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.

Thank you to all who responded and especially to Larry, Phil J, Jim L,
and Gerry. I am still not sure what the answer is but each response shed
some light on the issue.


Larry Dighera: This is what I originally thought but I didn't consider
that in stable flight, the CG and Cp may be at the same point.


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.

Phil J: Great thought experiment. Posed like Einstein used to.

I am not an engineer, so I am going add to your question. Imagine
that you had a couple of tall jack stands that you could place under
the wings to elevate the airplane a foot or so off the ground. Let's
say you place the stands under the wings just back from the CG such
that you have to press down on the tail to keep the nosewheel off the
ground. This is similar to the condition of flight since the center
of lift is aft of the center of gravity. Now if you push down on the
tail, the airplane will rotate about the center of lift. Wouldn't it
work the same way in the air?

Jim L: Key insight is in a regime of stabile flight, in which case, the
cl and cg are at the same point. This makes the books correct (they all
say the aircraft rotates through the CG and this would explain why its
true in stable flight.

As I understand it, the force of the tail plane's elevators typically
moves the center of lift forward and backward along the airplane's
axis as the elevators are moved up and down (as well as changing the
lift magnitude a little - though that is secondary). One presumably
enters stable flight when the center of lift is moved to coincide with
the center of gravity.

Thank you all again.

--
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 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

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?

I believe that is technically correct - external forces will either rotate
an object about its center of gravity and/or cause translational movement
of the object's center of gravity. But from a frame of reference relative
to the earth an object can be made to appear to rotate around any point
inside (or outside) that object.

The complication is that, unlike the idealized case of a body in a vacuum
in free fall, an aircraft in the earth's atmosphere shouldn't generally be
treated as an isolated system and the earth and its atmosphere treated as
an immobile frame of reference.

On Jan 25, 5:22*pm, 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..

this is an aviation group, most of us are pilots or kooks (or both)
not injuneers,. My understanding is that any force on the airpcraft
will cause a moment around the center of gravity of the aircraft,
through which rotation will occur if those moments are not balanced.
the center of pressure concept as i was taught it was just where on
the wing the lift acted through. It is just one of several forces on
the aircraft, other forces such as thrust and drag act through other
points, and in terms of what causes an aircraft to pitch by pulling
the stick back the force on the horizontal tailplane is far more
important. but all of these forces will just result in a net moment
around the center of gravity, where rotation occurs.
terry