Aerodynamic question for you engineers

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?

Most aircraft have the CG located slightly forward of the center of
pressure ( CP or center of lift) for positive pitch stability. I was
wondering if the actual point of rotation is displaced somewhat aft of
the CG, someplace close to the CG but in fact some small distance
towards the CP.

When the aircraft is subject only to the force of gravity, any
displacement will cause it to rotate around the cg but in flight its
subject to gravity as well as the aerodynamic forces which act through
the CP, suggesting to me that the point of rotation is not quite on the CG.



Thanks

--
Peter D. Brown
http://home.gci.net/~pdb/
http://groups.yahoo.com/group/akmtnsoaring/


Going home after a long day
http://farm2.static.flickr.com/1415/...22928754_b.jpg

The fleet at Summit. Mt. McKinley is about 45nm away at 20,320 msl.
http://farm1.static.flickr.com/187/4...cb8d2482_b.jpg

The 170B at Bold near Eklutna Glacier
http://farm1.static.flickr.com/168/4...a216d7bb75.jpg

On Thu, 24 Jan 2008 21:22:23 -0900, Pete Brown wrote in
:


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?

Most aircraft have the CG located slightly forward of the center of
pressure ( CP or center of lift) for positive pitch stability. I was
wondering if the actual point of rotation is displaced somewhat aft of
the CG, someplace close to the CG but in fact some small distance
towards the CP.

When the aircraft is subject only to the force of gravity, any
displacement will cause it to rotate around the cg but in flight its
subject to gravity as well as the aerodynamic forces which act through
the CP, suggesting to me that the point of rotation is not quite on the CG.



That's an interesting question. I hadn't thought about it before.

First let me say, that I'm not an aeronautical engineer, but
intuitively I figure it this way.

In stable, level flight lift (acting through the center of pressure) =
weight (acting through the center of gravity), so it would seem that a
downward force on the tail would cause the aircraft to rotate on its
lateral axis through a point midway between the center of
lift/pressure and center of gravity. But that's a guess, and it
doesn't consider the displacement of the center of pressure forward
with the increase in angle of attack.

On Jan 25, 10:22 am, Pete Brown wrote:
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?

Most aircraft have the CG located slightly forward of the center of
pressure ( CP or center of lift) for positive pitch stability. I was
wondering if the actual point of rotation is displaced somewhat aft of
the CG, someplace close to the CG but in fact some small distance
towards the CP.

When the aircraft is subject only to the force of gravity, any
displacement will cause it to rotate around the cg but in flight its
subject to gravity as well as the aerodynamic forces which act through
the CP, suggesting to me that the point of rotation is not quite on the CG.


It's tempting to think that it could be somewhere near the center of
the balance arm but it's hard to logically argue against the CG, isn't
it?

Ramapriya
[not an aero engineer]

D Ramapriya wrote in
:

On Jan 25, 10:22 am, Pete Brown wrote:
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?

Most aircraft have the CG located slightly forward of the center of
pressure ( CP or center of lift) for positive pitch stability. I was
wondering if the actual point of rotation is displaced somewhat aft
of the CG, someplace close to the CG but in fact some small distance
towards the CP.

When the aircraft is subject only to the force of gravity, any
displacement will cause it to rotate around the cg but in flight its
subject to gravity as well as the aerodynamic forces which act
through the CP, suggesting to me that the point of rotation is not
quite on the CG.


It's tempting to think that it could be somewhere near the center of
the balance arm but it's hard to logically argue against the CG, isn't
it?


They're the same thing.


Bertie

On Jan 25, 7:19 pm, Bertie the Bunyip wrote:
D Ramapriya wrote :



On Jan 25, 10:22 am, Pete Brown wrote:
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?

Most aircraft have the CG located slightly forward of the center of
pressure ( CP or center of lift) for positive pitch stability. I was
wondering if the actual point of rotation is displaced somewhat aft
of the CG, someplace close to the CG but in fact some small distance
towards the CP.

When the aircraft is subject only to the force of gravity, any
displacement will cause it to rotate around the cg but in flight its
subject to gravity as well as the aerodynamic forces which act
through the CP, suggesting to me that the point of rotation is not
quite on the CG.

It's tempting to think that it could be somewhere near the center of
the balance arm but it's hard to logically argue against the CG, isn't
it?

They're the same thing.


By balance arm, I meant the distance between CG and CP. Missing
something, am I?

Ramapriya

D Ramapriya wrote in
:

On Jan 25, 7:19 pm, Bertie the Bunyip wrote:
D Ramapriya wrote
innews:8e2786b5-d92c-4fb3-b950-2d9346494a87@
1g2000hsl.googlegroups.com
:



On Jan 25, 10:22 am, Pete Brown wrote:
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?

Most aircraft have the CG located slightly forward of the center
of pressure ( CP or center of lift) for positive pitch stability.
I was wondering if the actual point of rotation is displaced
somewhat aft of the CG, someplace close to the CG but in fact some
small distance towards the CP.

When the aircraft is subject only to the force of gravity, any
displacement will cause it to rotate around the cg but in flight
its subject to gravity as well as the aerodynamic forces which act
through the CP, suggesting to me that the point of rotation is not
quite on the CG.

It's tempting to think that it could be somewhere near the center
of the balance arm but it's hard to logically argue against the CG,
isn't it?

They're the same thing.


By balance arm, I meant the distance between CG and CP. Missing
something, am I?

Well, that'd be a misdefinition. Also, CP really should read CL


Bertie

On Jan 25, 8:29 pm, Bertie the Bunyip wrote:
D Ramapriya wrote :


It's tempting to think that it could be somewhere near the center
of the balance arm but it's hard to logically argue against the CG,
isn't it?

They're the same thing.

By balance arm, I meant the distance between CG and CP. Missing
something, am I?

Well, that'd be a misdefinition. Also, CP really should read CL

Bertie


But we need some term for what I referred to as the balance arm, i.e.
the distance between CG and CL (CP).

Ramapriya

D Ramapriya wrote in
:

On Jan 25, 8:29 pm, Bertie the Bunyip wrote:
D Ramapriya wrote

m:


It's tempting to think that it could be somewhere near the
center of the balance arm but it's hard to logically argue
against the CG, isn't it?

They're the same thing.

By balance arm, I meant the distance between CG and CP. Missing
something, am I?

Well, that'd be a misdefinition. Also, CP really should read CL

Bertie


But we need some term for what I referred to as the balance arm, i.e.
the distance between CG and CL (CP).


No we don't, you just defined it.



Bertie

On Jan 25, 9:10 pm, Bertie the Bunyip wrote:

It's tempting to think that it could be somewhere near the
center of the balance arm but it's hard to logically argue
against the CG, isn't it?

They're the same thing.

By balance arm, I meant the distance between CG and CP. Missing
something, am I?

Well, that'd be a misdefinition. Also, CP really should read CL

Bertie

But we need some term for what I referred to as the balance arm, i.e.
the distance between CG and CL (CP).

No we don't, you just defined it.

Bertie

Are you saying that CG and CL are the same? I definitely remember
reading that they're different.

Ramapriya

D Ramapriya wrote in news:8ff767b1-b1ff-44c4-
:

On Jan 25, 9:10 pm, Bertie the Bunyip wrote:

It's tempting to think that it could be somewhere near the
center of the balance arm but it's hard to logically argue
against the CG, isn't it?

They're the same thing.

By balance arm, I meant the distance between CG and CP. Missing
something, am I?

Well, that'd be a misdefinition. Also, CP really should read CL

Bertie

But we need some term for what I referred to as the balance arm,
i.e.
the distance between CG and CL (CP).

No we don't, you just defined it.

Bertie

Are you saying that CG and CL are the same?

No.


I definitely remember
reading that they're different.


Excellent first step. Read some more. Make sure it isn't the Sun.


Bertie