Aerodynamic question for you engineers

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?

"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...

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 -