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 | | From: | Ron Freimuth | | Subject: | Center of pressure vs. aerodynamic center | | Date: | Wed, 29 Dec 2004 09:48:45 -0800 |
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> I have a small problem to understand the difference of center of pressure
> and aerodynamic center.
> I've read in several books, but I don't really understand.
> Does anybody have an easy explanation?
I'd say the Aerodynamic Center is a defined reference point near, but
not necessarily at, the Center of Pressure. I'm also assuming the Center of
Pressure is the same as the "Center of Lift".
The "Aerodynamic Center" is usually taken as 25%* of the Wing's MAC,
and it's "Waterline". Which is near the Center of Lift for typical wings
before compressibility has an effect.
* Supersonic wings may be based on 50% MAC since the actual Center of
Lift is often near that location in the Supersonic realm.
The Actual Center of Lift of a wing varies a bit from 25% MAC, depending
on camber, etc.. However, that variation is taken care of by adding a "Zero
Lift Pitching Moment": Cmo. Note a moment moves the Center of Lift
forward/aft without changing its magnitude.
Further, The Center of Lift varies with Angle of Attack. That is
conventionally handled by extending the Pitching Moment to cover a range of
AoA, Alpha.
The "Center of Lift" above is relative to the complete AC, not an
isolated wing. However, 25% of the wing's Mean Aerodynamic Chord is still
used for this reference.
Further, the horizontal stabilizer may have positive or negative life,
and that moves the effective Center of Lift. I'm a bit confused on
details on the nomenclature, though I understand the effect. ;)
Now AC longitudinal stations (distances) in real AC are based on a
somewhat arbitrary "Reference Datum". That is rarely at the Aerodynamic
Center, rather it may be at or near the tip of the nose. Basic analysis is
best done relative to the 'Aerodynamic Center'.
Regardless, for a wing alone, 25% MAC is commonly assumed to be the
Center of Lift, and small variations from that reference are accounted for
by adding a Pitching moment. Airfoil polars in "Theory of Wing Sections"
are based on that longitudinal reference and any variation from that point
is accounted for by the corresponding Pitching Moment curve.
RAF
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