Aerodynamic Center
HazelNuts,
Ahhh, now that I read it, I see it !
You are right, that was confusing !
What I really wanted to say was actually the Pitching Moment at the Center of Pressure usually increases with AoA. At the Aerodynamic Center it remains constant. Thanks for pointing that out !
Ahhh, now that I read it, I see it !
You are right, that was confusing !
What I really wanted to say was actually the Pitching Moment at the Center of Pressure usually increases with AoA. At the Aerodynamic Center it remains constant. Thanks for pointing that out !
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Originally Posted by Mr. Irons
In simple terms can someone please explain what the Aerodynamic Center of an aerofoil is.
Thanks
Thanks
I certainly learned something.
I'm too lazy now to write more about aerodynamics...but, just to give some assistance wrt visualizing these concepts look up---[not wiki]....ZLA and CGT ...zero lift axis,....CoG track....that should help sort it out for everyone...not involved in {injunearin}
CliveL,
Yes, the exact 3D rendering of the gross airfoil characteristics are indeed the summation of section charteristics along dy....
Folks, Pay a little more attention to this man...
Sorry guys, but that is not what I said. If you take moments about the point where the lift due to incidence acts then you will get a constant pitching moment as incidence varies and this point is generally close to 25% chord. So the aerodynamic centre is the location where the lift due to incidence acts and is independent of incidence (more or less).
Centre of pressure on the other hand varies with incidence. At zero lift the centre of pressure is at plus/minus infinity if there is any wing camber, because the pitching moment at zero lift is produced by a couple. As you increase incidence (and lift) the centre of pressure will move towards the 25% chord point but it will never get there.
Centre of pressure on the other hand varies with incidence. At zero lift the centre of pressure is at plus/minus infinity if there is any wing camber, because the pitching moment at zero lift is produced by a couple. As you increase incidence (and lift) the centre of pressure will move towards the 25% chord point but it will never get there.
Yes, the exact 3D rendering of the gross airfoil characteristics are indeed the summation of section charteristics along dy....
Folks, Pay a little more attention to this man...
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As for the aerodynamic moment, which confuses me:
- This moment is like that of a couple of forces, isn't it? the moment is always the same no mater what point you choose, right? It's like a "pure" moment.
- Why is it always zero when the airfoil is symmetrical?
- Does it changes with Lift in cambered airfoils? Why?
- This moment is like that of a couple of forces, isn't it? the moment is always the same no mater what point you choose, right? It's like a "pure" moment.
- Why is it always zero when the airfoil is symmetrical?
- Does it changes with Lift in cambered airfoils? Why?
Aerodynamic (pitching) moment is made up of two bits:
The pitching moment at zero lift = Cmo
plus
The pitching moment due to the lift acting at the aerodynamic centre which is normally close to 25% chord, but which may be distant from the point to which moments are referred = CL*(moment arm between a.c and moment reference point)
Cmo is a constant. It does not change as AoA changes and it does not change with pitching moment reference point. It depends on the shape of the camber line, both in height and location of the 'peak'. For a symmetric airfoil (obviously) the camber is zero and Cmo is also zero.
The lift bit is with you always, whatever the camber, so symmetric airfoils are no different to cambered airfoils. But if you choose to measure pitching moments about 25% chord (as is often the case) then the pitching moment due to lift will be zero because there isn't any moment arm. Camber simply shifts the datum AoA at which you get zero lift.
So in the special case of a symmetric airfoil measured about 25% chord the aerodynamic moment will always be zero.
Hope that helps!
CliveL
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Still puzzled about it.
Is the aerodynamic moment a "theoretical" thing?
¿Can we use any point of reference we want?
Let's assume an airplane with the CG exactly in the CP, in trim, steady level flight. How much tail lift does it have? How much is the ANU moment it has to balance? Does it depend on what point we choose for the moments? That would make no sense...
When I think of a moment, I think of a force and an arm. Moments will vary depending on the point selected, of course. When talking about the aerodynamic center, Where is the force?
Is the aerodynamic moment a "theoretical" thing?
¿Can we use any point of reference we want?
Let's assume an airplane with the CG exactly in the CP, in trim, steady level flight. How much tail lift does it have? How much is the ANU moment it has to balance? Does it depend on what point we choose for the moments? That would make no sense...
When I think of a moment, I think of a force and an arm. Moments will vary depending on the point selected, of course. When talking about the aerodynamic center, Where is the force?
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Yes nice one LM.
Note that at the aerodynamic center the pitching moment does not vary but doesn't have to be zero. Graph shows it to be slightly negative which is reasonable for a normally cambered wing section.
Wikipedia has same answer ...
Aerodynamic center - Wikipedia, the free encyclopedia
I guess the next question is do all sections have an aerodynamic center? Are there sections for which the PM is never constant no matter where it's suspended?
Note that at the aerodynamic center the pitching moment does not vary but doesn't have to be zero. Graph shows it to be slightly negative which is reasonable for a normally cambered wing section.
Wikipedia has same answer ...
Aerodynamic center - Wikipedia, the free encyclopedia
I guess the next question is do all sections have an aerodynamic center? Are there sections for which the PM is never constant no matter where it's suspended?
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> When talking about the aerodynamic center, Where is the force?
A pitching moment is a torque. The interesting thing about a torque is that it's not a unique force acting at unique distance.
For example if I tell you the torque is 12 units you have no way of knowing if that's created by a 12 unit force at a 1 unit distance or by a 1 unit force at a 12 unit distance or any other combination (2 x 6 or 3 x 4 etc). It's all the same.
Where does the pitching moment come from? Perhaps think of it like this.. On a wing section each part of the chord contributes a bit to the pitching moment. A little bit of lift generated by a patch near the trailing edge on the top surface might contribute a pitch down, a little bit near the leading edge a pitch up etc. To work out the overall pitching moment the lot has in effect to be summed or integrated.
Most conventional sections have a negative (nose down). Consider what you need for a non swept tailless aircraft - you need a positive pitching moment. To turn a conventional section into one suitable for a tailless aircraft the trailing edge it typically "bent up". That modifies the overall pitching moment for the wing and provides stability (unfortunately it also reduces lift).
A pitching moment is a torque. The interesting thing about a torque is that it's not a unique force acting at unique distance.
For example if I tell you the torque is 12 units you have no way of knowing if that's created by a 12 unit force at a 1 unit distance or by a 1 unit force at a 12 unit distance or any other combination (2 x 6 or 3 x 4 etc). It's all the same.
Where does the pitching moment come from? Perhaps think of it like this.. On a wing section each part of the chord contributes a bit to the pitching moment. A little bit of lift generated by a patch near the trailing edge on the top surface might contribute a pitch down, a little bit near the leading edge a pitch up etc. To work out the overall pitching moment the lot has in effect to be summed or integrated.
Most conventional sections have a negative (nose down). Consider what you need for a non swept tailless aircraft - you need a positive pitching moment. To turn a conventional section into one suitable for a tailless aircraft the trailing edge it typically "bent up". That modifies the overall pitching moment for the wing and provides stability (unfortunately it also reduces lift).
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Still puzzled about it. Is the aerodynamic moment a "theoretical" thing?
¿Can we use any point of reference we want?
¿Can we use any point of reference we want?
Yes, but it is usual to use 25% mean aerodynamic chord in the wind tunnel and aircraft CG for anything else
Let's assume an airplane with the CG exactly in (i.e. at) the CP, in trim, steady level flight. How much tail lift does it have? How much is the ANU moment it has to balance? Does it depend on what point we choose for the moments? That would make no sense...
Dunno - what is ANU moment?
When I think of a moment, I think of a force and an arm. Moments will vary depending on the point selected, of course. When talking about the aerodynamic center, Where is the force?
CliveL
Yes, but it is usual to use 25% mean aerodynamic chord in the wind tunnel and aircraft CG for anything else
It seems that further explanation is required. There is no way I can give you what amounts to three hours of tuition on this subject on a thread such as this.
Even John Farley will be unable to do this.
However, I will try if the moderators will accept a very big post.
....(you never know, even PPop might learn something - and that should get a reponse!)......
If you are out there John................................
Can we use any point of reference we want?
We may use any point aft of the wing trailing edge and any point forward of the wing leading edge.
I use the trailing edge and the leading edge simply because my students grasp that more easily.
Regards to all, LM.
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Sorry CliveL, we can.
We may use any point aft of the wing trailing edge and any point forward of the wing leading edge.
We may use any point aft of the wing trailing edge and any point forward of the wing leading edge.
Question: So we can use any reference point we like right?
Answer: Yes
Pray tell me how that is in any way in conflict with anything you have written about being able to take moments anywhere from in front of the LE to behind the TE
What I wrote is consistent with an earlier post. This time I merely added that in practice aerodynamicists generally use 25% mac as a reference in the wind tunnel and aircraft CG in trim or stability work. Would you disagree with that?
CliveL
Last edited by CliveL; 11th Jan 2011 at 23:41. Reason: correction
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Sorry - I have total understanding failure.........
The tailplane force has absolutely nothing to do with this argument.
The tailplane force has absolutely nothing to do with this argument.
CliveL
Last edited by CliveL; 11th Jan 2011 at 23:44. Reason: addition