Aerodynamic question for you engineers

On Fri, 25 Jan 2008 11:14:25 -0800 (PST), terry
wrote:

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

correct terry.

Stealth Pilot

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

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

Bertie the Bunyip wrote:
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.


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.

Just checked one of my references[*] for proper terminology - where I used
"center of lift" it uses the phrase "total lift" with the symbol L. For the
lift of the main wings it uses Lw and for the lift of the tail it uses Lt.
[*] "Aerodynamics, Aeronautics, and Flight Mechanics" by Barnes W.
McCormick.

Jim Logajan wrote in
:

Bertie the Bunyip wrote:
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.


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.

Just checked one of my references[*] for proper terminology - where I
used "center of lift" it uses the phrase "total lift" with the symbol
L. For the lift of the main wings it uses Lw and for the lift of the
tail it uses Lt.


Sounds about right. I haven't read that stuff in years, though.


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

D Ramapriya wrote in
:

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.


Yeh, go with that...

Where're those aspirin?


Bertie

On Fri, 25 Jan 2008 19:15:13 -0800 (PST), D Ramapriya
wrote:



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

totally wrong.

Stealth Pilot

On Jan 26, 4:12 pm, Stealth Pilot
wrote:
On Fri, 25 Jan 2008 19:15:13 -0800 (PST), D Ramapriya

wrote:

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

totally wrong.

Stealth Pilot


While the CG is unchanging - ignoring CG travel due to fuel burn and
pax moving around - the CP (CL) changes with the AoA. I think it keeps
moving forward as the AoA increases. Thus, so long as the CP (CL) is
close to the CG, stable flight should be possible and their
coincidence isn't a sine qua non.

Still all wrong?

Ramapriya