Aerodynamic question for you engineers

[Pete Brown]

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/1415/1325102827_f322928754_b.jpg

The fleet at Summit. Mt. McKinley is about 45nm away at 20,320 msl.
http://farm1.static.flickr.com/187/437346531_a9cb8d2482_b.jpg

The 170B at Bold near Eklutna Glacier
http://farm1.static.flickr.com/168/437324742_a216d7bb75.jpg

[Larry Dighera]

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.

[D Ramapriya]

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]

[Bertie the Bunyip[_22_]]

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

[D Ramapriya]

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

[Bertie the Bunyip[_22_]]

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

[D Ramapriya]

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

[Bertie the Bunyip[_22_]]

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

[D Ramapriya]

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

[Bertie the Bunyip[_22_]]

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

[Jim Logajan]

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.

[terry]

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

[Phil J]

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

[Blueskies]

"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/1415/1325102827_f322928754_b.jpg
>
> The fleet at Summit. Mt. McKinley is about 45nm away at 20,320 msl.
> http://farm1.static.flickr.com/187/437346531_a9cb8d2482_b.jpg
>
> The 170B at Bold near Eklutna Glacier
> http://farm1.static.flickr.com/168/437324742_a216d7bb75.jpg
>

[Tina]

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/187/437346531_a9cb8d2482_b.jpg
>
> The 170B at Bold near Eklutna Glacierhttp://farm1.static.flickr.com/168/437324742_a216d7bb75.jpg

[[email protected]]

>
> 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.

[[email protected]]

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

[Jim Logajan]

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.

[Bertie the Bunyip[_22_]]

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

[D Ramapriya]

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

[Tina]

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/187/437346531_a9cb8d2482_b.jpg
>
> The 170B at Bold near Eklutna Glacierhttp://farm1.static.flickr.com/168/437324742_a216d7bb75.jpg

[Pete Brown]

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/1415/1325102827_f322928754_b.jpg

The fleet at Summit. Mt. McKinley is about 45nm away at 20,320 msl.
http://farm1.static.flickr.com/187/437346531_a9cb8d2482_b.jpg

The 170B at Bold near Eklutna Glacier
http://farm1.static.flickr.com/168/437324742_a216d7bb75.jpg

[Bertie the Bunyip[_22_]]

"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

[Bertie the Bunyip[_22_]]

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

[Stealth Pilot[_2_]]

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

[Stealth Pilot[_2_]]

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

[Stealth Pilot[_2_]]

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

[Stealth Pilot[_2_]]

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.

[Stealth Pilot[_2_]]

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).

[Stealth Pilot[_2_]]

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?

[D Ramapriya]

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

[Blueskies]

"Jim Logajan" > wrote in message ...
> 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.



You are in stable flight when the forces are balanced. You do not want the CG and CP to coincide if you want positive
stability on the airframe. With a conventional aircraft, the CG is forward of the CP and the horiz tail provides a down
force to hold the nose up. The elevator is a wing adjuster and basically sets the AOA. At a high AOA the CP moves
forward and at a low AOA the CP moves back. The rotation still occurs around the CG. This is why the nose pitches down
during a 'stall'. The C/L deteriorates in the stalled condition so it is no longer holding the nose up, and the up force
from the horiz tail pulls it up and the nose rotates down...

When the CG and CL coincide, the elevator forces are very light and the airplane is at best neutrally stable. Because
the CP moves with AOA (therefore C/L) if a gust upsets the aircraft and pitches it up for instance, the CP moves
forward, the plane rotates around the CG as the tail goes down, and there is no self stabilizing force to bring the tail
back up. If left uncorrected the wing will stall. This movement of the CP is just one of the Wright brothers many
insights that allowed them to make an airplane that could be maneuvered.

Now, if you want to be efficient, you load the airplane so the CG is at or just forward of the aft cg limit (and make
sure it will be there as the flight progresses). This way the down force is minimized ands you can fly at a lower AOA,
lower C/L, and therefore drag. I think some of the airliners put fuel in the horiz tail so the CG can be tweaked while
in flight to keep the CG back and maintain higher efficiency.

A canard design is just the opposite. The CG is behind the CP, and when the canard stalls, the nose drops because their
is no longer any lift to hold it up. This is why canards must always stall the front wing first.

[Blueskies]

"Stealth Pilot" > wrote in message ...
> 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.
>>


You are confusing the forces here. The CG is the center of mass, period. The only way it moves is if the masses on the
aircraft physically change position (burning or transferring fuel, PAX moving, etc). Moving the elevator does not change
the CG, it changes the AOA (if the plane is flying) and that change in AOA causes and change in C/L which moves the CP.




>>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.
>>
>>


When watching the Blue angels perform in hte F-18, it becomes apparent that the aircraft is rotating around the CG, and
the movement of the CG defines the flight path. You can see the controls wiggling and the nose moving up and down, but
the plane stays in the same relative position to the others. All those aerodynamic effects do not change the CG of the
aircraft. This is one of the reasons I liked watching them fly trhe A4s better. The F-18 has negative stability and
needs to always be seeking controlled flight, while the A4 was a positive stability craft and looked like it was on
rails...


>>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?

If the plane was moved out of the atmosphere it would tumble around the CG. Same answer here...

[[email protected]]

On Jan 26, 5:19 am, Stealth Pilot >
wrote:
> On Fri, 25 Jan 2008 17:03:04 -0800 (PST),
> wrote:

> > 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).

You are assuming that the aircraft rotates about its CG as it
would in space. But we're NOT in space, and lift and drag and various
vectors all come into play here. As the aircraft rotates nose-up, CL
(coefficient of lift) increases; CP shifts forward; the relationship
between CG and CP changes because of the CP shift, and especially so
if the CG is well above or below the CP; the tail's downforce
increases, adding another vector to the whole thing. Then we have
thrust and drag trying to rotate the airplane around some other point.
We can't consider mass alone if we're trying to figure a rotational
point. If mass is all you're concerned with, then the CG is good
enough. I'd rather have more than mass acting on my airplane; it flies
better that way.
Kerschner likely has something to say on it. I'll have a look.

Dan

[Phil J]

On Jan 25, 7:31*pm, Jim Logajan > wrote:
> 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.

Actually as I understand it in stable flight the CL is aft of the CG.
The airplane remains level not because these two are in line, but
because the tail is pressing down to counterbalance the offset of the
CL.

After thinking about this question some more, it strikes me that this
situation is equivalent to a lever and fulcrum. The lever doesn't
rotate around it's CG, it rotates around the fulcrum point. In an
airplane, this point is the center of lift.

Regarding the CL moving around, I think even given that complication
the airplane would still rotate around the CL.

Phil

[Blueskies]

> wrote in message ...
> On Jan 26, 5:19 am, Stealth Pilot >
> wrote:
>> On Fri, 25 Jan 2008 17:03:04 -0800 (PST),
>> wrote:
>
>> > 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).
>
> You are assuming that the aircraft rotates about its CG as it
> would in space. But we're NOT in space, and lift and drag and various
> vectors all come into play here. As the aircraft rotates nose-up, CL
> (coefficient of lift) increases; CP shifts forward; the relationship
> between CG and CP changes because of the CP shift, and especially so
> if the CG is well above or below the CP; the tail's downforce
> increases, adding another vector to the whole thing. Then we have
> thrust and drag trying to rotate the airplane around some other point.
> We can't consider mass alone if we're trying to figure a rotational
> point. If mass is all you're concerned with, then the CG is good
> enough. I'd rather have more than mass acting on my airplane; it flies
> better that way.
> Kerschner likely has something to say on it. I'll have a look.
>
> Dan
>

All those aerodynamic forces act on the CG (center of mass) to move it in a different direction, but it is still that
center of mass that needs to move, so the aircraft (or any other free body for that matter) will move around that point.

[Dudley Henriques[_2_]]

Stealth Pilot wrote:

> clueless. you'd never have understood the propeller like the wrights
> did because you cant simplify situations until the problem becomes
> solveable.

I believe you have just stated (magnificently I might add :-))) the
antithesis of Occam's Razor, which is most certainly a "can" and not a
"can't. :-))

In other words, it is a basic principle of mathematics to reduce the
problem to it's simplest form and then solve it. :-))


--
Dudley Henriques

[Jim Logajan]

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.

[Tina]

An object's motion can surely be described as the translation of its
center of gravity and rotation around it, but the point many are
missing is, those simplifications hold best when there are no external
forces acting on the body.

In the case of an airplane entering a climb, there are external
forces. If allowed to specify those forces I can make it rotate around
any point. Consider a modification of an example given earlier -- with
supports under the wings. Of course if you then rock the airplane it
will rotate around that pivot point. If you put the support under the
engine, it will rotate around that point. That support is applying an
external force.

If the elevator is applying a force, and the wings are applying
forces, as is gravity, with the correct application of those forces
the center of rotation can be anywhere!


On Jan 26, 9:40*am, "Blueskies" > wrote:
> "Stealth Pilot" > wrote in messagenews:sc9mp3hsojk4d6dluj7pe5q4h2l1hrgfb9@4ax .com...
> > 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.
>
> You are confusing the forces here. The CG is the center of mass, period. The only way it moves is if the masses on the
> aircraft physically change position (burning or transferring fuel, PAX moving, etc). Moving the elevator does not change
> the CG, it changes the AOA (if the plane is flying) and that change in AOA causes and change in C/L which moves the CP.
>
> >>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.
>
> When *watching the Blue angels perform in hte F-18, it becomes apparent that the aircraft is rotating around the CG, and
> the movement of the CG defines the flight path. You can see the controls wiggling and the nose moving up and down, but
> the plane stays in the same relative position to the others. All those aerodynamic effects do not change the CG of the
> aircraft. This is one of the reasons I liked watching them fly trhe A4s better. The F-18 has negative stability and
> needs to always be seeking controlled flight, while the A4 was a positive stability craft and looked like it was on
> rails...
>
> >>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?
>
> If the plane was moved out of the atmosphere it would tumble around the CG.. Same answer here...- Hide quoted text -
>
> - Show quoted text -

[Jim Logajan]

Phil J > wrote:
> Actually as I understand it in stable flight the CL is aft of the CG.
> The airplane remains level not because these two are in line, but
> because the tail is pressing down to counterbalance the offset of the
> CL.

I should have used the term "total lift" so as to avoid confusion with the
lift generated only by the main wings. What you state above appears
internally consistent and correct with the definitions you are using.

> After thinking about this question some more, it strikes me that this
> situation is equivalent to a lever and fulcrum. The lever doesn't
> rotate around it's CG, it rotates around the fulcrum point. In an
> airplane, this point is the center of lift.

Whether you are talking about center of total lift (that generated by the
main wings, tail or canards, and fuselage) or center of lift of the main
wings, what you state above is _incorrect_.

I know that what you wrote sounds plausible, but the problem is that the
main wings are no more a fulcrum than the tail wings. Suppose the main wing
and the tail wing are very nearly the same size and produce nearly the same
lift and all have elevator controls? Which line is the fulcrum point now
about which the airplane rotates?

> Regarding the CL moving around, I think even given that complication
> the airplane would still rotate around the CL.

Here's a NASA web link that explains where the rotation point is:

http://www.grc.nasa.gov/WWW/K-12/airplane/acg.html

Try to find some books on flight mechanics and look for the chapters or
sections that appear to discuss longitudinal static stability of aircraft.
They should all say that the aircraft rotates about the center of gravity.

[Jim Logajan]

D Ramapriya > wrote:
> 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.

If the total lift vector does not pass through the center of gravity, then
the resulting moment will rotate the aircraft. That is not considered a
stable situation. Here's what I hope is considered an authoritative web
site that discusses this issue:

http://www.grc.nasa.gov/WWW/K-12/airplane/acg.html

See also any text on flight mechanics and aerodynamics that has sections on
the subject of longitudinal static stability.

[Phil J]

On Jan 26, 1:50*pm, Jim Logajan > wrote:
> Phil J > wrote:
> > Actually as I understand it in stable flight the CL is aft of the CG.
> > The airplane remains level not because these two are in line, but
> > because the tail is pressing down to counterbalance the offset of the
> > CL.
>
> I should have used the term "total lift" so as to avoid confusion with the
> lift generated only by the main wings. What you state above appears
> internally consistent and correct with the definitions you are using.
>
> > After thinking about this question some more, it strikes me that this
> > situation is equivalent to a lever and fulcrum. *The lever doesn't
> > rotate around it's CG, it rotates around the fulcrum point. *In an
> > airplane, this point is the center of lift.
>
> Whether you are talking about center of total lift (that generated by the
> main wings, tail or canards, and fuselage) or center of lift of the main
> wings, what you state above is _incorrect_.
>
> I know that what you wrote sounds plausible, but the problem is that the
> main wings are no more a fulcrum than the tail wings. Suppose the main wing
> and the tail wing are very nearly the same size and produce nearly the same
> lift and all have elevator controls? Which line is the fulcrum point now
> about which the airplane rotates?
>
> > Regarding the CL moving around, I think even given that complication
> > the airplane would still rotate around the CL.
>
> Here's a NASA web link that explains where the rotation point is:
>
> http://www.grc.nasa.gov/WWW/K-12/airplane/acg.html
>
> Try to find some books on flight mechanics and look for the chapters or
> sections that appear to discuss longitudinal static stability of aircraft.
> They should all say that the aircraft rotates about the center of gravity.

Ok, I think it see it. There is a difference between the center of
lift and the location of the total lift vector (I guess you could call
this the net lift). In a non-canard airplane, the main wing is
pushing upward and that is the center of lift we have been
discussing. But the stabilizer is pushing downward. The net effect
of these two forces is to move the location of the total lift vector
forward to the CG location, and that results in stable flight. So a
rotational force will rotate the airplane around that point just like
a lever rotates on a fulcrum. The same thing would happen in a
canard, except that the location of the total lift vector would be
between the two wings since they both push upward.

Phil

[Bertie the Bunyip[_22_]]

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

[Blueskies]

"Marc J. Zeitlin" > wrote in message ...
> Blueskies wrote:
>
>> A canard design is just the opposite. The CG is behind the CP, and
>> when the canard stalls, the nose drops because their is no longer
>> any lift to hold it up. This is why canards must always stall the
>> front wing first.
>
> Your explanation of everything else was good, but this is incorrect.
> For ALL aircraft - conventional, canard, tandem - doesn't matter - the
> CG MUST always be ahead of the aerodynamic center (center of lift,
> center of pressure, neutral point - all terms are sometimes used
> interchangeably) for positive static stability in the pitch direction
> to exist. Put the CG behind the AC and the plane becomes unstable in
> pitch - the further back, the more unstable and harder to fly.
>

Eikes! You are correct of course. I know this, and that is part of the reason the canard configuration is more
efficient; all the flying surfaces are countering gravity...
Well, I get a B- on this one ;-)


> In ALL aircraft, the front wing must stall first to avoid deep stalls
> and maintain control in the stall to allow recovery. In canards (one
> of which I built and fly), as in all planes, the stall is not a
> complete loss of lift, but either a leveling off or a slight drop in
> lift as the AOA increases.
>
> This has been a bizarre discussion for this engineer, because by definition, if you separate translations from
> rotation, all rotation occurs about the CG of a mass.
>
> And with respect to the four-bar linkages in cabinet hinges (and car trunk hinges, etc.), depending upon the design,
> center of rotation of the moving member (door, trunk, etc.) can be continually changing.
>
> And for our friend from OZ, statements like "totally wrong", "clueless" and your insult of Gerry Caron don't really
> add anything to the conversation.
>
> --
> Marc J. Zeitlin
> http://www.cozybuilders.org/
> Copyright (c) 2008 http://www.mdzeitlin.com/Marc/

[Bertie the Bunyip[_22_]]

D Ramapriya > wrote in news:7e76f9d7-32f5-4585-
:

> 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? :)
>

Look, you've obviously got a narrow grasp on this, but you're all over
the place with definition. Now, to be fair, there are conflicting texts,
(lots of them) but it all works the one way.
You're trying to split hairs with a chainsaw.
In short, you've got a slim but perverted grasp of the physics but
you're not speaking the right language to learn any more here.
Go get a decent book on it if you real ywant to know and then come back
and ask again. You need to swap the chainsaw for a scalpel


Bertie

[Bertie the Bunyip[_22_]]

wrote in news:532e84e3-c288-47df-b2dc-
:


> Kerschner likely has something to say on it. I'll have a look.

Kershner is great. I used to teach from his book, but it's not a physics
book...


Bertie

[[email protected]]

On Jan 26, 9:52 am, Phil J > wrote:
> After thinking about this question some more, it strikes me that this
> situation is equivalent to a lever and fulcrum. The lever doesn't
> rotate around it's CG, it rotates around the fulcrum point. In an
> airplane, this point is the center of lift.
>
> Regarding the CL moving around, I think even given that complication
> the airplane would still rotate around the CL.


This might hold if the CL (or CG as some would argue) is
rigidly fixed. But in flight, we're in a rather elastic medium, and
things move around, even leaving out forward motion.
Imagine, for example, two kids on a seesaw or teeter-totter or
whatever name by which you know that playground thing. Two kids, same
distance from the pivot, same weight. The board rotates around the
pivot. The CG is at the pivot. No argument there. But suppose we had a
different mounting for that pivot, one where the pivot was suspended
by a couple of springs. Same kids, same weight, board level and kids
motionless. (Yeah, right: motionless kids.) Now I walk up to one kid
and shove down on him; where will the board *really* pivot? As i push
down, the pivot point will move down some, too, because of the mass
and inertia of the kid at the other end. Now the real point of
rotation is somewhere along the board between the pivot and the far
kid, and it'll move back toward the pivot as that kid starts to move
upward. At any instant in this process it's somewhere besides the
original CG.
We could complicate things: A heavier kid near the pivot, a light
kid at the other end, but this light kid is a little too light, so we
have a small spring pulling down under his seat, just enough to keep
the seesaw level. Just like the engine in our airplane (big kid near
the pivot), the mass of the airplane behind the CG (light kid) and the
elevator's downforce (little spring). The main pivot, still on big
springs (wing in the air) will still move downward at the instant I
shove down on the light kid and the real rotational point will be
somewhere on the big kid's side of the pivot.
Rotation about the CG works if we ignore all the other
variables. Trouble is, those variables are with us every time we fly.
We can watch an aerobatic airplane twisting around in the air,
appearing to rotate around its CG, but is it really? Can we see the
small displacement of that point (do we even know exactly where it is
just by looking at the airplane?) at the instant of any change in tail
forces or flight path?
Like I said earlier, CG is probably good enough for our puddle-
jumper purposes, but I think the guys who study advanced aerodynamics
would have something to add to it. I don't think it's really all that
simple.

Dan

[Stealth Pilot[_2_]]

On Sat, 26 Jan 2008 10:00:43 -0800, "Marc J. Zeitlin"
> wrote:


>
>And for our friend from OZ, statements like "totally wrong",
>"clueless" and your insult of Gerry Caron don't really add anything to
>the conversation.

did Gerry?

[Blueskies]

> wrote in message ...
> On Jan 26, 9:52 am, Phil J > wrote:
>> After thinking about this question some more, it strikes me that this
>> situation is equivalent to a lever and fulcrum. The lever doesn't
>> rotate around it's CG, it rotates around the fulcrum point. In an
>> airplane, this point is the center of lift.
>>
>> Regarding the CL moving around, I think even given that complication
>> the airplane would still rotate around the CL.
>
>
> This might hold if the CL (or CG as some would argue) is
> rigidly fixed. But in flight, we're in a rather elastic medium, and
> things move around, even leaving out forward motion.
> Imagine, for example, two kids on a seesaw or teeter-totter or
> whatever name by which you know that playground thing. Two kids, same
> distance from the pivot, same weight. The board rotates around the
> pivot. The CG is at the pivot. No argument there. But suppose we had a
> different mounting for that pivot, one where the pivot was suspended
> by a couple of springs. Same kids, same weight, board level and kids
> motionless. (Yeah, right: motionless kids.) Now I walk up to one kid
> and shove down on him; where will the board *really* pivot? As i push
> down, the pivot point will move down some, too, because of the mass
> and inertia of the kid at the other end. Now the real point of
> rotation is somewhere along the board between the pivot and the far
> kid, and it'll move back toward the pivot as that kid starts to move
> upward. At any instant in this process it's somewhere besides the
> original CG.
> We could complicate things: A heavier kid near the pivot, a light
> kid at the other end, but this light kid is a little too light, so we
> have a small spring pulling down under his seat, just enough to keep
> the seesaw level. Just like the engine in our airplane (big kid near
> the pivot), the mass of the airplane behind the CG (light kid) and the
> elevator's downforce (little spring). The main pivot, still on big
> springs (wing in the air) will still move downward at the instant I
> shove down on the light kid and the real rotational point will be
> somewhere on the big kid's side of the pivot.
> Rotation about the CG works if we ignore all the other
> variables. Trouble is, those variables are with us every time we fly.
> We can watch an aerobatic airplane twisting around in the air,
> appearing to rotate around its CG, but is it really? Can we see the
> small displacement of that point (do we even know exactly where it is
> just by looking at the airplane?) at the instant of any change in tail
> forces or flight path?
> Like I said earlier, CG is probably good enough for our puddle-
> jumper purposes, but I think the guys who study advanced aerodynamics
> would have something to add to it. I don't think it's really all that
> simple.
>
> Dan



In a sense it is that simple. The CG does move due to accelerations of the aircraft in flight (your spring analogy is
close), but the aircraft still rotates around the center of mass at any given moment (no pun intended!).

Dan also...d²

[[email protected]]

On Jan 26, 11:50 pm, Nomen Nescio > wrote:

> From:
>
> > Like I said earlier, CG is probably good enough for our puddle-
> >jumper purposes, but I think the guys who study advanced aerodynamics
> >would have something to add to it. I don't think it's really all that
> >simple.
>
> Yea, it's REALLY all that simple!
>
> There's a flaw in your seesaw example that makes it distinctly different
> from an aircraft. Figure out the flaw, and reality will fall right into place.

You're no help at all. Maybe you could point out the flaw: I
would be pleased to know what it is so I can retract my analogy if it
IS wrong.

Dan

[[email protected]]

On Jan 27, 1:40 pm, Nomen Nescio > wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
>
> From:
>
> > You're no help at all. Maybe you could point out the flaw: I
> >would be pleased to know what it is so I can retract my analogy if it
> >IS wrong.
>
> Flaw:
> A pilot sits either in or on the plane.
> You're observing the seesaw at a distance.
> Look at the same seesaw, but this time....SIT ON IT!
>
> Now what do you see?
>
> I'm purposely NOT spelling it out for you.
> If you realize what I'm getting at, on your own, you'll never see the
> world the same way, again.

So, where's the flaw? Does anyone else see the flaw? Is there
a flaw? Is Nescio here the only one who sees a flaw? A flaw he won't
identify? Is this a serious, enlightening discussion or some sort of
aviation Trivial Pursuit?

Dan

[[email protected]]

On Jan 27, 1:40 pm, Nomen Nescio > wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
>
> From:
>
> > You're no help at all. Maybe you could point out the flaw: I
> >would be pleased to know what it is so I can retract my analogy if it
> >IS wrong.
>
> Flaw:
> A pilot sits either in or on the plane.
> You're observing the seesaw at a distance.
> Look at the same seesaw, but this time....SIT ON IT!
>
> Now what do you see?
>
> I'm purposely NOT spelling it out for you.
> If you realize what I'm getting at, on your own, you'll never see the
> world the same way, again.

I went back and looked at one of your first posts in this thread,
and this is what I found:


From: Pete Brown >

>>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?

Nomen nescio writes:

>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.

And that is exactly what I was saying: the CG will move down
briefly when the tail is forced down. What, exactly, was your
disagreement with my analogy based upon?

Dan

[Tina]

It is NOT that simple.

Allow me to put an external force, applied with an elevator or a
thruster or an engine and I can turn it around any center you choose,
including one external to the airframe.

Tell us, please, where is the center of rotation in an airplane
flying a loop?

Where is the center of rotation of the orbitor, flying nose first, as
it circles the earth at constant altitude and attitude relative to
the local horizon?

One of the principles underlying scientific 'theory' is, if one
example show the theory is flawed, it's the theory, not the
observation, that should be abandoned.

I submit the center of rotation of the airplane flying the loop is the
center of the loop, and the center of rotation of the orbitor is some
4000 miles from its center of gravity. I'd appreciate knowing the
flaws in these examples.




On Jan 27, 1:50*am, Nomen Nescio > wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
>
> From:
>
> > * * *Like I said earlier, CG is probably good enough for our puddle-
> >jumper purposes, but I think the guys who study advanced aerodynamics
> >would have something to add to it. I don't think it's really all that
> >simple.
> ing o
> Yea, it's REALLY all that simple!
>
> There's a flaw in your seesaw example that makes it distinctly different
> from an aircraft. Figure out the flaw, and reality will fall right into place.
>
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> Version: N/A
>
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[Bertie the Bunyip[_22_]]

Tina > wrote in
:

> It is NOT that simple.
>
> Allow me to put an external force, applied with an elevator or a
> thruster or an engine and I can turn it around any center you choose,
> including one external to the airframe.
>
> Tell us, please, where is the center of rotation in an airplane
> flying a loop?
>
> Where is the center of rotation of the orbitor, flying nose first, as
> it circles the earth at constant altitude and attitude relative to
> the local horizon?
>
> One of the principles underlying scientific 'theory' is, if one
> example show the theory is flawed, it's the theory, not the
> observation, that should be abandoned.
>
> I submit the center of rotation of the airplane flying the loop is the
> center of the loop, and the center of rotation of the orbitor is some
> 4000 miles from its center of gravity. I'd appreciate knowing the
> flaws in these examples.

I'd love to meet the guy who can fly a loop that round!

The center of rotation will be around the CG of the airplane.
That doesn't mean that if, for example, the airplane were in a spin, that
the center line of the rotation will be around the CG. It won't. But the
airplane is still, nonetheless, rotating on it's CG.
It's what the CG "is"


Bertie

[Tina]

Let's talk about something a little more simple. Consider a horizontal
loop, let's call it a coordinated turn. Let's be even more specific,
and talk about the pilot's frame of reference, and make it a turn
around a pylon.

Where is the center of rotation of the airplane. What, if the pilot
is skilled, will appear to be unmoving when SHE is flying the turn?
That is the center of rotation in the pilot's frame of reference.


Don't like the coordinated turn? Fly a loop around a little cloud
(question -- what would the loop diameter have to be for it to be
legal?). The pilot would be looking 'up' and trying to keep that cloud
in a fixed position -- SHE is turning around it, it is the center of
rotation.

Let's not resort to that old idea, where if one's idea is wrong the
solution is to shout.

Here's an even more simple minded, but harder to do in real life,
idea, but it makes the point. Consider an airplane with a sturdy 10
foot mast extending up from its center of gravity, and in straight and
level flight, deply a big airbrake from the top of the mast (think if
it as a big off center thruster). Make it big enough so that when it
opens the top of the mast is stopped, right there. Where is the
center of rotation? If you can accept that the center of rotation is
at the top of the mast, then you have to accept that smaller forces,
deployed not thru the center of the gravity will also cause rotation
about a point not at the center of gravity. Think smaller and smaller
airbrakes, closer to the center of gravity., There is no discontinuity
in the Newtonian physics governing the motions, so the center of
rotation will move closer to the cg, but never get there!

I am very sure you are a much better pilot than I am, and if it helps
you to be better by thinking all rotations are about the center of
gravity of the airplane, by all means do so. You don't have to
understand the physics to be a good pilot (unless you're really
messing with the far edges of the envelope).



Q (as MX might say) ED.

[Bertie the Bunyip[_22_]]

Tina > wrote in news:c1b2a8de-4715-4731-9148-
:

> Let's talk about something a little more simple. Consider a horizontal
> loop, let's call it a coordinated turn. Let's be even more specific,
> and talk about the pilot's frame of reference, and make it a turn
> around a pylon.
>
> Where is the center of rotation of the airplane.


CG


What, if the pilot
> is skilled, will appear to be unmoving when SHE is flying the turn?
> That is the center of rotation in the pilot's frame of reference.
>
>
> Don't like the coordinated turn? Fly a loop around a little cloud
> (question -- what would the loop diameter have to be for it to be
> legal?). The pilot would be looking 'up' and trying to keep that cloud
> in a fixed position -- SHE is turning around it, it is the center of
> rotation.
>
> Let's not resort to that old idea, where if one's idea is wrong the
> solution is to shout.

Shout? I don't do that.
I'm proven wrogn al the time.


>
> Here's an even more simple minded, but harder to do in real life,
> idea, but it makes the point. Consider an airplane with a sturdy 10
> foot mast extending up from its center of gravity, and in straight and
> level flight, deply a big airbrake from the top of the mast (think if
> it as a big off center thruster). Make it big enough so that when it
> opens the top of the mast is stopped, right there. Where is the
> center of rotation? If you can accept that the center of rotation is
> at the top of the mast, then you have to accept that smaller forces,
> deployed not thru the center of the gravity will also cause rotation
> about a point not at the center of gravity. Think smaller and smaller
> airbrakes, closer to the center of gravity., There is no discontinuity
> in the Newtonian physics governing the motions, so the center of
> rotation will move closer to the cg, but never get there!
>

Nope, the center of rotation is at the CG still. The airplane may
describe an arc at the smae time, but it's rotating about it's CG.

> I am very sure you are a much better pilot than I am, and if it helps
> you to be better by thinking all rotations are about the center of
> gravity of the airplane, by all means do so. You don't have to
> understand the physics to be a good pilot (unless you're really
> messing with the far edges of the envelope).

I never said that all rotations ar about the CG. I said the airplane
rotates around it's CG.
In a spin, the airplane is rotating around it's CG. That does not mean
that the CG is describing a vertical line. It isn't.

But the airplane is rotating around it's mass. It can't do anything
else.


Bertie

[Jim Logajan]

Tina > wrote:
[ Elided since this doesn't directly deal with any specifics of Tina's
posts, but rather the whole context of this thread. ]

(1) Yes, an aircraft can be made to appear to rotate around any point in
space. The center of gravity is (obviously) not a constraint per se to
those rotations.

(2) But for the purposes of predicting an aircraft's motion due to external
forces, engineers must deal with an extended rigid body. The center of
gravity is almost essential to successfully determining how it rotates
_and_ translates in response to those forces.

So when a pilot makes an aircraft do turns around, say, a pylon, the
engineer says that the aircraft is _rotating_ around its c.g. at the same
time that the c.g. of the aircraft is _translating_ around the pylon. But
why? Well:

(a) The reason why the engineer says the aircraft is _rotating_ around its
c.g. is because the aircraft is a single solid extended body.

(b) The reason why the engineer says the aircraft is _translating_ around
the pylon is because the pylon and aircraft are independent objects.

Hmmm. There is more I could say on the subject to clarify the above points,
but I'm not sure that investment of time would accomplish much.

[Tina]

Sorry. Rigid bodies do NOT rotate around their cg if an external force
is applied whose vector goes thru it.

Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
hits a table. The center of rotation as a reaction to that force is
the table edge.

You may write an equation that descibes rotation around its cg, and
another that describes translation, but a center of rotation, to many
who deal with such things, is that point on a rotating body whose
translational motion does not include rotation, the body appears to
rotate around it.

In the case I just described, such a point is at the end of the
yardstick.

You are obviously defining center of rotation differenrtly than I am,
but my American Institute of Physics Handbook on page 2-9 talks about
rotation "in which some axis or point remains fixed in space". That is
the center of rotation. In the several examples I've given that axis,
the center of rotation, is not at the center of gravity.

I am sure the math and classical physics folks use the same
definition. It's perfectly fine to talk abou other ways of describing
rotation, but engineers who think about it a little, even if they are
pilots, would tend, I expect, tend to agree with AIP handbook if they
are trying to communicate with other engineers.
..
As I claimed earlier, if allowed thusters on a rigid body, I can make
it rotate around ANY point. The table edge in my example could be
replace by such a thruster.

Now, if the forces are removed, you will get no argument from me that
rotation is about the CG. The forces are not removed in the OP's
question.

[[email protected]]

On Jan 27, 5:12 pm, Tina > wrote:
> Sorry. Rigid bodies do NOT rotate around their cg if an external force
> is applied whose vector goes thru it.
>
> Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
> hits a table. The center of rotation as a reaction to that force is
> the table edge.
>
> You may write an equation that descibes rotation around its cg, and
> another that describes translation, but a center of rotation, to many
> who deal with such things, is that point on a rotating body whose
> translational motion does not include rotation, the body appears to
> rotate around it.
>
> In the case I just described, such a point is at the end of the
> yardstick.
>
> You are obviously defining center of rotation differenrtly than I am,
> but my American Institute of Physics Handbook on page 2-9 talks about
> rotation "in which some axis or point remains fixed in space". That is
> the center of rotation. In the several examples I've given that axis,
> the center of rotation, is not at the center of gravity.
>
> I am sure the math and classical physics folks use the same
> definition. It's perfectly fine to talk abou other ways of describing
> rotation, but engineers who think about it a little, even if they are
> pilots, would tend, I expect, tend to agree with AIP handbook if they
> are trying to communicate with other engineers.
> .
> As I claimed earlier, if allowed thusters on a rigid body, I can make
> it rotate around ANY point. The table edge in my example could be
> replace by such a thruster.
>
> Now, if the forces are removed, you will get no argument from me that
> rotation is about the CG. The forces are not removed in the OP's
> question.

I think I see where Bertie's coming from. Rotation is
about the CG, while that CG is moving along some line due to external
forces.

Dan

[Bertie the Bunyip[_22_]]

Tina > wrote in news:ba6ee109-c6e5-4633-ae25-
:

> Sorry. Rigid bodies do NOT rotate around their cg if an external force
> is applied whose vector goes thru it.
>
> Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
> hits a table. The center of rotation as a reaction to that force is
> the table edge.
>
> You may write an equation that descibes rotation around its cg, and
> another that describes translation, but a center of rotation, to many
> who deal with such things, is that point on a rotating body whose
> translational motion does not include rotation, the body appears to
> rotate around it.
>
> In the case I just described, such a point is at the end of the
> yardstick.
>
> You are obviously defining center of rotation differenrtly than I am,
> but my American Institute of Physics Handbook on page 2-9 talks about
> rotation "in which some axis or point remains fixed in space". That is
> the center of rotation. In the several examples I've given that axis,
> the center of rotation, is not at the center of gravity.
>
> I am sure the math and classical physics folks use the same
> definition. It's perfectly fine to talk abou other ways of describing
> rotation, but engineers who think about it a little, even if they are
> pilots, would tend, I expect, tend to agree with AIP handbook if they
> are trying to communicate with other engineers.
> .
> As I claimed earlier, if allowed thusters on a rigid body, I can make
> it rotate around ANY point. The table edge in my example could be
> replace by such a thruster.

I doidn't say you couldn't.
>
> Now, if the forces are removed, you will get no argument from me that
> rotation is about the CG. The forces are not removed in the OP's
> question.


Sigh.
Ok'



Bertie
>

[Bertie the Bunyip[_22_]]

wrote in news:c3beb705-9fa5-424c-a65e-
:

> On Jan 27, 5:12 pm, Tina > wrote:
>> Sorry. Rigid bodies do NOT rotate around their cg if an external
force
>> is applied whose vector goes thru it.
>>
>> Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
>> hits a table. The center of rotation as a reaction to that force is
>> the table edge.
>>
>> You may write an equation that descibes rotation around its cg, and
>> another that describes translation, but a center of rotation, to many
>> who deal with such things, is that point on a rotating body whose
>> translational motion does not include rotation, the body appears to
>> rotate around it.
>>
>> In the case I just described, such a point is at the end of the
>> yardstick.
>>
>> You are obviously defining center of rotation differenrtly than I am,
>> but my American Institute of Physics Handbook on page 2-9 talks about
>> rotation "in which some axis or point remains fixed in space". That
is
>> the center of rotation. In the several examples I've given that axis,
>> the center of rotation, is not at the center of gravity.
>>
>> I am sure the math and classical physics folks use the same
>> definition. It's perfectly fine to talk abou other ways of
describing
>> rotation, but engineers who think about it a little, even if they are
>> pilots, would tend, I expect, tend to agree with AIP handbook if they
>> are trying to communicate with other engineers.
>> .
>> As I claimed earlier, if allowed thusters on a rigid body, I can make
>> it rotate around ANY point. The table edge in my example could be
>> replace by such a thruster.
>>
>> Now, if the forces are removed, you will get no argument from me that
>> rotation is about the CG. The forces are not removed in the OP's
>> question.
>
> I think I see where Bertie's coming from. Rotation is
> about the CG, while that CG is moving along some line due to external
> forces.
>

What I mean is, is tht the CG is the CG.no matter what. While other
forces may alter the center of rotation ( my spin example, for example)
by application of an eccentric force, the CG is stil the center of that
mass's universe and is ultimately the governer of the rotation.
The spin is a perfec example. the airplane is not sliding down it's CG,
but a point smewhere out along the stalled wing, more than likely.
However, it is still, at the same time, rotating about it's CG.
The earth rotates about it's axis, bvut it also rotates along many
others dictated by their mass. We wobble consderably because of the
moon's pull, for instance, but we're still hinged on our own CG.

Sorry, I;m making it sound more compicated than it is.



Bertie

[Tina]

Your making it sound more complicated and you are, I am afraid, wrong.

Did someone steal Berite's id? Are you Mx hiding in Bertie garb?

Of course the cg remains the cg, it's a convenient way of describing
the mass as a point, not unlike assuming a spherical cow or MX, for
example. But airplanes are extended bodies. Point masses are ok for
translattional movements, but one needs to be careful about making
assumptions as to what happens to them if forces operate off center.

I continue to assert it's a matter of defining 'rotation' rather than
cg, and you choose to define it with what could be called from a
physics point of view an interesting frame of reference. That's fine,
in pilot to pilot speak, but not so fine if you actually want to crank
some numbers (but it is pretty good for cranking someone's tail!).

Now here's the other thing, and it will shock you. You are arguing
with a woman, and you can have the last words, even if they are not
(as I am sure you have been trained to say) "Yes, dear."





On Jan 27, 10:15*pm, Bertie the Bunyip > wrote:
> wrote in news:c3beb705-9fa5-424c-a65e-
> :
>
>
>
>
>
>
>
> > On Jan 27, 5:12 pm, Tina > wrote:
> >> Sorry. Rigid bodies do NOT rotate around their cg if an external
> force
> >> is applied whose vector goes thru it.
>
> >> Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
> >> hits a table. The center of rotation as a reaction to that force is
> >> the table edge.
>
> >> You may write an equation that descibes rotation around its cg, and
> >> another that describes translation, but a center of rotation, to many
> >> who deal with such things, is that point on a rotating body whose
> >> translational motion does not include rotation, the body appears to
> >> rotate around it.
>
> >> In the case I just described, such a point is at the end of the
> >> yardstick.
>
> >> You are obviously defining center of rotation differenrtly than I am,
> >> but my American Institute of Physics Handbook on page 2-9 talks about
> >> rotation "in which some axis or point remains fixed in space". That
> is
> >> the center of rotation. In the several examples I've given that axis,
> >> the center of rotation, is not at the center of gravity.
>
> >> I *am sure the math and classical physics folks use the same
> >> definition. It's perfectly fine to talk abou *other ways of
> describing
> >> rotation, but engineers who think about it a little, even if they are
> >> pilots, would tend, I expect, tend to agree with AIP handbook if they
> >> are trying to communicate with other engineers.
> >> .
> >> As I claimed earlier, if allowed thusters on a rigid body, I can make
> >> it rotate around ANY point. The table edge in my example could be
> >> replace by such a thruster.
>
> >> Now, if the forces are removed, you will get no argument from me that
> >> rotation is about the CG. *The forces are not removed in the OP's
> >> question.
>
> > * * * * * * *I think I see where Bertie's coming from. Rotation is
> > about the CG, while that CG is moving along some line due to external
> > forces.
>
> What I mean is, is tht the CG is the CG.no matter what. While other
> forces may alter the center of rotation ( my spin example, for example)
> by application of an eccentric force, the CG is stil the center of that
> mass's universe and is ultimately the governer of the rotation.
> The spin is a perfec example. the airplane is not sliding down it's CG,
> but a point smewhere out along the stalled wing, more than likely.
> However, it is still, at the same time, rotating about it's CG.
> The earth rotates about it's axis, bvut it also rotates along many
> others dictated by their mass. We wobble consderably because of the
> moon's pull, for instance, but we're still hinged on our own CG.
>
> Sorry, I;m making it sound more compicated than it is.
>
> Bertie- Hide quoted text -
>
> - Show quoted text -

[Dudley Henriques[_2_]]

Bertie the Bunyip wrote:
> Tina > wrote in news:ba6ee109-c6e5-4633-ae25-
> :
>
>> Sorry. Rigid bodies do NOT rotate around their cg if an external force
>> is applied whose vector goes thru it.
>>
>> Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
>> hits a table. The center of rotation as a reaction to that force is
>> the table edge.
>>
>> You may write an equation that descibes rotation around its cg, and
>> another that describes translation, but a center of rotation, to many
>> who deal with such things, is that point on a rotating body whose
>> translational motion does not include rotation, the body appears to
>> rotate around it.
>>
>> In the case I just described, such a point is at the end of the
>> yardstick.
>>
>> You are obviously defining center of rotation differenrtly than I am,
>> but my American Institute of Physics Handbook on page 2-9 talks about
>> rotation "in which some axis or point remains fixed in space". That is
>> the center of rotation. In the several examples I've given that axis,
>> the center of rotation, is not at the center of gravity.
>>
>> I am sure the math and classical physics folks use the same
>> definition. It's perfectly fine to talk abou other ways of describing
>> rotation, but engineers who think about it a little, even if they are
>> pilots, would tend, I expect, tend to agree with AIP handbook if they
>> are trying to communicate with other engineers.
>> .
>> As I claimed earlier, if allowed thusters on a rigid body, I can make
>> it rotate around ANY point. The table edge in my example could be
>> replace by such a thruster.
>
> I doidn't say you couldn't.
>> Now, if the forces are removed, you will get no argument from me that
>> rotation is about the CG. The forces are not removed in the OP's
>> question.
>
>
> Sigh.
> Ok'
>
>
>
> Bertie
>

There was an optimist, a pessimist, and an engineer. The optimist said,
"This bottle is half full"
The pessimist said,
"This bottle is half empty"
The engineer said;
"Yo!...... Will one of you PLEASE call those idiots over in management
and tell them this F*****g bottle is twice as big as it has to be!"

--
Dudley Henriques

[Stella Starr]

Tina wrote:
Fly a loop around a little cloud
> (question -- what would the loop diameter have to be for it to be
> legal?)

Ooh, ooh...4000 feet plus cloud diameter?
Or "clear of" plus that diameter, which gets pretty vague...

Egad, I haven't flown in too long.

[Bertie the Bunyip[_22_]]

Tina > wrote in news:3cae129f-1ac7-4a13-b1cf-
:

> Your making it sound more complicated and you are, I am afraid, wrong.
>
> Did someone steal Berite's id? Are you Mx hiding in Bertie garb?
>
> Of course the cg remains the cg, it's a convenient way of describing
> the mass as a point, not unlike assuming a spherical cow or MX, for
> example. But airplanes are extended bodies. Point masses are ok for
> translattional movements, but one needs to be careful about making
> assumptions as to what happens to them if forces operate off center.
>
> I continue to assert it's a matter of defining 'rotation' rather than
> cg, and you choose to define it with what could be called from a
> physics point of view an interesting frame of reference. That's fine,
> in pilot to pilot speak, but not so fine if you actually want to crank
> some numbers (but it is pretty good for cranking someone's tail!).


And where did you get the idea that I disagreed with that?
>
> Now here's the other thing, and it will shock you. You are arguing
> with a woman, and you can have the last words, even if they are not
> (as I am sure you have been trained to say) "Yes, dear."


OK, we're throught the looking glass now.


And even there the airplane still rotates arounf it's CG.


Bertie

[Bertie the Bunyip[_22_]]

Dudley Henriques > wrote in
:

> Bertie the Bunyip wrote:
>> Tina > wrote in news:ba6ee109-c6e5-4633-ae25-
>> :
>>
>>> Sorry. Rigid bodies do NOT rotate around their cg if an external
force
>>> is applied whose vector goes thru it.
>>>
>>> Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
>>> hits a table. The center of rotation as a reaction to that force is
>>> the table edge.
>>>
>>> You may write an equation that descibes rotation around its cg, and
>>> another that describes translation, but a center of rotation, to
many
>>> who deal with such things, is that point on a rotating body whose
>>> translational motion does not include rotation, the body appears to
>>> rotate around it.
>>>
>>> In the case I just described, such a point is at the end of the
>>> yardstick.
>>>
>>> You are obviously defining center of rotation differenrtly than I
am,
>>> but my American Institute of Physics Handbook on page 2-9 talks
about
>>> rotation "in which some axis or point remains fixed in space". That
is
>>> the center of rotation. In the several examples I've given that
axis,
>>> the center of rotation, is not at the center of gravity.
>>>
>>> I am sure the math and classical physics folks use the same
>>> definition. It's perfectly fine to talk abou other ways of
describing
>>> rotation, but engineers who think about it a little, even if they
are
>>> pilots, would tend, I expect, tend to agree with AIP handbook if
they
>>> are trying to communicate with other engineers.
>>> .
>>> As I claimed earlier, if allowed thusters on a rigid body, I can
make
>>> it rotate around ANY point. The table edge in my example could be
>>> replace by such a thruster.
>>
>> I doidn't say you couldn't.
>>> Now, if the forces are removed, you will get no argument from me
that
>>> rotation is about the CG. The forces are not removed in the OP's
>>> question.
>>
>>
>> Sigh.
>> Ok'
>>
>>
>>
>> Bertie
>>
>
> There was an optimist, a pessimist, and an engineer. The optimist
said,
> "This bottle is half full"
> The pessimist said,
> "This bottle is half empty"
> The engineer said;
> "Yo!...... Will one of you PLEASE call those idiots over in management
> and tell them this F*****g bottle is twice as big as it has to be!"



Mmm, k. I presume that this is a variation of the three blind guys and
the elephant?

Bertie.
>

[Dudley Henriques[_2_]]

Bertie the Bunyip wrote:
> Dudley Henriques > wrote in
> :
>
>> Bertie the Bunyip wrote:
>>> Tina > wrote in news:ba6ee109-c6e5-4633-ae25-
>>> :
>>>
>>>> Sorry. Rigid bodies do NOT rotate around their cg if an external
> force
>>>> is applied whose vector goes thru it.
>>>>
>>>> Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
>>>> hits a table. The center of rotation as a reaction to that force is
>>>> the table edge.
>>>>
>>>> You may write an equation that descibes rotation around its cg, and
>>>> another that describes translation, but a center of rotation, to
> many
>>>> who deal with such things, is that point on a rotating body whose
>>>> translational motion does not include rotation, the body appears to
>>>> rotate around it.
>>>>
>>>> In the case I just described, such a point is at the end of the
>>>> yardstick.
>>>>
>>>> You are obviously defining center of rotation differenrtly than I
> am,
>>>> but my American Institute of Physics Handbook on page 2-9 talks
> about
>>>> rotation "in which some axis or point remains fixed in space". That
> is
>>>> the center of rotation. In the several examples I've given that
> axis,
>>>> the center of rotation, is not at the center of gravity.
>>>>
>>>> I am sure the math and classical physics folks use the same
>>>> definition. It's perfectly fine to talk abou other ways of
> describing
>>>> rotation, but engineers who think about it a little, even if they
> are
>>>> pilots, would tend, I expect, tend to agree with AIP handbook if
> they
>>>> are trying to communicate with other engineers.
>>>> .
>>>> As I claimed earlier, if allowed thusters on a rigid body, I can
> make
>>>> it rotate around ANY point. The table edge in my example could be
>>>> replace by such a thruster.
>>> I doidn't say you couldn't.
>>>> Now, if the forces are removed, you will get no argument from me
> that
>>>> rotation is about the CG. The forces are not removed in the OP's
>>>> question.
>>>
>>> Sigh.
>>> Ok'
>>>
>>>
>>>
>>> Bertie
>>>
>> There was an optimist, a pessimist, and an engineer. The optimist
> said,
>> "This bottle is half full"
>> The pessimist said,
>> "This bottle is half empty"
>> The engineer said;
>> "Yo!...... Will one of you PLEASE call those idiots over in management
>> and tell them this F*****g bottle is twice as big as it has to be!"
>
>
>
> Mmm, k. I presume that this is a variation of the three blind guys and
> the elephant?
>
> Bertie.
>

Well, one could I guess, make a comparison based on an incomplete and
partial picture denies the whole truth scenario, but on the other hand,
in the world I know anyway, I've never met an engineer who wouldn't
swear they were right about the whole, even if they hadn't touched the
elephant at all:-)))

--
Dudley Henriques

[Bertie the Bunyip[_22_]]

Dudley Henriques > wrote in
:

> Bertie the Bunyip wrote:
>> Dudley Henriques > wrote in
>> :
>>
>>> Bertie the Bunyip wrote:
>>>> Tina > wrote in news:ba6ee109-c6e5-4633-ae25-
>>>> :
>>>>
>>>>> Sorry. Rigid bodies do NOT rotate around their cg if an external
>> force
>>>>> is applied whose vector goes thru it.
>>>>>
>>>>> Drop a yardstick, cg at the 18 inch mark, so that its zero inch
edge
>>>>> hits a table. The center of rotation as a reaction to that force
is
>>>>> the table edge.
>>>>>
>>>>> You may write an equation that descibes rotation around its cg,
and
>>>>> another that describes translation, but a center of rotation, to
>> many
>>>>> who deal with such things, is that point on a rotating body whose
>>>>> translational motion does not include rotation, the body appears
to
>>>>> rotate around it.
>>>>>
>>>>> In the case I just described, such a point is at the end of the
>>>>> yardstick.
>>>>>
>>>>> You are obviously defining center of rotation differenrtly than I
>> am,
>>>>> but my American Institute of Physics Handbook on page 2-9 talks
>> about
>>>>> rotation "in which some axis or point remains fixed in space".
That
>> is
>>>>> the center of rotation. In the several examples I've given that
>> axis,
>>>>> the center of rotation, is not at the center of gravity.
>>>>>
>>>>> I am sure the math and classical physics folks use the same
>>>>> definition. It's perfectly fine to talk abou other ways of
>> describing
>>>>> rotation, but engineers who think about it a little, even if they
>> are
>>>>> pilots, would tend, I expect, tend to agree with AIP handbook if
>> they
>>>>> are trying to communicate with other engineers.
>>>>> .
>>>>> As I claimed earlier, if allowed thusters on a rigid body, I can
>> make
>>>>> it rotate around ANY point. The table edge in my example could be
>>>>> replace by such a thruster.
>>>> I doidn't say you couldn't.
>>>>> Now, if the forces are removed, you will get no argument from me
>> that
>>>>> rotation is about the CG. The forces are not removed in the OP's
>>>>> question.
>>>>
>>>> Sigh.
>>>> Ok'
>>>>
>>>>
>>>>
>>>> Bertie
>>>>
>>> There was an optimist, a pessimist, and an engineer. The optimist
>> said,
>>> "This bottle is half full"
>>> The pessimist said,
>>> "This bottle is half empty"
>>> The engineer said;
>>> "Yo!...... Will one of you PLEASE call those idiots over in
management
>>> and tell them this F*****g bottle is twice as big as it has to be!"
>>
>>
>>
>> Mmm, k. I presume that this is a variation of the three blind guys
and
>> the elephant?
>>
>> Bertie.
>>
>
> Well, one could I guess, make a comparison based on an incomplete and
> partial picture denies the whole truth scenario, but on the other
hand,
> in the world I know anyway, I've never met an engineer who wouldn't
> swear they were right about the whole, even if they hadn't touched the
> elephant at all:-)))


You flatter me sir!

Bertie
>

[Jim Logajan]

Tina > wrote:
> Sorry. Rigid bodies do NOT rotate around their cg if an external force
> is applied whose vector goes thru it.

No need to be sorry. I agree with that and not sure what I wrote that
would imply otherwise.

> Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
> hits a table. The center of rotation as a reaction to that force is
> the table edge.
>
> You may write an equation that descibes rotation around its cg, and
> another that describes translation, but a center of rotation, to many
> who deal with such things, is that point on a rotating body whose
> translational motion does not include rotation, the body appears to
> rotate around it.
>
> In the case I just described, such a point is at the end of the
> yardstick.
>
> You are obviously defining center of rotation differenrtly than I am,
> but my American Institute of Physics Handbook on page 2-9 talks about
> rotation "in which some axis or point remains fixed in space". That is
> the center of rotation. In the several examples I've given that axis,
> the center of rotation, is not at the center of gravity.

I can't speak for anyone else posting to this thread, but I don't believe
I used the term "center of rotation" as such. And I don't disagree with
anything you've written above.

> I am sure the math and classical physics folks use the same
> definition. It's perfectly fine to talk abou other ways of describing
> rotation, but engineers who think about it a little, even if they are
> pilots, would tend, I expect, tend to agree with AIP handbook if they
> are trying to communicate with other engineers.
> .
> As I claimed earlier, if allowed thusters on a rigid body, I can make
> it rotate around ANY point. The table edge in my example could be
> replace by such a thruster.

Definite agreement. But if you were given only one thruster and it is at
the end of your yardstick pointing downwards like so:

|
=====================V

The yardstick would rotate around the CG when the thruster is turned on.

> Now, if the forces are removed, you will get no argument from me that
> rotation is about the CG. The forces are not removed in the OP's
> question.

Okay. The original post asked whether pulling back the stick causes the
plane to rotate about the CG or some other point. That is a tad more
analogous to my yardstick drawing above with only one thruster than the
other situations mentioned. Of course there is the complications of the
motion through the air and what happens as the angle of attack of the
wings is changed as the rotation starts to occur. But that resulting
complex motion first begins with the plane starting its rotation about
its center of gravity.

My BSc in physics may be a bit rusty, and I don't try to presume to speak
for engineers, but I'm not sure there is much disagreement left here
worth arguing about. And even if there were, I'm not sure it would
accomplish anything anyway! :-)

[Larry Dighera]

On Mon, 28 Jan 2008 07:20:09 +0100 (CET), Nomen Nescio
> wrote in
>:

>Your inability to distinguish rotation from translation.

I'm unfamiliar with the term 'translation' as it refers to this issue.
I presume you are referring to one of these definitions:

(1) : a transformation of coordinates in which the new axes are
parallel to the old ones

(2) : uniform motion of a body in a straight line

Can I impose on your MIT BSME to elaborate as to which definition it
is to which you are referring? Thank you.

Let me (and Merriam-Webster) take a guess first. Rotation is the
action or process of rotating on or as if on an axis or center, and
translation in this case is the movement or change of location of the
CG.

Am I helping or hurting?

[Bertie the Bunyip[_24_]]

Larry Dighera > wrote in
:

> On Mon, 28 Jan 2008 07:20:09 +0100 (CET), Nomen Nescio
> > wrote in
> >:
>
>>Your inability to distinguish rotation from translation.
>
> I'm unfamiliar with the term 'translation' as it refers to this issue.
> I presume you are referring to one of these definitions:
>
> (1) : a transformation of coordinates in which the new axes are
> parallel to the old ones
>
> (2) : uniform motion of a body in a straight line
>
> Can I impose on your MIT BSME to elaborate as to which definition it
> is to which you are referring? Thank you.



What, you couldn't find anything in the FARs about it?


>
> Let me (and Merriam-Webster) take a guess first. Rotation is the
> action or process of rotating on or as if on an axis or center, and
> translation in this case is the movement or change of location of the
> CG.
>
> Am I helping or hurting?
>
>

You're an idiot.


Bertie

[Bertie the Bunyip[_24_]]

Nomen Nescio > wrote in
:

> -----BEGIN PGP SIGNED MESSAGE-----
>
> From:
>
>> So, where's the flaw? Does anyone else see the flaw? Is there
>>a flaw? Is Nescio here the only one who sees a flaw? A flaw he won't
>>identify?
>
> I did identify the flaw.
> You can't seem to grasp it.
>
>> Is this a serious, enlightening discussion or some sort of
>>aviation Trivial Pursuit?
>
> A little of both, IF......you actually want to learn something.
>
> Try this one:
> You launch a missile at the moon. Halfway there you check it's
> free trajectory and find that it will hit dead center in crater X.
> Suddenly it explodes into a million pieces, all spinning off into
> space in all directions. What happens to the center of mass? :)

Depends, Plain or peanut center?

Bertie

[Stealth Pilot[_2_]]

>> >> As I claimed earlier, if allowed thusters on a rigid body, I can make
>> >> it rotate around ANY point. The table edge in my example could be
>> >> replace by such a thruster.
>>

no you cant.

an off centre force applied to a body resolves as a couple about the
cg and a force through the CG
the force through the CG pushes it along but the couple result in a
rotation about the cg.


in one of my old engineering texts the gentre of gravity is defined as
the point within a body about which all forces resolve.

the CG isnt a little sticker placed somewhere within a body. it is the
centre of the mass of the thing and it varies with the movement of
parts of that mass, be it evaporation of fuel in a carby which is
burnt and jetisoned overboard as gasses coming out the exhaust or
sweat evaporating from the brow of the pilot.

all forces resolve around the centre of gravity of the body.
it is an inherent attribute of all matter as seen through the eyes of
an engineer.

you really need to go out and fly a little aeroplane with not much
inertia. (like a tailwind) you will see that all gusts and control
inputs result in movements about the CG. it is what makes the
behaviour predictable and the damn things flyable (and the predictive
stress calculations possible)

Stealth Pilot

[[email protected]]

On Jan 28, 1:10 am, Nomen Nescio > wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
>
> From:
>
> > So, where's the flaw? Does anyone else see the flaw? Is there
> >a flaw? Is Nescio here the only one who sees a flaw? A flaw he won't
> >identify?
>
> I did identify the flaw.
> You can't seem to grasp it.
>
> > Is this a serious, enlightening discussion or some sort of
> >aviation Trivial Pursuit?
>
> A little of both, IF......you actually want to learn something.
>
> Try this one:
> You launch a missile at the moon. Halfway there you check it's
> free trajectory and find that it will hit dead center in crater X. Suddenly
> it explodes into a million pieces, all spinning off into space in all directions.
> What happens to the center of mass? :)

I gotcha now. No problem. No more argument.

Dan