Aerodynamic Center / Center of Pressure

[Sidious]

I am confused about the terms Aerodynamic Center and its relationship with Center of Pressure. I know the definition of it however Im confused when we use the terms with regards to longitudinal stability.

The PHAK only mentions Center of Pressure and its movement, however every other aerodynamic book I read uses Aerodynamic Center. What is the deal?

Does lift act through the CP or the AC? The Aerodynamic Center should produce a nose down pitch on its own on a cambered airfoil thus making it stable unless we put the CG so far rearward that it overcomes the nose down tendency and actually makes a nose up pitch....

But realistically we could place the CG aft of the Aerodynamic Center right?

Help... And any references would be great!

[tgrayson]

Originally Posted by Sidious

The PHAK only mentions Center of Pressure and its movement, however every other aerodynamic book I read uses Aerodynamic Center. What is the deal?

Don't say "other" aerodynamics books....the PHAK isn't one. Generally, only pilots talk about the CP, S&C (stability and control) engineers talk about AC.

Does lift act through the CP or the AC?

The conceptual problem here is the idea that lift acts anywhere in particular. Where does your weight act when you stand on the floor? It acts all over the soles of your feet, but we can pretend that it acts at your CG, for some purposes. But that's not really where the weight is in your body. It's a mathematical abstraction that usefully represents reality.

Same thing with CP and AC. Lift is the net pressure differences over the airfoil. It acts everywhere on the airfoil. The CP and AC are just mathematical figments of our imagination that make calculations easier.

The Aerodynamic Center should produce a nose down pitch on its own on a cambered airfoil thus making it stable unless

The AC doesn't produce anything. It's a location. And the existence of a nose down moment isn't stability, it's a trim issue. The essence of stability is what happens when there is an AOA change; does the airfoil tend to correct it?

we put the CG so far rearward that it overcomes the nose down tendency and actually makes a nose up pitch....

Yes, you could, and you could conceivably trim the airplane that way. But it would tend to pitch up with an AOA increase, making it unstable.

But realistically we could place the CG aft of the Aerodynamic Center right?

If you wanted an unstable airplane. Point of nomenclature: the aerodynamics books often point to the AC of the wing and say the CG must be in front of that. Not entirely true. The CG needs to be in front of the AC of the entire airplane, and that will likely be behind the AC of the wing. We call the AC of the entire airplane the neutral point, and its location is affected by the fuselage and the horizontal stabilizer.

[sdfcvoh]

You gonna be in Tucson anytime soon? Everytime I read your posts I want to hire you for a day or two in prep for my CFI checkride!

Originally Posted by tgrayson

Don't say "other" aerodynamics books....the PHAK isn't one. Generally, only pilots talk about the CP, S&C (stability and control) engineers talk about AC.

The conceptual problem here is the idea that lift acts anywhere in particular. Where does your weight act when you stand on the floor? It acts all over the soles of your feet, but we can pretend that it acts at your CG, for some purposes. But that's not really where the weight is in your body. It's a mathematical abstraction that usefully represents reality.

Same thing with CP and AC. Lift is the net pressure differences over the airfoil. It acts everywhere on the airfoil. The CP and AC are just mathematical figments of our imagination that make calculations easier.

The AC doesn't produce anything. It's a location. And the existence of a nose down moment isn't stability, it's a trim issue. The essence of stability is what happens when there is an AOA change; does the airfoil tend to correct it?

Yes, you could, and you could conceivably trim the airplane that way. But it would tend to pitch up with an AOA increase, making it unstable.

If you wanted an unstable airplane. Point of nomenclature: the aerodynamics books often point to the AC of the wing and say the CG must be in front of that. Not entirely true. The CG needs to be in front of the AC of the entire airplane, and that will likely be behind the AC of the wing. We call the AC of the entire airplane the neutral point, and its location is affected by the fuselage and the horizontal stabilizer.

[tgrayson]

Originally Posted by sdfcvoh

hire you for a day or two in prep for my CFI checkride!

Thank you, but you'd probably regret it. The CFI checkride caters mostly to rote memorization; if you try to go in too deep on stuff like this, unless you really have a strong command of the topic, you're likely to get yourself into some deep doo-doo. (Can I say "doo-doo" here?)

[sdfcvoh]

Thats okay. I couldn't pay anyway .

Originally Posted by tgrayson

Thank you, but you'd probably regret it. The CFI checkride caters mostly to rote memorization; if you try to go in too deep on stuff like this, unless you really have a strong command of the topic, you're likely to get yourself into some deep doo-doo. (Can I say "doo-doo" here?)