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Aerodynamic question for you engineers
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/...22928754_b.jpg The fleet at Summit. Mt. McKinley is about 45nm away at 20,320 msl. http://farm1.static.flickr.com/187/4...cb8d2482_b.jpg The 170B at Bold near Eklutna Glacier http://farm1.static.flickr.com/168/4...a216d7bb75.jpg |
#2
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Aerodynamic question for you engineers
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. |
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Aerodynamic question for you engineers
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] |
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Aerodynamic question for you engineers
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 |
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Aerodynamic question for you engineers
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 |
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Aerodynamic question for you engineers
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 |
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Aerodynamic question for you engineers
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 |
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Aerodynamic question for you engineers
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 |
#9
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Aerodynamic question for you engineers
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 |
#10
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Aerodynamic question for you engineers
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