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AC location

AC location

(OP)
Hi experts,

I am a CFD consultant and I have a customer who studies birds flights. For one of the 3D model we have (a very specific and unusual flight configuration of the bird), we know where is the center of mass. But we are now looking for a way to determine the torque in the 3 axis generated by this very disymmetric configuration. For this, we first need to determine first where could be located the aerodynamic center - which can't be done analyticaly considering the complexity and the specific wings retractation profile (the cord ratio rule is simply not applicable here)

My approcah is currently to trace one of the torque value for consecutive arbitrary points along the length of the bird and for various angle of attacks. Location of the AC should then coincide with non-changing value point. I would like first to know if this approach makes sense to you? If not, is there a practical way to determine it so I can reproduce the experiment digitally?

Thanks in advance for any support.

RE: AC location

given that the aerodynamic profiles of the bird's wings can change, i think the AC can change (rather than be "invariant").

if you know the CG of the bird, and if you know the inertial state of the bird (straight and level flight?) then you know where the AC is. if the bird is pitching then you know the AC is not in line with the CG.

another day in paradise, or is paradise one day closer ?

RE: AC location

(OP)
Dear rb,

The shape is indeed not fixed, but it is for the 3D model I am studying. We know where is the CG and at which speed/altitude he is flying. According to you, this should be enough to tell me where the AC is located? I would need more explanation if you don't mind.

BTW, is my practical approach (scanning for the invariant torque location) makes sense?

Thanks for your time.

RE: AC location

what is the flight path of the bird ? if straight and level then AC is inline with CG

another day in paradise, or is paradise one day closer ?

RE: AC location

(OP)
Mhmmm... inline OK, but not collocated. Flight is straight and stable in our simulation, yes.
Section 3.2 here (http://www.aerostudents.com/files/flightDynamics/t...) as in other documents I read report that the more the AC is aft in respect to the CG, the more stable is the flight. I am sure Mother Nature designed things very well, so I am not convinced these points should be superimposed :)

RE: AC location

in an airplane the AC of the wing is behind the CG of the plane because the H.Stab is providing a balancing load.

Is your AC for the bird or for the wing ?

if for the bird, then in straight and level flight the CG and the AC align, since there's no pitching moment.

if for the wing only, then it can be anywhere, dependent on the H.Stab (tail wing) balancing load.

another day in paradise, or is paradise one day closer ?

RE: AC location

(OP)
The AC is for the bird.
Actually CFD software needs the precise location of the AC to compute lift and drag coefficients. I was concerned about choosing the CG by default. Is the bird tail different from the plane's stabs? I would be interested in reading some related articles if you have - I mean the AC for the whole plane

Thanks

RE: AC location

I doubt anyone has much on the AC of an entire plane, I expect they work as wing and H.Stab.

The AC means Cm = 0, yes? I'd've thought that a CFD program should be able to tell you where the AC is, or you can get there from trial and error (looking at Cm).

another day in paradise, or is paradise one day closer ?

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