Aerodynamic question for you engineers

There's a physics issue here called 'frame of reference.'

Think about an external stationary (with respect to the air mass)
observer, and the airplane is made to fly a loop. The center of
rotation to that obsever is the center of the loop.


Or, if the observer tracks the airplane, or even its CG (assume it's
marked on the airplane) he'll see it move laterally at airspeed, then
at the start of the climb will probably see it dip a little bit, then
start assending.

I'm having a hard time imagining a frame of reference where the
airplane would appear to rotate about its CG, where I take 'rotate' to
mean a point about which the tail end goes down and the other end goes
up, because the CG itself will be moving, first down a little (I
think) then up. Down first because the elevator is adusted so it loses
upward lift of increases downward thrust, effectively making the
airplane heavier. As the aoa increases the wings (making a huge number
of assumptions (assume a spherical cow?) increase lift. Note also that
the 'center of lift' of a wing may change with aoa, so even that model
-- all effective lift concentrated at a fixed point -- may fail.

A more minor point, (but why not pick nits?) is that it's unlikely the
CG, center of lift of the wing, and center of lift of the elevator are
all in a straight line. In a high winged airplane the center of lift,
about a third of the way back from the front of the wing, and probably
pretty close to the wing's underside skin, is well above the CG. That
vertical displacement will not affect computing moments for horizontal
flight, but will as directions of flight different from horizontal
take place. Think for a moment or two about a helocopter in horizontal
flight transitioning to a nose up attitude. When I've seen that, it
appears the center of rotation is well above the hellcopter.


I know, there's nothing like adding some mud to the water.








flies a loop
On Jan 25, 1:22*am, Pete Brown wrote:
If a conventional aircraft is in stable level flight and the stick if 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

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

"Gerry Caron" wrote in
:


"Bertie the Bunyip" wrote in message
.. .

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

That's incorrect. Cl (lower case L) is the coefficient of lift. The
center of pressure on an aircraft is referred to as the aerodynamic
center (ac).

OK, I stand corrected, again!

Bertie

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 21:51:33 -0500, "Gerry Caron"
wrote:


"Bertie the Bunyip" wrote in message
. ..

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

That's incorrect. Cl (lower case L) is the coefficient of lift. The center
of pressure on an aircraft is referred to as the aerodynamic center (ac).

Gerry
an aero engineer


expanding ... " Centre of Pressure really should read Centre of Lift"

what part of that did your aero engineering brain not understand?

the original post was correct, your comment not.

Stealth Pilot

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 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 Fri, 25 Jan 2008 16:40:07 -0800 (PST),
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 XXXX RESULTANT!
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 Fri, 25 Jan 2008 17:03:04 -0800 (PST),
wrote:

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

clueless. you'd never have understood the propeller like the wrights
did because you cant simplify situations until the problem becomes
solveable. finding a solution to a problem is often a matter of
thinking about it with just the relevant factors at play.

the answer to your door problem lies in understanding that there are
multiple hinge points that each act in a simple manner.

the frame of reference is moving at 100 knots. the aircraft rotates
about it's centre of gravity (centre of mass).

On Fri, 25 Jan 2008 19:33:58 -0800 (PST), Tina
wrote:

There's a physics issue here called 'frame of reference.'

Think about an external stationary (with respect to the air mass)
observer, and the airplane is made to fly a loop. The center of
rotation to that obsever is the center of the loop.


Or, if the observer tracks the airplane, or even its CG (assume it's
marked on the airplane) he'll see it move laterally at airspeed, then
at the start of the climb will probably see it dip a little bit, then
start assending.

I'm having a hard time imagining a frame of reference where the
airplane would appear to rotate about its CG, where I take 'rotate' to
mean a point about which the tail end goes down and the other end goes
up, because the CG itself will be moving, first down a little (I
think) then up. Down first because the elevator is adusted so it loses
upward lift of increases downward thrust, effectively making the
airplane heavier. As the aoa increases the wings (making a huge number
of assumptions (assume a spherical cow?) increase lift. Note also that
the 'center of lift' of a wing may change with aoa, so even that model
-- all effective lift concentrated at a fixed point -- may fail.

A more minor point, (but why not pick nits?) is that it's unlikely the
CG, center of lift of the wing, and center of lift of the elevator are
all in a straight line. In a high winged airplane the center of lift,
about a third of the way back from the front of the wing, and probably
pretty close to the wing's underside skin, is well above the CG. That
vertical displacement will not affect computing moments for horizontal
flight, but will as directions of flight different from horizontal
take place. Think for a moment or two about a helocopter in horizontal
flight transitioning to a nose up attitude. When I've seen that, it
appears the center of rotation is well above the hellcopter.


I know, there's nothing like adding some mud to the water.



no mud at all.
consider another situation that may provide some insight.
when an aircraft is falling in a spin what does it spin about?