I have highly cambered airfoil as shown in the attached picture. I'm wondering would the aerodynamic center be quite far from the 1/4 chord point? Will the arodynamic center be still on the chord line?I assume you mean the 1/4 chord line. The definition of aerodynamic center is "the point at which the moment coefficient is constant regardless of the angle of attack. When you look at the moment coefficient on a NACA airfoil chart, there are two of them; one is CMc/4, and the other is CMa.c.. CMc/4 varies a little, while CMa.c. is almost constant, implying that the a.c. moves a little depending on aoa. In fact, the a.c. moves aft as aoa decreases and moves forward as aoa increases, which is why lower air speeds tend to make an aircraft less stable and high speeds make it more stable. You can calculate the location of the a.c, but it is really unnecessary since the moment about the quarter chord is more convenient for design calculations. For symmetric airfoils the a.c. is the 1/4 chord point, and the above doesn't apply.
I never thought about it until now. From this site, it seems the Aerodynamic Center would more or less follow the chord line and move down with more camber (from extending flaps down). Extending the flaps down also moves it forward.Anywhere I could find a clue regarding the vertical position?
I think she's talking about the aerodynamic center (the point about which Cm is constant), not center of pressure (the point about which Cm is zero)... two different things.A positively cambered airfoil moves the aerodynamic center of pressure upward. Its precise location is given by integration of the pressure distributions...
Curious as to why one would like to understand the physics rather than compute the desired result... onder:
You don't use C/4 as the a.c.; you use C/4 because (or when) the airfoil data gives lift and drag through and moment around that point. C/4 is used because it's usually close to the a.c. so the moment changes will be small for small changes in AOA. And it's easier since C/4 doesn't move if, for example, you're comparing different airfoils.When people design aircraft, do they just simply use 1/4 chord point as the aerodynamic center to calculate the position of the neutral point?
For my case, as my highly cambered airfoil is not quite conventional, I’m afraid the 1/4 chord rule won’t work for my case. I have the CFD simulation results of my wing for different AoA. I’m trying to look for a way to give more precise location of the aerodynamic center or at least confirm that 1/4 chord rule also works for my case.
If the vertical location of aerodynamic center could also change, I’m wondering how shall I calculate the aerodynamic center location? Any formula or reference on that?
It would be if such an exaggerated airfoil were pivoting in the spanwise direction. Esp. if it were free pivoting. The location of that rotation axis on the ideal a.c. would set the neutral point for its AoA.The vertical location of the a.c. doesn't generally matter as for stability calculations since it's relatively close to the chord line so any moment seen by assuming a different point (drag times the vertical distance) will be relatively small.
There is one thing you should consider also, the AC does not mean the same in American English and British English, they both sometimes exchange the CP with the AC. For example, in the US the AC is defined as the point on the section side view where most pitching moments are constant. However, if we read books written in English in Europe or UK (for example books that Darrol Stinton wrote) we might see that the AC (aerodynamic centre) is actually the center of pressure, not the neutral point of an airfoil. The same goes for some other differences between aerodynamic terminology coming from English and French, decalage has a different meaning depending on where you live and which books you started to read first. There are other confusing elements and quite a few books have the same problem with the AC, CP, CL (not to be confused with the Coeficient of Lift), NP and CG. The good thing is that CG is the same everywhere, but the others tend to vary depending on geographical region.I'm wondering is the aerodynamic center always on the chord line? What if it is a highly cambered airfoil?
I was looking through some old notes and ran across these NACA standardized airfoil characteristic charts from the 1930s that show the X,Y position of the aerodynamic center. Looking at the catalog that I scanned these from it appears that for cambered sections, at least for the NACA 5 digit series, the AC is always above the chord line and usually between the mean line and the top surface. They even marked the AC outside the airfoil for some of the 5 digit sections. They also marked a few symmetrical airfoil sections this way. I haven't seen this notation on any charts other than those from NACA Langley nor have I seen any explanation of how they arrived at this but if it's simply a mater of integrating the pressure distribution then Xfoil, XFLR5, or any other program that plots the the pressure distribution, should be able to locate the X,Y position of the AC.sorry that the airfoil picture I attached is misleading. I just use it as an example to tell that the chord line could be out of the airfoil geometry and asking would there be a vertical location for the a.c.