Aerodynamic question for you engineers

On Jan 27, 1:40 pm, Nomen Nescio wrote:
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From:

You're no help at all. Maybe you could point out the flaw: I
would be pleased to know what it is so I can retract my analogy if it
IS wrong.

Flaw:
A pilot sits either in or on the plane.
You're observing the seesaw at a distance.
Look at the same seesaw, but this time....SIT ON IT!

Now what do you see?

I'm purposely NOT spelling it out for you.
If you realize what I'm getting at, on your own, you'll never see the
world the same way, again.

So, where's the flaw? Does anyone else see the flaw? Is there
a flaw? Is Nescio here the only one who sees a flaw? A flaw he won't
identify? Is this a serious, enlightening discussion or some sort of
aviation Trivial Pursuit?

Dan

On Jan 27, 1:40 pm, Nomen Nescio wrote:
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From:

You're no help at all. Maybe you could point out the flaw: I
would be pleased to know what it is so I can retract my analogy if it
IS wrong.

Flaw:
A pilot sits either in or on the plane.
You're observing the seesaw at a distance.
Look at the same seesaw, but this time....SIT ON IT!

Now what do you see?

I'm purposely NOT spelling it out for you.
If you realize what I'm getting at, on your own, you'll never see the
world the same way, again.

I went back and looked at one of your first posts in this thread,
and this is what I found:


From: Pete Brown

If a conventional aircraft is in stable level flight and the stick is
pulled back, all of the texts I have read indicate that the aircraft
pitches up, rotating through the CG.

Is this exactly correct or is it a very useful approximation good for
all practical purposes?

Nomen nescio writes:

The aircraft will rotate EXACTLY at the CG.
As a side note, the CG will actually lose a little altitude until it stabilizes
at the new attitude.

And that is exactly what I was saying: the CG will move down
briefly when the tail is forced down. What, exactly, was your
disagreement with my analogy based upon?

Dan

It is NOT that simple.

Allow me to put an external force, applied with an elevator or a
thruster or an engine and I can turn it around any center you choose,
including one external to the airframe.

Tell us, please, where is the center of rotation in an airplane
flying a loop?

Where is the center of rotation of the orbitor, flying nose first, as
it circles the earth at constant altitude and attitude relative to
the local horizon?

One of the principles underlying scientific 'theory' is, if one
example show the theory is flawed, it's the theory, not the
observation, that should be abandoned.

I submit the center of rotation of the airplane flying the loop is the
center of the loop, and the center of rotation of the orbitor is some
4000 miles from its center of gravity. I'd appreciate knowing the
flaws in these examples.




On Jan 27, 1:50*am, Nomen Nescio wrote:
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From:

* * *Like I said earlier, CG is probably good enough for our puddle-
jumper purposes, but I think the guys who study advanced aerodynamics
would have something to add to it. I don't think it's really all that
simple.
ing o
Yea, it's REALLY all that simple!

There's a flaw in your seesaw example that makes it distinctly different
from an aircraft. Figure out the flaw, and reality will fall right into place.

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Tina wrote in
:

It is NOT that simple.

Allow me to put an external force, applied with an elevator or a
thruster or an engine and I can turn it around any center you choose,
including one external to the airframe.

Tell us, please, where is the center of rotation in an airplane
flying a loop?

Where is the center of rotation of the orbitor, flying nose first, as
it circles the earth at constant altitude and attitude relative to
the local horizon?

One of the principles underlying scientific 'theory' is, if one
example show the theory is flawed, it's the theory, not the
observation, that should be abandoned.

I submit the center of rotation of the airplane flying the loop is the
center of the loop, and the center of rotation of the orbitor is some
4000 miles from its center of gravity. I'd appreciate knowing the
flaws in these examples.

I'd love to meet the guy who can fly a loop that round!

The center of rotation will be around the CG of the airplane.
That doesn't mean that if, for example, the airplane were in a spin, that
the center line of the rotation will be around the CG. It won't. But the
airplane is still, nonetheless, rotating on it's CG.
It's what the CG "is"


Bertie

Let's talk about something a little more simple. Consider a horizontal
loop, let's call it a coordinated turn. Let's be even more specific,
and talk about the pilot's frame of reference, and make it a turn
around a pylon.

Where is the center of rotation of the airplane. What, if the pilot
is skilled, will appear to be unmoving when SHE is flying the turn?
That is the center of rotation in the pilot's frame of reference.


Don't like the coordinated turn? Fly a loop around a little cloud
(question -- what would the loop diameter have to be for it to be
legal?). The pilot would be looking 'up' and trying to keep that cloud
in a fixed position -- SHE is turning around it, it is the center of
rotation.

Let's not resort to that old idea, where if one's idea is wrong the
solution is to shout.

Here's an even more simple minded, but harder to do in real life,
idea, but it makes the point. Consider an airplane with a sturdy 10
foot mast extending up from its center of gravity, and in straight and
level flight, deply a big airbrake from the top of the mast (think if
it as a big off center thruster). Make it big enough so that when it
opens the top of the mast is stopped, right there. Where is the
center of rotation? If you can accept that the center of rotation is
at the top of the mast, then you have to accept that smaller forces,
deployed not thru the center of the gravity will also cause rotation
about a point not at the center of gravity. Think smaller and smaller
airbrakes, closer to the center of gravity., There is no discontinuity
in the Newtonian physics governing the motions, so the center of
rotation will move closer to the cg, but never get there!

I am very sure you are a much better pilot than I am, and if it helps
you to be better by thinking all rotations are about the center of
gravity of the airplane, by all means do so. You don't have to
understand the physics to be a good pilot (unless you're really
messing with the far edges of the envelope).



Q (as MX might say) ED.

Tina wrote in news:c1b2a8de-4715-4731-9148-
:

Let's talk about something a little more simple. Consider a horizontal
loop, let's call it a coordinated turn. Let's be even more specific,
and talk about the pilot's frame of reference, and make it a turn
around a pylon.

Where is the center of rotation of the airplane.


CG


What, if the pilot
is skilled, will appear to be unmoving when SHE is flying the turn?
That is the center of rotation in the pilot's frame of reference.


Don't like the coordinated turn? Fly a loop around a little cloud
(question -- what would the loop diameter have to be for it to be
legal?). The pilot would be looking 'up' and trying to keep that cloud
in a fixed position -- SHE is turning around it, it is the center of
rotation.

Let's not resort to that old idea, where if one's idea is wrong the
solution is to shout.

Shout? I don't do that.
I'm proven wrogn al the time.



Here's an even more simple minded, but harder to do in real life,
idea, but it makes the point. Consider an airplane with a sturdy 10
foot mast extending up from its center of gravity, and in straight and
level flight, deply a big airbrake from the top of the mast (think if
it as a big off center thruster). Make it big enough so that when it
opens the top of the mast is stopped, right there. Where is the
center of rotation? If you can accept that the center of rotation is
at the top of the mast, then you have to accept that smaller forces,
deployed not thru the center of the gravity will also cause rotation
about a point not at the center of gravity. Think smaller and smaller
airbrakes, closer to the center of gravity., There is no discontinuity
in the Newtonian physics governing the motions, so the center of
rotation will move closer to the cg, but never get there!


Nope, the center of rotation is at the CG still. The airplane may
describe an arc at the smae time, but it's rotating about it's CG.

I am very sure you are a much better pilot than I am, and if it helps
you to be better by thinking all rotations are about the center of
gravity of the airplane, by all means do so. You don't have to
understand the physics to be a good pilot (unless you're really
messing with the far edges of the envelope).

I never said that all rotations ar about the CG. I said the airplane
rotates around it's CG.
In a spin, the airplane is rotating around it's CG. That does not mean
that the CG is describing a vertical line. It isn't.

But the airplane is rotating around it's mass. It can't do anything
else.


Bertie

Tina wrote:
[ Elided since this doesn't directly deal with any specifics of Tina's
posts, but rather the whole context of this thread. ]

(1) Yes, an aircraft can be made to appear to rotate around any point in
space. The center of gravity is (obviously) not a constraint per se to
those rotations.

(2) But for the purposes of predicting an aircraft's motion due to external
forces, engineers must deal with an extended rigid body. The center of
gravity is almost essential to successfully determining how it rotates
_and_ translates in response to those forces.

So when a pilot makes an aircraft do turns around, say, a pylon, the
engineer says that the aircraft is _rotating_ around its c.g. at the same
time that the c.g. of the aircraft is _translating_ around the pylon. But
why? Well:

(a) The reason why the engineer says the aircraft is _rotating_ around its
c.g. is because the aircraft is a single solid extended body.

(b) The reason why the engineer says the aircraft is _translating_ around
the pylon is because the pylon and aircraft are independent objects.

Hmmm. There is more I could say on the subject to clarify the above points,
but I'm not sure that investment of time would accomplish much.

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.

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.

Dan

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