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#21
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Aerodynamic question for you engineers
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/187/4...cb8d2482_b.jpg The 170B at Bold near Eklutna Glacierhttp://farm1.static.flickr.com/168/437324742_a216d7bb75.jpg |
#22
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Aerodynamic question for you engineers
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/...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 |
#23
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Aerodynamic question for you engineers
"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 |
#24
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Aerodynamic question for you engineers
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 |
#25
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Aerodynamic question for you engineers
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 |
#26
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Aerodynamic question for you engineers
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 |
#27
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Aerodynamic question for you engineers
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 |
#28
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Aerodynamic question for you engineers
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#29
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Aerodynamic question for you engineers
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#30
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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? |
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