition. It's perfectly fine to talk abou other ways of describing
rotation, but engineers who think about it a little, even if they are
pilots, would tend, I expect, tend to agree with AIP handbook if they
are trying to communicate with other engineers.
.
As I claimed earlier, if allowed thusters on a rigid body, I can make
it rotate around ANY point. The table edge in my example could be
replace by such a thruster.

I doidn't say you couldn't.
Now, if the forces are removed, you will get no argument from me that
rotation is about the CG. The forces are not removed in the OP's
question.


Sigh.
Ok'



Bertie


There was an optimist, a pessimist, and an engineer. The optimist said,
"This bottle is half full"
The pessimist said,
"This bottle is half empty"
The engineer said;
"Yo!...... Will one of you PLEASE call those idiots over in management
and tell them this F*****g bottle is twice as big as it has to be!"

--
Dudley Henriques

Tina wrote:
Fly a loop around a little cloud
(question -- what would the loop diameter have to be for it to be
legal?)

Ooh, ooh...4000 feet plus cloud diameter?
Or "clear of" plus that diameter, which gets pretty vague...

Egad, I haven't flown in too long.

Tina wrote in news:3cae129f-1ac7-4a13-b1cf-
:

Your making it sound more complicated and you are, I am afraid, wrong.

Did someone steal Berite's id? Are you Mx hiding in Bertie garb?

Of course the cg remains the cg, it's a convenient way of describing
the mass as a point, not unlike assuming a spherical cow or MX, for
example. But airplanes are extended bodies. Point masses are ok for
translattional movements, but one needs to be careful about making
assumptions as to what happens to them if forces operate off center.

I continue to assert it's a matter of defining 'rotation' rather than
cg, and you choose to define it with what could be called from a
physics point of view an interesting frame of reference. That's fine,
in pilot to pilot speak, but not so fine if you actually want to crank
some numbers (but it is pretty good for cranking someone's tail!).


And where did you get the idea that I disagreed with that?

Now here's the other thing, and it will shock you. You are arguing
with a woman, and you can have the last words, even if they are not
(as I am sure you have been trained to say) "Yes, dear."


OK, we're throught the looking glass now.


And even there the airplane still rotates arounf it's CG.


Bertie

Dudley Henriques wrote in
:

Bertie the Bunyip wrote:
Tina wrote in news:ba6ee109-c6e5-4633-ae25-
:

Sorry. Rigid bodies do NOT rotate around their cg if an external
force
is applied whose vector goes thru it.

Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
hits a table. The center of rotation as a reaction to that force is
the table edge.

You may write an equation that descibes rotation around its cg, and
another that describes translation, but a center of rotation, to
many
who deal with such things, is that point on a rotating body whose
translational motion does not include rotation, the body appears to
rotate around it.

In the case I just described, such a point is at the end of the
yardstick.

You are obviously defining center of rotation differenrtly than I
am,
but my American Institute of Physics Handbook on page 2-9 talks
about
rotation "in which some axis or point remains fixed in space". That
is
the center of rotation. In the several examples I've given that
axis,
the center of rotation, is not at the center of gravity.

I am sure the math and classical physics folks use the same
definition. It's perfectly fine to talk abou other ways of
describing
rotation, but engineers who think about it a little, even if they
are
pilots, would tend, I expect, tend to agree with AIP handbook if
they
are trying to communicate with other engineers.
.
As I claimed earlier, if allowed thusters on a rigid body, I can
make
it rotate around ANY point. The table edge in my example could be
replace by such a thruster.

I doidn't say you couldn't.
Now, if the forces are removed, you will get no argument from me
that
rotation is about the CG. The forces are not removed in the OP's
question.


Sigh.
Ok'



Bertie


There was an optimist, a pessimist, and an engineer. The optimist
said,
"This bottle is half full"
The pessimist said,
"This bottle is half empty"
The engineer said;
"Yo!...... Will one of you PLEASE call those idiots over in management
and tell them this F*****g bottle is twice as big as it has to be!"



Mmm, k. I presume that this is a variation of the three blind guys and
the elephant?

Bertie.

Bertie the Bunyip wrote:
Dudley Henriques wrote in
:

Bertie the Bunyip wrote:
Tina wrote in news:ba6ee109-c6e5-4633-ae25-
:

Sorry. Rigid bodies do NOT rotate around their cg if an external
force
is applied whose vector goes thru it.

Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
hits a table. The center of rotation as a reaction to that force is
the table edge.

You may write an equation that descibes rotation around its cg, and
another that describes translation, but a center of rotation, to
many
who deal with such things, is that point on a rotating body whose
translational motion does not include rotation, the body appears to
rotate around it.

In the case I just described, such a point is at the end of the
yardstick.

You are obviously defining center of rotation differenrtly than I
am,
but my American Institute of Physics Handbook on page 2-9 talks
about
rotation "in which some axis or point remains fixed in space". That
is
the center of rotation. In the several examples I've given that
axis,
the center of rotation, is not at the center of gravity.

I am sure the math and classical physics folks use the same
definition. It's perfectly fine to talk abou other ways of
describing
rotation, but engineers who think about it a little, even if they
are
pilots, would tend, I expect, tend to agree with AIP handbook if
they
are trying to communicate with other engineers.
.
As I claimed earlier, if allowed thusters on a rigid body, I can
make
it rotate around ANY point. The table edge in my example could be
replace by such a thruster.
I doidn't say you couldn't.
Now, if the forces are removed, you will get no argument from me
that
rotation is about the CG. The forces are not removed in the OP's
question.

Sigh.
Ok'



Bertie

There was an optimist, a pessimist, and an engineer. The optimist
said,
"This bottle is half full"
The pessimist said,
"This bottle is half empty"
The engineer said;
"Yo!...... Will one of you PLEASE call those idiots over in management
and tell them this F*****g bottle is twice as big as it has to be!"



Mmm, k. I presume that this is a variation of the three blind guys and
the elephant?

Bertie.


Well, one could I guess, make a comparison based on an incomplete and
partial picture denies the whole truth scenario, but on the other hand,
in the world I know anyway, I've never met an engineer who wouldn't
swear they were right about the whole, even if they hadn't touched the
elephant at all:-)))

--
Dudley Henriques

Dudley Henriques wrote in
news
Bertie the Bunyip wrote:
Dudley Henriques wrote in
:

Bertie the Bunyip wrote:
Tina wrote in news:ba6ee109-c6e5-4633-ae25-
:

Sorry. Rigid bodies do NOT rotate around their cg if an external
force
is applied whose vector goes thru it.

Drop a yardstick, cg at the 18 inch mark, so that its zero inch
edge
hits a table. The center of rotation as a reaction to that force
is
the table edge.

You may write an equation that descibes rotation around its cg,
and
another that describes translation, but a center of rotation, to
many
who deal with such things, is that point on a rotating body whose
translational motion does not include rotation, the body appears
to
rotate around it.

In the case I just described, such a point is at the end of the
yardstick.

You are obviously defining center of rotation differenrtly than I
am,
but my American Institute of Physics Handbook on page 2-9 talks
about
rotation "in which some axis or point remains fixed in space".
That
is
the center of rotation. In the several examples I've given that
axis,
the center of rotation, is not at the center of gravity.

I am sure the math and classical physics folks use the same
definition. It's perfectly fine to talk abou other ways of
describing
rotation, but engineers who think about it a little, even if they
are
pilots, would tend, I expect, tend to agree with AIP handbook if
they
are trying to communicate with other engineers.
.
As I claimed earlier, if allowed thusters on a rigid body, I can
make
it rotate around ANY point. The table edge in my example could be
replace by such a thruster.
I doidn't say you couldn't.
Now, if the forces are removed, you will get no argument from me
that
rotation is about the CG. The forces are not removed in the OP's
question.

Sigh.
Ok'



Bertie

There was an optimist, a pessimist, and an engineer. The optimist
said,
"This bottle is half full"
The pessimist said,
"This bottle is half empty"
The engineer said;
"Yo!...... Will one of you PLEASE call those idiots over in
management
and tell them this F*****g bottle is twice as big as it has to be!"



Mmm, k. I presume that this is a variation of the three blind guys
and
the elephant?

Bertie.


Well, one could I guess, make a comparison based on an incomplete and
partial picture denies the whole truth scenario, but on the other
hand,
in the world I know anyway, I've never met an engineer who wouldn't
swear they were right about the whole, even if they hadn't touched the
elephant at all:-)))


You flatter me sir!

Bertie

Tina wrote:
Sorry. Rigid bodies do NOT rotate around their cg if an external force
is applied whose vector goes thru it.

No need to be sorry. I agree with that and not sure what I wrote that
would imply otherwise.

Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
hits a table. The center of rotation as a reaction to that force is
the table edge.

You may write an equation that descibes rotation around its cg, and
another that describes translation, but a center of rotation, to many
who deal with such things, is that point on a rotating body whose
translational motion does not include rotation, the body appears to
rotate around it.

In the case I just described, such a point is at the end of the
yardstick.

You are obviously defining center of rotation differenrtly than I am,
but my American Institute of Physics Handbook on page 2-9 talks about
rotation "in which some axis or point remains fixed in space". That is
the center of rotation. In the several examples I've given that axis,
the center of rotation, is not at the center of gravity.

I can't speak for anyone else posting to this thread, but I don't believe
I used the term "center of rotation" as such. And I don't disagree with
anything you've written above.

I am sure the math and classical physics folks use the same
definition. It's perfectly fine to talk abou other ways of describing
rotation, but engineers who think about it a little, even if they are
pilots, would tend, I expect, tend to agree with AIP handbook if they
are trying to communicate with other engineers.
.
As I claimed earlier, if allowed thusters on a rigid body, I can make
it rotate around ANY point. The table edge in my example could be
replace by such a thruster.

Definite agreement. But if you were given only one thruster and it is at
the end of your yardstick pointing downwards like so:

|
=====================V

The yardstick would rotate around the CG when the thruster is turned on.

Now, if the forces are removed, you will get no argument from me that
rotation is about the CG. The forces are not removed in the OP's
question.

Okay. The original post asked whether pulling back the stick causes the
plane to rotate about the CG or some other point. That is a tad more
analogous to my yardstick drawing above with only one thruster than the
other situations mentioned. Of course there is the complications of the
motion through the air and what happens as the angle of attack of the
wings is changed as the rotation starts to occur. But that resulting
complex motion first begins with the plane starting its rotation about
its center of gravity.

My BSc in physics may be a bit rusty, and I don't try to presume to speak
for engineers, but I'm not sure there is much disagreement left here
worth arguing about. And even if there were, I'm not sure it would
accomplish anything anyway! :-)

On Mon, 28 Jan 2008 07:20:09 +0100 (CET), Nomen Nescio
wrote in
:

Your inability to distinguish rotation from translation.

I'm unfamiliar with the term 'translation' as it refers to this issue.
I presume you are referring to one of these definitions:

(1) : a transformation of coordinates in which the new axes are
parallel to the old ones

(2) : uniform motion of a body in a straight line

Can I impose on your MIT BSME to elaborate as to which definition it
is to which you are referring? Thank you.

Let me (and Merriam-Webster) take a guess first. Rotation is the
action or process of rotating on or as if on an axis or center, and
translation in this case is the movement or change of location of the
CG.

Am I helping or hurting?