NEGATIVE AOA???

[PILOT]

Is it possible to have a negative angle of attack? (not angle of incidence)

[Stone Cold]

Sure! How do you think aerobatic airplanes stay aloft during inverted flight?

[meritflyer]

tgrayson in 5..4..3..2..1..

[USMCmech]

Originally Posted by Stone Cold

Sure! How do you think aerobatic airplanes stay aloft during inverted flight?

Negative,

When inverted, you still have a positive AOA. Only what was to top of the wing is now the bottom.

The airplane dosen't know or care what the picture looks like out the window. It only knows the forces acting on it.

[Stone Cold]

How is AOA measured? It does not revert to upside down if you are upside down. There are AOA indicators and they measure it one way, right? Show me one book where it says AOA can be measured backwards if upside down...

One more question...what happens if you push the yoke forward? AOA decreases, right? Well, if you have enough elevator movement and push forward quickly enough, you will go down very fast, right? What happens? AOA decreases. I would venture to say yes you can have a negative AOA even when upright, depending on the airplane.

One last thing...NASA says you can have a negative AOA: http://www.grc.nasa.gov/WWW/K-12/airplane/incline.html in their examples of lift and AOA.

[nosehair]

Yes, push over, and you have negative AoA. I can't say with sure, but I'm guessing when you get around zero g you are negative. I'd bet money you're negative AoA when you are negative g.

[tgrayson]

Originally Posted by Stone Cold

How is AOA measured?

Your original answer was right on, the rest of the discussion is a matter of semantics. Note that the negative vs. positive is somewhat arbitrary. Airfoils have an angle of attack that generates lift in *some* direction on an airplane. We normally define positive AOA as one that produces a force along the vertical axis in the negative direction, which, oddly, is normally "up". The horizontal stablizer normally generates a "down" force, a force in the positive z direction. We generally view this as a negative AOA.

When an aircraft is inverted, it may or may not have a negative AOA. In a normal loop, the AOA stays positive, but in sustained inverted flight, the aircraft will have a negative AOA. (In other words, the lift is being produced in a direction opposite from normal, with respect to he geometry of the airplane.)

[IcarusGR]

The Zero lift angle of attack is -4 Degrees .. so basically you always have a negative angle of attack.

[tgrayson]

Originally Posted by IcarusGR

The Zero lift angle of attack is -4 Degrees .. so basically you always have a negative angle of attack.

Depends on the airfoil, of course. The zero lift AOA is zero on a symmetrical airfoil.

[IcarusGR]

Originally Posted by tgrayson

Depends on the airfoil, of course. The zero lift AOA is zero on a symmetrical airfoil.

Yes I am talking about a asymmetrical airfoil. The decrease in pressure above and below the section are equal and no diffrential exists, unlike the Angle of Attack from 0 degrees to around 8 degrees.

[PILOT]

Thanks!

[seagull]

Originally Posted by IcarusGR

The Zero lift angle of attack is -4 Degrees .. so basically you always have a negative angle of attack.

I have been watching to see if there was going to be some elaboration of this, as it should be clear that the particular AoA in question would only pertain to a particular airfoil section. The above post appears to imply that this is a number that can be used for any asymmetric airfoil.

[tgrayson]

Originally Posted by seagull

I have been watching to see if there was going to be some elaboration of this, as it should be clear that the particular AoA in question would only pertain to a particular airfoil section. The above post appears to imply that this is a number that can be used for any asymmetric airfoil.

You have a sharp eye. I deliberately decided to let that comment slide, since he was conceptually in the ballpark. The zero lift AOA will vary among cambered airfoils; for instance, the zero lift AOA for the NACA 2412, used on the C172, is about -2 degrees. On the NACA 4412, it's about -4. Put in 60 degree flaps and it changes to about -15 degrees.

[IcarusGR]

Sorry guys for not being a little more specific on that. Of course it depends on the type of airfoil, I am talking on a broader scale. I learned all these things while doing my ATPL in Europe (JAA) so we were talking on a general scale, never specific on types of airfoils.