![]() |
If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
|
#1
|
|||
|
|||
![]()
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? |
#2
|
|||
|
|||
![]()
"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... |
#3
|
|||
|
|||
![]()
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 - |
Thread Tools | |
Display Modes | |
|
|
![]() |
||||
Thread | Thread Starter | Forum | Replies | Last Post |
a question for the aeronautical engineers among us | Tina | Piloting | 10 | November 4th 07 12:56 PM |
Are flight engineers qualified to fly? | Mxsmanic | Piloting | 14 | January 23rd 07 07:39 PM |
Aerodynamic Drag | Kyle Boatright | Home Built | 7 | September 26th 05 03:34 AM |
Aerodynamic Simulation of Standard Cirrus Glider | Jim Hendrix | Soaring | 13 | November 9th 04 11:38 PM |
Airfoil aerodynamic simulator... | Thomas J. Paladino Jr. | Piloting | 4 | May 17th 04 02:41 PM |