Aerodynamic question for you engineers

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

"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.

"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...

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

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

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.

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

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.

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 -

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.