Aerodynamic question for you engineers

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

On Jan 25, 12:22*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..

Thanks

--
Peter D. Brown

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?

Phil

"Pete Brown" wrote in message ...

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.


It rotates around the CG, not through it.


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

There's some great reading here

http://www.av8n.com/how/htm/aoastab.html

It may not answer your exact question, but you'll understand the
areodynamics a bit better.



On Jan 25, 1:22*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..

Thanks

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

Going home after a long dayhttp://farm2.static.flickr.com/1415/1325102827_f322928754_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 Glacierhttp://farm1.static.flickr.com/168/437324742_a216d7bb75.jpg

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.


I think this absolutely has to be the case.

CL is simply the place the overall lift averages out to. It's a FORCE
acting on a body. And so is any force acting on the horizontal tail
surfaces, or acting on any other place on the plane.

The body has a CG and it will rotate at the CG.

Displacement is another matter.

On Jan 25, 5:40 pm, wrote:
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.

I think this absolutely has to be the case.

CL is simply the place the overall lift averages out to. It's a FORCE
acting on a body. And so is any force acting on the horizontal tail
surfaces, or acting on any other place on the plane.

The body has a CG and it will rotate at the CG.

Displacement is another matter.

And if the CG is moving horizontally at 100 knots, where
is the rotation point now? As soon as rotation starts, the aircraft
begins to change its flight path, and any determination of rotation
point, whether it's the CG or CP or any other point, becomes very hard
to determine and might be irrelevant. I would prefer to think of the
fixed end of the flight path radius (which is also changing) as the
airplane rotates, just like one of those complicated cabinet door
hinges that has two arms and four pivot points. Where is the rotation
point of that door? There is no fixed point.

Dan

Phil J wrote:
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?

They aren't equivalent situations, mechanically speaking.

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.

Jim Logajan wrote in
:

Phil J wrote:
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?

They aren't equivalent situations, mechanically speaking.

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.


That's exactly the case if you include the stab in the CL equation. If
you're just referring to it on the wing itself, providing the AoA and speed
remain the same it doesn;t shift. It's a matter of definition.


Bertie

On Jan 26, 5:31 am, Jim Logajan wrote:

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.


Since the CL can be altered by the wing configuration - deployment/
retraction of flaps for a given pitch, e.g., I'm not sure that the CG
and CL need to necessarily coincide for stable flight. Also, for a
body such as an aircraft, I think the CG would theoretically be
somewhere within it while the CL is a point on the fuselage, so their
coincidence may even be an impossibility.

Ramapriya